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Individuals do not live in isolation...

• Individuals of the same species have similar requirements for survival, growth, reproduction
• demand for resource may exceed supply
• individuals may compete for resources...


Intraspecific Competition

= an interaction between individuals of the same species (intra) brought about by a
shared requirement for a resource in limited supply


Intraspecific Competition-
– Results in:

• Reduced survival, growth, and/or reproduction of some individuals
– possible reduction in fitness Competition acts as a selective pressure in nature


How does competition arise?

- Resources are not evenly distributed (distributed in patches)
- Aggregative Response
- competitive interactions


Aggregative Response

predators concentrate where prey density is high
o due to high consumption rates


Competitive Interactions

- can lower consumption rates
- patch with highest prey density is not always the best!


Types of Competition

1. Exploitation (Scramble) Competition
2. Interference (Contest) Competition


• Exploitation (Scramble) Competition

– an individual responds to a decreased level of a limited resource remaining after it has been
exploited by other individuals
– individuals donotinteractdirectly


• Interference (Contest) Competition

– an individual actually prevents another from exploiting a limited resource within a portion
of the habitat
– individuals interactdirectly


Intraspecific Competition What is the result of Competition?

net energy gain - energy loss ↑ +/or energy gain ↓ due to competitive interactions
-Consumption rates decrease with increasing numbers of competitors


Low competition

Reduced Growth


Moderate competition

Reduced Growth, Reproduction


High competition

Reduced Growth, Reproduction & survival


How do individuals balance the attraction to patches of high prey abundance and the repulsion by the presence of competitors?

Use Behavioural Ecology to study how an individual’s behaviour is adapted to it’s environment


Ideal Free Distribution Model Assumptions:

1. There are a number of prey patches that vary in quality
2. Competitors are ‘free’ to exploit all patches (can move ‘freely’ among
3. Individual prey consumption rates decline with increasing numbers of
competitors in the patch increases
4. Individuals have equal competitive abilities
5. Animals distribute themselves ‘ideally’ among patches to obtain highest
consumption rates (maximize net energy gain to ensure high fitness)


As competition ^ in high quality patch

the patch quality v (leading to lower consumption rates)


Individuals in an area reach an equilibrium distribution, where:

• all patches occupied
• more individuals in higher quality patches...


At the equilibrium distribution...

the consumption rates of individuals are equal for all competitors in all patches


Can be a dynamic equilibrium

– individuals are constantly moving among the available patches
... but individuals in the lowest quality patch will not gain access to more prey if they move to a higher quality patch


Generalized Experimental Design:

• Prey arrives in the patch at a particular rate
• Prey is consumed immediately
• Scramble/Exploitation Competition -competitors do not interact directly


Example – Harper 1982

Ducks on a lake with two feeding stations (station A and B)
• pre-cut, weighed bread pieces
• identify individual ducks
• same input rate at A and B

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