228 CHAPTER 11 Competition

Figure 11.1 Darwin's finches. The massive billof the large ground finch{Geospiza magnirostris) can open large, hard seeds (from Grant and Grant,2009).

In the 2004 drought, medium ground finches with

the largest bills were at a selectivedisadvantage

because they competed directly with the large

ground finch.There were two important conse

quences of this new situation. During the drought,

the billsize of the medium ground finch decreased

as selection pushed it away from competition with

the larger species.And this left the finch with only

small seeds, which were rapidly depleted. Second,

its population declined radically.

Thisexample raises many questions. Is this always

the effect of competition? Areother results pos

sible? Howdo the finches actually interact? Can

these species coexist?These are the questions we

address inthis chapter.As we workthrough them, we will ultimately be able

to answer the fundamental question: What aretheeffects of competition for

resources?

TIX

interspecific competition Theinteraction among two or more species

overa limiting resourcethat resultsinadecrease in the populationsizeofat ieast

one of the species.

intraspecific competition Competition

among members of the same species.

What Is Competition?The interaction of the large and mediumground finches on the Galapagos contains all the key elementsof interspecific competition: the interaction amongtwoor more species over a limitingresource that resultsinadecrease in thepopulation sizeofat least oneof the species. Central to this definition is thefact thatduring the drought, seeds became a limiting resource for both populations. Although both require oxygen, it is not limiting, and thus they are not in competition for it. Also important in this example is the fact that the abundance of thelimiting resource changes over time. Consequently, the degree to which it limitsboth species changes—there are times when competition for seeds isespeciallyintense, others when it is relaxed.

Many kinds of resources are limiting. For sessile organisms like plants orbarnacles, space on the substrate maybe the resource in short supply. Althoughoxygen is usually not limiting for terrestrial animals, it can be in aquatic ecosystems, where strong gradients in oxygen concentration occur. Sometimes theessential factoris the meansof access to another resource. For example, rainbowtroutcompete with Atlantic salmon when theyinhabit thesame streams (Blanchetet al., 2008). When trout are present, the salmon activity pattern shifts frommostly nocturnal to diurnal (daytime) activity (Figure 11.2). However, activitytime itselfisnot the resource; at stake is accessto rich feedingareas in thestream.Becausethe trout exclude the salmon from such sites,the salmonshift their activitypatterns to times when trout are lessactive.

Intraspecific competition occurs when individuals of the same species arelimited by the abundance of a key resource. In this case the effect is primarilyon the individual. Those individuals better able to obtain and use the resource

survive and reproduce at a higher rate than those who cannot. Intraspecific competition underlies manyof the density-dependenteffects, discussedin Chapter 9,that limit populationgrowth.

For plants, space is essential,because it ensures accessto nutrients and water.At high intraspecific density, the population size declines but the remaining

individuals grow larger, a phenomenon known as self-thinning (Figure 11.3). The decrease in density overtime occurswith a characteristic slope of —3/2.

Many species exhibit a decline in population densityunder intraspecific competition. The human parasitethat causes African sleeping sickness (Trypanosomabrucei) occurs in a number of genetically differentstrains. It iscommon foran individual to beinfected bymore than one strain ata time, which opens the possibilityofintraspecific competition among thetrypanosomesin their human host. Bulmer et al. (2009) have shownthatwhen multiple infections occur, competition amongthestrains reduces the population sizesattainedbyeach,effectively diminishing the virulenceof the infection.

How Do We Demonstrate ThatCompetition Is Occurring?We assume that competition is important to many species. Still, we needaprecise mechanism for demonstratingthat itisin fact operating. The definition of interspecificcompetition provides the keyto empirical tests ofcompetition: we need todetermine thataresource islimiting andthatdiminished access tothatresource negatively impactsthe putative competitors. This appears to beastraightforward procedure. However, ecological interactions are socomplex that it can be difficult to disentangle the effects ofother factors that negatively impact a species, suchaspredation, parasitism, andabiotic factors.

Astudy ofcompetition inbarnacles (Connell, 1961) remains one oftheclearest demonstrations ofcompetition ever documented. Inthe rocky intertidal, twospecies of barnacles, Semibalanus balanoides and Chthamalus stellatus, occur indifferent zones. The adults are sessile filter feeders—they filter detritus ormicroorganisms from thewater as itwashes over them. Chthamalus isfound higher intheintertidal zone, where itisexposed toairlonger andtowave action; Semibalanusisfound below themean tideline. The larvae ofbothspecies are released into thewater column anddriftforsome timebefore settling randomly ontothesubstrate,where they develop into the sessile adult form. Both species settle throughouttheintertidal, yettheir adult distributions differ. Connell asked thequestion: Istheadult distribution the result of competition or the different physical conditionshigh and lowin the intertidal?

Virtually all the rocksubstrate in this system is covered withbarnacles, suggesting that space ispotentially limiting. Connell addressed the question ofcompetition with two key experiments. First, he moved rocks bearing Semibalanusinto the upper intertidal to see if they could survive the conditions there. He alsodid the reciprocal experiment—he moved rocks bearing Chthamalus from theupper tothelower intertidal. Second, heperformed removal experiments to assessthe absence of one species on its potential competitor. He removed Chthamalusfrom rocks in the upper intertidal and followed the fate of Semibalanus there. Inthe reciprocal experiment he removed Semibalanus from rocks in the lowerintertidal and followed the fate of Chthamalus. He found that although Chthamaluscan survive in the lower intertidal, Semibalanus cannot tolerate the conditions inthe upper zone. Thus, Semibalanus is limited by the physical conditions. Experimental removal of Semibalanus in the lower zone resulted in Chthamalus estab

lishing there (Figure 11.4). Connell took aseries ofphotographs ofrocks in thelower intertidal that showed Semibalanus slowly growing under Chthamalus andprying the individuals from the rock. Thus, the distribution of Chthamalus islimited bycompetition from Semibalanus.

Low intraspecificcompetition

What IsCompetition?

High intraspecificcompetition

interspecificcompetition

229

Figure 11.2 Atlantic salmon activity patterns. The mean proportion(+/- Standard Error) of Atlantic salmon activeduring daytime (bluebars)and twilight (orange bars)when maintained at lowand high intraspecificdensity and with rainbow trout in (a)2005 and (b)2006 (fromBlanchet et al., 2008). Analyze: is this an example of direct or indirectcompetition for a resource?

self-thinning Aphenomenon inplantsinwhich individuals at high density havesmaller population size but larger individuals.

Density(number of plants

per m2)

Figure 11.3 Self-thinning. The process ofself-thinning in the grass Lolium. Each linerepresents a trajectory of weight per plantin a population over time. The straight lineof the shift in density over time for thesepopulations has a slope of -3/2 (fromLonsdale and Watkinson, 1983). Analyze:What would this graph look iike if intraspecific competition were not importantin these plants?

230 CHAPTER 11 Competition

Figure 11.4 Intertidal competition.When Semibalanus is removed from rocks

in the lower intertidal, Chthamaluspersists, indicating that competition fromSemibalanus limits Chthamalus to the

upper intertidal (fromConnell, 1961).Analyze: If there were no other evidencein this study, would this graph unequivocally demonstrate competition?

THINKING ABOUT ECOLOGY:

Imaginethat in Conneii'sexperiment

Semibalanus and Chthamalus were each

restrictedto a portion of the intertidal

bytheirtolerance of the physical factors.Thatis,neitherspeciescansurvive in theother'spreferred zone.Isit possible thatcompetition isstill ultimately responsiblefor their distribution?

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Demonstrating competition can be even more difficult if both intraspecificcompetitionand interspecific competition are affecting the species. Weexpectthat members ofthe same species aremore likely tobe" limitedbythe same limitingresource. In otherwords, because theyare more likely to use the same resource, we might expect intraspecific competition to be more intense thaninterspecific competition. Ofcourse, themoresimilar the competing species, themore likely it is that they, too,compete. How canwe assess the relative importance of intraspecific andinterspecific competition?

Forplants, which areimmobile andcanbe grown undercontrolled conditions,the analysis is relatively straightforward. For example, two species of oats, Avenafatua andA. barbata, inhabit the same grasslands in California. Marshall andJain(1969) used an experimental design known as a DeWitt replacement series tomeasurethe relative importanceof intraspecific and interspecific competition inthesetwospecies. In aDeWitt replacement series, thetwospecies aresown inasetofpotsatconstant totaldensity. However, across theseries, therelative numbers ofthe two species vary. A series ofpotswas established, each ofwhich contained atotalof128plants. Therelative numbers of the twospecies varied from0A.fatuaand 128A. barbata at one extreme to 128A.fatua and 0 A. barbata at the other.

Thus,each end of the spectrum representedpure intraspecific competition. At one extreme, A. barbata wascompeting only with itself; at the other, A.fatua wascompeting only with itself. Intermediate combinationsrepresented varying degrees of intraspecific and interspecific competition. Interspecificcompetitionwas mostintensein thepots with 64 individuals ofeachspecies.

MarshallandJain measuredthe survivalandreproduction of each species. Theygenerated a null hypothesis in this analysis: that competition within species isexactly equivalentto competitionbetweenspecies. Forexample, if A.fatua produces x seeds per plant whencompeting only with itself, under the null hypothesisthissamenumberofseeds shouldbeproducedifsomeofthe competing individuals areA. barbata. Thisis shownby the dashed lines in Figure 11.5. They then measuredthe reproductive output of each individual in each potandplottedit on the samegraph.Notein Figure11.5 thatA.fatuaproducedmore seedsthan predictedbythe nullhypothesis, especially at the most intense interspecificcompetition (64 individuals of each species). Thus, it

50 100

Number of barbataseeds sown

Figure 11.5 Replacement experiments. DeWittreplacement experiments between Avena fatuaand A.barbata. The dashed line representsthe nullhypothesis—the seed production of A fatua(left)and A barbata(right)in the absence of interspecificcompetitors.The actual performanceof each species isshown bythe solid line (from Marshalland Jain, 1969).Analyze:Whydoes the dotted line represent the nullhypothesis?

a Bridled goby

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Conspecific density

b Goldspotgoby

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EE

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What IsCompetition?

2 3 4

Conspecific density

231

Figure 11.6 Gobie growth rates. The growth rate of bridled gobies (a) and goldspot gobies (b) as functions of density. Blue circles are the results'from single reef populations; orange circles are for reefs occupied by both species (from Forresteret al., 2006). Analyze: Why do these datasuggest that intraspecific competition is more important tothese species than interspecific competition?

performed better in interspecific competition than in intraspecific competition: itwas limited more by members ofits own species than another. In contrast, A. barbata performed worst when interspecific competition was most intense. Thus, A.barbata was limited more by interspecific than intraspecific competition.

Demonstrating intraspecific and interspecific competition is more complex formobilespecies.Somesystems, however, are amenable to experimental manipulation.Forrester et al. (2006) studied the magnitude ofcompetition in two coral reeffish,the bridled goby (Coryphopterusglaucofraneum) and the goldspot goby (Gnatholepisthompsoni). These species inhabit smallpatch reefs that represent isolatedisland habitats. The researchers manipulated goby densities by stocking reefs with either onespecies or equal numbers ofthe two species. The growth rate ofUgged individualswas used to measure the impact ofconspecifics on the presence ofthe other species.Both gobies' growth rates were significantly affected by the presence oftheir ownspecies (Figure 11.6), indicating intense intraspecific competition. Growth ratesalso declined as afunction ofthe populationsize ofthe other species, but onlyabouthalf as much. Thus, these fish are affected by both intraspecific and interspecificcompetition, but interactions with their own species have the greater effect.

What Are the Mechanisms of Interspecific Competition?Access to aresource in short supply is central to the process ofcompetition. Anyadaptations that improve the species' ability to exploit alimiting resource conferaselective advantage. In general, the mechanisms that accomplish this fall intotwo broad categories. In exploitation competition, one species reduces theamount or availability of the limiting resource. This often occurs by adaptationsthat improve the efficiency with which the resource is used. The two competitorsmay never even encounter each other; one species simply exploits the resource soefficiently that the otherspecies is compromised. Inthe deserts ofthe southwesternUnited States, the seeds ofannual plants remain in the soil for years awaiting sufficient rainfall for germination. Two very different taxa exploit this seed resource:granivorous rodents such as kangaroo rats, and ants (Brown and Davidson, 1977)Each depends on this resource. To the extent that either does that more efficiently,

exploitationcompetition Amechanismofcompetition in which one speciesreduces the amount oravailability ofthelimiting resource.

232 CHAPTER 11 Competition

interference competition Amechanism

of competition in which one species

actively inhibits anotherfrom obtaining

the resource.

allelopathic Describes a plant thatproducesand releases chemicalsthatinhibit the growth ofnearby individualsof the same or another species.

Figure 11.7 Allelopathic chemicals.The creosote bush produces allelopathicchemicals that inhibit the growth ofother plants.

preemptive competition Amechanismofcompetition in which a plant establishes

access to resources byestablishing itselfand occupying space.

diffuse competition Thesummedeffectsofall competitors.

11*2

the population of the other is negatively affected, even though there is no directinteraction between them.

Alternatively, interference competition occurs when one species activelyinhibits another from obtaining the resource. Some desert plants, such ascreosote bush(Larrea tridentata), are allelopathic. They secrete chemicals into the soil that inhibitthe growth ofpotential competitors for scarce water and nutrients (Figure 11.7).Some plants seem to take this competitive advantage a step farther. The allelo-chemicals of the bitter vine (Mikania micrantha) not only inhibit the growth ofother plants, they improve the availability ofany mineral nutrients present inthesoil for theirexclusive benefit (Chenet al., 2009). Interspecific territoriality, theactive exclusion of other species from a portion of the habitat, is an importantmechanism ofinterferencecompetition in animals.

For many plant species, space is a critically important resource, because theamount ofspace an individual occupies in the habitat directly determines its access toessential resources. The amount ofsoil space occupiedbytheroots determines accesstowater or nutrients. Iflight is the limiting resource, there must be sufficient leafarea exposed to the sun for effective photosynthesis. Thus, for plants, preemptivecompetition is amechanism that ensures access to these resources. The plant establishes itselfand occupies space thatensures access tosufficient soil orlight resources.Inthis way itprevents—that is, preempts—its competitors' use ofthe resources.

More often than not, the competitive effects faced bya species do not comefrom asingle competitor species. The summed effect ofall competitors is knownas diffuse competition. Itmay be that no single competitor isimportant; rather,it is the combined effect of two or more competitors that is significant. For example, Brown and Davidson (1977) demonstrated diffuse competition in the ant-rodent desert system byexcludingrodents from experimentalplots andmeasuringthe resulting increase in ant density. However, their removal experiments eliminated not just a single rodent species butagroup ofspecies whose summed competitive effect was significant to the ants. A tree seedling in a mature forestcompetes for light not only with agroup ofother seedling species but with theadults ofa number ofspecies. Adult trees absorb light before it reaches the seedlings. Thus each adult fractionally reduces thelight available totheseedlings.

KEY CONCEPTS 11.1

0 Limited resources lead to competition between species as wellas within species.B Competition isempirically demonstrated by showingthat (a) the resource is limiting

and (b) the interactionbetween the species hasa negative impacton one or both ofthem. Competition forlimited resourcesmaybe between different species,membersof the same species, or a combination of the two.

QUESTION:

Why would youexpect interspecific competition between vastlydifferent taxa suchas

ants and rodents to be relatively rare?

What Determines the Intensity of Competition?Competition occurs when two species exploit the same resource. Thus, acrucialfactor is thedegree to which theyoverlap in theiruseand reliance on aparticularresource. For example, in tundra habitat in Alaska, plant production is limitedprimarily by soil nitrogen, which occurs in a variety of chemical forms, suchasammoniumnitrateand aspart offree amino acids such as glycine. Moreover, thetemporal and spatial availability of these nitrogen compounds varies. However,the intensityofcompetition fornitrogenismediatedby the specific form ofnitrogen theplants are adapted to utilize (McKane etal., 2002). The more similar the

what Determines the Intensity of Competition? 233

species use ofnitrogen forms, the more their production is reduced by competition (Figure 11.8). This suggests that it is important to be able to quantify thespecific limitingresource.

HowDo WeQuantify the Useof Resources?Early in this history ofecologyJoseph Grinnell (1917) described the functional niche as the ecological role that theorganism plays in the community. We have alluded to anumber offunctional niches in this chapter. For example,Darwin's finches and ants are granivores that inhabit variable environments. And the functional niche ofabarnacle-is asessile filter-feeding invertebrate in the rocky intertidal

Hutchinson (1957) extended this concept in amorequantitative form, today known as the ecological niche:the set of biological and physical resources that determine growth, survival, and reproduction. For any species, we can identify aset ofresources, each ofwhich isimportant to these vital processes. We represent each resource on a separateaxis. We can envision the niche as

!hZ1UmC tfinetby ^tderanCe UmitS aI°n§ each «* <Fi§ure 1W).B* %urehowS amchem three resource dimensions. However, in reality, each imporlntresource is included as an axis in multidimensional space. The niche is an objectknown as ahypervolume bounded by the use of those n-dimensions. The axes hatmake up the ecological niche are both physical factors and biological resources Forexample for atrout living in amountain stream, the physical factors that might de-tern^neits successpH. Prey size might be an important biological factor

Obviously, it is nearly impossible to identify and quantify each ofthe resourcenS:"TTn°r SPedes-/°r^ely, not every resource Lis thatel f fl hraitinS-For the ^udy of competition, we can often reduce theecology niche to the single axis ofthe resource that is most limiting. Figure 1110

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Figure 11.8 Competition for nitrogen. The production of tundraplants as afunction of their similarity in the form of nitrogen utilized(from McKane et al., 2002). Analyze: Why is the form of nitrogenusc-o by these species important to interspecific competition?

Resource

functional niche Adescriptive definitionofthe niche as the ecological role ofthespecies in thecommunity.

ecological (Hutchinsonian) niche Theset ofbiological and physical resourcesthat determine the growth, survival, andreproduction ofa species.

Figure 11.9 Ecological niche. An ecological niche based on three resource dimensions. Species are represented byspheres defined bythelimits of their tolerance and use ofeach resource. Analyze:What quantitative measures could you useto characterize the niches ofthese speciesand the relationships of their niches?

Figure 11.10 Niche parameters. The niche parameters of two speciesusing angle resource. Ana^e: Where is competition betweenthese twospeciesmostintense?

234 CHAPTER 11 Competition

niche breadth Thevariation amongindividuals in a population in theirresource use.

niche separation Thedifference betweentwo niches measured bythe difference inthe mean use of a resource.

niche overlap (a) Theextent ofcommonuse ofa resourcebytwo or morespecies.

THINKING ABOUT ECOLOGY:

The finches ofthe Galapagos areaffectedbythe declineintheirfoodsupplydue toperiodic drought. Why do you supposetheystili compete soeffectively? Why havetheynotadoptedthe niche ofa fugitivespecies?

fugitive species Poor competitors thatexploitdisturbances, where theyfacefewercompetitors.

shows the niches oftwospecies alonga singlelimiting-resource axis.From suchagraph,we can quantifykeyaspects of the nichesand their relationship. Not allindividuals inapopulation useprecisely the same resources. Nichebreadth(w)is a measureofthe variationamongindividualsin their use of the resource. Notethat in previous chapterswe used the same symbol, w, for relative fitness; here itmeasuresnichebreadth,an entirelynewquantity.Thedifference in thenichesofthe two species is measuredby the difference between the mean values of theirresource use, theniche separation (d). Theextentofcommon useoftheresourcebytwo ormore species isquantified bythe amount ofnicheoverlap (a).

Finally, densityfactors into the intensity of competition. If a potential competitor is rare, its impactwill be muchless than if it is abundant. In the nichesdepicted in Figure 11.10, thesizeofeachpopulation is depicted bythe area underits resourcecurve.Thus,species2 is lessabundant than species1.We mayexpectthat species2 willhavelessimpact on species 1than the reverse. When the largeground finch immigrated to Daphne Island, its populationgrewfor many yearsbefore the 2004 droughtbrought it into competitionwith the mediumgroundfinch. If itsnumbers hadbeenstill smallwhenthe droughtoccurred, itscompetitiveimpactonthe medium groundfinch wouldhave beenmuch smaller. In otherwords,competitionisa density-dependentphenomenon.

What Are the Effects of Disturbance on the Intensityof Competition?Manyspeciesexperience physicaldisturbance ofone form or another. The immediateeffectofphysicaldisturbancesuch as fire, volcaniceruption,or windstormisto reduce the population density. Wenow understand that in some habitats, disturbance isrelatively infrequent;in others, it iscommon.Forthosespecies whosehabitat is regularly disturbed, competition is a much lesspotent selective force.Colonistsofdisturbed systems inhabit a worldof high resource abundance andlowpopulationdensityoftheir ownand other species. Theyareadaptedtoexploitthese conditionswith rdpid growth and reproduction.As this occurs, thehabitatbecomes more saturated and competition increases. Disturbance-adapted species do not fare well as the intensity of competition increases. Eventually theiroffspring mustdisperseto anewlydisturbed habitat.

These differences between mature, undisturbed habitats and habitats that experience recurring disturbance underlie some of the life history differences weexplored in Chapter10. Specifically, the theoryofr-and K-selection suggests thatforspecies normally found athighdensity, thereproductive strategyemphasizesthe competitive abilityof the offspring. In contrast, species or populations thatarefrequently disturbed, and thus experience lowdensityandlittle competition,invest theirreproductive energyin large numbers ofoffspring.

In general, competition and disturbance workin opposition: if disturbanceis infrequent, densities arehighandcompetition increases. In contrast, ifdisturbance is frequent and densities are low, competition is weaker. Poor competitorsthatexploitdisturbanceswhere theyface fewer competitors are known as fugitivespecies. Their preferred habitat eventually disappears and they must colonizenewly disturbed sites. Dispersal ability isanimportant life history traitoffugitivespecies.

Experiments withtheannual plantArabidopsis thaliana show that thepatternofdisturbance determines competitive ability. Fakheranet al.(2010) establishedpopulations of Arabidopsis that experience either a high or low disturbanceregime. Static populations experienced no disturbance. The habitat of the dynamic populations was destroyed each generation. After five generations thepopulations had diverged: those from static populations tolerated competitionbetter than those from dynamic populations. Specifically, static conditions selected for more rapid growth andthus preemption ofresources. Dynamicpopulations were more sensitive to crowding. However, plantsfrom thedynamic regime

-..- _ ._ _ What Determines the Intensity ofCompetition?

ON THE FRONTLINE

Distinguishing Resource and Nonresource CompetitionPlants face competition from nearby plants above ground aswellas below ground. Below-ground competition is particularly hardto study. Plant roots face twoforms ofcompetition. One, knownas resource competition, is for nutrients and water. The other,nonresource competition, is over space. Obviously the two areintimately connected.

Messier et al. (2009) examined these forms ofcompetition infour tree species: a hybrid of cottonwood and balsam poplar(Populusdeltoides xP. balsamifera), paper birch (Betulapapyrifera),sugar maple (Acer saccharum), andwhite ash {Fraxinus americana).They addressed the question of the relationship between resource and nonresource competition among these species. Thesefour species differ in the kind of forests they inhabit. The hybridand birch arefound in disturbed forests; sugar maple andash arefound in mature forests that have not been disturbed forsometime. These differences led the researchers to analyze the effectof disturbance on competition.

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The researchers devised a clever system for testing the effectof below-ground competition. They planted the trees in largecontainers divided into two halves separated by a solid wall.Roots of the trees grew into each half of the container. In this waythey could manipulate the conditions the roots encountered inthe two compartments. Four variables were used in all possiblecombinations: no competition, competition from the roots ofgrasses, no fertilization, and fertilization. At the end of the experiment, they carefully removed the roots from the containersand measured their length, biomass, and branching. Asymmetrybetween the two halves of the container in root production orarchitecture was attributed to the different environments the rootsencountered. Thus, asymmetry between fertilized/nonfertilizedcontainers was ameasure of nonresource competition. Asymmetrybetween grass/nongrass roots measured the effect of competition.

PREDICTION: Thedisturbance species should have asymmetricrootproduction inthe competition and noncompetition halvesof the containers. Nondisturbance species shouldhavelessrootasymmetrybetweencompetitiveand noncompetitivetreatments.

Figure 1 Root production. The root length (y-axis) for four speciesexpressed asthe ratio ofvegetated halfmonvegetated half (that is,competition:no competition). Orange bars are nonfertilized experiments; blue bars arefertilized experiments (from Messier et al. 2009).Analyze: What dowe mean by "asymmetrical root production" inthisexperiment?

The accompanying figure shows the results of the experiment. The prediction was confirmed: there was asymmetricalroot production in the disturbance species exposed tograss andnongrass. This means that disturbance species compete poorlywith other species. Their roots avoided those of other speciesand grew less in the presence ofcompetitors. Maple and ash,species found in mature forests, had much less asymmetry incompetition.

These results make sense in light of the ecological nichesof the two groups of trees. In species whose niche centers ondisturbed sites, fine root growth that avoids competing rootsis an adaptation to maximize nutrient uptake for rapid growth.In essence these are fugitive species adapted to exploit noncompetitive environments. The root morphology of sugar mapleand white ash responds to competitors, an important adaptationif they are to establish roots in the face of competition.

eventually grew taller, which enabled their seeds to disperse farther from theadultplant (Figure 11.11).

KEYCONCEPTS 11.2

« The ecological niche is central to competition because it is based on the use of critical resources.

Resources exploited by aparticular niche may be biological, such as prey typesize, or physical, such as aspecific temperature regime. or

i II

236 CHAPTER 11 Competition

A

StaticDispersing

seedsare lost

DynamicNon-dispersing

seedsare lost

Generation t

Generation t

Non-dispersingseeds usedfor the nextgeneration

New soil

Dispersingseeds usedfor the nextgeneration

New soil

Generation t+1

Generation t+1

Figure 11.11 Diagrammatic (A) and photographic (B)depictions of Arabidopsis populations.Photo shows static (left)and dynamic (right) populations (from Fakheran et al.,2010). Analyze:How do the differences between these treatments affect seed dispersal?

Disturbance leads to higher resource abundance and lower competition. In undisturbed sites, the number of species and their densities increase, leading tocompetition for scarce resources. Species tend to be adapted to one or the othersituation.

QUESTION:

In what way is the use of a single-resource axis appropriate for the analysis of competi

tion between two or more species? In what way might it be inappropriate?

What Are theEcological and Evolutionary Consequences of Competition? 237

THE HUMAN IMPACT

Human activity moves many species around the planet, introducing new species to well-established communities of nativeorganisms. Plants and animals move ina variety ofways—seedscontaminate agricultural seed, marine organisms are releasedwith ballast water in ships, andsome aredeliberately introduced.Nonnative species often proliferate rapidly in their new com-,munities. Many have significant economic impact because theyinterfere with oroutcompete important native plants and animals.For example, invasive marine species introduced to the GreatLakes by oceangoing vessels probably cost the commercial andsportfishing industries more than $200 million a year. The totallosses in the United States from invasive species may beas highas $100billionper year.

Why are these speciesso devastatingto the local biota? Eachintroduction is in a sense ecologically unique. However, a fewgeneral principles have emerged. Because introduction by humansis rapid relative to natural invasions over longer evolutionary orgeological time spans, the native species do not have time toadapt tothe new competitor. If the invasive is asuperior competitor,itmay rapidly outcompete the native species. Research showsthatthenative species with anarrow ecological niche areparticularlyvulnerable (Bohn et al., 2008).

Many of the most successful invaders are species adaptedto disturbance. This is particularly important for agriculturalweeds. Invading plant species adapted for disturbance in theirnative range find an ideal habitat incultivated fields—a form ofartificial disturbance that opens the habitat and reduces competition. Velvetleaf (Abufj/on theophrasti), imported from China,now causes millions ofdollars ofdamage to corn and soybeancrops. Grazing land disturbed by cattle is also ideal habitat foralien fugitive species that thrive in theabsence ofcompetition.Cheat grass (Bromus tectorum) and leafy spurge (Euphorbiaesula) are both exotic species that colonize sites disturbed byheavy grazing.

There is also evidence that invasive species evolve rapidlyin the new community. The evolution of increased competitiveability (EICA) hypothesis suggests that because invasive plantsare released from their natural herbivores, they rapidly evolveincreased competitive ability, perhaps because they donotneed

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Plant characteristics

Leaf

damage

Figure 1 Invasive species. The performance ofSapium sebiferum fromaninvasive population relative to thenative population incompetitionwith oneanother. The performance ofthenative population was standardized as 1.0. Bars extendingabove the value 1.0 indicate that theinvasive population outperformed the native population (from Zouetal., 2007). Analyze: What do theseresults show about theadaptations of invasive species compared to natives?

to invest as much energy in predator defense. Zou et al. (2007)compared invasive populations of the plant Chinese tallow[Sapium sebiferum) with populations from the native range. Rapidevolutionary shifts have clearly occurred. The invasive populationsgrow more rapidly and outcompete noninvasive populations.They also have lost the mechanisms thatdeter herbivores, butthey tolerate herbivory better than the plants from the nativerange of the species.

What biological interactions besides competition might determinethe success ofan introducedspecies?

B What Are the Ecological and EvolutionaryConsequences of Competition?We consider the consequences ofcompetition on two different time scales.The immediate effects occur in ecological time—periods ofone or afew generations. They represent the immediate ecological impact of competition forresources. There are also long-term consequences that occur as a result ofnatural selection. These are multigenerational effects and thus occur on anevolutionary time scale.

"I

238 CHAPTER 11 Competition

fundamental niche The niche in the

absence of competition.

realized niche The subset of the

fundamental niche that results from

competition.

competitive exclusion Theextinctionofonespecies dueto competition fromanother.

competitive exclusionprincipleThe concept that two species cannot

coexist on a singlelimiting resource.

characterdisplacement Ashift inthenichesofcompetingspeciesthat reducescompetition among them. Naturalselection favors those individuals in each

specieswhose resource use overlaps lesswiththat of the other species.

What Is the Immediate Effect of Competition?The niche reflects the pattern of resource usebythespecies. Figure 11.10 showstherelationship between two species thatshare asingle limiting resource. Competition occurs where the two bell curves overlap. Both exploitation and interference competition can cause a change in the niches of competing species. Forexample, imagine aspecies that has the niche shown inFigure 11.12a. We refer toitsnichein theabsence ofcompetition asits fundamental niche.If a competitorarrives in thehabitat, thespecies may alterits resource use to avoid competition(Figure 11.12b). The resulting change initsniche is known as the realized niche.

In the scenario depicted in Figure 11.12, the two species both occupy thesame habitatbut their niche dimensions change to accommodate competition.However, there are limits tothis process. Iftheniche shrinks toomuch, the population may not survive. This principle was first elucidated inaset ofclassic experiments with Paramecium. Gause (1934) grew Paramecium aurelia andP. caudatuminseparate cultures. Each culture had the typical sigmoid growth pattern, described in Chapter 8,ofapopulation innew and resource-rich habitat. However,when thetwo species were grown inthe same culture, oneincreased toan asymptote whereas the other declined to extinction (Figure i 1.13). The results wereconsistent: P. aurelia persisted and P. caudatum always wentextinct.

Gause termed this phenomenon competitive exclusion: the extinction ofone species as aresult ofcompetition with another. This process is intimately tiedtotheoverlap inresource use. In theParamecium cultures, bothspecies were consuming bacteria as a food resource. With just that single resource and the twospecies' complete reliance onit,coexistence was not possible. The better competitoracquired sufficient bacteria to prosper; theother declined toextinction.

This led to the competitive exclusion principle, which states that no twospecies can long coexist on asingle limiting resource. If theniche overlap ofthetwo species issignificant, one will outcompete theother. Inother words, two species cannot occupy thesame ecological niche. Innature werarelyfind two speciesexploitingprecisely thesame limiting resource.When we look closely, we usually

Species 1—f ^V f ^— Species 2e

3 Realized / \ .

2u.

niche / V

8 10 12 14 16 18

DaysResource

Figure 11.12 Fundamental niche, (a) Afundamental niche in the absence of competition, (b) In the presence of a competitor, the realizedniche Isa subset of the fundamental niche such where competition isreduced. Analyze: Arethe two realized niches necessarily narrowerthan the fundamental niche?

Figure 11.13 Limits of niche competition. Growth curves ofParamecium aureliaand P.caudatumseparately (top) and together(bottom). (From Gause,1934.) Analyze: What would you condude ifin some trials P. caudatum won and in other trials P. aurelia won?

- - — - _ What Are the Ecological and Evolutionary Consequences ofCompetition? 239

find that there are subtle differences in their niches. Where more than one speciesseems to be using a resource, one of two mitigating factors generally occurs.Either the resource is abundant enough to support more than one species, or theresource isnot critically limiting.

What Is the Evolutionary Effect of Competition?The long-term effects of competition are due to selective forces acting on thecompeting species. One obvious result ofadaptive change isanincrease incompetitive ability. We have alluded to this in the discussion of the difference between populations that inhabit stable high-density habitats compared to thosethat experiencedisturbance.

There is ample evidence for the evolution of competitive ability. Mueller(1988) showed that populations ofDrosophila—the common fruit fly—could beselected for competitive ability. Mueller established two groups of populations:one set was maintained at high density, one at much lower levels. The high-density populations experienced food shortage for bothlarvae and adults. After 128generations, Mueller tested the two groups for their competitive ability. Whenplaced in direct competition for limited food resources, the flies that had experienced high density were significantly more competitive—they acquired thelimited food supplies more efficiently than the low-density flies. This phenomenon is certainly important innature. Many adaptations improve the efficiency ofresource exploitation or ensure exclusive access to resources by actively excluding other species.

Competition also causes adaptive shifts in the niche. Specifically, competitionmay cause the niches oftwo competing species to diverge, aphenomenon known ascharacterdisplacement. Consider two competing species as shown in Figure 11.14.Those individuals whose resource use overlaps with the other species will be at aselective disadvantage. If resource use has agenetic basis, directional selectionwill drive the niches of the two species in opposite directions, causing the nichesto diverge.

Once again Darwin's finches provide aclassic case of this important process.Tile medium ground finch (Geospizafortis) and the small ground finch (Geospizafulginosa) occur separately on Daphne Major and Los Hermanos Islands. Populations separated in this way are called allopatric. On Santa Cruz the two speciesoccur together, or sympatrically. This provides a natural experiment in whichwe can examinethe nichesofthe two species where they do and do not compete.Figure 11.15 shows their niches under these two conditions. In allopatry, the two species use similar seed resources; in sympatry, their seed-size niches havediverged by the process ofcharacter displacement.

Character displacement has been described for anumber of competing organisms and for a range ofcritical resources. For example, Tyerman et al. (2008)observed character displacement in strains ofthe bacterium E. coli competing for different carbon sources.When strains experienced competition, they diverged.When competition was relaxed, their niches convergedagain. Any critically limiting resource can result inadaptive shifts in the niche. Song is an important meansof communication in birds. For forest-dwelling birds,species with similar songs are at adisadvantage—theirsongs are less acoustically distinct, making them less effective. Kirschel et al. (2009) show that where morethan one species ofAfrican tinkerbirds (Pogoniulus sp.)occur, their songs diverge (Figure 11.16),.

Resource

Resource

Figure 11.14 Character displacement.The individuals of each species that overlap with the other in resource use are ata selective disadvantage(top). Overtime,directional selection willshift the niche ofeach such that competition is reduced(bottom). Anaiyze: What factors willdetermine how fast this shift can occur?

allopatric populations Populationsoccurringindifferent places.

sympatric Describes populations thatoccur in the same place.

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Figure 11.15 Allopatry and sympatry. Character displacement in thefinches Geospiza fulginosa and G. fortis. Where the two species are allopatric, their niches are similar. In sympatry, their niches have diverged(from Schluter etal., 1985). Analyze: What is the control for thisexperiment?

i

240 CHAPTER 11 Competition

THE EVOLUTION CONNECTION

Competition is also important in the phylogeny of a species, thebroad patterns and history of its evolution. There are two important mechanisms by which competition affects phylogenies. First,the mass movement of species into new habitats may put themin direct competition with other species. The formation of landbridges such as the one that connected North and South America3.5 million years ago in the Pliocene permitted mixing of previously separate biotas. In this case, placental mammals invadedSouth America, where many niches were occupied by marsupials.Many of the North American placental mammals, especially thecarnivores, were superior competitors and drove their SouthAmerican counterparts to extinction. In effect, this history reflects a series of competitive exclusion events.

Second, invasions can lead to the origin of new species. If aspecies or taxonomic group colonizes a new region with littlecompetition, the group may diversify into an array of new species,each of which occupies a new ecological niche, a process knownas adaptive radiation. Many examples of this phenomenon areknown from both the fossil record and extant species. When placental mammals invaded South America, one of the marsupialgroups driven to extinction was a set of small granivorous andinsectivorous species. The placental rodent family Cricetidae—including rats and mice—radiated into some 60 genera and300 species in South America.

The ecology of islands is conducive to adaptive radiation. Because islands are typically colonized by just a few species whomanage to disperse there, they face little competition. Asa result,there are many examples of adaptive radiation on islands.The finches of the Galapagos are just one of many such cases.Figure 1 shows the diversity of vangids on Madagascar. One ofthe most spectacular adaptive radiations of birds is the honey-creepers of the Hawaiian Islands. The diversity of niches occupiedby honeycreepers in Hawaii far exceeds that of their ancestorsthe cardueline finches. Character displacement appears to be akey component of this process. As species arise, interspecificcompetition leads to character displacement and ultimatelyadoption of new niches by the nascent species.

The fossil record documents the role of character displacement and competitive exclusion in the process of adaptive

Figure 1 Adaptive radiation. Adaptive radiation of the honeycreepersin Hawaii.

radiation. Although there are spectacular examples of adaptiveradiation in the Galapagos and Hawaii, there are taxa that colonized the islands that did not radiate as the finches and honeycreepers did. For example, mockingbirds reached the Galapagosbut did not diversify even though they inhabit islands with whatseem to be empty niches (Arbogast et al., 2006).

QUESTION:

What is the relationship between isolation of populations and

adaptive radiation?

competition coefficient A measure of

the competitive effect ofone species onanother, determined bytheoverlap intheirresource use.

What Ecological Conditions Permit Coexistence?We have described two important consequences ofcompetition: competitiveexclusion and character displacement. If niches are too similar, one speciesmay drive the other to extinction. Orselection may result inthe divergence ofthe niches, thus avoiding competitiveexclusion. How do we know which willoccur?

We can address this question using a setofgraphical models developed independently byLotka (1925) and Volterra (1926). These models are based on thefundamental growth equations for populations with overlapping generations(Chapter 8). However, we will be following the course oftwo competing populations, so we add aset ofsubscripts todenote each population. The growth rate ofspecies 1inthe absence ofcompetition is given by the equation

What Are theEcological and Evolutionary Consequences ofCompetition? 241

dNL =riN]KI-N1dt K,

Similarly, the growth ofspecies 2without competition isdN

dt K

2_ „ K2-N2

Now, ifspecies 1and 2compete, we modify their growth equations as follows:dN. '*,-VN,-tQdr='.N. K.

and

dN

dt 2 2

K.-qc^N.-R

K„

We have added the variable a and the population size of the competitor to eachequation. The variable a2] isthe competition coefficient, a measure ofthe competitive effect ofspecies 1on species 2(note the order ofthe subscripts). The carrying capacity, Kj; is decreased by two factors: the population size ofspecies 1(N )and the competitive effect ofspecies 2 on species 1times the population size ofspecies 2(a[2Nj. The larger the value ofa and the larger the competitor population, the more the competitor decreases the carrying capacity.

In the simplest case ofcoexistence, the populations ofthe two species arestable (and nonzero). In other words, for each species, dN/dt = 0. The easiest way

DO THE MATH

How to Quantify Niche Overlap

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Figure 11.16 Song competition. Character displacement in the songs of Africantinkerbirds. The arrows show the shift in

song frequency when populations aresympatric (from Kirschel et al., 2009).Analyze: What factors other than com

petition might explain these results?

The value a is the competition coefficient in the Lotka-Volterraanalysis. How do we calculate this variable such that it measuresthe competitiveeffect of one species on another?

If two species use parts of the same resource distributionand their resource curves are normal (bell shaped), their nicheparameters determine the value of a. Specifically,

-<?_a = e4w!.

Qualitatively, we see that if the separation of the curves (d) islarge and the variation(niche breadth (w) is small, the curves willhave littleoverlap—the value of a is small. This means that exploitation ofthe resource by species 1will have little impact onspecies 2.

Another way to think of the value of a is with direct measurement ofthe common uses ofa resource. Imagine, for example,a food-resource axis based on n different prey size categories.We identify the proportional use of prey items of different sizesas shown in Figure 1. We calculate a, the measure oftheoverlapin prey-size categories, with the equation

different they are, the smaller the value of a,2. We calculate a insimilar fashion. Note it is not necessarily the case that ct2 - ol Inother words, competitive impacts betweentwo species are notnecessarily symmetrical. This occurs if the two resource curvesdeviate from the assumption ofnormality.

«,-> =

The values plx and p2x refer to the use ofthe series ofprey sizecategories byspecies 1and species 2, respectively. If the valuesPi,and pQ are similar, thevalue ofa,2 approaches 1.0, because thenumerator and denominator become more similar. The more

Figure 1 Prey size categories. The use ofprey size categories by twospecies. Analyze: Why dowe depict thesedataasbargraphs ratherthan continuous curves?

242 CHAPTER 11 Competition

isocline(inthe Lotka-Volterra models)Theline on a graphofthe population sizeof species1and 2 that represents zeropopulation growth ofoneofthe species.

to describe that condition using these equations is to set the numerator term inthe growth equationequalto zero.For species 1,

(K.-OC^-N^O.

Ifthis term equals zero, the entire right side ofthe equation is zero and growthrate iszero (dN/dt == 0).

For species 2,

(K2 - ^.N, - N2) = 0.

Each ofthese equations represents astraight line interms ofthe population sizesofspecies 1and species 2. The first equation describes astraight line onwhich atall points, dN,/dt = 0; the second represents astraight line on which all pointsrepresent dN2/dt = 0. These lines that depict zero population growth for the species are known as isoclines.

We can represent an isocline on agraphwhose axes are the numbers ofindividuals ofspecies 1and species 2.An example ofsuch agraph isshown inFigure 11.17.For species 1, we obtain the intercepts bysetting Nj and then N2 = 0. Thus, for species 1, ifNj = 0,

or

and

If N, = 0,

K,/u.,<

—dN,/dt = 0

K,<

N,

^N,V-dN/dt=oX

K/a„ K,N.

Figure 11.17 Isoclines. The isoclines of two competing species. Ateachpoint (1,2, and 3), the trajectory of each population and their combinedtrajectories are shown. Atall points on the graph, the trajectory of thepopulations istowardN, - K, and N2 = 0. Analyze: ifthe speciesherewere P.caudatum and P.aurelia, which would be species 1 and whichwould be species 2?

K, = <xl2N2

aJ2

N, = K,.

We thushave thetwointercepts forthespecies 1isocline.Alinejoiningthesepointsrepresents the combinationsofnumbers ofspecies 1 andspecies 2 atwhichspecies 1is not changing. We do the same analysis for species 2and determine that the intercepts ofitsisoclines are K2and K2/oc2j. We plot that isocline onthe same graph.

With this graphical representation ofthe isoclines,we can determine the population trajectory for eachspecies at anypopulation size. If the populationofspecies1isbeyondits isoclines—thatis,ifthe valueonthex-axis is farther to the right than the correspondingpoint on its isocline—species 1 is beyond its carryingcapacityand declines (dNj/dt < 0). Ifthe population ofspecies 1 is below the correspondingpoint on its isoclines, it is below its carrying capacity and increases(dNj/dt > 0). We analyze species 2the same way usingthe y-axis. If the population of species 2 is larger thanthe correspondingpoint on its isocline—that is,higheron the y-axis—species 2 declines. If the population isbelowthe correspondingpoint on the isoclines, species2 increases.

-)^.^LAJ!^JJg_^colog'cal and Evolutionary Consequences ofCompetition? 243

K,/o

K,/a„.§

K./o

K./o.

{^Stableequilibrium

Now considerpoint 1on Figure 11.17. Bothspecies 1and 2 are beyond their isoclines; the numbers ofbothspecies decline. We calculate the combined trajectoryof the two species as the sum of two vectors—one depicts the decline ofspecies 1, the other depicts the decline ofspecies 2. The trajectory ofthe two populationsis the sum ofthese vectors—a line diagonally towardthe origin. Atpoint 2, both species are below their respective isoclines andsobothpopulations increase. Thesum ofthe two trajectories is a line diagonally awayfrom the origin. Now look at what happens atpoint 3.Here species 1is belowits isocline, and so its populationincreases. But species 2 is above its isocline, and itspopulation decreases. The net movement of the twopopulations is diagonally down and right, toward K.Ifthe trajectoryhits N2 = 0, onlyspecies 1remains, and itgrows until itreaches its carrying capacity, K,. Infact, atany point between the two isoclines, the trajectory ofthe two populations inevitably leads to N = K andN2 = 0. Species 2has gone extinct and species 1is atitsmaximumdensity.

There are four possible arrangements oftheisoclines(Figure 11.18) depending on the relative positions ofthe intercepts. Figures 11.18a and b depict conditionsthat result in the extinction ofone of the species. In fact, these arrangements ofthe isoclines represent competitive exclusion. In Figure 11.18a, species 1alwaysoutcompetes species 2.The reverse occurs inFigure 11.18b.

Figure 11.18c represents an interesting case. Where the two isoclines intersect, neither population is changing; this is an equilibrium point. Note, however,that in one ofthe regions created by the intersection, species 1declines to 0andspecies 2increases to K2. In other words, species 2competitively excludes species 1. In the other region, the reverse occurs. When the two species are eitherabove or below both isoclines, they move toward the isoclines. Ifthey happenedto hit the equilibrium point, the two species would coexist. However, any perturbation away from that equilibrium point results in competitive exclusion ofone ofthe species. Thus, Figure 11.18c represents an unstable equilibrium.

Now examine the trajectories of the two populations that occur in Figure11.18d. No matterwhere we start on the graph, the trajectories ofthe two populations lead to the equilibrium point at the intersection of the isoclines. Any perturbation awayfrom that equilibrium point leads to the immediate return tothatpoint. Thus, Figure 11.18drepresents the arrangement ofthe isoclines that alwaysleads tostable coexistence ofthe two species.

This graphical analysis allows us to identify the conditions that permit coexistence. We obtain stable coexistence when the isoclines are arranged as inFigure 11.18d. The algebraic relationship that leads to that arrangement is thattwo conditions hold:

Figure 11.18 Isocline arrangement The four possible arrangementsofthe isoclines. Analyze: How do the variables /(and a determinethe arrangement of the axes?

K KiK!<—*-andK2<-^-.a a21 12

What does this mean biologically? For both inequalities to hold, the valuesofa, the competitive effect ofeach species on the other, must be small. Also, thevalues ofK, and K2 must not be too different. The more different they are, the lesslikelyit is that bothinequalities can hold. The similarityofthe carrying capacitiesmakes sense as well Ifthe carrying capacity ofone species were radically smallerthan that of the other, it would be much easier for competition to negativelyimpact the species with thesmallervalue ofK, which inturn would make coexistenceless likely.

THINKING ABOUT ECOLOGY:

What fundamental assumption arewemaking about the nature ofthe carryingcapacity (K) inthisanalysis? What would agraph of/Cover time looklike in thisanalysis?

244 CHAPTER 11 Competition

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Figure 11.19 Empirical testing. Empirical measurement of the isoclinesfor Drosophilaserrataand D.pseudoobscura in the laboratory. Theactual equilibrium point occurs at the intersection of bowed isoclines(solid lines) below that for the theoretical isoclines (dashed lines).

(From Ayala, 1969.) Analyze: Why do bowed lines like this suggestthat a is density-dependent?

It has been difficult to test these predictions empirically with natural populations. This is largely becausedetailed quantitative information on growth rates,carrying capacity, and the values of a are extremelydifficult to obtain. In addition, populations are affectedby many interactions besides interspecific competitionfrom just one other species. Consequently, it is difficult to devise experiments that isolate this one factor.Where it has been possibleto experimentallyassess thisgraphical analysis in the laboratory, populations seemto operate as expected, with some modification.Forexample, Ayala (1969) showed that for two laboratorypopulations ofDrosophila, coexistenceoccurs at apointthat requires both isoclines to bend downward relativeto the predictions of the Lotka-Volterra equations(Figure 11.19). This kind of shift would occur if thevaluesof a are not fixedbut changewith density.This iscertainly biologically possible. This suggests that theLotka-Volterra models are correct in broad outline but

mayneed modification to include other factors.

KEY CONCEPTS 11.3

s Competition results in immediate and long-term effects on the species.ss Competitive exclusionis the most important immediate effect of competition. Ifthe

niches of two species are too close together, one drives the other to extinction.5 There are two important long-term (evolutionary) effectsof competition: (a)increased

competitive ability and (b)character displacement.3§ The Lotka-Volterra graphical analyses show that coexistence is possible ifthe niche

overlap of the two species is small and the carrying capacities for the two speciesare similar. Ifthese conditions do not hold, the result is competitive exclusion or anunstable equilibrium condition.

QUESTION:

One adaptive response to competition is niche specialization, a narrower niche breadth.

Explain why there is a limit to this response. Why is there a limit to specialization?

Putting It All TogetherDarwin's finches in the Galapagos inhabit an ecologically dynamic system.Theabiotic conditions, especially rainfall, change markedly and unpredictably overtime. Rainfalldeterminesplant production and thus the seed resourcesavailabletothefinches. However, itisnotjustweather andthefood resources that change;species that co-occur also change. Although the islands are separated bysomedistance, thefinches occasionally colonize a new island, suddenly throwing species into competition.

This isprecisely what happened tothemedium ground finch onDaphne. Notonlydid it have to cope with the vagaries of rainfall and seedproduction, in the2004drought it suddenly had to deal with competition from the large groundfinch. This was particularly significant because thelarge groundfinch hasamoremassive bill and isbetter able toopen the large seeds thatareleft after drought hasdepleted the total seed supply. Here is aprime example ofthe ongoing nature ofadaptive evolution. As the ecology ofthe Galapagos continues to change, so dothe adaptive responses of its inhabitants.

1

Summary

In this way, the answer to the central question of the chapter—what are theeffects ofcompetition for resources?—is remarkably complex. Interacting formsofcompetition such as interspecific and intraspecific competition and interference and exploitation competition may be superimposed on one another. Theseinteracting forces change the niche—sometimes temporarily, as in the shiftfrom the fundamental to the realized niche, and sometimes permanently, aswhen character displacement occurs. Ifthe niches are too similar, competitiveexclusion removes one species. Thus, the niche is central to the interacting effects ofcompetition. Ifthe niche ofthe large ground finch centered on the use ofinsects as food, its competitive effect on the medium ground finch would benegligible. Had it been even more aggressive or efficient in acquiring its seedresources, itmight have competitively excluded the medium ground finch fromDaphne.

Moreover, the nonequilibrium nature ofmany ecological systems, driven bythe prevalence of disturbance, opens an important set ofniches—those offugitive species. There is awhole set ofspecies that, rather than compete for scarceresources in relative stable systems, is adapted to recurring disturbance. Thesespecies invest energy differently—in rapid reproduction to take advantage ofthe abundant resources for which there is little competition, and in dispersalsystems to ensure that their offspring locate the next disturbed site. This, too,is an adaptive response to competition—their ecology, population dynamics,and life history strategies are geared toward noncompetitive, nonequilibriumconditions.

Summary11.1 What Is Competition?

a Competition occurswithin and among specieswhen oneormore crucial resources is limiting.

• Competition isempirically demonstrated byshowing that (a) the resource islimiting and(b) the interaction negatively affects oneor both species.

• Competition can be direct (interference competition)or indirect (exploitation competition).

• The negative impact ofcompetition may arisefrom an interaction with a single competitor orfrom the summedeffects of manycompetitors(diffuse competition).

11.2 What Determines the Intensityof Competition?• The ecological niche can be defined in several

ways:

• The ecological roleofthe species.• Thepattern of useofa critically limiting

resource.

• The summed use ofbiological and physicalresources (Hutchinsonian niche).

• Niches are characterized by niche breadth andtheirseparation from andoverlap with the nichesof other species.

a Species that inhabitsystems with frequentdisturbance often experience high resourcelevelsand few competitors. Where disturbanceis less frequent, competition isoften moreintense.

!1.3 What Are the Ecological andEvolutionaryConsequences of Competition?a Competition alters the niche inecological time

(short-term effects) andover evolutionary time(long-term effects).

R In ecological time, competition may reduce nichebreadth.This smaller niche, the realized niche, isasubset ofthe niche in theabsence ofcompetition(fundamental niche).

B Two speciescannot long coexiston the samelimiting resource. According to thecompetitiveexclusion principle,one will drive the other toextinction.

s Characterdisplacement isthe shift inone or morespecies'niches when natural selection shifts theniches such thatcompetition among themisreduced.

a Speciesare more likely to coexist ifthe valuesof a(overlap) aresmall and thecarrying capacities (K)are similar.

245

246

.

CHAPTER 11 Competition

Key Termsallelopathic p. 232allopatric populations p. 239character displacement p. 238competition coefficient p. 240competitive exclusion p. 238competitive exclusion principle p. 238diffuse competition p. 232ecological niche p. 233

exploitation competition p. 231fugitive species p. 234functional niche p. 233fundamental niche p. 238Hutchinsonian niche p. 233interference competition p. 232

interspecific competition p. 228intraspecific competition p. 228

isocline p. 242niche breadth p. 234niche overlap (a) p. 234niche separation p. 234preemptive competition p. 232realized niche p. 238self-thinning p. 229sympatric p. 239

Review Questions

1. Explain the potential changes in niche dimensions thatresult from interspecific competition.

2. Using Lotka-Volterra graphs, explain the relationshipbetween niche overlap and carrying capacity that leadsto (a) competitive exclusion and (b) coexistence.

3. What is the relationship between disturbance andcompetition?

4. Why do our analyses of competition tend to focus on(a) pairs of species such as Connell's barnacles and(b) a single-resource axis?

5. Why does intraspecific competition tend to increaseniche breadth whereas interspecific competition tendsto narrow it?

Further ReadingGoldberg, D.E., and A.M. Barton. 1992. Patterns and conse

quences of interspecific competition in natural communities: a review of field experiments with plants. AmericanNaturalist 139:771-801.

This paper provides an extensive review of the empiricalwork on interspecific competition in plants. The authorsquantitatively analyze the literature for experiments thatdemonstrate competitive effects and organize those resultsinto the forms and types of effects.

6. What is the relationship between fugitive species andthe reproductive life history strategy (Chapter 9)?

7. What is the relationship between the competitioncoefficient and niche breadth?

8. What is the null hypothesis in a DeWitt replacementseries? Why is it important to the analysis?

9. Is the shift from the fundamental to the realized niche

an evolutionary or an ecological effect?

10. In Gause's experiments with competition in Paramecium, the same species always drove the other toextinction. Why is this consistent result expected fromcompetition theory?

Weiner, J. 1995. The beak ofthe finch. New York:

This book is an excellent and thorough explanation of thelong-term studies of Darwin'sfinches in the Galapagos byPeter and Rosemary Grant. Although it was written for thegeneral public, it presents their scientific work in detail. Itexplains the range of work on the adaptive evolution andecology of this group. It also provides insight into the natureof fieldworkin ecology.

Bolnick,D.I.,etal.2010.Ecologicalreleasefrominterspecificcompetitionleadstodecoupledchangesinpopulationandindividualnichewidth.ProceedingsBiologicalSocietyB.London.277:1789-1797.

Thiselegantstudydemonstratestherelationshipbetweenintraspecificandinterspecificcompetitiononthenicheofsticklebacksinrelationtotroutandscuipincompetitors.Theyalsoaddressthetimescaleandmechanismsoverwhichnicheshiftsoccur.

FurtherReading

MacArthur,R.H.1958.Populationecologyofsomewarblersofnortheasternconiferousforests.Ecology39:599-619.Thisisaclassicpaperonnichedifferentiationduetocompetition.MacArthurstudiedthedetailsofthemicrohabitatandforagingnichesofagroupofwarblersthatcanbefoundforaginginthesametree.Hisobservationsdetailhowthespeciesdifferslightlyinwaysthat,heargues,allowthemtocoexist.

247