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There’s enough on this planet for everyone’s needs, but not enough for everyone’s greed. Gandhi Exploitation involves living off the land or seas, such that wild animals, plants, and their products are taken for purposes ranging from food to medicines, shelter, and fiber. The term harvesting is often used synonymously with exploitation, though harvesting is more appropriate for farming and aquaculture, where we reap what we sow. In a world that seems intent on liquidating natural resources, overexploitation has become the second most important threat to the survival of the world’s birds, mam- mals, and plants (see Figure 3.8). Many of these species are threatened by subsistence hunting in tropical regions, though others are also threatened in temperate and arc- tic regions by hunting, fishing, and other forms of exploitation. Exploitation is also the third most important driver of freshwater fish extinction events, behind the ef- fects of habitat loss and introduced species (Harrison and Stiassny 1999). Thus, while problems stemming from habitat loss and degradation quite rightly receive a great deal of attention in this book, conservationists must also contend with the specter of the “empty forest” and the “empty sea.” We begin this chapter with a brief historical context of exploitation, which also pro- vides an overview of some of the diverse reasons people have for using wild popula- tions of plants and animals. This is followed by reviews of recent impacts of exploita- tion on both target and nontarget species in a variety of habitats. To understand the responses of populations, we then review the theory behind sustainability. The chap- ter ends with a consideration of the culture clash that exists between people who are concerned with resource management and people who worry about extinction risk. History of, and Motivations for, Exploitation Humans have exploited wild plants and animals since the earliest times, and most contemporary aboriginal societies remain primarily extractive in their daily quest for food, medicines, fiber, and other biotic sources of raw materials to produce a wide 8 Overexploitation John D. Reynolds and Carlos A. Peres

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  • Theres enough on this planet for everyones needs, but not enough for everyonesgreed.


    Exploitation involves living off the land or seas, such that wild animals, plants, andtheir products are taken for purposes ranging from food to medicines, shelter, andfiber. The term harvesting is often used synonymously with exploitation, thoughharvesting is more appropriate for farming and aquaculture, where we reap what wesow.

    In a world that seems intent on liquidating natural resources, overexploitation hasbecome the second most important threat to the survival of the worlds birds, mam-mals, and plants (see Figure 3.8). Many of these species are threatened by subsistencehunting in tropical regions, though others are also threatened in temperate and arc-tic regions by hunting, fishing, and other forms of exploitation. Exploitation is alsothe third most important driver of freshwater fish extinction events, behind the ef-fects of habitat loss and introduced species (Harrison and Stiassny 1999). Thus, whileproblems stemming from habitat loss and degradation quite rightly receive a greatdeal of attention in this book, conservationists must also contend with the specter ofthe empty forest and the empty sea.

    We begin this chapter with a brief historical context of exploitation, which also pro-vides an overview of some of the diverse reasons people have for using wild popula-tions of plants and animals. This is followed by reviews of recent impacts of exploita-tion on both target and nontarget species in a variety of habitats. To understand theresponses of populations, we then review the theory behind sustainability. The chap-ter ends with a consideration of the culture clash that exists between people who areconcerned with resource management and people who worry about extinction risk.

    History of, and Motivations for, ExploitationHumans have exploited wild plants and animals since the earliest times, and mostcontemporary aboriginal societies remain primarily extractive in their daily quest forfood, medicines, fiber, and other biotic sources of raw materials to produce a wide


    John D. Reynolds and Carlos A. Peres

  • range of utilitarian and ornamental artifacts. Modernhunter-gatherers in tropical ecosystems, at varyingstages of transition to agriculture, still exploit a largenumber of plant and animal populations. By definition,these species have been able to coexist with some back-ground level of human exploitation. However, the ar-chaeological and paleontological evidence suggests thatpremodern peoples have been driving other species toextinction since long before the emergence of recordedhistory. Human colonization into previously unexploit-ed islands and continents has often coincided with arapid wave of extinction pulses resulting from overex-ploitation by novel consumers. Mass extinction events oflarge-bodied vertebrates in Europe, parts of Asia, Northand South America, Madagascar, and several archipela-gos have all been attributed to post-Pleistocene humanoverkill (Martin 1984; McKinney 1997). These are rela-tively well documented in the fossil and subfossil record,but many more obscure target species extirpated byhuman exploitation will remain undetected.

    In more recent times, extinction events induced by ex-ploitation have also been common as European settlerswielding superior technology expanded their territorialfrontiers and introduced market and sport hunting. Thedeath of the last Passenger Pigeon (Ecopistes migratorius)in the Cincinnati Zoo in 1914 provided a notorious ex-ample of the impact that humans can have on habitatsand species. The Passenger Pigeon probably was themost numerous bird in the world, with estimates of 15billion individuals (Schorger 1995). Hunting for sportand markets, combined with clearance of their nestingforests (Bucher 1992), had significant impacts on theirnumbers by the mid-1800s, and the last known wildbirds were shot in the Great Lakes region of the U.S. atthe end of the century. After that, it was simply a ques-tion of which of the birds in captivity would survive thelongest, and Martha in Cincinnati won.

    The decimation of the vast bison herds in North Amer-ica followed a similar time-line, but here hunting for meat,skins, or merely recreation, was the sole cause. In the1850s, tens of millions of these animals roamed the GreatPlains in herds exceeding those ever known for any othermega-herbivore, but by the centurys close, the bison wasall but extinct (Dary 1974). Following an expensive popu-lation recovery program, both the plains (Bison bison bison)and wood bison (Bison bison athabascae) are currently clas-sified by the 2003 IUCN Red List as Lower Risk/Conser-vation Dependent, although the wood bison is listed asendangered by the U.S. Endangered Species Act and Ap-pendix II of CITES.

    An example that is lesser known is the extirpation ofmonodominant stands of Pau-Brasil trees (Caesalpinia echi-nata) from eastern Brazil, a source of red dye and hard-wood for carving that gave Brazil its name. Pau-Brasil

    once formed dense clusters along 3,000 km of the Brazil-ian Atlantic forest, and the species sustained the firstmajor trade cycle between the new Portuguese colonyand European markets. It was exploited relentlessly from1500 to 1875, when it finally became economically extinct.Since the advent of synthetic dyes, the species has beenused primarily for the manufacture of high-quality violinbows, and Pau-Brasil specimens are currently largely con-fined to herbaria, arboretums, and a few small protectedareas (Dean 1996). These examples suggest that evensome of the most abundant populations can be driven toextinction in the wild by exploitation. Exploitation of bothlocally common and rare species thus needs to be ade-quately managed if populations are to remain demo-graphically viable in the long term.

    People exploit wild plants and animals for a varietyof reasons, which need to be understood if managementis to be effective. There may be more than food andmoney involved. The recreational importance of huntingand fishing in developed countries is well known. Forexample, hunting creates more than 700,000 jobs in theU.S. and a nationwide economic impact of about $61 bil-lion per year, supporting nearly 1% of the entire civilianlabor force in all sectors of the U.S. economy (LaBarbera2003). Over 20 million hunters in the U.S. spend nearlyhalf a billion days afield in pursuit of game, and feeslevied to game hunters finance a vast acreage of conser-vation areas in North America (Warren 1997).

    The importance of exploitation as a recreational activ-ity is not restricted to wealthy countries. For example, inreef fisheries in Fiji, capture rates are highest with spearsor nets, and while these techniques are used when fishare in short supply, the rest of the time people adopt amore leisurely pace with less efficient hand-lines, usingthe extended time for social and recreational purposes(Jennings and Polunin 1995). Cultural and religious prac-tices are often important. For example, feeding taboosswitch on or off among hunters in tropical forests accord-ing to availability of alternative prey species (Ross 1978;Hames and Vickers 1982). The fate of some endangeredspecies is closely bound to religious practices, as in thecase of the babirusa wild pig, Babyrousa babyrousa, in Su-lawesi. This species is consumed heavily in the Christian-dominated eastern tip of Sulawesi, but rarely consumedover the Muslim-dominated remainder of the island(Clayton et al. 1997).

    Impacts of Exploitation on Target SpeciesMany of the best-known impacts of exploitation on pop-ulations involve cases of direct targeting, whereby hunt-ing, fishing, logging, and related activities are selective,

    250 Chapter 8

  • aimed at a particular species. In this section we presentexamples from major ecosystems in both temperate andtropical areas.

    Tropical terrestrial ecosystemsTIMBER EXTRACTION Tropical deforestation is driven pri-marily by frontier expansion of subsistence agricultureand large-scale development programs involving im-proved infrastructure and access. However, animal andplant population declines are typically preceded by hunt-ing and logging activity well before the coup de grce ofcomplete deforestation is delivered. Approximately 5.8million ha of tropical forests are logged each year (Foodand Agriculture Organization 1999; Achard et al. 2002).Tropical forests account for about 25% of the global in-dustrial wood production, worth U.S.$400 billion orabout 2% of the global gross domestic product (WorldCommission of Forests and Sustainable Development1998). Much of this logging activity opens up new fron-tiers to wildlife and nontimber resource exploitation, andcatalyzes the transition into a landscape dominated byslash-and-burn and large-scale agriculture.

    Few studies have examined the impacts of selectivelogging on commercially valuable timber species, andcomparisons among studies are limited because theyoften fail to employ comparable methods that are re-ported adequately. The best case studies come from themost valuable timber species that have already declinedin much of their natural geographic distributions. For in-stance, the highly selective logging of broadleaf ma-hogany (Swietenia macrophylla) is driven by the extraor-dinarily high value of this species on internationalmarkets. These conditions make it lucrative for loggersto pay royalty payments as well as high transportationcosts of reaching remote wilderness areas. Selective log-ging of mahogany and other prime timber species affectsthe forest by creating canopy gaps and causing severecollateral damage due to construction of roads and skidtrails, particularly in the case of mechanized operations.

    One of the major obstacles to implementing a sustain-able forestry sector in tropical countries is the lack of fi-nancial incentives for producers to limit offtakes to sus-tainable levels and invest in regeneration (see Essay 8.1by Steve Ball for an example involving East Africanblackwood). Economic logic dictates that trees should befelled whenever their rate of timber volume incrementdrops below the prevailing interest rate (Pearce 1990).Postponing exploitation beyond this point would incuran opportunity cost because profits from logging couldbe reinvested at a higher rate elsewhere (see Chapter 5for a full discussion of economic discounting). This isparticularly the case where land tenure systems are un-stable, and where there are no disincentives against min-ing the resource capital at one site and moving else-

    where once this is depleted. This is clearly shown in amahogany study in Bolivia where the smallest treesfelled are about 40 cm in diameter, well below the legalminimum size (Gullison 1998). At this size, trees are in-creasing in volume at about 4% per year, whereas realmahogany price increases have averaged only 1%, sothat a 40-cm mahogany tree increases in value at about5% annually, slowing down as the tree becomes larger(Figure 8.1). In contrast, real interest rates in Bolivia inthe mid-1990s averaged 17%, creating a strong econom-ic incentive to liquidate all trees with any value regard-less of resource ownership. The challenges and prospectsof sustainable forestry in the Tropics are discussed inmore detail in a case study by Pinard and colleagues inCase Study 6.2

    SUBSISTENCE HUNTING Humans have been hunting wildlifein tropical forests for more than 100,000 years, but con-sumption has greatly increased over the last few decades.Exploitation of bushmeat (the meat from wild animals) bytropical forest dwellers has increased due to larger numbersof consumers, changes in hunting technology, scarcity of al-ternative sources of protein, and because it is often a pre-ferred food. Recent estimates of annual hunting rates are25,850 tons of wild meat in Sarawak (Bennett 2002),73,890181,161 tons in the Brazilian Amazon (Peres 2000a),and 13.7 million tons in Central Africa (Fa et al. 2001).

    Overexploitation 251






    00 40 80 120 160





    DBH (cm)

    Minimum harvestablesize with value

    Bolivian real interest rates



    Figure 8.1 Productivity of Bolivian mahogany trees as afunction of size expressed as tree diameter at breast height(DBH). Financial productivity is equivalent to size incrementscombined with a 1% real increase in price. Trees equal to orsmaller than 40 cm in DBH have no commercial value, but atthis size they increase at 5% in value per year. To maximizetheir profits, loggers should fell trees at a size when the finan-cial productivity drops below the prevailing interest rate,which in Bolivia was 17% in 19891994. The discrepancy be-tween the interest rates and tree growth rate provides astrong incentive to overexploit. (Modified from Gullison1998.)


  • Hunting rates are already unsustainably high across largetracts of tropical forests, averaging six times the maximumsustainable rate in Central Africa, for example (Figure 8.2).Consumption by both rural and urban communities isoften at the end of supply chains that are hundreds of kilo-meters long, and that extend into many previously inacces-sible areas (Milner-Gulland and Bennett 2003). The rapidacceleration in tropical forest defaunation due to unsus-

    tainable hunting occurred initially in Asia, is now sweepingthrough Africa, and is likely to move to even the remotestparts of the Neotropics. This pattern reflects human demo-graphics in different continents: There are 522 people perkm2 of remaining forest in South and Southeast Asia, 99 inCentral-West Africa, and 46 in Latin America.

    Subsistence game hunting affects the structure oftropical forest mammal assemblages, as revealed by vil-

    252 Chapter 8

    East African Blackwood ExploitationSteve M. Ball, Mpingo Conservation Project

    n The Miombo woodlands, whichcover a large swath of southern Africa,are a highly species-diverse ecosystemand the mammalian fauna requireslarge ranges. Thus, large contiguousareas of land must be protected for con-servation of biodiversity to be effective.Even if the protected area system werewell maintained, fragmentation wouldhave a significant impact on theintegrity of the conserved areas. Com-munity-based conservation could fillthe gap, providing buffer zones andcorridors between reserves, but thecommunities need adequate incentive.The East African blackwood (Dalbergiamelanoxylon) has the potential to do this.

    The tree gets its name from its beauti-ful dark heartwood, which is inky blackin the best-quality timber. The wood isused in the West to make woodwindinstruments, especially clarinets andoboes, and locally it is a prime choice forwood carvers. It is the most valuabletimber growing in the Miombo wood-lands of southern Tanzania (Figure A).Both the species and its habitat areunder threat from commercial exploita-tion and inward migration of people asa result of recent road improvements.The blackwoods high economic value,its status as Tanzanias national tree, andits cultural significance both in the Westand in Tanzania make it an ideal flag-ship species to justify conservation ofthe habitat (Ball 2004).

    Current exploitation levels are prob-ably unsustainable, with at least 50% oftrees being felled illegally without alicense. Local forestry officials whodepend on logging companies fortransport into the field do not have suf-ficient resources to enforce existing reg-

    ulations. Villagers generally knowwhen illegal logging is taking place andcould apprehend suspects, but theyhave no incentive to do so because allhardwood timbers remain the exclusiveproperty of the government. Intentionalfires are also thought to prevent regen-eration and facilitate heart-rot in adulttrees. Plantations are not an optionbecause of their high costs, uncertainreturn, and long rotation time (black-wood is estimated to take 70100 yearsto reach timber size).

    Community-based forest manage-ment is now a major theme of conser-

    vation and rural development activitiesin Tanzania. In largely deforested areasthese can succeed by focusing on fire-wood and water-catchment issues, butthis approach is unlikely to succeed insoutheastern Tanzania where forestcover currently exceeds 70%. Howevernew laws allow for villages to takeownership of local forest resources ifthey can produce a viable managementplan for the forest. This includes tenurerights over even high-value timbertrees, such as East African blackwood,which were previously government-reserved species. License fees for fellingtimber could significantly increase vil-lage income, giving local people anincentive to look after their naturalresource assets. Once schemes are wellestablished, rural communities coulduse the promise of future income as col-lateral against micro-finance loans toincrease the up-front benefits.

    Simple economic analysis suggeststhat sustainable, community-basedmanagement of East African blackwoodoffers a powerful argument to justifyconservation of the forest on publiclands and to complement the protectedareas system, providing income inbuffer zones and corridors betweenreserves. However, it will be importantto work with the suppliers of musicalinstrument manufacturers, and avoid asignificant increase to their costs. Ifthese challenges can be met, blackwoodcould be used as an economic key toconserveas managed areaslargetracts of Miombo woodland. Thiswould not happen through a system offorest reserves, but rather with a moreinclusive strategy of sustainableexploitation of the regions naturalresources. n

    ESSAY 8.1

    Figure A A mpingo, an African black-wood tree. (Photograph courtesy of SteveM. Ball.)

    fpo-hi res

  • lage-based kill profiles in Neotropical (Jerozolimski andPeres 2003) and African forests (Fa and Peres 2001). Thiscan be seen in the composition of residual game stocks atforest sites subjected to varying degrees of hunting pres-sure where overhunting often results in faunal biomasscollapses, mainly through declines and local extinctionsof large-bodied mammals (Bodmer 1995; Peres 2000a).

    NONTIMBER FOREST PRODUCTS Nontimber forest products(NTFP) are biological resources other than timber thatare taken from either natural or managed forests (Peters1994). Examples of exploited products (of whole plantsor plant parts) include fruits, nuts, oil seeds, latexes,resins, gums, medicinal plants, spices, dyes, ornamentalplants, and raw materials and fiber such as Desmoncusclimbing palms, bamboo, and rattan. Forest wildlife andbushmeat can also be considered as a prime NTFP, butfor a number of reasons they will be treated as a distinctcategory.

    The socio-economic importance of NTFP extraction toindigenous peoples cannot be underestimated. Manyethnobotanical studies have catalogued the wide variety

    of useful plants or plant parts used by different aborig-ine groups throughout the Tropics. For example, theWaimiri-Atroari Indians of central Brazilian Amazoniamake use of 79% of the tree species occurring in a single1-ha terra firme (upland) forest plot (Milliken et al. 1992),and out of the 16,000 species of angiosperms in India,6000 are used for Ayurvedic or other traditional medi-cine, and over 3000 are officially recognized by the gov-ernment for their medicinal uses.

    Exploitation of NTFPs often involves the systematicremoval of reproductive units from the population, butthe level of mortality in the exploited population de-pends on the method of extraction and whether vitalparts are removed. Traditional NTFP extractive practices,for either subsistence or commercial purposes, are oftenconsidered to comprise a desirable, low-impact econom-ic activity in tropical forests compared to alternativeforms of land use involving structural disturbance, suchas selective logging and shifting agriculture (Plotkin andFamalore 1992). As such, the exploitation of NTFPs isusually assumed to be sustainable and is viewed as apromising compromise between the requirements of bio-diversity conservation and those of extractive communi-ties under varying degrees of market integration.

    A recent study of Brazil nuts (Bertholletia excelsa) ques-tions the standard assumptions about sustainability ofNTFPs (Peres et al. 2003). Brazil nuts are the base of amajor extractive industry supporting over half of the trib-al and nontribal rural population in many parts of theBrazilian, Peruvian, and Bolivian Amazonia, either fortheir direct subsistence value or as a source of income.This wild seed crop is firmly established in domestic andexport markets, has a history of over 150 years of com-mercial exploitation, and is one of the most valuable non-timber extractive industries in tropical forests anywhere.Brazil nuts have been widely held as a prime example ofa sustainably extracted NTFP, yet the persistent collectionof B. excelsa seeds has severely undermined the patternsof seedling recruitment of Brazil nut trees. This has dras-tically affected the age structure of many natural popula-tions to the point where persistently overexploited standswill succumb to a process of senescence and demograph-ic collapse, threatening this cornerstone of the Amazon-ian extractive economy (Peres et al. 2003; Figure 8.3).Nevertheless, the concept of sustainable NTFP extractionis now so deeply entrenched into national resource usepolicy that extractive reserves (or their functional ana-logues) have become one of the fastest growing cate-gories of protected areas in tropical forests (Fagan et al.2005). The implicit assumption is that traditional meth-ods of NTFP exploitation have little or no impact on for-est ecosystems and tend to be sustainable because theyhave been practiced over many generations. However,virtually any type of NTFP exploitation in tropical forests

    Overexploitation 253

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    Figure 8.2 Hunting rates are unsustainably high across largetracts of tropical forests as seen in the relationship betweentotal extraction and total production of game meat through-out the Congo and Amazon basin (solid and open symbols,respectively) by mammalian taxa. The solid line indicateswhere extraction equals production; the dashed line indicatesexploitation levels at 20% of production, considered to be sustainable for long-lived taxa. Taxon symbols are as follows:ungulates (squares), primates (triangles), carnivores (circles), rodents (inverse triangles), and other taxa (diamonds). (Modi-fied from Fa et al. 2002.)


  • will have an ecological impact. The exact extent and mag-nitude of this impact depends on the accessibility of theresource stock, the floristic composition of the forest, thenature and intensity of extraction, and the particularspecies or plant part under exploitation (Peres and Lake2003).

    Few studies have quantitatively assessed the demo-graphic viability of nontimber plant products. A boom inthe use of homeopathic remedies sustained by over-col-lection of therapeutic and aromatic plants is threatening atleast 150 species of European wild plants and drivingmany populations to extinction (TRAFFIC 1998). Com-mercial exploitation of the pau-rosa or rosewood tree(Aniba rosaeodora), which contains linalol, a key ingredientin fine perfumes, involves a destructive technique that al-most invariably kills the tree. This species has conse-quently been extirpated from virtually its entire geo-graphic range in Brazilian Amazonia (Mitja and Lescure2000). Chanel N5 and other perfumes made with pau-rosa fragrance gained an enormous international marketdecades ago after being popularized by Hollywood starslike Marilyn Monroe, but the number of processing plantsin Brazil fell from 103 in 1966 to fewer than 20 in 1986, dueto the dwindling resource base. Yet French perfume con-

    noisseurs have been reluctant to replace the natural pau-rosa fragrance by synthetic substitutes, and the last unex-ploited populations of pau-rosa remain threatened. Thesame could be argued for a number of NTFPs for whichthe exploitation by destructive practices involves a lethalinsult to whole reproductive individuals, such as the ex-traction of fruits and palm-hearts in many arborescentpalms. For example, in the Iquitos region of Peru, thefruits of Mauritia flexuosa palm trees, a long-lived forestemergent, are often collected only once by felling wholereproductive adults (Vasquez and Gentry 1998).

    Enthusiasm for NTFPs in community developmentand conservation partly results from unrealistic studies re-porting their high economic value. For example, Peters etal. (1989) reported that the net present value of fruit andlatex extraction in the Rio Nanay of the Peruvian Amazonwas $6,330/ha, assuming a 5% discount rate and that 25%of the crop was not taken. This is in sharp contrast with a30-month study in Honduras that measured the localvalue of foods, construction and craft materials, and med-icines extracted from the forest by 32 Indian households(Godoy et al. 2000). The combined value of consumptionand sale of forest goods ranged from U.S.$18 toU.S.$24 per hectare per year, at the lower end of previousestimates (between U.S.$49 and U.S.$1,089 per hectare peryear). NTFP extraction thus cannot be seen as a solutionfor rural development and in many studies the potentialvalue of NTFPs is exaggerated by the assumption of un-realistically high discount rates, unlimited market de-mands, availability of transportation facilities, and ab-sence of product substitution.

    What, then, is the impact of NTFP extraction on thedynamics of natural populations? How does the impactvary with the life history of plants and animals? Are cur-rent extraction rates truly sustainable? These are allquestions that could steer a future research agenda butthe demographic viability of NTFP populations will de-pend on the species ability to recruit new adults eithercontinuously or in sporadic pulses while being subject-ed to repeated exploitation.

    Temperate terrestrial ecosystemsFORESTRY According to one assessment, only 22% of theworlds original forest cover remains in large, relativelynatural ecosystems, or so-called frontier forests(Bryant et al. 1997). These remaining frontier forests arepredominantly classed as either boreal (48%) or tropical(44%), with only a small fraction (3%) remaining in thetemperate zone. However, much like tropical forests,most of the frontier forests in mid to high latitude re-gions are also increasingly threatened by logging andagricultural clearing (World Resources Institute 1998).

    The overall impact of different sources of structuraldisturbance generated by modern forestry in the temper-

    254 Chapter 8

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    50 5




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    Figure 8.3 Relationships between historical levels of Brazilnut collection and mean tree size, expressed in terms of DBH,cm (A); and percentage of juvenile trees (B) in different popu-lations. Numbers above bars indicate the number of popula-tions studied throughout the Brazilian, Bolivian, and PeruvianAmazon. (Modified from Peres et al. 2003.)


  • ate and boreal zones may depend on the spatial scale andintensity of disturbance, the history of analogous formsof natural disturbance, the groups of organisms consid-ered, and whether forest ecosystems are left to recoverover sufficiently long intervals following a disturbanceevent such as commercial thinning or clearcutting. Thepatterns of landscape-scale human disturbance in theseforests can be widely variable in intensity, duration, andperiodicity, and are often mediated by economic incen-tives to cut timber from high-biomass old-growth forests,rather than natural regrowth or fast-growing tree planta-tions on a long rotation cycle.

    The expanding frontier of commercial forestry intohitherto remote, roadless wildlands often results in highlevels of forest conversion and fragmentation of remain-ing stands, with significant impacts to forest wildlife. A re-view of 50 studies in Canadian boreal forests on the effectsof postlogging silviculture on vertebrate wildlife conclud-ed that large impacts are universal when native forests arereplaced by even-aged stands of rapidly-growing nonna-tive tree species (Thompson et al. 2003). Loss of specialstructures, such as snags containing nest cavities and largedecaying woody debris, is particularly important in thedecline of forest species depending on those structures(McComb and Lindenmeyer 1999). Unlogged borealconiferous forests within protected areas in Finland,where population trends of land birds have been excep-tionally well documented, are extremely important to thenative old-growth avifauna, such as the Siberian Jay(Perisoreus infaustus) and the hole-nesting Three-toedWoodpecker (Picoides tridactylus), which have declined inareas under timber extraction (Virkkala et al. 1994). In-deed, logging is often considered to be the most importantthreat to species in boreal forests (Hansen et al. 1991; Im-beau et al. 2001). Some 50% of the red-listed Fennoscandi-an species are threatened because of forestry (Berg et al.1994). Forests actively managed for biodiversity couldsupport 100% of the species occurring in Washingtonstate, whereas timber management on a 50-year rotationat the landscape level could support a maximum of 87%and a mode of 64% of the species potentially occurring inforests (Carey et al. 1996).

    In summary, many of the detrimental impacts of for-est management on biodiversity are associated with thelarge-scale structural simplification of the ecosystem atall forest levels, age-class truncation, and other conse-quences of intensive forest management intended to in-crease the yield of the desired forest component.

    A reduction of this impact often involves modernprinciples of ecological forestry (Seymour and Hunter1999), which may include a partial removal of the stand,often in a patch mosaic using relatively small clear-cutsizes so that each stage of forest development is repre-sented somewhere in the landscape. As long as most

    stages are present at any one time, the requirements ofnearly all species can be met somewhere (although seeexamples in Chapter 7 on the negative effects of habitatfragmentation). Furthermore, clear-cutting may only beused when and where absolutely necessary. Other ex-ploitation methods include thinning and partial cutting,each with different effects on wildlife habitat. Land-scape-level decisions to maximize the biodiversity valueof the forest may include allocating different portions ofthe total area to successively longer rotations, rangingfrom 50 years for short-lived species up to 300 years forlate-successional habitat.

    HUNTING Large mammals, small game, and waterfowlare also major targets in temperate countries, whererecreational hunting can be a popular sport across alarge section of the constituents. Annual culls of largeungulates have been rather successful in North America,as shown by population trends, in both controlling pop-ulations and stocking the freezer of the average hunter.White-tailed deer (Odocoileus virginianus), the most com-mon and widespread of wild ungulates in the U.S., in-creased from fewer than 500,000 around the turn of lastcentury, to about 30 million today. Deer are among themost heavily hunted species in the U.S., with some 5million killed by over 10 million hunters every year, gen-erating about 20 billion dollars in hunting-related rev-enue in 2001 (U.S. Fish and Wildlife Service 2002). InTexas alone, it is estimated that the white-tailed deerpopulation numbered more than 3.1 million in 1991 inspite of heavy hunting pressure, and approximately474,000 animals were shot by hunters in that year. Resi-dent birds and small to mid sized mammals frequentlyhunted or trapped for sport are often referred to assmall game. These may include game birds such asquail, pheasant, partridge and grouse, rodents and lago-morphs such as squirrels, rabbits, and hares, and evencarnivores such as coyotes and raccoons. The effects ofdifferent hunting strategies upon populations of themost popular game bird in the U.S., the bobwhite quail,have been simulated showing that maximum yields canbe sustained from annual capture rates of about 55%(Roseberry 1979). Hunting at such high rates, however,leaves little room for error in calculated bag quotas be-cause it depresses the following spring populations by53% below unexploited levels.

    Ducks, geese, and swans are gregarious and often mi-gratory species that also attract enormous attention fromgame hunters. As such, the establishment of annual wa-terfowl hunting regulations is a complex procedureshared by various governmental levels and private or-ganizations, involving thousands of wildlife biologistsand habitat managers under the jurisdiction of wildlifeagencies. For example, retrieved duck and goose har-

    Overexploitation 255

  • vests during 2001 were 19.4 million in the U.S. andCanada, down by 6.6% on the total numbers bagged inthe previous year (Martin and Padding 2002). In the U.S.alone, in 2001, this involved annual sales of 1.66 millionfederal duck stamps to 1.59 million hunters who collec-tively spent nearly 15 million hunter-days in pursuit ofwaterfowl. Needless to say, seasonal hunting license feesand leases of private hunting areas generate welcomecash used for both population management and for pro-tection of habitats against other forms of land use.

    The relatively orderly use of wildlife in North Americais not necessarily the rule for all temperate regions. Illegaluse and commercialization of wildlife continue to generatea substantial clandestine trafficeven for species that arefully protected on paperin several mid- to high-latitudecountries ranging from Chile to Russia. The problem usu-ally lies not in the regulations, which are often already ex-tensive and strict, but rather in lack of law enforcementthat is often attributed to weak institutional capacity.

    Aquatic ecosystems MARINE ECOSYSTEMS The impacts of marine fisheries ontarget species are well known. Roughly three-quarters ofthe worlds fish stocks are considered to be fully fished oroverexploited (Food and Agriculture Organization 2002).Since the 1990s global catches have leveled off for the firsttime in human history, despite continuing advances incapture technology (Figure 8.4). Exploitation of aquaticanimals now seems to be following the pattern that oc-curred in many terrestrial ecosystems long ago, with re-liance on hunting of wild animals being supplementedby the captive rearing of domestic stock, through aqua-culture (see Figure 8.4). No one should delude them-selves into thinking that the move toward fish farming

    will save wild stocks, as many of the fish that are reared,such as salmon and trout, are fed with meal derived fromwild fish! Indeed, stocks of many wild fishes have con-tinued to decline. Recent surveys of 232 stocks showed amedian decline of 83% over the past 25 years (Hutchings2000, 2001; Hutchings and Reynolds 2004). A stock of At-lantic cod (Gadus morhua) off eastern Canada, which onceseemed absolutely limitless, has undergone a decline of99.9%, since the 1960s, leading to a designation of en-dangered under the Canadian Species at Risk Act (Com-mittee on the Status of Endangered Wildlife in Canada2003). This decision is not an isolated case; several otherspecies of commercially exploited fishes have been listedrecently as threatened with extinction (Musick et al. 2000;IUCN 2003). These developments show that some fish-eries concerns are moving from the traditional focus onsustainability into the realm of extinction risk (Reynoldset al. 2002; Dulvy et al. 2003).

    Concerns about extinction risk in the sea are most acutefor those targeted species that have combinations of traitsthat make them most susceptible to capture, and biologi-cally least productive (Reynolds et al. 2002; Dulvy et al.2003). For example, the marine species that are most likelyto be targeted are those that occupy shallow waters acces-sible to fishing gear, those that form dense shoals in pre-dictable places, or those that are most valuable. If a specieshas one or more of these characteristics in combinationwith long generation times, the results can be hazardousto the health of the population. The Chinese bahaba (Ba-haba taipingensis) is a fish that meets many of these criteria(Sadovy and Cheung 2003). It is a huge species of croaker(family Scienidae), which may exceed 2 m in length (Fig-ure 8.5). It has traditionally been caught along its coastalrange from Shanghai to Hong Kong. This species has been

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    1977 19













    Figure 8.4 Trends in global fisheries.The gray portion of the bars indicatecapture fisheries and the black portionof the bars indicate aquaculture. (FromHart and Reynolds 2002, based on datafrom FAO 2000; corrected for misre-porting of capture fisheries of China byWatson and Pauly 2001.)


  • popular for the medicinal properties of its swim bladder.Its numbers have declined to 1% of its abundance in the1960s. During this time its price skyrocketed to the pointwhere swim bladders in 2001 were worth seven times theprice of gold (Sadovy and Cheung 2003). Scientists do nothave enough information to say much about its popula-tion demography, but its large size invariably implies thatit will take many years to reach maturity. This biologicalfeature, combined with such strong economic incentivesfor people to catch it, have conspired to push the speciesto the edge of extinction.

    The same basic rules of vulnerability for fish speciesapply to other taxa. For example, species of abalonealong the Pacific coast of North America often occur inshallow waters, where they are readily accessible todivers (Figure 8.6). There has been serial depletion ofthese species, beginning with the most valuable andthen moving on to less valuable ones (Tegner et al. 1996).This is reminiscent of the pattern that occurred with the

    great whales before the International Whaling Commis-sions moratorium on commercial whaling in 1986. Thewhite abalone (Haliotus sorenseni) has been hit particu-larly hard. Ranging from California to Baja California,white abalone densities in some locations during theearly 1970s were estimated at 1,0005,000 per acre (Laf-ferty 2003). Commercial and recreational capture re-duced its numbers to less than one per acre by the 1990s.In May 2001 this species became the first marine molluscto come under the U.S. Endangered Species Act, with anestimated population in the wild of only about 3,000 in-dividuals. Most remaining animals are restricted to deepwaters beyond the reach of fishing (Lafferty et al. 2003).While the abalones value and accessibility provided themotive and the means for overexploitation, it has alsosuffered from an additional problem: the Allee effect.This is the phenomenon whereby per capita fitness de-clines as a population becomes smaller (see Chapter 12).The ensuing feedback can cause populations to becomemore vulnerable as they become increasingly rare, po-tentially spiraling to extinction. In the case of abalone,the Allee effect arises through the need for individuals tobe within 1 m of each other for a males sperm to reach afemale. Biologists have had some success with artificialfertilization in captivity, with the intention of rearingabalones for release into the wild.

    FRESHWATER ECOSYSTEMS Many freshwater taxa are sub-ject to exploitation for food and a variety of additionalproducts. In 2000, the global estimate for all inland cap-ture fisheries was estimated at 8.8 million tons (FAO

    Overexploitation 257

    Figure 8.5 Chinese bahaba (Bahaba taipingensis) caught as anincidental by-catch by a trawler west of Hong Kong. (Photocourtesy of Cheng Tai-sing.)

    fpo-hi res

    Figure 8.6 White abalone (Haliotis sorenseni) underside.(Photograph courtesy of K. D. Lafferty.)

    fpo-lo res

  • 2002). This figure is considerably less than the estimatefor inland aquaculture (22.4 million tons). In many tem-perate countries, recreational fishing in fresh waters is amajor past-time, yielding an estimated 2 million tons offish (Cowx 2002). In 1996 it was estimated that 35 millionpeople in the U.S. spent 514 million angler-days fishing,spending U.S. $38 billion in the process (U.S. Fish andWildlife Service 1997). Among 22 European countries,the number of anglers was estimated at 21.3 million(Cowx 1998).

    The worlds salmon species illustrate the vulnerabili-ty of fish species that spend all or part of their life cycle infreshwater. Populations of four of the seven species ofeastern Pacific salmon and trout in the genus Oncorhyn-cus are currently listed under the U.S. EndangeredSpecies Act. Over-fishing has contributed to severe de-clines in many populations, often in combination withhabitat loss through construction of dams that block theirmigrations, as well as degradation of spawning streamsdue to forest clearance and water abstraction (Lynch et al.2002). The plight of salmon species is particularly sober-ing when one considers the enormous amount of atten-tion that has been paid to these populations by scientistsand conservationists, as well as their high economic andcultural significance. The most recent response of the U.S.government at the time of writing (May 2004) has beenthe announcement of the intention to count hatchery-re-leased fish as wild (Kaiser 2004). Thus, the hundreds ofmillions of fish that are reared in captivity each year willelevate the population counts of many of the 27 popula-tions of Pacific salmon and cutthroat trout that are en-dangered. This can lead to their removal from protectionunder the Endangered Species Act and presumably, thediscontinuation of many habitat restoration programs.The decision ignores the fact that hatchery fish becomedomesticated rapidly, showing genetic and phenotypicdivergence from wild stocks (e.g., Fleming et al. 1994;Heath et al. 2003). Furthermore, there is little evidencethat hatcheries enhance the viability of wild stocks(Myers et al. 2004). Yet the regional director of the Na-tional Marine Fisheries Service, which has jurisdictionover salmon management, has declared that Just as nat-ural habitat provides a place for fish to spawn and to rear,also hatcheries can do that... Presumably, zoos can alsobe considered natural habitats.

    Fish are not the only taxa exploited from fresh waters.During the late 1800s and early 1900s millions of fresh-water mussels (family Unionidae and Margaritiferae)were collected from freshwaters of Canada and the U.S.,primarily to make buttons from their shells (Williams etal. 1993; Helfrich et al. 2003; Williams and Neves 2004).This exploitation was rarely sustainable, due to the slowgeneration times of these species. Mussels were savedfrom major impacts of exploitation by the switch to plas-

    tics for manufacturing buttons during the 1940s. How-ever, today several million kilograms of mussels are stillexported annually to Asia, where small beads are madefrom their shells and inserted into other bivalves for theproduction of pearls. Unfortunately, mussels now facemuch more critical threats from the usual suspects infreshwater conservation: pollution, damming, dredging,channelization, siltation, and competition with nonna-tive bivalves such as the Asian clam (Corbicula fluminea)and the zebra mussel (Dreissina polymorpha) (Strayer etal. 2004). These problems have led to 72% of the 297species that occur in the U.S. being considered endan-gered, threatened, or of special concern, including 21species that are extinct (Williams and Neves 2004).

    Impacts of Exploitation on NontargetSpecies and Ecosystems Most hunting, fishing, and collecting activities affect notonly the species that are the primary targets, but alsothose that are taken accidentally or opportunistically.Furthermore, exploitation may cause physical damageto the environment, and also may have ramifications forother species through cascading interactions, phaseshifts in the structure of the ecosystem, and changes infood webs. Here we discuss a few examples of how ex-tractive activities targeted to one or a few species candrastically affect the structure and functioning of terres-trial and aquatic ecosystems.

    Tropical terrestrial ecosystemsLOGGING AND FOREST FLAMMABILITY Even logging target-ed to a single timber species can puncture the forestcanopy and increase the density of treefall gaps. This in-crease can trigger major ecological changes by increasinglight and creating a warmer and drier microclimate in theunderstory, which thereby affects the dynamics of plantregeneration, and increases forest susceptibility to firedisturbance. In fact, even highly selective logging opera-tions with modest levels of incidental damage to nontar-get trees can generate enough structural disturbance togreatly augment understory desiccation and dry fuelloads, thereby breaching the forest flammability thresh-old (Holdsworth and Uhl 1999; Nepstad et al. 1999). Anysource of ignition during subsequent severe droughts caninitiate extensive ground fires that will dramatically re-duce the functional and biodiversity value of previouslyunburned tropical forests (Barlow and Peres 2004). Sur-face wildfires that are at least partly induced by loggingdisturbance currently threaten millions of hectares ofAmazonian, Mesoamerican, and Southeast Asian forests(Cochrane 2001; Siegert et al. 2001). Despite these unde-sirable direct and indirect effects, large-scale mechanized

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  • logging operations continue unchecked in many season-ally dry tropical forest regions.

    HUNTING AND LOSS OF SEED DISPERSAL SERVICES Successfulseedling recruitment in many flowering plants dependson seed dispersal services provided by large-bodied fru-givores (Howe 1984). In tropical forests, the proportionof plant species with an endozoochorous dispersalmode (bearing seeds dispersed by an animals digestivetract) is often more than 90% (Peres and Roosmalen2002). Undispersed seeds simply fall to the ground un-derneath the parents canopy and have a low survivalprobability (Augspurger 1984; Chapman and Chapman1996). For example, 99.96% of Virola surinamensis seedsthat drop under the parent are killed within 12 weeks(Howe et al. 1985). Many studies have shown lowerseed mortality rates caused by fungal attack or verte-brate and invertebrate seed predators are lower atgreater distances from parents. However, there havebeen only a few studies that have examined the effectsof removing seed dispersers are lower on the demogra-phy of gut-dispersed plants. Wright et al. (2000) ex-plored how hunting alters seed dispersal, seed preda-tion, and seedling recruitment for two palms (Attaleabutyraceae and Astrocaryum standleyanum) in Panama.They found that where hunters had not reduced mam-mal numbers, most seeds were dispersed away from theparent palms, but were subsequently eaten by rodents.Where hunters had reduced mammal abundance, fewseeds were dispersed, but these tended to escape rodentpredation. Thus, seedling density increased by 35-foldat heavily hunted sites compared to unhunted sites. Incontrast, Asquith et al. (1999) demonstrated that recruit-ment of Hymenaea courbaril required scatterhoarding oftheir large seeds by agoutis (Dasyprocta sp.), and re-cruitment rates of many plant species that produce verylarge seeds cached by rodents is likely to be very low inheavily hunted areas.

    Studies examining seedling recruitment under differ-ent levels of hunting pressure (or abundance of large-bod-ied seed dispersers) reveal very different outcomes. At thecommunity level, seedling density in overhunted forestscan be indistinguishable, greater, or less than that in theundisturbed forests (Dirzo and Miranda 1991; Chapmanand Onderdonk 1998; Wright et al. 2000), but the conse-quences of increased hunting pressure to plant regenera-tion is likely to depend on the target species. In persist-ently hunted Amazonian forests, where large primates areeither driven to local extinction or severely reduced innumbers, the probability of effective dispersal of large-seeded plants ingested primarily by these frugivores candecline by more than 60% compared to nonhunted forests(Peres and Roosmalen 2002). However, more conclusiveevidence is required before the importance of the loss or

    reduction of effective animal dispersal services can beproperly understood for different plant species.

    Temperate ecosystemsHigher order interactions resulting from selective ex-tinction or severe population declines of large mammalsthat play an important role as landscapers at the ecosys-tem scale have been documented in the temperate zone.Beavers (Castor canadensis) and their Eurasian congener(Castor fiber) are prime examples of ecosystem engineers,which can be defined as organisms that have the poten-tial to dramatically alter the structure and function ofecosystems at large spatial scales. Large ponds createdby the labor-intensive stream-damming activity ofbeaver colonies create large-scale semi-permanent flooddisturbance that drastically changes the structure andpatch dynamics of wetlands (Naiman et al. 1986; Wrightet al. 2002). Beavers were once locally abundant in manyparts of North America and Eurasia but their popula-tions plummeted due to the pelt trade and habitat con-version in the nineteenth and early twentieth century,radically changing wetland ecosystems. No one knowsthe precise extent of these changes but it is clear thatbeaver damming activity has profound effects on thebiogeochemistry of wetland systems (Naiman et al.1994), the dynamics of shifting successional mosaics ofaquatic patches (Johnston and Naiman 1990), and ulti-mately the population dynamics of other wetlandspecies including waterfowl (McCall et al. 1996). Beaversare now making a gradual comeback in many parts ofNorth America (although they were exterminated by1700 and are yet to be reintroduced in many parts of Eu-rope including Britain) but they often continue to be per-ceived as a nuisance requiring controversial measures ofpopulation control.

    In temperate terrestrial ecosystems, large mammalsthat once had profound large-scale effects on the struc-ture of plant communities but have been hunted to nearextinction in historic times include bison, bears, andwolves. Joel Berger and colleagues (2001) demonstrateda cascade of ecological events that were triggered by thelocal extinction of grizzly bears (Ursus arctos) and wolves(Canis lupus) from the Yellowstone ecosystem. These in-clude large increases in the population of a riparian-de-pendent herbivore, the moose (Alces alces), the subse-quent alteration of riparian vegetation structure anddensity by ungulate herbivory, and the coincident re-duction of Neotropical migrant birds in the affected wil-low communities, including riparian specialists such asGray Catbirds (Dumetella carolinensis) and MacGillivraysWarblers (Oporornis tolmiei).

    Where large predators (wolves, bears) have been re-moved through predator control programs, or otherforms of direct persecution, ungulate populations can

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  • greatly increase in size, with subsequent impacts onplant communities. White-tailed deer in the U.S. andreindeer in Europe were once controlled by wolves andbear, and despite annual harvests, their numbers arelarger than they were when controlled by native preda-tors. The result is that their larger populations may over-browse forests, which can cause extensive damage insome places (e.g., Vre et al. 1996 and Horsley et al.2003). Particularly in areas where hunting is prohibited,deer populations can become so large that most tree andherb seedlings are consumed, which may lead to achange in patterns of forest succession (e.g., Horsley etal. 2003 and Pedersen and Wallis 2004), or to decline ofrare species (e.g., Gregg 2004). Plant populations can beslow to recover from episodes of excessive herbivory.For example, a population of the rare orchid showy ladyslipper (Cypripedium reginae) in West Virginia lost up to95% of all stems during a 3-year period of excessive deerbrowsing, from which the population took 1112 yearsto recover (Gregg 2004). Beyond effects on plants, un-controlled herbivore populations can have indirect ef-fects on decomposer (Wardle and Bardgett 2004) andspider communities (Miyashita et al. 2004).

    Aquatic ecosystemsMARINE ECOSYSTEMS Marine fisheries are estimated tohave a global by-catch of roughly 27 million tons annu-ally, which is between one-third and one-fourth of thetotal marine landings (Alverson et al. 1994). This is prob-ably a conservative estimate. Most of these by-catcheswere from trawl fisheries, followed by drift nets and gillnets. Shrimp trawls, with their small mesh sizes, accountfor roughly one-third of the total by-catch, with ratios ofweight of discarded animals per weight of shrimpcaught typically about 5:1. It has been estimated thatshrimp trawlers in the Australian northern prawn fish-ery typically discard 70,000 individual organisms duringeach night of fishing.

    Some by-catches are threatening species with extinc-tion. All but two of the worlds 21 species of albatross areconsidered threatened with extinction, and most of thesehave been severely affected by longline fisheries in theSouthern Ocean (IUCN 2003). Albatrosses, as well asother seabirds such as petrels, drown when they takebaited hooks as the lines are being set in fisheries aimedat species such as the Patagonian toothfish (Dissostichuseleginoides). While the rate of accidental capture per hookis very low, the potential risks are enormous when scaledup by the total fishing effort, with individual vesselsoften setting many thousands of hooks each day, and atotal of over 250 million hooks being set each year sincethe 1990s south of 30o South (Tuck et al. 2003). Long-livedspecies such as seabirds have extremely low resilienceagainst elevated adult mortality, due to their very long

    generation times and low rates of productivity. Recent re-assessments by the IUCN (2003) have led to six species ofalbatross recently being upgraded toward more se-verely threatened status, with fisheries by-catch playinga significant role in each case. These problems are notconfined to the Southern Ocean. For example, it has beenestimated that 10,000 Black-footed Albatrosses (Phoebas-tria nigripes) may be killed each year in the central NorthPacific (Lewison and Crowder 2003). Measures adoptedto reduce mortality on seabirds include bird-scaring de-vices, the release of baited lines from below the watersurface (where albatrosses cannot reach them), use ofheavy lines that sink immediately, and use of fish oil onthe surface, which dissuades seabirds from landing onthe water.

    Sea turtles are caught incidentally by longlines andtrawlers. For example, it has been estimated that 20,000turtles have been killed each year in the Mediterraneanin longlines set for swordfish. Turtle excluder devices arenow in widespread use in trawl fisheries, and these re-duce turtle mortality by providing an exit flap that tur-tles can push through, while retaining the fish. Turtlesare not the only reptiles taken as by-catch. It has been es-timated that 120,000 sea snakes are taken annually byprawn trawlers in the Gulf of Carpentaria, Australia.

    There are mounting concerns about the impacts of by-catches on populations of many fish species as well. Thebest-known cases involve sharks, skates, and rays (seeCase Study 8.1 by Julia Baum). These fishes are towardthe seabirdturtle end of the life-history continuum. Forexample, the European common skate (Raja batis) doesnot reach maturity until it is 11 years old. They used tobe taken by the thousands as by-catch in prawn trawls inthe Irish Sea (Brander 1981), but only six individualswere captured by extensive government bottom fish sur-veys between 1989 and 1997 (Dulvy et al. 2000). In thissame region, there is evidence for complete local extinc-tion of one and possibly two more species.

    FRESHWATER ECOSYSTEMS In fisheries, it is easy to findfreshwater equivalents of the kinds of difficulties thatface many marine fish species. An example of nontargetfishes is the Mekong giant catfish (Pangasianodon gigas)(Figure 8.7), a species that is in a similar situation to theChinese bahaba discussed earlier. This fish is restricted tothe Mekong River Basin in Thailand (where no individu-als have been caught since 2001), Laos, and Cambodia.Studies by Zeb Hogan and colleagues (2004) have shownthat individuals can reach 3 m in length and weigh 300kg, and are therefore extremely valuable to fishers. Fur-thermore, they are migratory, thereby running a gauntletof nets set for other species. Finally, their only spawninggrounds, in whirlpools and rapids in the Chiang Khong-Chiang Saen region of the ThaiLaos border, are being

    260 Chapter 8

  • dynamited as part of a plan to enhance river navigation.Conservation biologists have set up a scheme wherebyfishers telephone them day or night if they catch aMekong giant catfish, which the biologists then purchaseat market price, tag, and release away from the nets. Thisis a stop-gap measure, being used to save a small numberof fish while working on longer-term conservation meas-ures. The prospects for this species do not look good, andits threat status has now been raised to critically endan-gered by the IUCN (2003).

    Overexploitation can also have unforeseen effects onbiodiversity through trophic interactions among speciesin an ecosystem. A freshwater example that is currentlyreceiving a great deal of attention involves the effects ofreduced numbers of Pacific salmon on productivity ofstreams and their riparian vegetation in northwesternNorth America (e.g., Cederholm et al. 1999). All nativePacific salmon species die after they spawn, and theirdecomposing bodies provide a large proportion of thenutrients received by streams in their range. These nu-trients have been acquired when the fish were growingduring the marine phase of their life cycle. Experimentalstudies have shown a variety of effects of nutrients from

    salmon carcasses. For example, carcass enrichment of ar-tificial streams has led to an eight-fold increase in densi-ties of macroinvertebrates, which also increased twenty-five-fold in artificially enhanced natural streams (Wipfliet al. 1998). These effects have been shown to have im-portant feedback to productivity of the streams forsalmonids themselves, because juvenile salmon prey onmany of these insects (Wipfli et al. 2003). Studies havealso shown higher growth of riparian trees such as whitespruce, Picea glauca (Helfield and Naiman 2002). Indeed,there has been preliminary success in matching recordsof the sizes of salmon runs in southeastern Alaska from1924 to 1994 with tree-ring growth in Pacific coastal rain-forests (Drake et al. 2002). All of these examples implythat stream ecosystems, as well as associated terrestrialcomponents, will have been affected strongly by themany extinctions and depletions of salmon populationsthat have occurred in recent decades.

    Biological Theory of Sustainable ExploitationThe examples in the preceding sections give a flavor ofthe great diversity of forms of exploitation and impactson species and ecosystems. Some of these forms of ex-ploitation, such as fisheries, hunting, and forestry in tem-perate countries, have been occurring under scientifical-ly informed management. Because this is a chapter onoverexploitation, most of the examples that we havechosen have not been very encouraging. In fact, speciesand ecosystems show a wide variety of responses to ex-ploitation. In this section we review the theory that hasbeen advanced to explain how populations and ecosys-tems respond to exploitation, and tie this to manage-ment options.

    Biological populations are by definition renewable,but why should we ever expect plant and animal popu-lations to be able to withstand the elevated mortality thatoccurs with most forms of exploitation? The key is theability of birth or death rates to compensate when we re-move individuals from the population. As populationsare reduced by exploitation, there may be reduced com-petition for food, territories, shelter, and a lower trans-mission rate of diseases. This can lead to greater birthrates or enhanced survival. The tendency for such com-ponents of fitness to limit populations at high density isknown as density dependence (Figure 8.8). This does notmean that populations do not experience episodes ofdensity-independent growth, as would occur after sud-den environmental changes, for example. But the ten-dency of populations to fluctuate around some sort ofmean value, rather than growing indefinitely, can be at-tributed to density dependence.

    Overexploitation 261

    Figure 8.7 Mekong giant catfish, Pangasianodon gigas, fromthe Mekong River. (Photograph courtesy of Zeb Hogan.)

    fpo-hi res

  • To understand sustainable and unsustainable levelsof exploitation, we need to ask whether removal of indi-viduals is equivalent to thinning the population, there-by allowing the survivors to grow more quickly or sur-vive better, or is the removal occurring too rapidly forpopulations to compensate.

    The simplest way to ask about the ability of a popula-tion to compensate for elevated mortality is to start withthe logistic model, which gives the number of individu-als at time t as:


    Here Nmax is the maximum population size (Figure 8.9A).This is often called the carrying capacity, or equilibri-um population size. None of these terms is meant toimply that populations remain stable, but they conveythe idea of some sort of average size over a given periodof time. N0 is the initial number of individuals. The pa-rameter r is the intrinsic rate of natural increase, that is,the difference between per capita birth and death rates atsmall population sizes where there is no density depend-ence. For many species we may want to substitute bio-mass for number of individuals, as in the case of fisheries,in which case we often see the terms B substituted for N.

    Figure 8.9A shows the classical population growthrate that matches the logistic equation. This is the sort oftrajectory that we would expect, for example, if we put afew Daphnia into a tank of water with phytoplankton as

    food. The population grows slowly at first, because thereare few individuals producing offspring, but growth ac-celerates up to a point where the animals start runningout of food, whereupon growth stops. In the real world,populations will be buffeted by changing environmentalconditions, interactions with their predators, and so on.These can cause population fluctuations, cycles, or crash-es. But the logistic is still a reasonable default option,conveying the essence of the potential for density-de-pendence. Specific details of the biology of species willdetermine whether the initial upward slope and final de-cline due to density dependence are steeper, shallower,or occur sooner or later than shown here.

    The growth rate of this population before exploitationcan be considered its surplus production, g(N), and isgiven as:

    [8.2]g N rN NN

    ( )max









    max1 10

    262 Chapter 8


    on BiologyG










    tion Reproduction


    EquilibriumPopulation density


    Figure 8.8 Density dependence stems from relationships be-tween population density and per capita rates of birth anddeath. The difference between these at a small population sizeis the intrinsic rate of natural increase, r, and the density atwhich these rates are balanced is the equilibrium populationsize.


    Sinauer Associates, Inc.Groom 3ePrinciples of Conservation BiologyGro0809.eps100% of size01.17.05















    Population number (N)

    Maximumsustainable yield






    Figure 8.9 (A) Logistic population growth of a populationup to a maximum population size, Nmax. (B) Sustainableyield, Y (surplus production) against population size for thelogistic case shown in (A). The maximum sustainable yield(YMSY) occurs at 50% of the maximum population size.


  • This indicates, sensibly, that as the number of individu-als, N, approaches the maximum population size, Nmax,there is no growth, and even more sensibly, that growthstops when the population size is zero.

    If the population in Figure 8.9A is being exploited at asteady rate, then its rate of change per unit time, dN/dtwill be the difference between its surplus productionand the yield, Y, for a given level of exploitation:


    The populations surplus production is the yield that canbe removed sustainably. We can plot this propertyagainst different population sizes (Figure 8.9B). Thisgives us the classic dome-shaped yield curve that under-pins all models of exploitation. This shows that the max-imum sustainable yield (YMSY) occurs at an intermediatepopulation size, which coincides with the inflection pointin the logistic curve shown in Figure 8.9A. This makes in-tuitive sense: We can take the most from a populationwhen it is at the size where it can grow most quickly. Thedome does not have to be symmetric; a failure of the lo-gistic assumption can cause it to lean to the left or to theright. For example, density-dependent processes such ascannibalism, competition, predation, or disease mayoccur at smaller or larger population sizes than depictedin Figure 8.9A (Sutherland and Gill 2001). If the curveleans to the right, we should allow the population to re-main at higher numbers.

    Stability of exploitationIn theory, we have discovered how many individuals wecan take from the population to maximize the yield. Butin practice, we will have a very difficult time taking ex-actly that number. For one thing, people rarely do whatthey are told, and common experience suggests that wehave to assume that the population will probably be ex-ploited at a higher rate than we recommend. Even thebest-controlled fisheries and hunts are subject to the va-garies of uncertainty in the population estimates, or un-foreseen circumstances such as weather conditions caus-ing populations to behave in unpredictable ways. Thequestion is, what happens when (not if) the populationis exploited at a rate that differs from the one that theorysuggests should be maximally sustainable? The answerdepends on how the exploitation rate is managed.

    Constant quota exploitationOur formulation implies a system of exploitation in whichthe number of individuals removed is independent of thepopulation size. That is, in Equation 8.3 we have subtract-ed Y from the surplus yield, rather than making Y pro-portional to N. This case is typically known as constant

    quota because the numbers removed are constant in thesense of being independent of the population size. Exam-ples include fisheries that set quotas on numbers of fishthat are caught, regardless of subsequent changes in thenumber of fish in the sea, or hunters who are given fixedlimits that do not vary as the number of target animalsgoes up or down over time. This may sound extreme, be-cause common sense suggests that quotas should be ad-justed according to such changes in the quarry. And it isextreme. And so was the collapse of Peruvian anchovy inthe 1970s, which occurred in part because people did notfully appreciate the implications of this assumption (seethe discussion later in this chapter).

    The constant quota situation is depicted in Figure8.10, where each of three potential quotas runs horizon-tally across the graph, implying no relationship withpopulation size. First, consider the high-quota line. Here,the yield exceeds the populations surplus productioncapability, perhaps because of illegal hunting activity, orbecause the populations resilience had been over esti-mated. The arrow beneath this line shows that with con-stant exploitation at this rate, the population will becomeextinct. Year after year the quota exceeds the popula-tions ability to keep up with the elevated mortality. Analternative that seems more sensible is the MSY quota.However, this is a very risky target because it is impossi-ble to get precise measurements of either the surplusproduction curve, or N. Furthermore, as we have ar-gued, even if this were possible, there is still a goodchance that we will be unable to set quotas sufficientlyaccurately to score a bulls eye when we shoot for theMSY point. If the population is at or smaller than NMSY,the quota will exceed its surplus production and it willdecline to zero. On the other hand, the NMSY point willbe stable if the population is initially higher, because al-


    rN NN




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    Population number (N)

    High quota

    MSY quota

    Low quota



    NmaxN1 N2

    Figure 8.10 Equilibria and population stability under con-stant quota exploitation. The arrows indicate the directions ofchange in population size for each quota.


  • though the MSY quota is initially higher than surplusproduction, as the population declines it will benefitfrom reduced density dependence. Still, given the diffi-culty of estimating MSY with precision, such a quota isdangerous. Finally, consider the low quota example. Ifthe population is initially below N1 it will still crash.However, if it is between N1 and N2, then its productionwill exceed the quota, and it will grow to N2. If it is ini-tially higher than N2, it will be driven down to N2 by thequota exceeding its surplus production. So N1 is an un-stable equilibrium and exploitation in that region shouldbe avoided. In contrast, as long as we are sure the popu-lation is well above N1, exploiting it with the low quotaindicated would allow it to grow to a stable equilibriumat N2. Here, finally, we have sustainable exploitation.

    Proportional (constant effort) exploitationA much more sensible way to set exploitation targets isto tie them directly to the size of the population. In prac-tice, this is always bound to happen to some extent, be-cause as plants and animals become scarce, people tendto switch to alternative species or other activities, even ifno one is forcing them to do so. For example, local de-mand for game species in tropical forests tends to in-crease sharply when catches in the marine fishing sectorfair poorly (Brashares et al. unpublished data), and trop-ical forest hunters become heavily reliant on smaller-bodied game species once large-bodied species are de-pleted (Jerozolimski and Peres 2003). Conservationistsand resource managers amplify this kind of commonsense by encouraging or forcing people to reduce pres-sures on populations that reach low abundance. So, un-like the case of constant quota above, we have exploita-tion effort that is proportional to the population size.

    In this scenario the size of the yield, Y, will be equal tothe exploitation rate, E, multiplied by the populationsize, N:

    Y = EN [8.4]

    From Equations 8.2 and 8.3 we know that a steadystate will occur between the yield and the surplus pro-duction when g(N) = Y. This means that g(N) = EN.Therefore,


    We can rearrange this equation to find the equilibriumpopulation size for any rate of exploitation, as long as Eis below r:


    These equilibria are shown in Figure 8.11. The advantageof this form of management is that as long as the ex-

    ploitation rate is below the intrinsic rate of natural in-crease, r, then all equilibria are stable. For example,whereas the high quota crashed the population underthe constant quota scenario in Figure 8.10, here we findthat if the population is initially below this removal rate,it will increase to N1, and if it is above this point, it willdecrease to N1. The MSY is also stable, and it still occursat half of the maximum population size.

    Figure 8.12 compares the risk of extinction from ex-ploitation based on either constant quotas or proportion-al rates for a study of the American marten (Martes amer-icana) in southern Ontario, Canada (Fryxell et al. 2001).The marten is a member of the weasel family (Mustel-idae) found primarily in coniferous forests where it preyson a wide variety of small vertebrates as well as some in-vertebrates. Marten are trapped for their fur and trappersare granted licenses for exclusive access to trappinggrounds. Fryxell et al. (2001) use a simulation model ofdata from commercial trapping from 1972 to 1991 to eval-uate effects of different types of harvest. This analysisshowed that whereas exploitation in proportion to thepopulation size has a negligible chance of causing localextinction, this risk becomes quite high for moderate lev-els of exploitation under a constant quota system thatdoes not track the population.

    Threshold exploitationThe final class of exploitation strategies involves the useof population size thresholds to determine not only therate of exploitation but also whether exploitation shouldtake place at all (Lande et al. 1997, 2001). All populationsare subject to random (stochastic) variations, for exam-

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    Population number (N)

    High exploitation

    Very high exploitation

    MSY exploitation

    Low exploitation

    0 NmaxN1 N2

    Figure 8.11 Equilibria and population stability under pro-portional exploitation (also called constant effort exploita-tion). Here, a constant fraction of the population is removed.The dashed lines have slopes E corresponding to differentrates of exploitation. The arrows indicate the directions ofchange in population size for each scenario.


  • ple, due to environmental variation. In the absence of ex-ploitation, they will sometimes exceed their carrying ca-pacities temporarily. We can take advantage of this bytaking the entire surplus whenever the population isabove its carrying capacity, but otherwise ceasing ex-ploitation completely. This would maximize the cumu-lative yield while minimizing the chances of collapse. Amore precautionary variant on this is to remove only aproportion of the surplus. This is an even safer versionof the proportional exploitation method outlined abovebecause it maintains the population near its carrying ca-pacity. However, while such low rates of exploitation areexcellent for conservation, it is difficult to convince peo-ple to accept such severe restrictions.

    BioeconomicsUnderstanding biological constraints is necessary toachieving a sustainable level of exploitation. But as wesaw earlier in the chapter, we also need to understand thehunters incentives and disincentives if we are to under-stand their impacts on the hunted, and provide manage-ment advice to mitigate such impacts. Bioeconomic mod-els incorporate the costs and benefits of exploitation. Evenin societies where exploitation is a necessity rather than asource of revenue, as in subsistence hunting, a costbene-fit framework can be very useful for understanding peo-ples behavior (see Essay 8.2 for an example).

    Open access and the tragedy of the commonsThe tragedy of the commons (Hardin 1968) providesa powerful explanation for a lot of the damage that peo-

    ple are doing to the environment and to each other.Imagine you have sole access to the trout in a lake onyour property. You will probably manage these troutquite carefully, because if you take too many, you will bethe one who suffers in future. But if the pond straddlesthe boundary with your neighbor, then each of youshould only take half the number you would take if youhad exclusive access. Will you both be so prudent? Un-less you get along very well with each other, probablynot, because each persons self-restraint can be exploitedby the other one. Imagine the outcome if we scale thisexample up to the North Sea, bordered by many coun-tries, each with thousands of fishers, all competing forthe same fish. The sea is a common fishing ground,and indeed, the massive over-fishing of the past centuryhas been tragic for both fishers and their prey.

    We can incorporate the tragedy of the commons intothe scheme developed in the previous sections by assum-ing that the benefits from exploitation are directly propor-tional to the yield. So the dome in Figure 8.13 representsthe benefits. Assume that the costs of exploitation are pro-portional to the effort necessary to obtain that yield. Forexample, in a fishery the more days spent at sea, thegreater the costs of fuel and labor. This is depicted by thestraight line rising from the origin. If we ignore the costs,the maximum benefits will be found at EMSY, or the effortthat provides the maximum sustainable yield. However,if the goal is to maximize profits, this would lead to alower exploitation rate corresponding to the effort atwhich the difference between benefits and costs is maxi-mized, at EP. But the most important fundamental truth of

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    0.0200 250 300 350 400 450






    Mean number of martens taken

    Proportional quota

    Constant quota

    Figure 8.12 Probability of overexploitation (local extinction)in relation to mean yield of marten in commercial trapping insouthern Ontario, Canada. The proportional quota line(black) refers to yields that track annual changes in the popu-lation whereas the constant quota line (gray) is based on thesame absolute number of individuals being trapped each yearregardless of population size.









    Exploitation effort






    EP EC

    Figure 8.13 The economics of exploitation, represented bycosts that are proportional to effort, and benefits proportionalto yields. At EP the profits are maximized, at EMSY the maxi-mum sustainable yield is obtained, and at EC the costs areequal to the benefits. Note that unlike Figures 8.10 and 8.11,the x-axis here represents fishing effort (which increases fromleft to right, corresponding to population number decreasingfrom left to right).


  • 266 Chapter 8

    Using Economic Analysis to Bolster Conservation EffortsMarine Aquaria and Coral Reefs

    Gareth-Edwards Jones, University of Wales, Bangor

    n Advances in technology combinedwith increased interest in marine sys-tems have contributed to a largeincrease in American householdskeeping marine aquaria. As part ofthe large global business associatedwith establishing and maintainingthese aquaria, approximately 350million fish are harvested annuallyfrom the wild and sold worldwidewith a value of $963 million (Young1997). This continued wild harvest isnecessary as some species of aquar-ium fish do not breed well in captiv-ity. Some of the more popular aquar-ium species are associated with coralreefs, and 85% of aquarium fishes arecaught from coral reefs around thePhilippines and Indonesia. Catchingthese fishes can be an importantsource of income for some communi-ties, but it is increasingly happeningon a large scale and there are 4000aquarium fish collectors in thePhilippines alone.

    A network of traders are involvedin the marketing of the fish; the fish-ermen themselves may sell to localtraders who then sell to exportersand so on. There are significantfinancial gains at each step in themarketing chain. For example, anorange and white striped clownfishcosts 10 cents when bought from aFilipino collector, but sells for $25 ormore in an American pet store (Simp-son 2001). Thus, traders in developedcountries typically make the largestgains, while collectors or even thefirst traders make pennies. Becausethe returns to collectors are so low,there is room for conservation alter-natives to be attractive; it does nottake as much return to exceed theearnings of the people supplying thefish.

    The impacts of collecting thesefishes from the wild are quite varied.When using traditional netting tech-niques for catching the fish the mostdirect impact is a reduction in the population density of the harvestedspecies, as shown for the Banggai cardinalfish (Pterapoggon kauderni)in Sulawesi, Indonesia (Kolm andBerglund 2003) (Figure A).

    While traditional forms of harvestingcan reduce species densities, an issue ofgreater concern is the use of cyanide tocatch the fish. Fishermen typically crushhydrogen cyanide tablets in a squeezebottle and squirt the resulting liquidinto the crevices where the reef fish

    hide. The cyanide triggers asphyxiationor muscle spasm in fish, stuns them,and makes them easier to catch. Thesemethods have been used since the 1950sand are believed to have quite a largeimpact on fishes and the coral reefsthemselves. Experiments have tested

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    (II)Figure A (I) Map of the BanggaiArchipelago of Indonesia, showingstudy sites as black circles. The Bang-gai cardinalfish is pictured in thelower left corner. (II) Correlation be-tween Banggai cardinalfish densityand the intensity of fishing at eightseparate sites in the Banggai Archipel-ago near Sulawesi, Indonesia. Becausethe cardinalfish is associated with seaurchins, their density is expressed perm2 of sea urchins on the sea bed. In-tensity of fishing was estimated frominterviews with local people. Banggaicardinalfish are caught by nets only,so reductions in density are solely re-lated to fishing pressure, not to reefdestruction (Modified from Kolm andBerglund 2003.)

  • Overexploitation 267

    the response of 10 species of coral toconcentrations of hydrogen cyanidelower than those typically used by fish-ers. Eight of the coral species diedimmediately, and the other two diedwithin three months. Death occurredbecause the cyanide disrupted the rela-tionship between the coral and its sym-biotic zooxanthellae. Doses as low of 50mg/L cause death of the zooxanthellae,and one spray (approximately 20 cc) cankill the corals over an area of 5 m2within 36 months. These impacts are ofconcern as Southeast Asia has 30% ofthe worlds coral reefs, and these havebeen declining in quality due to a vari-ety of factors, of which cyanide fishingis but one. Now only 4.7% of Philippineand 6.7% of Indonesian reefs are in per-fect condition and quite naturally theseare the reefs targeted by collectors. Thecyanide also affects fishes themselves.Half the poisoned fish die on the reef,and 40% of those caught alive die beforereaching an aquarium (Simpson 2001).

    Estimates of the economics of thispractice suggest that the profitability ofcyanide fishing to the fishermen isabout $33,000 per km2 over 25 years(10% discount rate). The direct losses tofisheries total $40,000 per km2 and thetourism losses can range between$3,000 and $436,000 per km2 dependingon location (Cesar 1997). The nonusecosts of losing the coral reef are likelyto be larger than these direct values.

    Solutions?This is an interesting situation wherepoor local people are seeking to harvesta local resource to sell it to richer West-erners, but a side effect of this harvestis the loss of another valuable resource.The marketed resource is nonessentialto Westerners, (i.e., we could all livewithout a marine aquarium if we hadto), but we like the aquariums, andmany conservationists may argue thatthrough keeping aquaria, peoplesinterests in marine conservation couldbe heightened. So what should we do?

    The solution seems to be to permit asustainable harvest of fishes caught in anondestructive manner. Attempts toachieve this can be legislative; forexample the application of internationallaw like CITES (Convention on theInternational Trade in Endangered WildFauna and Flora) to the relevant fishspecies could stop all trade in thesespecies. Alternatively, tighter importcontrols could be implemented in thedestination countries (e.g., in NorthAmerica and Europe) whereby onlyfish harvested to approved standards

    would be permitted entry. While somelegislative backing of this nature couldbe beneficial, in practice it will be veryhard to police.

    Alternatively, it may be possible toachieve more sustainable harvest byapportioning property rights to differ-ent sets of actors. Currently mostmarine fisheries are open access, andbecause of this are susceptible to the so-called tragedy of the commons. Intheory the solution to this is to allocateproperty rights to different groups ofpeople. In the presence of propertyrights, the fishermen should seek tomanage their resource with a long-termperspective, as opposed to the extremeshort-term perspective that open accessencourages (Ostrom and Schlager1996). The allocation of property rightsto fishermen is on-going around theworld; for example, the Chilean gov-ernment is offering the rights to harvesta benthic gastropod Concholepas conc-holepa, known locally as loco (Castillaand Fernandez 1998). This involvesallocating certain areas of seabed todefined groups of local people fromlocal communities who then managethem collectively.

    While the theory of property rightsis simple, recent work on the Chileanexperience shows that the allocation ofproperty rights is not a panacea andcan lead to considerable social tensionbetween different groups in a commu-nity. One important issue in all suchefforts relates to policing the agree-ments, as it is difficult for hard-pressedofficials to spot breaches of agreementsat sea. In the absence of official polic-ing, disagreements between groupsabout who can harvest where canbecome difficult, if not violent. Thesesorts of issues highlight the need forstrong, functional institutions to helpcoordinate and manage any network ofcommon property resources. While theneed for such institutions is clear fromtheory and practice, their developmentcan be a long and complex processrequiring considerable effort from allinvolved (Ostrom 1990).

    A third approach to dealing withthese issues is being tested by theMarine Aquarium Council. They aim toset up a certification scheme wherebyfish caught in traditional nets would belabeled in some way, which wouldenable consumers to buy these fish inpreference to cyanide-caught individu-als. If all consumers preferentially pur-chased certified fishes, then the advan-tage of using cyanide would disappear.To get such a certification scheme to

    work several issues have to be followedin parallel:

    There needs to be accurate identifi-cation of non-cyanide caught fishesnear the point of capture. This canbe done through testing samples offishes in export warehouses forcyanide exposure.

    Fisherman need to be informedabout the certification scheme andtrained to use alternative collectingtechniques, such as hand nets.

    Fishes caught without cyanideshould be labeled so that fish buyerscan choose to support practices thatpreserve reefs.

    Consumers should be educatedabout the environmental issues asso-ciated with the use of cyanide forcatching fish.

    Many such certification schemes arecurrently under development aroundthe globe, particularly for food and for-est products, and are easily linked toother management schemes such as theallocation of property rights. In mostsituations certified products will costmore than non-certified products. Thisis almost inevitable if, as in the case ofaquarium fishes, the damaging fishingtechniques are used becau