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Community Ecology
Competition: theory
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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. Outcomes of competition
5. Studies of competition
A. Lotka-Volterra
i. Gause’s lab experiments: competitive exclusion
B. Tilman’s resource-ratio hypothesis
6. Three things to remember about competition: ghosts, indirect effects, and apparent
competition
Terms/people:
intraspecific
interspecific
interference
exploitation
indirect effects
limiting similarity
G. Gause
amensalism
competitive exclusion principle
Lotka-Volterra
J. Connell
“ghost of competition past”
apparent competition (R. Holt)
character displacement
competition
resource-ratio hypothesis (D. Tilman)
“paradox of the plankton” (G. Hutchinson) niche partitioning
commensalism
niche differentiation Hutchinsonian ratio
diffuse competition
Types of biotic interactions that structure communities, determine diversity and abundance
(Odum 1969):
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
if +/- or -/+, is predation or parasitism
if +/0 or 0/+, is commensalism
if -/0 or 0/-, is amensalism
if 0/0, is neutral
if -/-, is competition
Of these types of interactions, competition has historically been considered to be the most
important mechanism in structuring communities (dictating diversity, abundance, and
distribution patterns). 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?]). 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).
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…)
2 types of competition:
1) intraspecific
2) interspecific
2 forms of competitive interactions:
1) interference
2) exploitative
To determine whether competition is at work, the usual strategy is to remove one of the
potential competitors; if there is a rise in the other competitor (in terms of numbers, fitness,
health, mass, etc.), then competition is assumed to have been occurring.
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)
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
character displacement (Brown and Wilson 1965) - see Stuart and Losos 2013 for a
nice history
Hutchinsonian ratio (1.3:1)
limiting similarity (MacArthur and Levins 1967)
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)
C) genetic change
3) neither species is clear “winner”
diffuse competition
Alfred J. Lotka (American biophysicist) and Vito Volterra (Italian mathematician) independently derived mass action equations (extensions of logistic growth eqns) to determine
outcome of competition:
 K  N1  N 2 
dN1

 r1 N1  1
dt
K1


 K  N 2  N1 
dN 2

 r2 N 2  2
dt
K2


where dN/dt = rate of change in pop. size over time, r = population growth rate, N = pop. size, K
= constant (carrying capacity), and  is a measure of the effect of one individual of species 2 on
the growth of species 1.
If then individuals of the 2 species are interchangeable. If  > 1, interspecific competition
is more influential than intraspecific competition; if  < 1, intra > inter (and species can coexist).
If = 0, there is no competition. Example: if  = 4, then a single individual of species 2
consumes 4 times the resources as a single individual of species 1.  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). (See handout.) 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.
Recall that this is an upper-division/graduate level ecology course–if you don’t know what
logistic growth is, or r, or K, or dN/dt, then you need to do some serious catch-up reading on
your own. Go to a basic ecology text, as by Molles, Ricklefs, or Begon et al.
There are MANY ways in which these equations can be modified to take other factors into
account (e.g. time lags). We will be dealing with the fundamental equations with respect to
simple competition.
Assumptions of L-V models:
- species are equal in population growth rate
- equilibrium (species reach carrying capacity)
- environment is ignored (assumed homogeneous)
- migration is negligible
- other biotic interactions (predation, mutualism, etc.) are ignored
Tested empirically (lab) by G.F. Gause 1932, 1934 - Paramecium aurelia and P. caudatum
Formulated the competitive exclusion principle: two species that overlap completely in
resource use cannot coexist indefinitely.
Then why isn’t Pc extinct in the wild? Role of spatial heterogeneity in promoting
coexistence:
e.g. Huffaker (1958)
e.g. G. Evelyn Hutchinson (1961) and the “paradox of the plankton”
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).
But is the competitive exclusion principle "trivial and untestable" (Sinclair et al. 2006)?
David Tilman (U. MN) - resource-ratio hypothesis (R* theory) for competition over essential
resources - Be sure to read the supplemental online materials on this carefully.
Grime vs. Tilman: is competition fiercest in resource-rich or resource-poor environments?
3 important things to consider about competition:
1) "ghost of competition past" - Connell 1980
2) indirect effects (see Strauss 1991, Werner and Peacor 2003)
3) apparent competition - Holt 1977
So with ghosts, indirect effects, and apparent competition, how can you tell if it really is
competition? Next lecture: experiments on competition.
References:
Abrams, P.A. 1983. The theory of limiting similarity. Ann. Rev. Ecol. Syst. 14:359-376.
Begon, M., C.R. Townsend, J.L. Harper. 2006. Ecology: From Individuals to Ecosystems.
Blackwell Publ., Malden, MA.
Bengtsson, J., T. Fagerstrom, and H. Rydin. 1994. Competition and coexistence in plant
communities. Trends Ecol. Evol. 9:246-250.
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.
Connell, J.H. 1983. On the prevalence and relative importance of interspecific competition:
evidence from field experiments. American Naturalist 122:661-696.
Connor, E.S., and M.A. Bowers. 1987. The spatial consequences of interspecific competition.
Ann. Zool. Fenn. 24:213-226.
Gause, G.F. 1932. Experimental studies on the struggle for existence. J. Experimental Biol.
9:389-402.
Gause, G.F. 1934. The Struggle for Existence. Williams and Wilkins, Baltimore, MD.
Grace, J.B. 1995. In search of the Holy Grail: explanations for the coexistence of plant species.
Trends Ecol. Evol. 10:263-264.
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.
Hopf, F.A., T.J. Valone, and J.H. Brown. 1993. Competition theory and the structure of
ecological communities. Evol. Ecol. 7:142-154.
Holt, R.D. 1977. Predation, apparent competition, and the structure of prey communities.
Theoret. Pop. Biol. 12:197-229.
Huffaker, C.B. 1958. Experimental studies on predation: dispersion factors and predator-prey
oscillations. Hilgardia 27:343-383. [a classic]
Hutchinson, G.E. 1961. The paradox of the plankton. American Naturalist 95:137-145.
Keddy, P.A. 1989. Competition. Chapman and Hall, London, UK.
Law, P., and A.R. Watkinson. 1989. Competition. Pp. 243-284 in: Ecological Concepts (J.M.
Cherrett, ed.). Blackwell Sci. Pub., Oxford, UK.
MacArthur, R.H., and R. Levins. 1967. The limiting similarity, convergence, and divergence of
coexisting species. Am. Nat. 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.
Am. Nat. 165:439-448.
Murray, B.G. 1986. The structure of theory and the role of competition in community dynamics.
Oikos 46:145-158.
Niemelä, J. 1993. Interspecific competition in ground-beetle assemblages (Carabidae): what have
we learned? Oikos 66:325-335.
Roughgarden, J. 1983. Competition and theory in community ecology. Am. Nat. 122:583-601.
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 Publ, Malden, MA.
Spencer, M., and J.E. Tanner. 2008. Lotka-Volterra competition models for sessile organisms.
Ecology 89:1134-1143.
Strauss, S.Y. 1991. Indirect effects in community ecology: their definition, study and
importance. TREE 6:206-210.
Stuart, Y.E., and J.B. Losos. 2013. Ecological character displacement: glass half full or half
empty? TREE 28:402-408.
Thompson, K. 1987. The resource ratio hypothesis and the meaning of competition. Functional
Ecology 1:297-303.
Thompson, K.M., and J.P. Grime. 1988. Competition reconsidered –– a reply to Tilman.
Functional Ecology 2:114-116.
Tilman, D. 1982. Resource Competition and Community Structure. Princeton University Press,
Princeton, NJ.
Tilman, D. 1987. On the meaning of competition and the mechanisms of competitive superiority.
Functional Ecology 1:304-315.
Tilman, D. 1988. Plant Strategies and the Structure and Dynamics of Plant Communities.
Princeton University Press, Princeton, NJ.
Trepl, L. 1994. Competition and coexistence: on the historical background in ecology and the
influence of economy and social sciences. Ecol. Model. 75/76:99-110.
Walter, G.H. 1988. Competitive exclusion, coexistence and community structure. Acta
Biotheoretica 37:281-313.
Welden, C.W., and W.L. Slauson. 1986. The intensity of competition versus its importance: an
overlooked distinction and some implications. Quart. Rev. Biol. 61:23-44.
Werner, E.E., and S.D. Peacor. 2003. A review of trait-mediated indirect interactions in
ecological communities. Ecology 84:1083-1100.
Wiens, J.A. 1977. On competition and variable environments. Am. Sci. 65:590-597. [a citation
classic]
Wilson, S.D., and D. Tilman. 1993. Plant competition and resource availability in response to
disturbance and fertilization. Ecology 74:599-611.