Resource competition

• Ideal free distribution• Dominance

– Correlates– Hierarchies– Hormonal effects

• Territoriality

Ideal free distribution

• Premise: animals sequentially fill available habitat starting with best patches

• Assumptions– “Ideal” by possessing perfect information about

resource quality– “Free” to disperse appropriately

• Expectation– Input matching rule: animals disperse to equalize

energy intake or reproductive success

Ideal Free Distribution

Ideal free distribution experimentsStickleback fish: 6 fish in a tank consume Daphnia introduced from the ends with an initial probability ratio of 2:1; after the start of feeding (first arrow) the number of fish in patch 2 quickly approaches the number predicted by the input matching rule (dotted line). At the second arrow, the patch probabilities are reversed (Milinski 1979).

Mallard ducks: similar experiment in which 33 ducks were fed pieces of bread from two stations with probability ratio of 2:1 around a pond. The number of ducks at the poorer station quickly approaches the input matching rule prediction (Harper 1982).

Deviations from IFD

• 16 of 20 studies show too many in poor habitat and too few in rich habitat (Sutherland 1986)

• Perceptual error– Expected if animals choose habitat at random

• Differences in competitive abilities– Dominants exclude subordinates

Competitive differences exist

Dominance - why?

• Regulates access to resources without continual fighting

• Should be tolerated if the potential risk of injury from fighting exceeds the potential gain from resource acquisition

Dominance - how?

• Ability to control access to a resource is termed resource holding potential (RHP)

• Correlates include– body size

– experience

– matrilineal relationships

– fat reserves

– prior success (or failure)

• Requires individual recognition or status badges

Dominance and size

Dominance and age

Dominance and residence

Matrilineal dominance

Daughters outrank older sisters in

Dominance and fat reserves

No effect of weightBut, males with moreenergy reserves win aerialchases to determineownership of matingterritories along streams

Dominance hiearchies

Linear, transitive

Linear hierarchies are difficult to explain in large groups because RHP should be difficult to rank accurately among average individuals.

Intransitive

Alliances

Mechanisms for linear hierarchies

• Animals recognize each other using signals that correlate with RHP - status badges– Harris sparrows, house sparrows

• Animals adjust their perceived status on the basis of the outcome of recent contests (winner-loser effects)– fishes

• Animals adjust their perceived status after observing contests involving others (bystander effects)– chickens

Status badges in Harris sparrows

Dominance correlates with size of black patch on head and chest

Why not fake it?

After

Before

Darkened birds were attacked more often!

Hormones and dominance

• Winners often have elevated testosterone levels

• Losers have elevated plasma corticosterone levels compared to winners

• In lobsters, losers can be made aggressive by infusing serotonin. This effect can be blocked by Prozac, a serotonin inhibitor

Effects of testosterone on male vertebrates

Steroid hormone synthesis

Stress hormone Female sexual function Male sexual function

Economics of territoriality

• Resource must be defensible– Renewable, e.g. nectar, emerging insects– not ephemeral or superabundant

• Benefits > Costs of defense– Energetic costs increase with

• Density of intruders• Territory size

– Benefits accrue by• Increasing energy intake rate• Reducing search costs• Reducing starvation risk

Territoriality scales with

body weight and diet

Optimal territory size

Territory size should be adjusted to maximize energy gain

© 1999 Bob Pelham

Territory size should depend on resource availability

Eastern Arizona NW California

Territories should occur at intermediate resource densities

Variable Salmon Creek

Doran Beach

Territory length

41 ± 10 82 ± 14

Intruder density

11.5 ± 7.4 0.5 ± 0.4

Prey density

580 ± 160 671 ± 130

Prey density -> Intruder pressure -> territory size (Myers)

Territory size should respond to competition

Testosterone increases competitive ability

Testosterone alters territoriality, at a cost, in juncos

Males defend larger territories Males feed nestlings less