Community Ecology
Competition: theory
Community Ecology
Competition: theory
Outline:
1. Types of biotic interactions (following Odum 1969)
A. Definition of competition
2. Two types of competition: intraspecific and interspecific
A. How to tell if competition is present
3. Two forms of competition: interference and exploitation
4. Two outcomes of competition
5. Studies of competition
A. Lotka-Volterra
i. Gause’s lab experiments: competitive exclusion
a. Importance of heterogeneity
B. Tilman’s resource-ratio hypothesis (R* theory) to explain coexistence of competitors
6. Some additional things to remember about competition: diffuse competition ghost of competition past, indirect effects, and apparent competition
Types of biotic interactions that structure communities, determine diversity and abundance:
expressed as effect on species 1 vs. effect on species 2, with + indicating benefit, - indicating deleterious effect, 0 indicating no/neutral effect
if +/+, is mutualism – example:
if +/- or -/+, is predation or parasitism – examples:
if +/0 or 0/+, is commensalism – example:
if -/0 or 0/-, is amensalism – example:
if 0/0, is neutral
if -/-, is competition
Four of these interactions have historically been considered to be the most important ones in structuring communities (dictating diversity, abundance, and distribution patterns): which ones do you think they are and why?
Of these types of interactions, competition has historically been considered to be the most important mechanism in structuring communities. The importance of competition is deeply seated in biology (is one of the premises of natural selection, is often witnessed, and is intuitive to grasp [but perhaps this says more about humans than about nature?]). Competition has traditionally been considered to be the most important biotic interaction shaping community structure for several reasons. (But hopefully your critical thinking skills are in play by now and you’re suspicious of sacred cows in this class…) The question is not whether competition exists. Rather, the question is, "How important is competition in structuring communities?"
Competition occurs over a shared limited and limiting resource; however, overlap in resource use does not necessarily mean competition is occurring because resource may not be limited (demand must > supply).
Limited means:
Limiting means:
2 types of competition:
1) intraspecific -
2) interspecific -
2 forms of competitive interactions:
1) interference -
2) exploitative -
Outcomes of competition:
1) competitive exclusion (local extinction) of at least one sp.:
A) species 1 "wins" (excludes species 2); i.e., limits 2 more than it limits self
B) species 2 wins
C) neither species wins (both lose, like the two cats from Kilkenny)
2) both species coexist (stable equilibrium); i.e., a species limits itself more than it limits the other species; occurs via niche partitioning/differentiation – how competing species can coexist
via character displacement (Brown and Wilson 1965) - see Stuart and Losos 2013 for a nice history
evidence: sympatric vs. allopatric species’ similarity
classic example: Darwin's finches of Galapagos islands (Grant 1986); also –sticklebacks (Schluter and McPhail 1992) (see Begon et al. 2006 pp. 251-253 for more examples)
Alfred J. Lotka (American biophysicist) and Vito Volterra (Italian mathematician) - independently derived mass action equations (extensions of logistic growth equations) to determine outcome of competition:
where dN/dt = rate of change in pop. size over time, r = population growth rate, N = pop. size, K = constant (carrying capacity), and a is a measure of the effect of one individual of species 2 on the growth of species 1.
If a = 1, then individuals of the 2 species are interchangeable. If a > 1, interspecific competition is more influential than intraspecific competition; if a < 1, intra > inter (and species can coexist). If a= 0, there is no competition. Example: if a = 4, then a single individual of species 2 consumes 4 times the resources as a single individual of species 1. b is the same coefficient with respect to the effect of species 1 on species 2. These two equations can be used to determine the number of individuals of each species that can coexist at equilibrium.
When the L-V equations are given various values and graphed, a very informative series of figures can be generated (plotting number of species 2 vs. species 1). The space within a figure represents a combination of abundances of species 1 and 2. The graphs can then be used to determine which species would win in competition.
Zero net growth isocline (ZNGI) -
See the supplementary online materials on Lotka-Volterra.
There are MANY ways in which these equations can be modified to take other factors into account (e.g. time lags). But ultimately, the equations are phenomenological and do not elucidate the mechanisms of competition. They are useful in illustrating that two species can coexist only if intraspecific competition is stronger than interspecific competition for both species.
L-V was tested empirically (lab) by G.F. Gause 1932, 1934 - Paramecium aurelia and P. caudatum
Formulated the competitive exclusion principle (Gause’s principle): two species that overlap completely in resource use cannot coexist indefinitely.
So why isn’t P. caudatum extinct in the wild?? Competitive exclusion will only occur in a homogenous environment. In a heterogeneous environment competing species may be distributed in an aggregated manner (species 1 may be dominant in certain patches where it is a superior competitor whereas species 2 prefers other patches where it is superior to species 1, resulting in coexistence). Thus, is the competitive exclusion principle "trivial and untestable" (Sinclair et al. 2006)?
One very important thing missing from the Lotka-Volterra equations is what the species are competing over: resources! And the focus has been on competitive exclusion rather than on coexistence.
David Tilman - developed a general theory and series of models to relate competition among plant species over limiting resources to overall patterns of species coexistence and diversity. He considers plants to follow an essential resource form of resource limitation, in which levels of one resource cannot substitute for deficiencies of some other resource in the environment; species are limited by a series of resources (e.g. nutrients), each of which is essential to growth and reproduction at some level. His theory models the use of resources that will produce a population equilibrium (i.e., dN/dt = 0); the dynamics of resource consumption and depletion in the environment, balanced by resource supply/renewal rates, determine the overall levels of limiting resources at which a population is in equilibrium. Each species consumes resources in a particular ratio, hence, the resource-ratio hypothesis (sometimes called the R* theory). Be sure to read the supplementary online materials on resource-ratio theory on this carefully.
Under this hypothesis, for two species to coexist, it is necessary that:
- the species differ in resource ratios, and
- each species reduces the resource that limits its own growth more than the resource that limits the growth of the other species
These criteria mean that the species that can survive at the lowest levels of a limiting resource will be the best competitor for that resource. Species dominance varies with the ratio of the availabilities of multiple resources, which are determined by resource supply rates. The number of coexisting species will necessarily be < than the number of limiting resources. The highest diversity of competing species will occur at an intermediate ratio of the availability of resources.
Because plant species can therefore coexist by virtue of small differences in resource ratios, there can be many more species in an area than there are types of resources. The model also predicts that species diversity will be greater in resource-poor than in resource-rich environments (explains "paradox of eutrophication").
Tilman’s theory has generated considerable discussion and response, especially from a group of ecologists in the UK led by Philip Grime (see Grime 1979 for more information: plant spp. have “strategies” as competitors, ruderals, or stress tolerators). In Grime’s view, a good competitor for light (for example) will expand and also be a good competitor for soil nutrients or moisture (i.e., a strong competitor for one resource is strong for all resources). In this view, competition is strongest in resource-rich environments. But Wilson and Tilman (1993) showed that competition is strongest when resources are scarce, because species differ in their ability to acquire different resources (can’t be best at everything because there are tradeoffs in competitive ability for resources [good at one means poor at another]); this is evidence in support of Tilman rather than Grime.
Both the L-V and Tilman models can be made more sophisticated/realistic = more complicated; see Gotelli (2008) and Mittelbach and McGill (2019) for examples.
Some additional things to consider about competition:
1) diffuse competition -
2) "ghost of competition past" - Connell 1980
3) indirect effects (see Strauss 1991, Werner and Peacor 2003)
4) apparent competition - Holt 1977
So how can you tell if it really is competition? Next lecture: experiments on competition.
References:
Begon, M., C.R. Townsend, J.L. Harper. 2006. Ecology: From Individuals to Ecosystems. Blackwell Publ., Malden, MA.
Brown, W.L., and E.O. Wilson. 1956. Character displacement. Systematic Zoology 5:49-65.
Connell, J.H. 1980. Diversity and coevolution of competitors, or the ghost of competition past. Oikos 35:131-138.
Gause, G.F. 1932. Experimental studies on the struggle for existence. Journal of Experimental Biology 9:389-402.
Gause, G.F. 1934. The Struggle for Existence. Williams and Wilkins, Baltimore, MD.
Gotelli, N.J. 2008. A Primer of Ecology, 4th ed. Oxford University Press, Oxford, UK.
Grant, P.R. 1986. Ecology and Evolution of Darwin's Finches. Princeton University Press, Princeton, NJ.
Grime, J.P. 1979. Plant Strategies and Vegetation Processes. John Wiley & Sons, New York, NY.
Holt, R.D. 1977. Predation, apparent competition, and the structure of prey communities. Theoretical Population Biology 12:197-229.
Holt, R.D., and M.B. Bonsall. 2017. Apparent competition. Annual Review of Ecology, Evolution, and Systematics 48:447-471.
MacArthur, R.H., and R. Levins. 1967. The limiting similarity, convergence, and divergence of coexisting species. American Naturalist 101:377-385.
Miller, T.E., J.H. Burns, P. Munguia, E.L. Walters, J.M. Kneitel, P.M. Richards, N. Mouquet, and H.L. Buckley. 2005. A critical review of twenty years’ use of the resource-ratio theory. American Naturalist 165:439-448.
Mittelbach, G.G., and B.J. McGill. 2019. Community Ecology, 2nd ed. Oxford University Press, Oxford, UK.
Schluter, D., and J.D. McPhail. 1992. Ecological character displacement and speciation in sticklebacks. American Naturalist 140:85-108.
Sinclair, A.R.E., J.M. Fryxell, and G. Caughley. 2006. Wildlife Ecology, Conservation, and Management (2nd ed.). Blackwell Publishers, Malden, MA.
Strauss, S.Y. 1991. Indirect effects in community ecology: their definition, study and importance. Trends in Ecology and Evolution 6:206-210.
Stuart, Y.E., and J.B. Losos. 2013. Ecological character displacement: glass half full or half empty? TREE 28:402-408.
Tilman, D. 1982. Resource Competition and Community Structure. Princeton University Press, Princeton, NJ.
Tilman, D. 1988. Plant Strategies and the Structure and Dynamics of Plant Communities. Princeton University Press, Princeton, NJ.
Werner, E.E., and S.D. Peacor. 2003. A review of trait-mediated indirect interactions in ecological communities. Ecology 84:1083-1100.
Wilson, S.D., and D. Tilman. 1993. Plant competition and resource availability in response to disturbance and fertilization. Ecology 74:599-611.