This article was originally published in the Encyclopedia of Inland ... - Miami … et... · 2009. 3. 27. · This article was originally published in the Encyclopedia of Inland Waters

This article was originally published in the Encyclopedia of Inland Waters published by Elsevier, and the attached copy is provided by Elsevier for the

author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in

instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator.

All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open

internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through

Elsevier's permissions site at:

http://www.elsevier.com/locate/permissionusematerial

Vanni M J, Duncan J M, González M J and Horgan M J. (2009) Competition Among Aquatic Organisms. In: Gene E. Likens, (Editor) Encyclopedia of Inland

Waters. volume 1, pp. 395-404 Oxford: Elsevier.

Author's personal copy

Competition Among Aquatic OrganismsM J Vanni, J M Duncan, M J Gonzalez, and M J Horgan, Miami University, Oxford, OH, USA

ã 2009 Elsevier Inc. All rights reserved.

Introduction

Competition can be defined as a negative interac-tion between organisms resulting from a sharedrequirement for a resource that is in limited supply.When competing, individuals use limiting resourcesthat would otherwise be available for other indivi-duals. Consequently, competing individuals obtainresources at lower rates, and are likely to growmore slowly, have fewer offspring, and have a lowerchance of surviving than they would in absence ofcompetition.Ecologists classify competition based on the iden-

tity of interacting individuals. Intraspecific competi-tion occurs between individuals of the same species,while interspecific competition occurs between indi-viduals of two or more species. Competition is alsocategorized according to the mechanism by which itoccurs. Interference competition occurs when an indi-vidual directly prevents another from obtainingresources. For example, a fish that aggressively chasesaway other fish to monopolize a nesting site is aninterference competitor. In contrast, individuals canalso compete via exploitative competition, whichoccurs when individuals deplete resources thatwould otherwise be available to others; in this casethere is no direct agonistic interaction. For instance,the depletion of nutrients by an individual algal cellreduces nutrient availability to other algal cells.Both intraspecific and interspecific competition can

occur simultaneously and both can occur via exploit-ative or interference mechanisms. Most groups ofaquatic organisms (bacteria, protists, algae, plants,and animals) compete both intraspecifically andinterspecifically, and via both mechanisms, althoughcompetition does not always occur. Because competi-tion occurs within a network of species interactions(Figure 1), the outcome of competition can depend onthe presence of other species, especially predators ofpotential competitors.The study of competition among freshwater organ-

isms has a long history, parallel with that of competi-tion studies in terrestrial environments. Aquaticecologists were also among the first to study thecomplexities of competitive interactions, includingcompetition in size-structured populations and vari-able environments and the interactions between com-petition and predation.

Encyclopedia of Inland Wat

Intraspecific Competition

Intraspecific competition is a common and importantinteraction for many aquatic species. A classic labora-tory study by L. B. Slobodkin showed reduced growth,survival, and reproduction of Daphnia when popula-tion size was high, as a result of exploitative competi-tion, and served as the basis of subsequent studies oncompetition in zooplankton. One outcome of intra-specific competition is logistic population growth(called sigmoidal or S-shaped growth); populationgrowth is nearly exponential when numbers are low,but then growth rate is reduced progressively as thepopulation expands, and eventually the populationapproaches its carrying capacity. Logistic growth ofaquatic populations has been demonstrated repeat-edly in laboratory studies of aquatic algae, bacteria,protozoans, and metazoans. It has been demonstratedless often in the field, probably because it is difficult toobserve colonization events that usually precede logis-tic growth.

Interference competition can also be an importantmechanism of intraspecific competition. Many zoo-plankton taxa make autotoxins, which are chemicalsthat inhibit feeding or increase mortality in conspeci-fics. For example, individuals of the rotifer Synchaetapectinata produce an autotoxin that reduces growthrate and increases mortality of other individuals ofthe same species. Autotoxin effects have also beendemonstrated in a marine phytoplankton species. Itis likely that autotoxic effects are common amongfreshwater organisms, but little research has beendirected toward this phenomenon.

One consequence of intraspecific competition isstunted growth of fish in dense populations. Fisheriesmanagers observed long ago that fish in a crowdedpopulation (or with low food availability) often showlow (stunted) growth rates and thus are much smallerthan individuals growing in a population with fewindividuals (or with abundant resources). Stuntedgrowth has many implications. Small and large fishoften rely on different food resources, so a stuntedpopulation may have food web effects different fromthose of a population with larger individuals. In addi-tion, smaller individuals may be more vulnerable topredators, especially other fish that are gape-limited.Stunted populations also may be less desirable forrecreational and commercial harvest.

395ers (2009), vol. 1, pp. 395-404

Potentialcompetitors

Keystonepredation

Intraguildpredation

Apparentcompetition

or mutualismExploitativecompetition

Interferencecompetition

Intraspecificcompetition

Predators

Resources

Figure 1 Diagram representing different kinds of competitive interactions. Potential competing species are shown in the middlerow, their resources in the bottom row, and their predators in the top row. Arrows indicate direction of consumptive flows. The curved

arrow for intraspecific competition denotes competition among individuals of the same species, and the two-headed arrow for

interference competition indicates direct agonistic competition. Modified from Blaustein L and Chase JM (2007) Interactions between

mosquito larvae and species that share the same trophic level. Annual Review of Entomology 52: 489–507.

396 Biological Integration _ Competition Among Aquatic Organisms


Intraspecific competition can also lead to increasedvariability in body size. Competition is often highlyasymmetric, meaning that it affects some individualsmuch more than others. This could be because someindividuals are inherently better competitors, orbecause some individuals arrive at a site (or areborn) earlier than others and thus preempt resources.Superior or early-arriving individuals may reach arelatively large size while inferior competitors orlate arrivers suffer reduced body size. Often there isa gradient in competitive ability or arrival times, anda population growing under intraspecific competitiondisplays a wide distribution of sizes among indivi-duals of equal age. Such asymmetries have beendemonstrated in fish, amphibians, and insects. Differ-ences in size initiated by intraspecific competition canbecome magnified over time by size-dependent com-petitive superiority. An individual that gains an initialadvantage (e.g., by arriving early or by having aslightly larger initial size) will grow more rapidlythan the average individual. This individual may usea wider range of resources (e.g., larger fish can con-sume a wider range of prey items), leading to a furthergain in size relative to other individuals. This differ-ence in size may become more pronounced over time.Size differences can also set up hierarchies in which

large individuals are superior via interference compe-tition because larger individuals may be better atguarding territories, gaining access to mates, or sur-viving aggressive interactions with conspecifics.

Interspecific Competition

The study of interspecific competition among aqua-tic species has an accomplished history. G. F. Gause

Encyclopedia of Inland Water

(1910–1986), who studied competition among aquaticprotozoans (Paramecium and others), showed thatfor some pairs of species, a superior competitoralways reduced the inferior species to extinction. Inother cases, two species coexisted but neverthelessshowed reduced stable population sizes when grownin each other’s presence, compared with when eachwas grown alone. These studies had a large effect onthe growing field of ecology and stimulated muchexperimental work on competition in aquatic andterrestrial communities. In the 1950s, P. W. Frank’slaboratory studies of Daphnia populations wereamong the first experimental studies of interspecificcompetition in animals. They revealed competitiveasymmetry between two similar species, causing onespecies consistently to outcompete the other.

Early laboratory studies such as Frank’s laid thegroundwork for more sophisticated studies of inter-specific competition within food webs of dozens orhundreds of potentially interacting species. Modernstudies include many factors that can affect competi-tion, such as the size structure of populations, abioticconditions, and the abundance, ratio, and temporalvariation of limiting resources. Much progress hasbeen made in understanding these and other aspectsof competition in freshwater ecosystems. Studies ofcompetition among aquatic organisms have contrib-uted greatly to our understanding of aquatic commu-nities and have enriched the broader field of ecology.

Body Size and Competition

The species with the larger body size is often compet-itively dominant over smaller species. In a highlyinfluential study published in 1965, J. L. Brooks and

s (2009), vol. 1, pp. 395-404

Biological Integration _ Competition Among Aquatic Organisms 397


S. I. Dodson proposed the ‘size-efficiency hypothesis’for freshwater zooplankton, which states that largerspecies outcompete smaller species because the for-mer are more efficient grazers of phytoplankton.Thus, when fish, which preferentially consume largerzooplankton species, are scarce, large species shoulddominate because they competitively exclude smallerspecies. Field studies have repeatedly shown thatlarge zooplankton species usually dominate whenthe intensity of fish predation is low, while smallspecies are common only when fish predation isintense and large zooplankton are scarce. Experimen-tal studies also show that large cladoceran speciesshow positive population growth at lower food con-centrations than do smaller cladocerans and rotifers;specifically, they are more tolerant of low food con-centrations that can be produced during competitivesituations.While much evidence supports the size-efficiency

hypothesis, other research shows that larger speciesdo not always prevail in competitive situations. Forexample, small-bodied cladocerans and rotifers oftencoexist, even though they differ greatly in size. Otherresearch shows that multiple kinds of environmentalconditions determine whether the large or small spe-cies is competitively dominant. For example, thedominant species amongDaphnia species of differentsize depends on food abundance and quality. Also,high concentrations of inorganic particles (e.g., siltand clay) or filamentous cyanobacteria can changethe outcome of competition; in general, these parti-cles favor small species because they reduce the feed-ing efficiency of large species. Several studies alsoshow that the dominance of large zooplankton spe-cies, and the associated scarcity of small species, inlakes with low fish predation may be caused by pre-dation of larger zooplankton on smaller zooplanktonrather than competitive exclusion based on sharedfoods. Thus, while large zooplankton species areoften better competitors than small species, this isnot always true, and it is not clear to what extentinterspecific competition can explain the dominanceof large zooplankton in lakes where planktivorousfish are scarce.Larger fish often are better competitors than their

smaller counterparts in the absence of predation.However, because body size distributions and feedinghabits overlap to a great extent among species, theoutcome of competition can depend on the size dis-tributions of competing species (Figure 2). Newlyhatched fish are small, and even species that differgreatly in maximum size have offspring that are ofsimilar size. As fish grow they feed on larger prey, sospecies that differ greatly in maximum size can over-lap extensively in diet, especially as juveniles. Evena species that is prey to another fish species can


compete with its predator (a piscivore) under someconditions because juveniles of the piscivore speciesmay share the same food resources as adults of theprey species. For example, juvenile bass and adultsunfish both consume invertebrates. Under these con-ditions, the prey species (sunfish) may actually be thebetter competitor and thereby limit the survival ofyoung predator (bass) individuals (Figure 2). This cancreate a bottleneck that limits the survival of juvenilepiscivores, preventing the piscivore from establishing aviable population. In contrast, if the piscivore popula-tion is dominated by large individuals predation maylimit the number of prey individuals surviving, therebyreducing competition between adults of the prey spe-cies and juveniles of the piscivore. Such interactions arelikely to be important among most species, but havebeen studied primarily for fish and amphibians.

Among aquatic primary producers, the importanceof size in competition may depend on the limitingresource. For example, aquatic plants (includingmacrophytes and algae) can intercept light, thusshading out smaller species. Even so, sometimesbeing large is disadvantageous for benthic (bottomdwelling) algae because of increased susceptibility oflarge algal mats to grazing or physical scouring. Also,for phytoplankton and bacteria, smaller species maybe competitively superior when nutrients are limitingbecause small size provides a higher ratio of surfacearea to volume, which facilitates uptake of limitingnutrients.

Priority Effects

A priority effect occurs when a competitively domi-nant species is not predetermined according to speciesidentity, but rather by the order of birth or arrival.This has been shownmost convincingly in amphibiansof temporary ponds, where body size plays a key role.The first species arriving at a pond to breed has thecompetitive advantage because its tadpoles are largerrelative to tadpoles of competitors. Therefore, thecompetitive dominant depends on body size, whichis strongly influenced by arrival time. In amphibiansof temporary ponds, body sizemay bemore importantthan species identity in determining competitive dom-inance. Priority effects have also been observed inphytoplankton, zooplankton, and insects.

Competition and Niche

A species’ ecological niche can be defined as the rangeof resources and conditions allowing the species tomaintain a viable population. Theoretically, if twospecies have the same niche, one species will excludethe other. The corollary is that the niches of coexistingspecies must differ. Niches of coexisting species can

ers (2009), vol. 1, pp. 395-404

Adultbass

Juvenilebass

Juvenilebluegill

Juvenilebluegill

Juvenilepumpkinseed

Adultpumpkinseed

Adultbluegill

Adultbluegill

Invertebrates

Zooplankton Littoralinvertebrates

Snails

Figure 2 Competition in size-structured fish populations. Consumptive flows are indicated by the solid arrows and competitive

interactions by the dashed, two-headed arrows. The top diagram shows size-specific interactions between bluegill (Lepomis

macrochirus) and largemouth bass (Micropterus salmoides). The bottom diagram shows size-specific competition between bluegill and

pumpkinseed (Lepomis gibbosus). On the right are photographs of largemouth bass (top), bluegill (middle), and pumpkinseed (bottom).Diagrams based on information from Olson MH, Mittelbach GG, and Osenberg CW (1995) Competition between predator and prey:

resource-based mechanisms and implications for stage-structured interactions. Ecology 76: 1758–1771; Osenberg CW, Mittelbach,

GG, and Wainwright PC (1992) Two-stage life histories in fish: The interaction between juvenile competition and adult performance.

Ecology 73: 255–267; and Persson L (1988) Asymmetries in competitive and predatory interactions in fish populations. In: Ebenman Band Persson L (eds.) Size-Structured Populations: Ecology and Evolution, pp. 203–218. Berlin: Springer.



be similar, but not identical. Such ‘niche partitioning’has been shown in many aquatic organisms. Nichepartitioning of fish species provides examples. Forinstance, sunfish (family Centrarchidae) preferen-tially feed in a pond habitat (e.g., littoral vegetationor open water) where they gain the most energy perunit effort. Habitat preference (hence niche occu-pancy) depends, however, on the presence of othercompetitors; a species may have a wide niche inabsence of competitors but a narrow niche in thepresence of a competitor. Furthermore, the niche ofa species often varies predictably with age (size). Such‘ontogenetic niche shifts’ often are driven by changesin food, as discussed earlier. Within a species, smallindividuals may have a different feeding niche thanlarger individuals, and feeding often is associatedwith habitat preference. For example, small fish mayfeed on zooplankton in open waters while larger fish


may feed on benthic invertebrates near shore. Asmentioned earlier, the strength and outcome of inter-specific competition may depend on the size (age)structure of the populations. Thus, the niche dimen-sions of a species may depend on the presence orabsence of a competing species and the size distribu-tions of competing species. Furthermore, habitatselection also is strongly influenced by predation. Inthe absence of predators, small individuals may for-age where the rate of food intake is optimal. In thepresence of a predator, these individuals may feedwhere risk of predation is lower.

Resource-Ratio Competition

D. Tilman developed a model of competition amongphytoplankton that has influenced ecological researchto a great extent over the past 25 years. The model

s (2009), vol. 1, pp. 395-404



uses the minimum resource requirements (R*) of spe-cies and the ratio at which resources are supplied (theresource ratio) to predict competitive outcomes. Animportant prediction of the model is that the resourceratio determines the outcome of competition, i.e.,which species will eliminate competitors or whetherspecies will coexist.As an example, two species of algae may be com-

peting for nitrogen and phosphorus (Figure 3(a)). Ifone species has a lower minimum requirement (alower R*) for both N and P, that species will alwaysdrive the other to extinction via exploitative competi-tion. When the two species are better competitors fora different resource, the resource ratio determines theoutcome (Figure 3(b)). In this case, one species (spe-cies A) is the better competitor for P and competi-tively excludes the other species (species B) whenP is in short supply, i.e., when the N:P supply ratiois high. When N is limiting (i.e., the N:P supply ratiois low), species B will exclude species A. When theresource ratio is intermediate, the two species coexistbecause each species is limited by a different resource(Figure 3(b)). Thus, when two species (A and B)compete for two resources and the two are superiorcompetitors for different resources, three outcomes

NA*

NB*

Resour

Res

ourc

e 1

(e.g

., N

)

PA* PB*

(b)Resource 2 (e.g., P)

Res

ourc

e 1

(e.g

., N

)

A

B

PA* PB*

NA*

NB*

S2

(a)

S1

Figure 3 Resource-ratio competition. (a) Species A is the better cominimum concentrations of N and P. When competition is for P (Sup

towards their carrying capacities and reduce concentrations of N and

P concentration is driven below Species B’s minimum P concentratio

exhibits positive population growth, but Species B declines, and SpeP. Because Species A has a lower minimum concentration for N, it wil

Thus, Species A is always the superior competitor, regardless of the ra

competitor for one of the resources, and the outcome of competitionthe better competitor for P (PA*<PB*), and thus will outcompete Spe

supply ratio is high (Supply Point S1). When N is limiting (i.e., the N:P s

win because it is the better competitor for N, i.e., NB*<NA*. The two sp

P supply ratio falls within the triangle indicated by the dashed lines, apromote species coexistence. Here, five species (A–E) compete for t

species can coexist and all others go extinct. The identity of the coex

ratio. However, when the N:P supply ratio varies temporally, several

dominant changes as the supply ratio varies. Temporal variation in thecoexistence regions of all five species. Thus, all five species can pers


are possible: species A wins, species B wins, or A andB coexist. The outcome depends on the resource ratio;the outcome does not depend on the absolute concen-trations or supply rates, because the organisms reduceresources to equilibrium concentrations defined byR*, regardless of supply rates.

The Tilman resource-ratio model has been appliedextensively to phytoplankton, and it is now generallyaccepted that resource ratios are important in deter-mining phytoplankton community composition. Lab-oratory competition experiments generally confirmthe veracity of the model under equilibrium condi-tions, although surprisingly few studies have explic-itly tested the model’s predictions. For example, whendiatom species compete for silica (Si) and phosphorus(P), the outcome of competition depends on theSi:P ratio. Field surveys also offer support for theresource-ratio model. Different diatom species domi-nate at different Si:P ratios, as predicted by laboratorycompetition experiments. Similarly, low N:P supplyratios can favor N-fixing cyanobacteria (taxa forwhich N* is essentially zero because they can utilizeatmospheric N2) whereas higher N:P ratios oftenfavor taxa other than cyanobacteria. In addition, atleast one study has successfully used the resource

Resource 2 (e.g., P)

Res

ourc

e 1

(e.g

., N

)

D

ABC

EA

B + C

A + B

C + DD + E

B

CDE

(c)

S2

S3

S1

ce 2 (e.g., P)

B

A

mpetitor for both resources because it can survive at lowerply Point 1, or S1), under equilibrium conditions the species grow

P (indicated by the arrow emanating from S1). Eventually the

n (PB*) but remains above PA*. When this happens, Species A still

cies A eventually excludes Species B via competition forl also outcompete Species B when N is limiting (Supply Point S2).

tio at which resources are supplied. (b). Each species is the better

depends on the ratio at which N and P are supplied. Species A iscies B when P is in short supply relative to N, i.e., when the N:P

upply ratio is low, as indicated by Supply Point S2), Species B will

ecies will coexist if the resource ratio is intermediate, i.e., if the N:

nd illustrated by Supply Point S3). (c). Nonequilibrium conditionswo resources. When the supply ratio is constant, only one or two

isting species (i.e., A, AþB, B, etc.) depends on the N:P supply

species can coexist because the identity of the competitive

N:P supply ratio is indicated by the oval, which encompasses theist.

ers (2009), vol. 1, pp. 395-404



ratio model to predict the competitive outcomebetween two rotifer species feeding on two phyto-plankton species.

Diatomsand

greenswin

No blooms

Critical depth

Criticalturbulence

Microcystis winsTurb

ulen

t diff

usio

n (c

m2 /

s−1)

100

10

1

0.1

0.010.1 1 10 100 1000

Water-column depth (m)

Figure 4 Effects of a variable light climate on phytoplankton

competition. The diagram shows model predictions, defined by

the zones labeled ‘diatoms and greens win,’ ‘Microcystis wins,’and ‘No blooms.’ The shaded region indicates conditions that are

predicted to lead to coexistence of the two algal groups. Diatoms

and green algae are predicted to competitively excludeMicrocystis when light intensity is variable (i.e., when turbulent

diffusion, or lake mixing, is high), whereas Microcystis is

predicted to win when conditions are calm and variability in light

intensity is low. The points and error bars show the outcome,under normal conditions (open circle) and when the lake was

artificially mixed to increase turbulence (solid circle). From

Huisman J, Sharples J, Stroom JM, et al. (2004) Changes in

turbulent mixing shift competition for light betweenphytoplankton species. Ecology 85: 2960–2970.

Competition under Variable Conditions

Many competition models assume equilibrium condi-tions and many experimental studies employ suchconditions. Yet in nature, equilibrium conditionsrarely occur, so it is important to understand howcompetition plays out under variable conditions.Ecologists have begun this quest with both theoryand experiments. The general consensus is that spa-tial or temporal variability tends to increase thenumber of species that can coexist. Furthermore, innature many species often coexist. G. E. Hutchinson(1903–1991) coined the famous term ‘paradox of theplankton,’ which asks why well-mixed pelagic envir-onments of lakes or oceans maintain a large numberof phytoplankton species (tens or hundreds), eventhough the component species largely require thesame resources. Therefore, niche diversificationamong species in mixed water should be minimal.Hutchinson suggested that under variable conditions,the identity of the competitive dominant changes,usually too quickly for competitive exclusion tooccur.Disturbances, i.e., discrete events that impose mor-

tality (such as storm flushing a stream or wind mixinga lake), can also reduce the severity of competition byreducing populations to densities below those atwhich competition occurs. The role of disturbancemay be particularly important in streams, wherehigh flow events can impose considerable mortalityon organisms, effectively ‘re-setting’ competitiveinteractions.The resource-ratio model can be extended to vari-

able conditions (Figure 3(c)). When the resource sup-ply ratio varies temporally, several species can coexistbecause the identity of the competitive dominantchanges when the supply ratio varies. If the N:Pratio changes more rapidly than the time needed forcompetitive exclusion to occur, more species can per-sist than under equilibrium conditions. Experimentalwork supports this hypothesis. For example, pulsednutrient supply (Si and P) increased the number ofcoexisting phytoplankton species when comparedwith supply at steady state.Exposure to light is also variable in mixed water;

phytoplankton experience high light intensity whenthey circulate near the surface and low lightwhen they are in deeper water. Coexistence dependson the species’ reactions to nutrients and light. Forinstance, when air was bubbled into the bottom of alake to increase turbulence (mixing), dominance


shifted from the buoyant, toxin-producing cyanobac-terium Microcystis to preferable diatom and greenalgal species because diatoms and green algaerespond better than cyanobacteria to fluctuatinglight (Figure 4). In addition to demonstrating theimportance of temporal variation in mediating com-petition, this example shows how knowledge ofcompetition can be used to enhance water quality.

Interference Competition

Many aquatic species compete with each other viaagonistic (aggressive) interactions. For example, manyspecies, including many fish and insects, activelyexclude other individuals (conspecifics as well asother species) from territories. In streams, somebenthicinsects (e.g., caddisflies and blackflies) defend feedingterritories where they capture suspended particles.Their densities can be quite high, leading to simulta-neous exploitative and interference competition.Indeed, many benthic species (plants or animals) prob-ably simultaneously compete via both interferencecompetition for space and exploitation for food. Forexample, zebra and quagga mussels (Dreissena poly-morpha andD. bugensis) can grow densely on shells oflarger bivalve species, leading to reduced growth or

s (2009), vol. 1, pp. 395-404



mortality of the larger bivalve species via both interfer-ence and exploitative competition.Some zooplankton species mechanically interfere

with each other in a size-dependent manner.Daphniacan capture rotifers while feeding and in the processrotifers may be damaged or even ingested (Figure 5).Generally, such mechanical interference is importantonly between very large Daphnia and small rotifers.When the size difference is less or when the rotifer hasspines or armor, interference competition rarelyoccurs. Thus, both interference and exploitativemechanisms can contribute to the competitive superi-ority of large zooplankton.Allelopathy can be important among aquatic organ-

isms, particularly in algae, plants, and bacteria. Allmajor groups of aquatic primary producers generateallelochemicals that act against competitors, espe-cially interferingwith enzymeactionorphotosynthesis.Emergent wetland plants use allelochemicals againsteach other. Floating macrophytes such as Eichhorniaare allelopathic as they need to compete with otherprimary producers for nutrients dissolved in the water.Rooted macrophytes often get nutrients from sedi-ments, but they can be shaded by epiphytic algae thatgrow on them or phytoplankton that grow abovethem. Thesemacrophytes often use allelopathy to com-pete with primary producers that interfere with theirability to gather light. Such interactions are involved inabrupt shifts between phytoplankton dominance and

200 µm

D. p.

S. p.S. o.

K. cr.K. c. P. r.

K. t.

K. b.

C. u.

A. p. A. e.

Figure 5 Interference competition between Daphnia and rotifers (dr

of the rotifer taxa to interference competition with Daphnia are shownto interference from Daphnia pulex. Ecology 69: 1826–1838.


macrophyte dominance in shallow lakes. Allelopathyalso occurs among benthic and pelagic microalgaeand cyanobacteria. Allelopathy appears to be strongestin still waters where released chemicals can affect neigh-boring competitors. In streams and rivers, allelochem-icals would be washed away without providing benefitto the organism that produced them.The exceptionmaybe localized interactions among stream benthic algae.

Competitive Effects of Exotic Species

Exotic species are those introduced by human activ-ities to areas where they are not native (Figure 6).Some exotics have significant effects on native spe-cies, including those mediated by competition. Non-native zebra and quagga mussels (Dreissena spp.)can competitively exclude or reduce populations ofmollusks native to North America, as mentionedearlier. Via their filtering, dreissenids have greatlyreduced phytoplankton in some environments (e.g.,the Hudson River, New York, USA), and this has pro-bably reduced the abundance of other species relyingon phytoplankton. In addition to dreissenids, exam-ples of competitive effects of exotics on native speciesinclude fish (e.g., effects of brown trout on galaxiidsin New Zealand streams), crustaceans (e.g., effects ofexotic crayfishes on native crayfishes around theworld), and macrophytes (e.g., effects of water hya-cinth Eichornia on native plants).

Rel

ativ

e su

scep

tibili

ty

3.3

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0(2) (2) (1) (2) (3) (2) (2) (3) (2) (2) (1)

Kel

licot

tia

bos

toni

ensi

sA

spla

nchn

a

prio

dont

aC

onoc

hilu

s

uni

corn

is

Ker

atel

la c

rass

a

Pol

yart

hra

rem

ata

Ker

atel

la te

stud

o

Syn

chae

ta p

ectin

ata

Ker

atel

laco

chle

aris

f. ty

pica

Asc

omor

pha

ecau

dis

Syn

chae

ta o

blon

ga

Syn

chae

taob

long

a (ju

veni

les)

Rotifer

awn to the same scale at left). At right, the relative susceptibilities

. From Gilbert JJ (1988) Susceptibilities of ten rotifer species

ers (2009), vol. 1, pp. 395-404

(a) (c)

(b) (d)

Figure 6 Representative invasive exotic species that negatively affect native species via competition. (a) The macrophyte Eichornia

crassipes (water hyacinth), which can outcompete native plants for light and nutrients; (b) the zebra mussel (Dreissena polymorpha)

growing on top of a native bivalve; (c) brown trout (Salmo trutta), which often reduces the abundance of native fishes; and (d) the rustycrayfish (Orconectes rusticus), which has numerous effects on native crayfish and other benthic invertebrates.



Evolutionary Consequences of Competition

Over many generations competition can cause sub-populations to diverge morphologically, ecologically,and genetically as each group becomes specialized inways that reduce competition. Genetic differencescan proceed to speciation, meaning that the two sub-populations become two reproductively isolated spe-cies. A familiar example is the spectacular adaptiveradiation of cichlids in ancient tropical lakes. Overmillions of years, dozens to hundreds of species haveemerged, each with slightly different niches, pre-sumably as a consequence of historical competitiveinteractions. Speciation events have also occurredin sticklebacks in small temperate lakes. Within alake, two different species of sticklebacks can existbecause one is a pelagic specialist feeding on zoo-plankton and the other is a benthic specialist. Inlakes with just one species, that species exhibits char-acteristics intermediate of the two specialist species.Detailed experimental and molecular studies have


verified that recent speciation produced these species,and that speciation is strongly driven by competition.

When competition is intense, natural selection pro-motes divergence of competing species to minimizecompetition. Given such an evolutionary history,extant species may show little evidence of competi-tion. Even so, a lack of present-day competition couldalso mean that the species never have competed. Ecol-ogists favoring the former explanation for lack ofcompetition, without necessarily having adequateevidence, have sometimes been accused of invokingthe ‘ghost of competition past.’ Recently, ecologistshatched Daphnia from dormant resting eggs in lakesediments and found that clones isolated from oldersediments (several decades) were strongly influencedby interspecific competition, while clones hatchedfrom more recent sediments showed reduced compe-tition and could coexist with other extant Daphniaspecies. These results are consistent with the hypoth-esis that natural selection typically reduces the inten-sity of interspecific competition.

s (2009), vol. 1, pp. 395-404



Competition within Food Webs

Competition occurs within food webs through directand indirect species interactions (Figure 1). Thus, thenature and outcome of competition often depends onspecies other than those competing for resources.

Effects of Predation on Competition

Many studies show that predation can reduce theabundance of potential competitors (Figure 1), thusincreasing resources for the competitors. For exam-ple, predators can depress herbivores, leading to anincrease in growth of plants. This process is knownas a trophic cascade. Even if the resources of theprey do not increase because of predation, each sur-viving individual of the prey species will have moreresources. Thus, predation can reduce intensity ofcompetition among prey species.If predators selectively prey on a competitive domi-

nant, predation can promote coexistence among preyspecies. Such ‘keystone predation’ was first describedby R.T. Paine in his classic studies in the marine inter-tidal zone, but has also been shown in freshwatercommunities. For example, zooplankton can promotespecies diversity of phytoplankton by grazing mostheavily on the superior competitors. A recent syn-thesis suggests that predation is most likely to pro-mote coexistence of prey if competitors (1) competefor space; (2) are efficient consumers of their ownresources; (3) are consumed by several specialist pre-dators; or (4) show a trade-off between competitiveability versus defense against predation. One or moreof these conditions is often met in aquatic commu-nities. Predation does not always decrease the inten-sity of competition, but rather can have various effectson it. For example, effects of predation may interactwith environmental conditions such as productivity,and may affect superior and inferior competitorsdifferently.Pathogens can have effects similar to those

described earlier for predators. For example, fungalpathogens can alter competition between amphibiansby inducing species-specific effects on tadpole growthrate and time to metamorphosis. The interactionbetween competition and disease can also go bothways – for example, larval mosquitoes whose growthis reduced by interspecific competition may showincreased prevalence of viral pathogens.Predators often induce behavioral changes in prey

that indirectly affect competitive interactions amongprey. When predators are present, prey often mustoccupy habitats that are suboptimal for resource acqui-sition but offer protection frompredators. The prey thusmay suffer increased competition, leading to decreasedgrowth, reproduction, or survival. For example, when


predation in intense, herbivorous stream insects spendmore time on the bottom of rocks where the abundanceof algae (their food) is much lower than on the uppersurface. Similarly, small fish sometimes aggregate inlittoral vegetation to avoid piscivores. In these cases,prey experience increased competition because of theirhigh densities in a habitat that offers them protectionfrom predators.

Intraguild Predation

A species may both compete with and prey uponanother species (Figure 1). This phenomenon isknown as ‘intraguild predation’ because predatorsoften belong to the same ecological guild. Intraguildpredation is common among aquatic organisms andoften is mediated by interactions that vary with size.For example, juveniles of a piscivorous fish can com-pete with adults of prey fish (as discussed earlier).Similarly, juvenile copepods (nauplii) compete withherbivorous zooplankton, but adults of the samecopepod species prey on herbivorous zooplankton.Intraguild predation is common in many interactionsbetween size-structured species, including fish,insects, crustaceans, and amphibians.

Apparent Competition and Apparent Mutualism

When competitor species share a common predator,they may interact indirectly through the predator,positively or negatively (Figure 1). If the presence ofprey species A causes a numerical increase of thepredator, this may negatively affect prey speciesB. This is called ‘apparent competition’ because thenegative relationship between the two prey speciesmay be mistaken for interspecific competition. Thisinteraction between prey species may, however, bepositive over shorter time scales. If the predatorpreys heavily on prey species A, in the short term thiscan reduce predation on prey species B. This interac-tion is referred to as ‘apparent mutualism.’ In tempo-rary ponds, prey species may be apparent mutualistswith each other early in the season, but apparentcompetitors later in the season, i.e., after the predatorshows a numerical response that subsequentlydepresses the abundance of one of the competitors.

Competition and Regime Shifts

Competition for light and nutrients also can play arole in determining alternate ecosystem states, orregime shifts, in shallow lakes. When phytoplanktonare abundant, they can reduce the amount of lightreaching macrophytes. Such a turbid lake may havelittle littoral vegetation. In contrast, when phyto-plankton are scarce, macrophytes may have more

ers (2009), vol. 1, pp. 395-404



light and become more abundant. In clear lake con-ditions, macrophytes sequester nutrients and can alsosuppress phytoplankton via allelopathy, as discussedearlier. In contrast, under turbid lake conditions, phy-toplankton sequester nutrients that would otherwisebe locked up in macrophytes and can outcompetemacrophytes for light. It is often extremely hard toforce a lake to change state. Competition is one ofmany mechanisms that can be important in initiatingand maintaining alternate states for lakes.

Conclusions

Competition influences the organization of aquaticcommunities. Early laboratory studies showed thepotential importance of interspecific competitionand laid the groundwork for subsequent theory andfield-based studies. Future studies will continue toelucidate the ways in which competition operates,and more effectively incorporate competition intoissues of environmental concern such as the competi-tive effects of exotic species, the interactions betweencompetition and disease, and the role of competitionin conservation of biodiversity.

Glossary

Allelopathy – The production and release of chemicalsubstances by an organism that inhibit the growth,survival, or reproduction of another organism.

Apparent competition – An interaction betweentwo potentially competing species that share a pred-ator, whereby one potential competitor speciescauses the predator to increase in abundance, lead-ing to a decrease in the other potential competitorspecies.

Apparent mutualism – An interaction between twopotentially competing species that share a predator,whereby the predator prefers to feed on one poten-tial competitor species, thereby alleviating preda-tion on, and hence benefiting, the other potentialcompetitor species.

Exploitative competition – A form of competitionin which competing individuals do not directlyinteract, but rather compete by consuming sharedlimiting resources.

Interference competition – A form of competition inwhich competing individuals directly interact witheach other agonistically.

Interspecific competition – Competition betweenindividuals of different species.


Intraguild predation – A predator–prey interaction inwhich the predator and prey also compete with eachother.

Intraspecific competition – Competition betweenindividuals of the same species.

Keystone predation – Predation on the dominantcompetitor that alleviates competition among preyspecies.

Niche – The range of conditions and resources allow-ing a species to maintain a viable population.

Priority effect – A competitive interaction inwhich thespecies arriving (or being born) first outcompetesthe other species.

Further Reading

Blaustein L and Chase JM (2007) Interactions between mosquito

larvae and species that share the same trophic level. AnnualReview of Entomology 52: 489–507.

Chase JM, Abrams PA, Grover JP, et al. (2002) The interaction

between predation and competition: A review and synthesis.

Ecology Letters 5: 302–315.Ebenman B and Persson L (1988) Size-Structured Populations:

Ecology and Evolution. Berlin: Springer.Gilbert JJ (1988) Suppression of rotifer populations by Daphnia:

A review of the evidence, the mechanisms, and the effects onzooplankton community structure. Limnology and Oceanogra-phy 33: 1286–1303.

Gross EM (2003) Allelopathy of aquatic autotrophs. CriticalReviews in Plant Sciences 22: 313–339.

Holt RD and Lawton JH (1994) The ecological consequences of

shared natural enemies. Annual Review of Ecology and System-atics 25: 495–520.

Miller TE, Burns JH, Mungia P, et al. (2005) A critical review oftwenty years’ use of the resource ratio theory. American Natu-ralist 165: 439–448.

Olson MH, Mittelbach GG, and Osenberg CW (1995) Competi-tion between predator and prey – Resource-based mechanisms

and implications for stage-structured dynamics. Ecology 76:

1758–1771.

Passarge J, Hol S, Escher M, and Huisman J (2006) Competitionfor nutrients and light: Stable coexistence, alternative stable

states, or competitive exclusion? Ecological Monographs 76:

57–72.

Polis GA, Myers CA, and Holt RD (1989) The ecology and evolu-tion of intraguild predation – Potential competitors that eat each

other. Annual Review of Ecology and Systematics 20: 297–330.Schluter D (2001) Ecology and the origin of species. Trends in

Ecology and Evolution 16: 372–380.

Simon KS and Townsend CR (2003) Impacts of freshwater invaders

at different levels of ecological organisation, with emphasis on

salmonids and ecosystem consequences. Freshwater Biology 48:982–994.

Tilman D (1982)Resource Competition and Community Structure.Princeton, NJ: Princeton University Press.

s (2009), vol. 1, pp. 395-404

Documents

This article was originally published in the Encyclopedia of Inland ... - Miami … et... · 2009. 3. 27. · This article was originally published in the Encyclopedia of Inland Waters