Factors Limiting Distribution: Habitat Selection – Chapter 5

Habitat – any part of the biosphere where a particular species can live, either temporarily or permanently.

Habitat Selection – typically thought of only with respect to animals that can in some sense choose where to live by moving among habitats.

Habitat Selection is a process that operates at the level of the individual:

At A – go to northern or southern Mexico?

At B – Woodland or shrub habitat?

At C – Which type of tree?

Most habitat selection studies have been done with birds:

Three species of hawks all eat much of the same prey but nest in different habitats.

Red-tailed hawk: wings less suited for soaring and tend to search for prey from a perch so they tend to select habitats with several trees or bluff’s.

Swainson’s hawk: Wings most suited for soaring so select areas with very few trees.

Ferruginous hawk: Wings intermediate for soaring, but still avoid areas with a lot of trees.

Foraging behavior determines habitat selection in this example.

The tree pipit and meadow pipit have similar requirements (both nest on the ground) except the tree pipit only breeds in areas near tall trees.

Therefore, only meadow pipits are found in treeless areas except near telephone poles: The tree pipit finishes its song in a tree and the meadow pipit finishes its song on the ground.

Again – behavior is determining habitat choice.

Although habitat selection is partly genetic in birds, it can be influenced by early experiences (may be the reason for a slow response of birds to human induced changes – Killdeer; brown pelican).

Time spent (%) in

Chipping sparrows Pine Oak

Wild caught adults 71 29

Lab-reared, no foliage 67 33

Lab-reared, only oak exposed

46 54

Again, even though these birds are able to survive in either habitat, they are demonstrating specific habitat selection.

Why Has Habitat Selection Evolved?

• Natural Selection:– Individuals that use the habitats in which the

most progeny can be raised successfully are favored by natural selection

– Individuals that choose marginal habitats may not leave as many offspring.

– However, marginal habitats may continue to be populated by an ‘outflow’ from preferred habitats.

• Natural selection may act on a particular behavior that chooses a habitat or it may act on the capacity to learn which habitat is appropriate for a given situation.

Behavior: Nest Site Choices

Theoretical Model

Overtime, selection (in this case directional selection) will favor the behavior that selects the successful habitat

Blue Winged Teal Breeding Success

This actual data reflects the previous theoretical graph. If this type of data continues for several generations, then this may lead to directional selection.

A Theory of Habitat Selection-used to illustrate how habitat selection may operate in a natural population

• For a particular species, habitat is defined as any place on Earth where that particular species can live (temporarily or permanently)

• Suitability – an index of habitat quality– In this example there are three different levels of

suitability• Preferred, Intermediate, Poor

• Suitability is = to fitness in this example– Females produce more young in more suitable

habitat

Suitability Is Not Constant In This Model

• Can be affected by food supply, shelter, and predators

• Suitability is usually a function of the density of other conspecifics– In other words, as population increases habitat

suitability decreases

• Assume Fretwell’s (1972) ideal free distribution:– Individuals are free to move into any habitat

without constraint

Model

• Assumes three habitat suitabilities:A. Good

B. Intermediate

C. Poor

• Also assume’s Fretwell’s ideal free distribution– As one habitat type fills up, individuals will use the

next habitat type

Ideal Free Distribution Model Of Habitat Selection

At very high densities, all habitats have equal suitability.

Yellow-legged Gulls• Prefer to nest in shrubs and are forced to

marginal grass habitats when all shrub habitat is taken

• However, relative fitness was similar between the two habitats.

Ideal Despotic Model

• Territorial / aggressive animals

• Aggressive behavior of individuals forces others to marginal habitats– Density not necessarily lower in marginal

habitats

• Predicts that fitness will be lower in the marginal habitats

One More Model

• Organisms are attracted to areas where conspecifics are present

• Tame flightless mallard ducks attracted wild mallards to breeding lakes

• Individuals may benefit because they know the habitat is suitable– At least safe from predators