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
Competition
A mutually negative interaction among spp at
the same trophic level directly or indirectly
resulting in reduced fitness
How might competition take place (2 main classes)?
Cameos of competition (mechanisms cf Schoener 1983)
Consumptive: Ants and rodents
(Brown and Davidson 1977)
Pre-emptive: Barnacles
(Connell 1961)
Overgrowth: Tree seedlings, (Chapman 1945)
Chemical: Allelopathy
(Lankau et al. 2009)
Territorial: Birds (Manorina)
(Loyn et al. 1983)
Encounter: Beetles
(Park et al. 1965)
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 %
Chemical interference
competition in garlic
mustard
Garlic mustard produces gylcosinolates
that kill mycorrhizas in the soil and
inhibit seed germination of neighboring
native plants
Glucosinolate production declines
as garlic mustard populations age.
Why? Competing native plants
recover as glucosinulate
concentration declines
(walnut in this case).
Territorial competition
Bell 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)
Experimental Test: First ‘remove’ 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
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
Single species logistic growth curve
dN/dt = rN(K-N/K)
What about the “N” of other spp??
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 αpurple, green = 4
Read α1,2 as effect of species 2 on species 1
Lotka (1925) and Volterra (1926) used the logistic equation to describe competition between two species
Species 1: dN1/dt = r1N1((K1-N1-α12N2)/K1)
Species 2: dN2/dt = r2N2(( K2-N2-α21N1)/K2)
Where αij = 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, αij If αij = αji = 1, then effect of individuals of each species are
the same, and species are _________________
Competitive interactions need not be symmetrical:
If αij <1, and αji <1, 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 (N>0)?
Set differential equation to zero
0=(K1-N1- α12N2) Gives us zero-growth isoclines:
So Equilibrium N1 = K1- α12N2; Then, equilibrium N2 = K2- α21N1
Can we calculate equilibrial N1 without knowing equilibrial N2?
Example of N1
Substitute:
N1 = K1 - α12(K2 - α21N1) rem: equilibrial N2 = K2- α21N1
Multiply out:
N1 = K1- α12K2/1- α12 α21
For N1 to have an equilibrium population size >0, the
denominator must be >0 (ie product α12 α21 is <1).
MUCH CLEARER USING STATE SPACE GRAPH!
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 αpurple, green = 4
Read α1,2 as effect of species 2 on species 1
Zero growth isoclines - combination of abundances of
N1 and N2 at which growth of one species is zero
=1000/0.6
+ve
-ve
K1=1000
a1,2 = 0.6
Zero growth isoclines - combination of abundances of
N1 and N2 at which growth of one species is zero
=1000/0.6
+ve
-ve
K1=1000
a1,2 = 0.6
No competitor (N2) so population
Of N1 will stop growing at K1
Zero growth isoclines - combination of abundances of
N1 and N2 at which growth of one species is zero
=1000/0.6
+ve
-ve
K1=1000
a1,2 = 0.6
No N1 so N2 will grow to
a maximum size
determined by how much
of K1 each individual of
N2 uses. Zero growth isoclines - combination of abundances of
N1 and N2 at which growth of one species is zero
=1000/0.6
+ve
-ve
For any starting value of N1
and N2 what is the predicted
equilibrium population sizes??
Predicted popn sizes
What is the outcome of competition now?
K1/a1,2 =1000/0.5
a12 =0.5
a21 =1.2
K1 = 1000
K2 = 1000
K2/a21 =1000/1.2 =833
(here we just switched the comp coefficients around) a21 =1.2
a12 =0.5
K1 = 1000
K2 = 1000
a1,2 ~ 1.1
a2,1 ~ 1.2
Competitive exclusion principle
Volterra model predicts that species can only coexist
if intraspecific competition is stronger than
interspecific competition. (i.e. both competition
coefficients < 1 for similar K)
Volterra models had a major influence through
1930’s 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?
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). 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”