Download General Ecology: EEOB 404

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
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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)
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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
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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
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Lab studies
Tribolium species (see text)--mutual antagonism
 Paramecium species studied by Gause (aurelia, caudatum,
bursarum
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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 in Paramecium
species
Citrus scale insects
(Aonidiella) in California
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
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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
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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 rselected; 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
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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
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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; a 1,2 =
competitive effect of species 2 on 1
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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:
= 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
 dN1/dt
Lotka-Volterra competition model,
cont.
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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
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Model cannot be solved explicitly, but we can use isocline
analysis to study its behavior
Graphical analysis of LotkaVolterra 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 N1 phase plane graphs
Identification of four configurations
of isoclines-->four outcomes
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Graphical analysis of 4 possible configurations in species 2by-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)
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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:
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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)
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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
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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?”
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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
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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:
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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