SUCCESSION, PHENOMENON OF

SUCCESSION,PHENOMENON OF

H. H. ShugartUniversity of Virginia

I. IntroductionII. Organic Explanations of Ecological Succession

III. Mechanistic Explanations of Ecological Suc-cession

IV. Succession and Biodiversity

GLOSSARY

autotrophic successions These generate energy frominternal processes (photosynthesis).

climax community According to some theories of suc-cession, the end result of succession in which succes-sional change ends with a community that does notchange and which is in equilibrium with the climate.

disturbance Major alternations of vegetation due toevents such as wildfires, hurricanes, landslides, andhuman clearing.

heterotrophic successions Dependent on already fixedenergy, such as the successional of communities as-sociated with decomposition of dead logs.

individualistic view of succession Concept that suc-cession is a consequence of species interacting withone another and their environment.

primary succession Succession on newly exposed sub-strates such as a sandbar or rubble at the foot of areceding glacier.

progressive succession Successions in which the dy-namic changes are in the directions of increasing

Encyclopedia of Biodiversity, Volume 5Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 541

species diversity, structural complexity, greater bio-mass, and increased stability. Retrogressive succes-sions are in the opposite directions.

secondary succession Succession on existing substrate(soil) following a disturbance.

succession The pattern of change expected in a com-munity over time after a disturbance or after newsubstrate has been exposed.

ECOLOGICAL SUCCESSION is an ordered progressionof structural and compositional changes in ecosystemstoward an eventual stable condition. Descriptions ofsuccession involve the nature of the changes and thefactors that cause the changes. Ecologists have debatedwhether the succession is a community process or thesummation of the consequences of individual speciesand their interactions with each other and the environ-ment. Most ecologists doing research in this area cur-rently favor the latter view. Investigations of the typesof interactions among species have led to an increasedinterest in the mosaic nature of vegetation and to theapplication of computer models to project the expectedpatterns of change in vegetation over time. The biodi-versity of landscapes may be highest when there is anintermediate level of disturbance. The appearance ofspecies at different times in succession appears to beidiosyncratic to the particular vegetation; the rate of

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loss of species from communities seems to decreaselogarithmically.

I. INTRODUCTION

Ecological succession is an ordered progression ofstructural and compositional changes in ecosystems to-ward an eventual stable condition. Primary successionsare initiated on new substrates, such as a new volcanicisland, a new sandbar in a river, or rubble fields at thefoot of a receding glacier. Secondary successions involvethe recovery of vegetation on established soils from landabandonment and from disturbances such as wildfires,hurricanes, or human alterations to vegetation. Othersignificant dichotomies used to categorize successioninvolve whether a given successional sequence is

1. Progressive (dynamically changing in the direc-tions of increasing species diversity, structural com-plexity, greater biomass, and increased stability) or ret-rogressive (in the opposite directions)

2. Autotrophic (generating energy from internalprocesses) or heterotrophic (dependent on already fixedenergy, such as the successional of communities associ-ated with decomposition of dead logs)

3. Autogenic (changing due to interactions from in-side the system) or allogenic (changing in response tochanges in external variables)

Ecological succession is an early concept in ecologyand was essential in the early definitions of ecologicalcommunities. Although the basic concepts of succes-sion are easily understood, debates about succession(Box 1) have spawned considerable confusion and dis-cussion. The mechanisms that drive ecological succes-sion and its very existence as a natural phenomenonhave been the subject of continual debate among ecolo-gists. McIntosh (1985) identifies two contrasting viewsthat typify traditional natural history and also can befound in current discussions of theoretical ecology. Thisdichotomy, which can be used to organize theoriesabout ecological succession, contrasts mechanistic ex-planations and organic (holistic) explanations of thecauses of succession. For mechanistic theories aboutsuccession, known laws explain the actions of the indi-vidual parts of a system and the whole system is thesum of these parts and their interactions. In the caseof organic or holistic explanations, the whole system,its existence and design, explains the actions of theparts. Mechanistic explanations often are taken as themore modern interpretation of succession, but these

‘‘modern’’ views may have been the first developed andcertainly developed as early as the organic views. Or-ganic explanations of succession were most popular, atleast in the United States, between the 1920s and early1950s and have had a considerable impact on land useand conservation policies.

II. ORGANIC EXPLANATIONS OFECOLOGICAL SUCCESSION

Organic explanations seek to understand succession interms of principles that operate at the level of the wholesystem. In the 1920s and 1930s, F. E. Clements andparticularly John Phillips were ascribing to ecologicalcommunities the attributes of a superorganism—ahighly organized and coevolved assemblage of plantsand animals interacting in a dynamic system. The eco-system concept had its roots in debates regarding theorganization and dynamics of natural systems. It wasTansley’s negative view of Clements’ and Phillips’ inter-pretation of the community as a superorganism that in1935 inspired his development of the ecosystem con-cept as an alternative to the organic term ‘‘community.’’

A. Clementsian Concepts ofEcological Succession

Between 1905 and 1935, F. E. Clements promoted adynamic plant ecology built around a ‘‘supraorga-nismic’’ view of the ecological community. At the timeof their conception, Clements’ ideas represented a sig-nificant emphasis on the dynamics of vegetation andwere an important early attempt to develop a formaltheory predicting the pattern and expected change inecological communities. The underlying conviction wasthat evolution and internal interactions would producea homogeneous regional ‘‘climax’’ vegetation or commu-nity of regular species composition. The developmentof this ‘‘climatic climax’’ community was ecological suc-cession, which was viewed as the community analogof the embryological interactions that produce an or-ganism. In this view, succession was typified by a pro-gressive sequence of seral stages or seres, communitiesthat sequentially replaced one another over time untilthe climax stage was reached. Successional develop-ment was the result of a set of processes:

1. Nudation—the creation of a bare area (or par-tially bare area) to initiate succession

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Box 1

Complex Debates Arising in a Simple Definition

2. Migration—the arrival of organisms at the lo-cation

3. Ecesis—the establishment of organisms at the lo-cation

4. Coaction—the interactions, particularly compe-tition, among the organisms

5. Reaction (or facilitation)—the modification ofthe site by the organisms and the subsequent changein the relative abilities of the organisms to establishand survive

6. Stabilization—the development of a stable com-munity called the climax community


Stabilization is less a process and more a consequenceof the iterative reapplication of the migration, ecesis,coaction, and reaction processes until a stable commu-nity is reached. Clements’ concept of the climax com-munity was that there was only one stable vegetationtype that was in equilibrium with the regional climate.Succession was the orderly, predictable, progressive,and linear development toward this climax community.

B. Application of Clementsian ConceptsThe Clementsian succession paradigm had a pro-nounced effect on ecology in the United States in thefirst half of the twentieth century (much less so inEurope), and it shaped many of the laws and policieson the use of public lands. Earlier ecologists developedsome of these ideas, and others evolved over Clements’and associates’ scientific careers. Important aspects inthe Clemetisian paradigm, many of which continue tobe a part of natural land management today, are

Community-level management: The idea that onecould use the state of ecological communities to evalu-ate their past and present conditions and to predicttheir future is a significant contribution of Clementsand associates. Although it has been established bypaleoecological studies that communities are not neces-sarily stable over century and millennial timescales,short-term changes and disturbances are often seen aschanges in communities. Conservation groups oftenmake considerable effort to preserve unique communi-ties (as well as unusual or important species). Wildlifeand endangered species management is often based onmaintaining particular communities or habitat typesappropriate to the survival of focal species of animalor plants.

Indicator species concept: Clements believed that thepresence or absence of particular species could be usedto assess the state of a community and its potential foragricultural conversion. For example, the presence ofpoisonous or distasteful plants on a range indicatesovergrazing, the occurrence of certain species of Lupine(Lupinus plattensis) in a Nebraska prairie connotes adeep soil suitable for tilling and agriculture, and landwith plants such as Salicornia might be so loaded withsalt as to be unreclaimable. One could use indicatorspecies to determine the past history of a landscape andto predict its future changes and potential uses.

Climax community concept: The climax communityis the community that is stable in a given climate condi-tion and is the ultimate product of successional pro-cesses. Current vegetation maps, particularly for large

regions, display the expected climax community or po-tential natural vegetation expected in a region ratherthan the actual vegetation found there. Parks and wil-derness areas are managed to maintain the natural cli-max community that is typical of the region. As anissue in conservation of diversity, often considerableeffort is made to preserve unique communities (as wellas species).

Progressive nature of succession: Clements viewedsuccession as being progressive (moving in a positivedirection) toward the climax community. The succes-sion progressed toward more diverse, more stable, andmore desirable communities. This is a concept associ-ated with eighteenth-century intellectuals and tied toother concepts such as the divine design of naturalsystems and ideas about the ‘‘balance of nature’’ andof the antiquity of certain ecological communities.Whether from Clements or earlier sources, these ideashave considerable influence on the aesthetics of conser-vation in the valuation of wilderness and the assessmentof the importance of preserving certain species ratherthan others.

Perhaps the most enduring legacy from Clements ishis pioneering attempt to synthesize quantitative obser-vations about succession into a unified theory. Evenstrong detractors of the details of Clements’ conceptsare still involved in understanding succession as a gen-eral phenomenon.

C. Alternative Organic Theorieson Succession

Clements’ theory is the most frequently presented or-ganic succession theory, so much so that textbook writ-ers often term Clementsian theory to be ‘‘classic’’ succes-sion theory. However, all American and Britishecologists did not accept these ideas. This rejectionwas certainly the case for ecologists from continentalEurope. Some of these ecologists emphasized a moreindividual species-oriented mechanistic description ofsuccession. Others shared an interest in the holisticcauses of successional change but differed from Clem-ents in significant details. H. C. Cowles, who in 1899was one of the first American ecologists to study ecolog-ical succession, characterized succession as ‘‘a variableapproaching a variable’’: Succession was considered thechange of a system perturbed away from—but movingtoward—an equilibrium that is itself changing. Cowlesbelieved that the climax community was never reached.In 1935, A. G. Tansley contrasted Clements’ climatic


climax or monoclimax theory with a polyclimax theorythat had the climax vegetation of a region as a mosaicof local vegetation climaxes related to local conditionsand disturbance history. In 1913, W. S. Cooper studiedthe forest succession on Isle Royale (Michigan) andfound that the mature forest was a mosaic of patchesof different ages and not the uniform climax communityexpected in Clementsian succession. He also believedthat succession took multiple pathways and was not alinear progression of changes in seral stages. Similarly,in 1901, Cowles noted, ‘‘Succession is not a straight-line process. Its stages may be slow or rapid, direct ortortuous and often they are retrogressive’’ (Bot. Gazette31, 73–108, 145–182). Recognizing that there is andhas been considerable difference in opinion among ecol-ogists who use organic explanations of ecological suc-cession, there is an even stronger contrasting of con-cepts between Clementsian succession and mechanisticexplanations of ecological succession emphasized bysome ecologists.

III. MECHANISTIC EXPLANATIONS OFECOLOGICAL SUCCESSION

Some of the debate about the nature of succession con-cerns the degree to which the vegetation can be arrangedinto communities that are natural units of biologicalorganization. Introductions of modern biology or ecol-ogy texts often have diagrams of biological organizationthat illustrate an organizational hierarchy from cells totissues to individuals to populations to communities,etc. Such progressions convey the idea that the commu-nity is a unit of organization as demonstrable at a levelsuch as the existence of the liver or spleen as an organi-zational unit is at some other level.

The American H. A. Gleason (in 1926) and the Rus-sian L. G. Ramensky (in 1924) emphasized that vegeta-tion was mostly the consequence of the chance arrivalof species at a location and the subsequent interactionsamong the available species to produce the observedpattern of relative species abundance. These interac-tions did not produce distinct unit communities. Thevegetation was believed to vary continuously withchanges in the underlying environmental conditions.Under this ‘‘individualistic’’ view of ecological succes-sion, the process of succession was a consequence ofspecies interacting with one another in the context ofthe environment to produce vegetation dynamics orsuccessional change. Today, most ecologists subscribeto this individualistic view of ecological succession.

A. Descriptive Mechanistic Modelsof Succession

When one compares modern descriptive models of suc-cession with the Clementsian model, it is apparent thatthere is a substantial difference in the spatial scale con-sidered by the two schools. Clements’ climax commu-nity was considered by him to be a phenomenon thatoccurred over large areas. This is evident by the fact thatClementsian succession proceeded toward a regional-scale climax. Also, the union of the climax community(vegetation) and associated animals was a ‘‘biome’’ ofwhich there were thought to be only 13 in nontropicalNorth America. Clements believed that when the com-munity in a given location was too small to have all ofthe species represented, succession might take differentcourses. However, he also believed that the successionprocesses he had described (nudation, ecesis, coaction,etc.) would still operate at these smaller scales.

In 1987, Pickett and colleagues produced a tableof mechanisms and causes of succession that can becompared directly to earlier writing of Clements (TableI). In this comparison, one sees that many of the pro-cesses deemed important by Clements (notably, nuda-tion, migration, ecesis, coaction, and reaction) are alsorepresented by a mechanistic explanation of succession,albeit with different names and differing emphases. Thedifferences are most pronounced with regard to theimportance of the reaction (facilitation). Certainly inmany primary successions, the changes produced byone set of species appear necessary for the success ofthe next. For example, the rapidly growing willows(Salix sp.) that stabilize sandbars in rivers seem to bea necessary preamble to the success of subsequent spe-cies; the ability of alder (Alnus sp.) to symbiotically fixnitrogen increases the fertility of sites uncovered byreceding glaciers; and the organic acids produced byblue-green algae, lichens, and mosses speed the break-down of granite and the development of a thin soilto support grasses, herbs, and even trees in primarysuccessions on granite outcrops.

However, some species appear to block the successof others and hold sites against species that might other-wise succeed them. In some secondary successions, allthe species involved in the succession are present asseeds or other propagules from the initiation. In thesecases, the familiar successional sequence of grasses andherbs yielding to shrubs and then to trees may reflecta difference in rate of growth of individuals presentfrom the start. Evolutionarily, it is difficult to explainwhy a species would evolve to help another take overa site it could otherwise occupy.


TABLE I

Clementsian Succession Processes Compared with a Mechanistic Explanation of the Causes of Successiona

ContributingGeneral causes of processes of Clementsian succession

succession succession Modifying factors analog processes

Site availability Coarse-scale Size, severity and timing of disturbance, dispersion of species Nudation and migrationdisturbance

Differential species Dispersal Landscape configuration, dispersal agents Migrationavailability Propagule pool Time since last disturbance, land use conditions Migration and ecesis

Availability of Soil condition, topography, microclimate, site history Ecesisresources

Differential species Ecophysiology Germination requirements, photosynthesis rates, growth rates, pop- Coactionperformance ulation differentiation

Life history Photosynthesis allocation pattern, timing of reproduction, mode ofreproduction

Competition Hierarchy of competitive interactions, the presence of competitors,identity of competitors, within-community disturbances, preda-tors, herbivores, resource base

Herbivory, pre- Climate cycles, predator cycles, plant vigor, plant defenses, com-dation, munity composition, patchinessdisease

Environmen- Climate cycles, site history, prior occupants Reaction (in part)tal stress

Allelopathy Soil chemistry, soil structure, microbes, neighboring species ?

a The first three columns are from a review by Pickett et al. (1987).

Connel and Slatyer (1977) developed a descriptivemodel of the succession processes based on a mechanis-tic understanding of succession (Fig. 1). They usedthe reaction/facilitation issue to frame three models ofsuccession based on mechanisms of interaction (thefacilitation model, tolerance model, and inhibitionmodel). In Fig. 1, the facilitation model is most likeClementsian succession as typically interpreted. Thethree models are different and have different implica-tions for land management and particularly land recla-mation. If one had the objective of restoring degradedlandscapes, then one might speed the restoration byeliminating established species in the case of the inhibi-tion model, but this would be ill advised in the facilita-tion model (Fig. 1).

B. The Landscape as a MosaicIn addition to an expansion of the facilitation conceptassociated with Clementsian succession, there has alsobeen an emphasis on understanding the mosaic natureof vegetation. Historically, this emphasis derives fromearlier concepts of ‘‘cyclical succession’’ (exemplifiedby the cyclical replacement series involving a forest

canopy gap) and the ‘‘polyclimax’’ (the mature forestas a mosaic trees, gaps, and recovering gaps). Theoriesabout the dynamics of landscape mosaics have beendeveloped in forests to a significant extent, probablybecause, regardless of other sources of spatial heteroge-neity, a forest canopy is a mosaic of tree crowns (Fig. 2).

Starting with a small plot of land in a mature forestdominated by a single large tree, the large tree shadesthe ground and reduces the survival of smaller treesand seedlings below. There may be a few smaller shade-tolerating trees that survive under the large tree, butthese are strongly suppressed in their rate of growth.The large tree dominates the resources (light, water,and nutrients) that are available at the site and blocksother trees from growing at the location. When thistree dies, the forest floor (where there had previouslybeen little chance of a young tree’s survival) becomesa nursery for small seedlings and saplings. There isadequate light and other resources, and hundreds ofsmall trees survive and begin to grow toward the can-opy. As these trees grow, they begin to compete withone another. Some of the trees lose to more vigorouslygrowing competitors. Eventually one tree manages towin the race to be the local canopy tree and begins to


FIGURE 1 Mechanistic models of ecological succession (reproduced with permission from Con-nell and Slatyer, 1977).

eliminate the others. This represents the closure of thecycle with a large tree again dominating the site. Overthe course of time this cycle is reinitiated by the deathof the new dominant replacement tree. The implicationsof the cyclical nature of small-scale forest dynamicswere clearly elucidated by Cooper in 1913 and byA. S. Watt in a classic paper in 1947.

The expected changes in the amount of living mate-rial (biomass) over multiple iterations of a gap genera-

tion and filling should create a ‘‘saw-toothed’’-shapedcurve. This curve drops abruptly with the death ofa canopy dominant and then builds biomass as theregenerating trees grow, compete, and occupy the site(Fig. 3, top). The distances between the ‘‘teeth’’ in thesaw-toothed, small-scale biomass curve are determinedby how long a particular tree lives and how much timeis required for a new tree to grow to dominate a can-opy gap.


FIGURE 2 The forest canopy as a mosaic (photograph of SouthAfrican rainforest from J. C. van Daalen).

The larger scale biomass dynamics (Fig. 3, bottom)is a simple statistical consequence of summing the dy-namics of the parts of the landscape mosaic. If therehas been a synchronizing event, such as a clear-cuttingor other disturbance, one would expect the landscapemosaic biomass curve to increase as all of its parts aresimultaneously covered with growing trees (Fig. 3,a). Ifthe trees over the area have relatively similar longevities,there is also a subsequent period when the deaths (andbiomass) on plots where a canopy tree happens to dieare balanced by those on plots where large trees arestill growing. During this period, the loss balances gainin biomass and the curve levels (Fig. 3,b). If there issufficient synchronization in the sawtooth curves of thecomponent plots, this is followed by a period duringwhich many of the pieces that comprise the forest mo-saic all have deaths of the canopy-dominant trees (Fig.3,c) and landscape biomass decreases. Over time, thelocal biomass dynamics become desynchronized andthe biomass curve varies about some level (Fig. 3,d)This mosaic of repairing gaps with all different stagesof recovery represented on the landscape can be takenas the mature forest.

The mature forest should have patches with all stagesof gap phase dynamics and the proportions of eachshould reflect the proportional duration of the differentgap replacement stages. T. C. Whitmore believed thatthis was the expected pattern and process for all forestsand asserted in 1982 that

forests of the world are fundamentally similar,despite great differences in structural complexity

and floristic richness, because processes of forestsuccession and many of the autecological proper-ties of tree species, worked out long ago in thenorth temperate region, are cosmopolitan. Thereis a basic similarity of patterns in space and timebecause the same processes are at work.

FIGURE 3 Biomass dynamics for an idealized landscape. The re-sponse is from a relatively large, homogeneous area composed ofsmall patches with gap phase biomass dynamics. (Top) The individualdynamics of the patches that are summed to produce the landscapebiomass dynamics. (Bottom) The landscape biomass curve rises (a)as all of the patches are simultaneously covered with growing trees.Next, local decreases in biomass are balanced by the continued growthof large trees at other locations and the curve levels at a maximumvalue (b). If the trees have relatively similar longevities, there is aperiod in which several (perhaps the majority) of the patches thatcomprise the forest mosaic all have deaths of the canopy-dominanttrees and the curve decreases (c). Eventually, the local biomass dy-namics become desynchronized (d) and the landscape biomass curvevaries about some level (e) (reproduced with permission from Shu-gart, 1998).


Whitmore is referring to mosaic forest canopies asa consequence of gap replacement processes. Theoccurrence of such patterns has been documented forseveral different kinds of forests. The presence ofshade-intolerant trees occurring in patches in matureundisturbed forest is another observation consistentwith the mosaic dynamics view of mature forests.The scale of the mosaics in many natural forests issomewhat larger than one would expect from gapfilling of single tree gaps, indicating an importanceof phenomena that cause multiple tree replacements.Also, relatively long records (approximately 40 yearsin most cases) of forest structure and compositionindicate a tendency for the forest composition tofluctuate with species showing periods of relativelyweak recruitment of individuals to replace large treesand strong recruitment in other periods.

C. Quantitative Mechanistic Modelsof Succession

A consequence of recognition of the mosaic natureof vegetation and an emphasis on more mechanisticrepresentation of the succession process has been thedevelopment of quantitative models that can predictchanges in vegetation structure (Shugart, 1998). Sev-eral of these models simulate the successional dynam-ics by accounting for the birth, growth, death, andinteractions with the environment for the hundredsof individual plants living on a small plot of land.The predictions of hundreds of these plots are thencombined to obtain a prediction of the change inecological landscapes. Because they simulate the fatesof each of the millions of plants involved in a landscapesuccession, these models are called ‘‘individual-based’’model. These models require considerable computa-tion to solve but they can be solved relatively easilyas a consequence of the increased computationalpower of modern computers.

An advantage of individual-based models is that thefollowing implicit simplifying assumptions associatedwith other modeling approaches (e.g., the Markov pro-cess or differential equation-based models) are not nec-essary: (i) The unique features of individuals are suffi-ciently unimportant to the degree that individuals areassumed to be identical, and (ii) the population is ‘‘per-fectly mixed’’ so that there are no local spatial interac-tions of any important magnitude. Most ecologists areinterested in variation in individuals (a basis for thetheory of evolution and a frequently measured aspect

of plants and animals) and appreciate spatial variationas being quite important. These assumptions seem par-ticularly inappropriate for trees which are sessile andwhich vary greatly in size over their life span. This maybe one of the reasons that tree-based forest models areamong the earliest and most widely elaborated of thisgenre of models.

One group of individual-based models simulates theestablishment, diameter growth, and mortality of eachtree in an area the size of a gap left by the death of acanopy tree and is called gap models. Gap models canbe used as an example of the more general individual-based modeling approach. In most gap models, calcula-tions are on a weekly to annual time step. Early gapmodels were developed for a size unit (approximately0.1 ha) approximately that of a forest canopy gap. Gapmodels feature relatively simple protocols for estimatingthe model parameters. For many of the more commontemperate and boreal forest trees, there is a considerableamount of information on the performance of individ-ual trees (growth rates, establishment requirements,and height/diameter relations) that can be used in esti-mating the parameters of such models. Gap models havesimple, general rules for interactions among individuals(e.g., shading and competition for limiting resources)and equally simple rules for birth, death, and growthof individual trees (based on the natural history ofeach species).

Gap models differ in their inclusion of processes thatmay be important in the dynamics of particular sitesbeing simulated (e.g., hurricane disturbance, flooding,and formation of permafrost) but share a common setof characteristics. These latter characteristics involvean emphasis on the demography and natural history ofplant species, relatively general rules for physiologicaltradeoffs among species, and an emphasis on the under-standing of successional processes at the whole plantlevel. Each individual plant is simulated as an indepen-dent entity with respect to the processes of establish-ment, growth, and mortality. This feature is common tomost individual tree-based forest models and providessufficient information to allow computation of species-and size-specific demographic effects. Gap model struc-ture emphasizes two features important to a dynamicdescription of vegetation pattern: (i) the responses ofthe individual plant to the prevailing environmentalconditions and (ii) how the individual modifies theseenvironmental conditions. The models are hierarchicalin that the higher level patterns observed (i.e., popula-tion, community, and ecosystem) are the integration ofplant responses to the environmental constraints de-fined at the level of the individuals.


IV. SUCCESSION AND BIODIVERSITY

Since the initial formulation of a progressive, holisticconcept of ecological succession, there has been a ten-dency to associate positive attributes to mature commu-nities. Hence, there is an expectation for increasingsuccessional age to be associated with increased bioticdiversity. There are certainly ecological systems (nota-bly many forest systems) that demonstrate this pattern,but there are other examples of ecological successionsthat show highest levels of species diversity (measuredas the number of species or by standard indices ofdiversity) at intermediate successional ages (e.g.shortgrass prairie in Colorado) or even at initial succes-sional stages (e.g., boreal forests in areas of Canada).Succession on sand dunes on the coast of Queensland,Australia, has the highest species richness in shrub-dominated communities at the beginning and end. Thepattern of diversity in different successions seems to bean idiosyncratic consequence of the attributes of theparticipating species and the environmental conditionsat a given location.

A. Biodiversity on Disturbed LandscapesAt the landscape level, the overall biodiversity can berelated to the frequency and intensity of disturbanceand there is evidence from both theoretical work andobservations that an intermediate level of ecologicaldisturbance can produce the most diverse landscapes(Huston and Smith, 1987). This occurs in part becausedisturbed landscapes have a mixture of species able tosuccessfully occupy the differently aged patches createdby the disturbance history of a given landscape. Onewould generally expect highly heterogeneous land-scapes to be more diverse. Disturbances also preventparticularly well-adapted species from occupying theentire landscape.

In 1979, J. P. Grime developed a ‘‘triangle’’ based onthree primary plant response strategies that can be usedto develop rules that can be used to predict the propor-tions of each strategy (and associated life-forms) ex-pected under a particular environmental regime. Grimerecognized two types of external factors limiting thebiomass of plants. The first was stress, involving theconditions that restrict plant productivity (shortage oflight, H2O, mineral nutrients, etc.). The second wasdisturbance, involving partial or total destruction ofplant biomass (activities of herbivores, diseases, fire,frosts, etc.). These two external factors can operateindependently so that there are four possible combina-tions of high or low stress and high or low disturbance.

Grime reasoned that the combined action of highstress and high disturbance created a condition fromwhich the vegetation could not regenerate. Low-stressand low-disturbance environments would ultimately fa-vor species that were able to compete effectively againstother species (competitor strategy), high-stress andlow-disturbance environments should be dominated byplants of species able to tolerate the particular stress(stress-tolerator strategy), whereas low-stress and high-disturbance environments should favor short-lived,fast-growing species (ruderal strategy). The generalproblem exemplified by Grime’s work in developmentof these primary plant strategies is one of identifyingplant functional types. An essential basis of this as wellas other functional classifications of plants is that of‘‘tradeoffs’’—the idea that, due to underlying rules thatderive from the species physiology and natural history,a single species cannot simultaneously be the best as astress tolerator, a competitor, and a ruderal species.

B. The Gain and Loss of Specieswith Succession

One can view the richness of species on a parcel of landwith the same disturbance history as a consequence ofthe gain and loss of species. The gain of species involvesfactors such as the migration and establishment of spe-cies (or the species gaining sufficient abundance or sizeso it can be sampled). There has been debate as towhether the gain of species at a site over succession isa consequence of the different species present at thesite growing and developing at different rates or is actu-ally due to species establishing themselves in an orderedsequence. The succession is a consequence of differentgrowth rates of an initial innoculum of seeds of all thespecies involved germinating and growing at differentrates and is sometimes called the initial compositionmodel (Egler, 1954). The idea that one set of speciesis added to the community after a previous set hasmodified the site and lost the site has been termed therelay floristics model (Egler, 1954). To separate thesetwo ‘‘models,’’ a particular succession study has to besampled intensively enough to actually detect all thespecies at a location, which rarely occurs. Consideringa wide range of successions of plant communities, ani-mals associated with succession, and heterotrophic suc-cessions (such as the progression of changes in a de-caying log), one finds that the appearance of specieswith abundances sufficient to be counted varies withthe particular collection of species and with the changesin the physical environment associated with the suc-cession.


FIGURE 4 Loss rates of species from different successional sequences (reproduced with permission from Shugart and Hett,1973).

The loss of species from successional communitiestends to be somewhat more regular in its pattern ofvariation. Considering a wide range of communities(both autotrophic and heterotrophic successions), onefinds that the rate of species local extinction throughsuccession tends to decrease logarithmically over suc-cessional time. Successional sequences are often sam-pled and reported using a more or less logarithmicsampling regime (e.g., communities in a successionalstudy might be sampled at 1, 2, 5, 10, 17, 35, 60, and100 years). This reflects the pattern that one tends tosample successional sequences using designs where aproportion of the species in a site of a given age werefound in the younger sites. Figure 4 illustrates thispattern for five different successional sequences in dif-ferent locations. The initial loss rates of species are onthe order of approximately 10% of the species found ona plot disappearing each year. In the later successionalstages, this decreases to less that 0.1% species lost per

year (Fig. 4). Because this pattern occurs across a widerange of successional sequences, this does not appearto be a consequence of the later successional speciesliving longer (as is often the case in forest successions).

See Also the Following ArticlesDISTURBANCE, MECHANISMS OF • ECOSYSTEM, CONCEPTOF • ECOSYSTEM FUNCTION, PRINCIPLES OF • INDICATORSPECIES • LANDSCAPE DIVERSITY

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