Transcript Chp5

Factors Limiting Distribution:
Habitat Selection
– Chapter 5
Habitat – any part of the biosphere where a
particular species can live, either temporarily or
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
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
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
Wild caught adults
Lab-reared, no foliage
Lab-reared, only oak
Again, even though these birds are able to survive in
either habitat, they are demonstrating specific habitat
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
• 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
Overtime, selection
(in this case
selection) will favor
the behavior that
selects the
successful habitat
Theoretical Model
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
• Preferred, Intermediate, Poor
• Suitability is = to fitness in this example
– Females produce more young in more suitable
Suitability Is Not Constant In This Model
• Can be affected by food supply, shelter, and
• 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
– Individuals are free to move into any habitat
without constraint
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
• 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