Competition

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Competition
Not until we reach the extreme confines of
life, in the arctic regions or on the borders of
an utter desert, will competition cease Darwin
1859 Origin of Species
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Competition

• A mutually negative interaction among spp at
  the same trophic level directly or indirectly
  resulting in reduced fitness
• How might competition take place?

Direct (contest) or indirect (consumption of a
shared resource)?
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Cameos of competition (mechanisms cf Schoener
1983) Consumptive Under what conditions would
consumptive competition likely to be
important? Pre-emptive Barnacles (Connell
1961) Overgrowth Tree seedlings, (Chapman
1945) Chemical Tadpoles (Griffiths et al.
1993) Territorial Birds (Manorina) (Loyn et al.
1983) Encounter Beetles (Park et al. 1965)
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Consumptive competition between seed-eating
rodents and ants in Sonoran Desert
Strong resource limitation - seeds are primary
food of many taxa (rodents, birds, ants,
beetles) Almost complete overlap in the size of
seeds consumed by ants and rodents
Long-term exclosure experiments - fences to
remove rodents, insecticide to remove ants.
Recensuses of ant and rodent populations. What
do you predict?
Remove rodents number of ant colonies increased
70 Remove ants rodent biomass increased 24
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Chemical interference competition of Rana on Bufo
Rana temporaria
Bufo calamita
Observation Growth of Bufo is inhibited in
presence of Rana faeces even when food is
abundant. Inhibition mediated by an alga
Prototheca sp. Replication of Prototheca
requires gut passage (needs to be consumed by
faeces-eating tadpoles) Consumption of
Prototheca induces change in foraging behaviour
Small tadpoles (Bufo) are attracted to feed on
faeces and are diverted away from feeding on high
quality resources. Large tadpoles (Rana)
produce large numbers of algae but are not
diverted away from feeding on higher quality
food.
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Territorial competitionBell miners vs. the rest
Flocks of Australian Bell Miners defend patches
of Eucalyptus forest against other (larger
birds)
BM territories are often full of sick Eucalyptus
trees - covered in lerps (secretions of the red
gum lerp psyllid, Glycaspis brimblecombei
(Homoptera Psyllidae)
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Experimental Test First remove (ieshoot) the
BMs Methods Count the birds, measure foliage
Results Invasion by guild of 11 spp of
insectivorous birds 3-fold increase in lerp
removal rate 15 increase in epicormic
foliage
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Descriptive competition models

• When would competition lead to species
  coexistence or species local extinction
  (exclusion)?

Non-mechanistic - competition modeled as direct
influence of the abundance of one species on
another.
Assume population growth is dependent on pop
size, N, and limited resources ensure that there
is a max. pop. size, K (i.e., carrying capacity)
dN/dt rN(K-N/K) where r per capita rate of
increase
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Single species logistic growth curve
dN/dt rN(K-N/K)
What about the N of other spp??
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Competition coefficients (Gotelli 2001)
K species 1
Proportion of species 1 resources used
by individual of sp 1(purple) or sp 2 (green)
Individuals of species 2 (green) consume 4 times
as much of the resources available to the purple
species as does species 1 (purple)
itself. Competition coefficient ?purple, green
4 Read ?1,2 as effect of species 2 on species 1
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Lotka (1925) and Volterra (1926) used the
logistic equation to describe competition
between two species Species 1 dN1/dt
r1N1((K1-N1-?12N2)/K1) Species 2 dN2/dt
r2N2(( K2-N2-?21N1)/K2) Where ?ij competition
coefficient for the effect of species j on
species i Overall Proximity of each species i
to its carrying capacity, K is dependent upon its
current population size, Ni, and the population
size of its competitor, Nj, weighted by the
competition coefficient, ?ij If ?ij ?ji 1,
then effect of individuals of each species are
the same, and species are competitively
equivalent
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Competitive interactions need not be
symmetrical If ?ij lt1, and ?ji lt1, then
interspecific competitors have a weaker effect
than intraspecific competitors
What conditions lead to stable coexistence for
two species?
Same as asking under what conditions will the
growth rates (dN/dt) of both species 0 for
population sizes (Ngt0)? Set differential
equation to zero 0(K1-N1- ?12N2) Gives us
zero-growth isoclines So Equilibrium N1 K1-
?12N2 Then, equilibrium N2 K2- ?21N1
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Can we calculate equilibrial N1 without knowing
equilibrial N2? Example of N1 Substitute N1
K1 - ?12(K2 - ?21N1) rem equilibrial N2 K2-
?21N1 Multiply out N1 K1- ?12K2/1- ?12
?21 For N1 to have an equilibrium population
size gt0, the denominator must be gt0 (ie product
?12 ?21 is lt1). MUCH CLEARER USING STATE SPACE
GRAPH!
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Zero growth isoclines - combination of abundances
of N1 and N2 at which growth of one species is
zero
1000/0.6
-ve
ve
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Predicted popn sizes
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K1/a1,2 1000/0.5
a12 0.5 a21 1.2
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K2/a21 1000/1.2 833
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(No Transcript)
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a1,2 1.1 a2,1 1.2
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Competitive exclusion principle

• Volterra model predicts that species can only
  coexist if intraspecific competition is stronger
  than interspecific competition. (i.e. both
  competition coefficients lt 1 for similar K)
• Volterra models had a major influence through
  1930s and beyond on concept of the niche
• However Volterra leaves unanswered How different
  do species need to be to coexist, or is there a
  limit to similarity of competitors for
  coexistence?

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Multispecies Volterra

• Can generate a multispecies Volterra model with
  growth equations for each species
• Each species growth is determined by the additive
  effects of ai,j competition coefficients
• Assumes no higher order interactions (ie
  competition coefficients are fixed).

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Volterra model
Paradox of the plankton Hutchinson (1961) How
do 30-40 spp of plankton in temperate
lakes coexist when they all compete for the same
resources? Still a paradox today this is the
Grail of community ecology