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Where’s the Biology?
Wildebeest populations growth
competition for grass occurs
Individuals are energy stressed
Lions kill off weak individuals
1 dN = r (K-N)
N dt
K
??Energy/stress??
From a single species to
two species
Effect * of species 1 on species 2
-
+
-
COMPETITION
PREDATION
+
PREDATION
MUTUALISM
Effect* of
species 2
on species 1
* On per capita growth rate
What is interspecific competition?
Phenomenological: it is two species
(interspecific competition) that
have a negative effect on each other’s
per capita growth rate
Mechanistically this may involve:
Mutual depletion of a limiting resource
(exploitation), be it food, shelter, space
Defense of a resource (Interference)
Allelopathy – the production of toxins
(or otherwise modify conditions) to
the detriment of another species
What are its effects and what evidence do we have that it occurs??
Gause’s famous experiments with Paramecium showed for the first time
experimentally that one species could eliminate (exclude) another species
from its habitat
Balanus outcompetes Chthamalus for space and restricts it to
the upper tidal zone where Balanus is subject to desiccation
Similarly, many species of ungulates occur on the African
savannah, but appear to occupy different habitats
Zebras in the old, tall grass
Thompson’s gazelles in the
young, succulent grass
Removal of dominant competitor (K-rats)
leads to ecological release for weak
competitors
Removal of
K-rats
Leads to rapid
increase in
small seed-eaters
Gerbillus pyramidum
Egyptian Gerbil
~40 g
Gerbillus allenbyii
Allenby’s Gerbil
ARE THEY
GONE YET??
Let’s kick some
Allenbyii butt
Activity
Pattern
Time (hrs after sunset)
~20 g
Control
Gp removed
We know from an immense set of observations
and experiments that:
(1) Species can reduce each other’s abundance
(2) Species can force one another to extinction,
thereby influencing its geographic range, and
limiting its realized niche
But is competition simply haphazard??
Or is is Repeatable and Predictable
That is, is there anything similar between:
Paramecium aurelia vs. P. caudatum
G. pyramidum vs G. allenbyii
Balanus vs. Chthamalus
We have two options for attempting to answer this question:
(1) Study as many examples as possible and
try to formulate generalizations
(2) Construct a model to help conceptualize
the process of Competition
Logistic growth embodies intraspecific (within a species) competition
1 dN = r (K-N) = r (1 - N )
N dt
K
K
-X
denotes a negative effect
of another spp
denotes a negative effect
Of a species on itself
INTRASPECIFIC COMPETITION
Lotka-Volterra Competition Equations also embody
interspecific competition
1 dN1 = r
N1
N2
(1

)
1
12
N1 dt
K1
K1
1 dN2 = r
N2
N1
(1

)
2
21
N2 dt
K2
K2
exponential
growth
Limits to growth:
Spp reduces their
own growth rate
Other species
reduce the
growth rate
of competitors
Note:
is the same as:
1 dN1 = r
N1
N2
(1

)
1
12
N1 dt
K1
K1
1 dN1 = r
1 (K1 – N1 – 12N2)
N1 dt
K1
1 dN1
N
N
= r1 (1 - 1 - 12 2 )
N1 dt
K1
K1
There is only one new term: 12 and 21 are called
the Competition Coefficients
12 = the relative effect of an individual of species 2 on
species 1’s per capita growth rate
For instance, grasshoppers and zebras compete for grass. One zebra
eats as much as 1,000 grasshoppers. Therefore ZG = 0.001. Does
GZ = 1,000 ?? It may, but not necessarily.
Also, the ’s represent the per individual competitive effect, not the
population effect. If there are 3,000 more grasshoppers than zebras
grasshoppers have a small per individual effect, but a larger
population effect.
N2
K2
Species 2 drives
Species 1 to extinction
K1
12
K1
12
Species 1 drives
Species 2 to extinction
K2
K1
K1
12
K2
21
N1
Stable coexistence
at reduced density
K2
21
K2
K1
12
K2
K1
K2
21
K1
One of 2 states arise based
on starting conditions:
Species 1 alone at K1
Or
Species 2 alone at K2
K2
21
K1
To download a version of Populus:
http//www.cbs.umn.edu/populus/download/download.html
LV Competition
- Only 1 (in 4) solutions yields coexistence – Is coexistence the
unexpected outcome?
- Is competitive exclusion random or does the theory indicate
some property (not identity) of the competing species that can
guide us?
 Yes, as ’s get closer to 1 the conditions for coexistence get more restrictive
– the biological interpretation is the more similar two species are the less likely
they are to coexist
 Two species that occupy the same niche (they are identical) cannot coexist =
Competitive Exclusion Principle
Competition as a structuring force in biological communities
Small seed
specialist
Large seed
specialist
Frequency
In the diet
small
Seed Size
Overlap = competition
large
Medium seed
eater
Small seed
specialist
Large seed
specialist
Frequency
In the diet
small
large
Seed Size
(2) Niche partitioning – there is a limit on the amount of
similarity between adjacent species that results in a regular spacing
of species’ morphology
X
Medium seed
eater
Small seed
specialist
Large seed
specialist
Frequency
In the diet
small
large
Seed Size
Competition works as a biological filter to
remove or prevent the invasion of species
into biological communities
Niche partitioning in two
desert rodent communities
Wildcat
These coexisting
Felids in Israel
Caracal
Diameters of canines for 3 species of coexisting cats in Israel
(after Dayan et al. 1990)
Wildcat, fem.
Wildcat, male
Jungle cat, fem.
Caracal, fem.
Jungle cat, male
Caracal, male
5mm
6
7
8
Niche partitioning in the feeding appendages of Felids
9
Small seed
specialist
Medium seed
eater
Frequency
In the diet
small
large
Seed Size
Small seed
specialist
Medium seed
eater
Frequency
In the diet
small
large
Evolution
Frequency
In the diet
small
large
If Competition is an evolutionary force in species’ ecology:
Predictions:
(1) Species will differ greatly where the share an evolutionary history
(2) Species will not differ much when they have not shared a evo. history
Differences in species’ morphology will be
greater in sympatry than in allopatry
sympatry = occur together
allopatry = occur separately
(4) Character Displacement
in Darwin’s Finches
Conclusions:
(1) In Ecological Time competition is a Species Taker
(2) Competition is a structuring force – dissimilar species can coexist
more easily
(1) Influences geographic distributions
(2) Niche partitioning
(3) Niche expansion when there is release from competition - Ecology
(4) Character Displacement - Evolution