Where animals are found is obviously an important component of all subsequent studies of behavior.

How do we describe patterns of dispersion?

Snap-shot vs continous views of distribution and abundance

Many behaviors relate specifically to movements... and understanding movement can provide insites into subsequent patterns of dispersion.

Kinesis: movement as a function of stimulus strength but no directionality

e.g. wood lice.

Taxis: Movements towards (positive) or away(negative) from stimuli.... implies detection of a gradient.

Phototaxis (light)

Chemotaxis (chemical)

Phonotaxis (sound)

Rheotaxis (current)

Geotaxis (gravity)

Migration: Large scale, punctuated movements.

The costs of moving are presumably outweighed by either the advantages available at a new location or the costs of remaining at a particular location.

 

Patterns of dispersion generate a spatial and temporal "template" for behavioral interaction among animals.

Three basic patterns:

Random dispersion is least common - suggests underlying organizing factors...

What might they be?

Examining non-random dispersion in terms of per capita payoffs.

"Passive" vs. "active" determinants of dispersion

 

Passive: Animals do not seek or avoid others specifically, they simply go where payoffs are highest. Typically, all individuals, on average, receive the same payoff (no per capita benefit of grouping) and resources and/or predation are the determinants of dispersion.

 

Active: Animals actively seek out or avoid others and receive a per capita benefit from joining a group. Resources or predation may play a role, but dispersion cannot be explained simply by passive conditions.

 

The challenges of either obtaining food or avoiding becoming someone else's dinner provides a good logical framework for understanding how animals distribute themselves in space and time.

 

Getting food and group living

 

Passive settlement onto patchy resources: Ideal Free Distribution (Fretwell, 1972)

 

Assume

a) a habitat contains patchy resources

b) animals have perfect knowledge of their environment

c) they can move freely between resource patches

d) all animals are identical (no competitive advantages)

 

Predict

a) for a population within a habitat, animals will choose patches on the basis of relative patch quality and number of animals present on patches. Thus....

b) the fraction of the population on a particular patch (N) will equal the fraction of total resources within the patch (proportional dispersion)

c) at equilibrium, all individuals will achieve identical payoffs (1/N_x for individual in patch x)

 

A population dispersed in this manner matches the Ideal Free Distribution (IFD)

 

Violating the assumptions may bring the model closer to reality:

 

1) High interference among foragers (individuals get less than 1/N) as density within a patch increases: predict fewer animals in richer patches than IFD but all animals will receive equal payoffs

2) Low interference: crowding helps extract resource: more animals on rich patches than IFD, but equal payoffs

3) Patch switching: animals will stay longer on rich patches, causing more than predicted on rich patches, but still equal payoffs

4) Conspecific cuing: animals using the presence of others to decide where to feed will lead to larger than predicted groups in rich patches with lower than predicted payoffs.

5) Limited sensory capabilities: Animals may have a harder time distinguishing between two rich patches rather than two poor patches (relative differences). This leads to fewer animals on rich patches and higher than predicted payoffs there.

6) Competitive differences: dominant individuals may limit access of subordinates to rich patches. This would lead to fewer (dominant) individuals with high payoffs on on rich patches and more (subordinate) individuals with low payoffs on poorer patches.

 

Active mechanisms for group formation to increase foraging efficiency (the first two are covered in the text)

Information transfer

Coordinated hunting

Reduced path overlap

Positive prey response to foraging by predator

Risk minimization

 

Avoiding being food

 

Passive models are just like food, IFD with resource being refugia or protected sites

Selfish herd: No net benefit, but center is safer (the group itself is the "refuge")

A video of "selfish herd" behavior in sheep

 

Active models for group formation to reduce predation

Risk of predation is a function of two things:

Risk that group is discovered (encounter rate)

Fraction of group that is consumed by predatory after encounter (consumption rate)

 

Reducing encounter rate (Click here to see a more detailed description of these ideas)

If predator does not respond preferentially to groups, then grouping may reduce encounter

Conspicuousness of groups often increases with group size

Smell and sound are intensity cues, these increase linearly with group size

Vision is related to contrast and size, which may scale more slowly with group size

Thus, for predators using smell or sound to detect their prey, small prey groups are less likely to be detected

Grouping may be more advantageous for visually hunted prey, depends on type of hunter (ground vs air) and the way the group is dispersed

 

Reducing the consumption rate

Diliute risk: Predator saturation (swamping)

Predator confusion (e.g., starling murmurations)

Cooperative defense

Improved vigilance: More eyes/ears/noses, etc....

 

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