Competition
involves resource exploitation or interference by individuals of another species.
By what mechanism does competition operate?
What is the result of competition?
a reduction
in fecundity
survivorship
growth
Two types of competition
Interspecific competition-between individuals of ____________________species
Intraspecific competition-between individuals of the ___________ species
Examples of interspecific competition
a) Gause experiments: studying competition in the laboratory using three species of
Paramecium
All 3 species grew well alone, reaching stable carrying capacities consuming
bacteria or yeast cells.
when P. caudatum was
grown with P. aurelia P.
aurelia always won.
Gause’s experiments
when P. caudatum was
grown with P. bursaria they
coexisted at stable but lower
densities.
b) using two species of diatoms
(Asterionella formosa and
Synedra ulna): need for silicate
b) using two species of
diatoms (Asterionella
formosa and Synedra ulna)
Both algal species require
silicate
Competing for a similar resource
when grown separately each reached a stable carrying capacity while maintaining
the silicate at a constant low concentration
when the two species were grown together,
Mathematical representation of Interspecific competition
Question: What are the general conditions which permit the coexistence of
competitors and what conditions lead to competitive exclusion?
dn k N
= rN
dt k
Logistic growth occurs when resources are limited
How do we determine the effect of one species over another?
By adding conversion factors a, and b
a = effect of species 2 on species 1
b = effect of species 1 on species 2
If two species are competing then for species 1
for species 2
Meaning of symbols
N1 = population size for species one
K1= carrying capacity for species one
r1 =intrinsic rate of increase for species one
When a and b are both > 1 then the outcome of the competitive interaction
would depend on the densities of both populations.
Under what circumstances does species 1 increase or decrease in abundance?
Under what circumstances does species 2 increase or decrease in abundance?
What is a zero isocline?
According to the Lotka Volterra model,
there are four possible outcomes:
1) species 1 wins and species 2 becomes extinct
along the zero isocline dN1/dt = 0
K1>K2/b, K2K1/a
This means: the inhibitory intra-specific effects that species 2 exerts on itself are
greater than the inter-specific effects that species 1 can exert on species 2
3) Either species can win depending on the ecological variables operating at a
given time
K1>K2/b, K2>K1/a
Competitive exclusion
Either species wins depending on initial densities and the environment.
Individuals of a species compete more strongly with individuals of the other
sp.than they do among themselves.
4) Neither species wins, and they eventually coexist, dividing the resources
between them
K1 1 then species two has a greater effect on individuals of species one
When a 1 then species one has a greater inhibitory effect on
individuals of species two. etc.
Some major assumptions of the model
the environment is homogeneous and stable without any fluctuations
migration is unimportant
the effect of competition is instantaneous
competition is the only important biological interaction
Interspecific competition:lab experiments by Thomas Park
Two flour beetles, Tribolium castaneum and Tribolium confusum were used
Large colonies of beetles were reared in small containers of flour that provided
fundamental and realized niches for eggs, larvae, pupae, and adults
He used large numbers of replications
The beetles preyed on each other
Larvae and adults ate eggs and pupae cannibalizing their own species as well as
attacking the other species
Beetles of each species ate more individuals of the other species than they did of
their own
Both species were more affected by inter- than intra-specific competition
The outcome is thus strongly dependent on the relative abundances of the
competing species
Conclusions from Thomas Park’s experiments
Problem
o Assume two species are competing. Species 1 has a competition
coefficient (b) = 1.5 and a carrying capacity, K1 = 1000 and species 2 has
a competition coefficient (a) = 2.0 and a carrying capacity , K2 = 900.
Graph the isoclines describing competition between these two species.
What will be the outcome of this competition?