Competition For Resources - look at notes Flashcards
What resources?
- Food
- Space (territory / shelter / nesting site)
- Scarce materials (e.g. nesting materials; nutrients)
- Mates
Darwin’s theory of evolution by natural selection -
Only a proportion of individuals survive to reproductive age (Winners = superior competitors)
Interference:
Reduction in an individual’s rate of prey intake as a result of competition
Simple exploitation:
resources removed by other competitors (which do not even need to meet each other)
Scramble:
◦ competitors see each other using the same resource
◦ interference over each resource item
◦ fastest responder wins
◦ no aggression
Contest:
◦ competitors interact aggressively over resource items
◦ winner (‘despot’) may be able to exclude access by others
◦ can lead to…
‣ Resource defence ➔ economically defendable resources.
The importance of spatial distribution of resources:
Interference competition more likely…
◦ when resources are clustered
◦ with increasing density of competitors
= Competitors forced into closer contact; Increased chance of >1 competitor deciding simultaneously to ‘go for’ same prey item.
= Also increases chance that:
◦ aggression may occur ➔ contest;
◦ resources may become economically defendable
Example of the importance of spatial distribution of resources:
Brown hares
• Normally solitary/pairs, but will form feeding aggregations to give improved vigilance
• Fed apple pieces,
‣ either in a single clump
‣ or spaced at 1 m intervals
• Amount of aggression recorded (one hare displacing another from a food item)
◦ Low levels with dispersed food
◦ Much increased with clumped food, and increasing with group size
The importance of the temporal distribution of resources:
- Renewable resources: appear sequentially = spaced
- Competitors forced to interfere over each resource item if all food introduced together = ‘clumped’
Temporal clumping of resources reduces competition, Spatial clumping of prey increases competition
The Ideal Free Distribution (IFD):
= Model to describe the distribution of competitors in a patchy environment.
Assumptions:
◦ ‘Ideal’ – each animal has complete information regarding the availability of resources across patches;
◦ ‘Free’ – all animals are free to move around between patches without constraint or restriction.
➔ allowing movement to a location that maximises returns.
• Under IFD, the number of competitors in each patch balances so that no individual can better its return by moving to another patch
- Reward per individual diminishes as number of competitors increases
- Rich habitats fill up quickly, so payoff decreases
- May become viable to move to poorer, but less exploited habitats
Is Ideal Free Distribution (IFD) realistic?
In some cases, yes:
◦ Algal grazing catfish:
‣ Algae grew 6x quicker in sunny pools than in shaded pools;
‣ Catfish were 6x more abundant in sunny pools than in shaded ones.
- But mostly, resources become depleted over time
- Complicates IFD predictions
• Other assumptions of the simplest IFD model
◦ Resources are all equally valuable: no variation in size / quality = unrealistic
◦ Competitors are equal = untrue!
The Despotic Distribution:
- When individuals show aggressive guarding of resources; territories form in high quality habitat
- As territories in high-quality habitat fills, no more competitors can enter (a)
- Further competitors forced into low-quality habitat
- When this fills up (b), further competitors become floaters (non-territory holders)
- Not ‘free’ to move around
- Predictions of IFD may not apply
Economic defendability of a resource:
- Despotic distributions only develop when resources can be monopolised:
- Resource defence confers benefits to the defender, by guaranteeing resources (food, mates, shelter etc)
- Also costly – energy expenditure, risk of injury
- Territorial behaviour should be favoured when costs are outweighed by the benefits of resource defence
- Need to quantify the energetic costs and benefits of territory defence
