Interspecific Competition Topics for this class: • What is interspecific competition? • What are some mechanisms of competition in nature? • What are the outcomes of interspecific competition? • How do ecologists model interspecific competition, and what do these models tell us about the phenomenon? • What kinds/amounts of difference between species allows their coexistence?
Population growth rate depends on ecological conditions--e.g., two grain beetle species
Many observations in nature suggest that interspecific competition is widespread • include patterns where closely related species appear to avoid competition • Geographical allopatry (species replace each other in space or time); e.g., some Darwins Finches don’t co-occur • Different habitats, foraging methods (e.g., spider species in bottomland hardwoods) • Niche expansion in absence of competitors (cocos finches) • BUT these observations could be explained by mechanisms other than interspecific competition, and could result from evolutionary forces in past, and not from ecological interactions today
We’ll formalize competition concept, study it empirically & theoretically • individuals of one species suffer reduction in fecundity, survivorship, and/or growth due to exploitation or interference by individuals of second species over limited resources • Minus-minus interaction: both species potentially hurt • Mechanisms • Exploitation--getting resources most efficiently • Interference--indirect competition over resources • Aggression, fighting • Territoriality • Overgrowth, allelochemicals • Preemption--gaining access to resources by controlling space
Outcomes are also diverse • Competitive exclusion: one species out-competes and replaces a second species • Invasives • Mutual antagonism (Tribolium sp. eat each other’s eggs) • Indeterminacy: which competitor wins is not predictable, and depends on initial conditions (e.g., starting densities) • Coexistence: species co-occur due to niche differences Let’s look at some of these outcomes in lab, field...
Competitive exclusion is exhibited in a number of examples • Lab studies • Tribolium species (see text)--mutual antagonism • Paramecium species studied by Gause (aurelia, caudatum, bursarum • Field studies • Species of Aphytis (parasitic wasps) feeding on citrus scale (Aonidiella) in Southern California • Fire ants (Solenopsis invictus) have replaced most species of native ants over large areas of Southeastern United States • Interference competition--e.g., using allelochemicals (e.g., CA sagebrush; Centaurea -knapweed--see text ch. 1) • Lots of other examples involving invasive exotic (recently introduced) species
Competitive exclusion: successive replacements of Aphytis (parasitic wasp) species feeding on Aonidiella
Interference competition: allelochemicals by sagebrush shrubs, California chaparral Close-up view Aerial overview of sage (dark) invading California grassland
Other outcomes of interspecific competition • Outcome depends on environmental conditions • Tribolium flour beetles,studied by Parks in laboratory • Which species wins in competition depends on temperature, humidity of medium • T. castaneum winner in wet, hot conditions • T. confusum winner in dry, cold conditions • Genetic strain of Tribolium also influences outcome • Indeterminacy = stochasticity in outcome • T. castaneum & T. confusum flour beetles in early studies by Parks
Competitive coexistence via niche differentiation • Connell’s Barnacles • Coexistence of Panicum (grass) & Glycine (a legume) in Australia when Glycine has Rhizobium as a nitrogen source • Groundsel & bluegrass plant weeds (both r-selected; work of Bergelson) • Orb web spiders in Louisiana, based on subtle web differences • Beaks of Darwins Finches
Coexistence via niche differences in Joseph Connell’s barnacles; rocky intertidal zone, Europe; based on experimental removals, exclusion cages
Realized niche (range of conditions) of Chthamalus smaller than its fundamental niche; no difference in these niches of Balanus • Mechanism: interference by Balanus individuals (dominant competitor) • Asymmetric competition
Coexistence of two plant weeds • Work of Bergelson (in Kareiva text, pp. 65-70) • Common groundsel and annual bluegrass (Poa annua) coexist throughout Eurasia, U.S. (r-selected) • How do they coexist? • Groundsel gains some advantage in competition by early emergence in Spring (shown in experiments with different genotypes) • Bluegrass can outcompete groundsel across generations via its leaf litter, that inhibits groundsel seedling growth • Gap-colonization theory explains coexistence in disturbed environment: groundsel does best in gaps, where bluegrass cannot inhibit groundsel’s growth • Gaps presumably created by snow, ice, fire, etc.
Overview of empirical studies of interspecific competition • Literature review studies by Connell, Schoener • Interspecific competition widespread in nature (55%-75% of studies) • Varies by trophic level • Kind of environment (e.g., greater in marine than terrestrial environments) • Greater in some kinds of organisms than others (e.g., vertebrates compared with invertebrates) why?
Lotka-Volterra model of interspecific competition starts with logistic model, assumes exploitation mechanism; a1,2 = competitive effect of species 2 on 1
Lotka-Volterra model: interspecific competition • Assumptions of model: r’s, K’s, a’s are all constants; environment constant (no disturbances), homogeneous; no differences among individuals. • No mechanism of competition specified • Model equations: • dN1/dt = r1*N1*(K1 - N1 - a12*N2)/K1 • dN2/dt = r2*N2*(K2 - N2 - a21*N1)/K2 • Define a12 =“competition coefficient”, in the first case competitive effect of species 2 on species 1; -a21 = effect of species 1 on species 2. -Suppose that it takes two individuals of species two to have the same effect on an individual of species 1 as one ind. of species 1 on species 1; then a12 = 0.5 • Subscripts indicate species-specific population growth rates, population sizes, carrying capacities, competition coefficients
Lotka-Volterra competition model, cont. • Note that competitive effect is to reduce growth rate of each species in proportion to alpha and population size of other species • If alpha = zero, then this model collapses to two (independent) logistic models without any interspecific competition • Model cannot be solved explicitly, but we can use isocline analysis to study its behavior
Graphical analysis of Lotka-Volterra competition model • Interesting cases found when term in bracket = zero. - this situation defines equation for a line (Y = mX + b) • These lines, one per species, termed “zero isoclines” = no population growth. Population of that species declines above isocline, increases below it, doesn’t change on line • E.g., Pop. 1: (K1 - N1 - a12*N2) = 0 ===> N1 = K1 - a12*N2; we can rewrite this (algebraic manipulation) as N2 = -(1/ a12)*N1 + K1/ a12 (form of Y = mX + b); Pop. 2: (K2 - N2 - a21*N1) = 0 ===> N2 = - a21*N1 + K2 • These equations lead to N2 X N1phase plane graphs
Identification of four configurations of isoclines-->four outcomes • Graphical analysis of 4 possible configurations in species 2-by-species 1 phase-plane, using isoclines (see lecture, text) • Species 1 outcompetes species 2 (spp. 1 isocline above spp.2) • Species 2 outcompetes species 1 (spp. 2 isocline above spp.1) • Either species 1 outcompetes species 2 or vice versa, depending on initial conditions (isoclines cross in particular configuration) • Both species coexist (isoclines cross) • Conditions for stable coexistence? • K1/a12 > K2 and K2/a21 >K1 (from graphs discussed in class) • In words, each species must limit its own population growth (Ki) more than it limits growth of the other species (Kj/aji). • Simplest such case: K1 = K2 ==> a12 , a21 both < 1!
Conclusions from Lotka-Volterra Model: • This simple model, based on exploitation competition via competition coefficients (alphas) leads to four qualitatively different outcomes of competition, much like the outcomes we identified using empirical examples, above • Species 1 outcompetes species 2, and vice versa • Outcome indeterminate…depends on initial conditions • Stable coexistence because niches of two species are different (each species limits its own population more than it limits abundance of competitor) • Conditions for coexistence emerge as a conclusion of model: individuals of both competitor species must inhibit their own population growth more than they inhibit growth of the other population
Francisco Ayala’s test of Lotka-Volterra competition model with fruit flies: model could not work with linear isoclines; this suggests that competitive coefficients change with population densities
Criticisms of Lotka-Volterra Model • Difficult to test explicitly, except in laboratory (e.g., Ayala’s test with Drosophila fruit flies) • Rarely do alphas remain constant at all population densities…this is probably explanation of non-linear isoclines in Ayala’s study • Does not model systems well when interference mechanisms involved • Model does not specify any mechanism of interaction between consumers and resources (it probably best exemplifies exploitation competition)
Tilman’s model of interspecific competition explicitly specifies consumer-resource dynamics Resource level (R)
In this example, species B outcompetes species A because B can drive resources to lower level (it is more efficient at exploiting sparse resources--see previous two slides)
Competitive exclusion principle • Gause’s principle: “complete competitors cannot coexist”; i.e., some niche difference necessary for coexistence • Arose out of generality of laboratory, field experiments, showing competitive displacement of ecologically similar species • Holds theoretically, even with predator (e.g., a 3-species model (two identical competitors, one predator) not stable • Problems in applicability of competition? • Can always find niche differences between two species; are these what allow coexistence? Not range expansion etc.etc. ---We cannot find out easily without large research program • If no differences found, can never be certain that none exist (thus principle is impossible to falsify, or prove wrong)
Competitive exclusion principle leads to question, “How different must coexisting competitors be?” • Some theory has been developed to address this question • E.g., Hutchinsonian ratios (after G.E. Hutchinson) • Involve morphological traits, e.g., beak size, body size, ovipositor length (latter in parasitoid insects) • These differences (1:1.28 average; 1.1 minimum) presumably related to resource (niche) differences allowing coexistence • Not well supported by field studies, but still debated • Species may coexist in spatially heterogeneous habitats via inclusive niche pattern: Locally inferior competitor coexists with dominant species via broad niche, including conditions (refuge) dominant cannot tolerate (e.g, Connell’s barnacles). Such “asymmetric” competition may be frequent in nature
Conclusions: • Interspecific competition involves diverse mechanisms • Diverse outcomes, too, from exclusion of one often closely related species by another, to indeterminate (stochastic) outcome, to coexistence via niche differences • Lotka-Volterra Competition Model has its weaknesses, but specifies multiple outcomes of competition, depending on relative values of carrying capacities, competition coefficients--much as seen in nature • Condition for species coexistence in model (and nature) is that each species must inhibit its own growth more than that of other species • Ecologists now asking how different species must be to coexist, and what are the mechanisms