Week 7: Habitat selection

Question 1

Habitat

Answer 1

A type of place that provides more or less favorable characteristics for an organisms’ activity

The habitat of an organism is the physical environment it is found in.

Question 2

Animals have 4 basic habitat needs

Answer 2

1. water
   - weather and climate
2. food
   - evaluate habitat on basis of food requirement
3. cover
   - thermal: shade in summer, shelter from cold in winter
   - hiding: protection from predators
4. space
   - breeding and nesting

Question 3

Niche

Answer 3

the ecological niche of an organism is the position it fills in its environment, comprising the conditions under which it is found, the resources it utilizes and the time it occurs there.

Each habitat provides numerous niches

term related to species

Question 4

Home range

Answer 4

an area that an animal occupies but does not defend

term related to individual

Question 5

Territory

Answer 5

an area that an animal defends against intruders

term related to individual

Question 6

Habitat selection

Answer 6

preference for specific habitat type

In choosing where to live, many animals actively select certain places over others.

If living in a certain kind of habitat enhances fitness, then individuals able to occupy preferred habitats should have higher fitness than their competitors.

Question 7

Ideal free distribution

Answer 7

C = Q(i)/n(i) or n(i) = Q(i)/C

The number of competitors in each patch is proportional to patch quality

Question 8

Assumptions of IFD

Answer 8

IFD can be applied whenever there is competition over a resource which is patchily distributed.

Assumptions:
- individuals are ‘ideal’ in assessing patch quality (complete information about the availability of resources)
- individuals are ‘free’ to enter or leave any patch of their choice
- patch quality declines with increasing competitor density
- all individuals select the instantaneously most profitable patch
- all individuals are of equal competitive ability

Predictions:
- competitors will distribute themselves such that the number of individuals per patch is proportional to the fraction of resources in that patch
- the intake per individual is equal across all patches

Question 9

IFD with two habitats of different quality

Answer 9

The first arrivals will go to the rich habitat.

The more competitors the lower the rewards per individuals.

At a point the poor habitat will be equally attractive.

Thereafter the habitats should be filled so that the rewards per individuals are the same in both.

Question 10

Aphids

Answer 10

In a cottonwood poplar tree, each female aphid
- selects a leaf
- settles by its midrib, almost always near the base
- induces the formation of a hollow ball of tissue, a gall
- in which she lives with the offspring

about 20 aphids for every large leaf –> territorial dispute

best position is the bottom of the leaf.

Question 11

Habitat preference for gall-forming aphids

Answer 11

Individuals able to occupy preferred habitats have higher fitness than their competitors.

Defeated individuals settle on the best of the lower-value habitats

Gall aphids fight for access to big leaves, which yields higher reproductive success

Loser has two options
- take secondary position on the big leaf
- move on to a smaller leaf

Question 12

Cost-benefit analysis of territoriality

Answer 12

Costs:
1. energetic costs
- time
- energy
- physiological costs
2. risk costs
- injury or exhaustion
3. opportunity costs

Benefits:
- exclusive access to mates or food

Question 13

Dispersal

Answer 13

movement to a new habitat
- typically associated with crowding
- usually once in a lifetime event

(A) Some birds stayed for months near the place where they had been captured.

(B) Others alternated of staying put with substantial dispersal movements through unfamiliar terrain.

Being in a new area boosted the risk of being killed by predators at least threefold compared with birds stayed in familiar areas.

Question 14

Sex-biased dispersal in mammals

Answer 14

In mammals, the female progeny tend to be philopatric, and the male progeny tend to disperse.

Why female mammals are philopatric?
- The benefits of remaining on familiar ground are greater for females than for males. Ex.: hunting grounds and safe breeding dens)

Question 15

Belding’s ground squirrel

Answer 15

This species live on mountains in the western United States.

This extensively-studied rodent has a gray back with a buff-colored belly and sides, and is about 25 cm long.

It burrows where the earth is dry and soft enough to dig.

It often is seen sitting on its hind legs, watching for predators.

Question 16

Sex-biased dispersal in ground squirrels

Answer 16

When young squirrels reach about two months old, they leave the burrow where they were born.

Males go much farther on average from their natal burrows than females.

Question 17

Why sex-biased dispersal? Inbreeding avoidance hypothesis:

Answer 17

Inbreeding avoidance hypothesis:

• When two closely related individuals mate, the offspring they produce are more likely to carry damaging recessive alleles in double doses than are offspring produced by unrelated pairs.
• High juvenile mortality does indeed occur in inbred populations of many animals.
• Problem: Inbreeding can be avoided if either male or female offspring disperse. Then, one might expect as many female ground squirrels as males to travel 150 meters from their natal burrow. But they do not.

Question 18

Why sex-biased dispersal? mate competition hypothesis:

Answer 18

Males, not females, fight with one another for access to mates. Therefore, loser males may find it advantageous to move away from same-sex rivals that they cannot subdue.

In lions, when new males take over a pride, they violently displace the previous pride masters and chase off the subadult males in the pride as well. → support for this hypothesis

However, young males often leave anyway, without any coercion from adult males. → support for the inbreeding avoidance hypothesis

Question 19

Migration

Answer 19

the regular movement back and forth between two relatively distant locations

a special form of dispersal in which the migrant eventually returns to the place it left.

Most migrations are associated with food supply, reproduction or changes in ambient temperature.

The migratory route of arctic terns. These birds fly from high in the Northern Hemisphere to Antarctica and back each year, completing a roughly 40,000 km.

Question 20

Cost of migration

Answer 20

• time and energy expended in flight
• extra weight that the migrant has to gain
• running out of energy before destination

The costs of migration are not trivial. Migrants will evolve tactics that reduce the costs of the trip, which obviously include the energy expended in flight.

Birds flying in V-formation can take advantage of the updrafts created by the wingbeats of their companions.

Question 20

Red-eyed vireos

Answer 20

migrating in the fall from the eastern US to the Amazon basin

two flight routes
- trans-Gulf flight: shorter, but over the open sea
- overland flight: longer, but no risk of drowning at sea

birds with:
low fat reserves → head west
high fat reserves → orient due south

Question 21

Benefits of migration

Answer 21

Nearly half of all the breeding birds of North America are migrants.

North America (breeding) <—> Mexico, Central America or South America (wintering)

In areas with higher latitudes:
- immense populations of protein-rich insects
- many hours of summer daylight

Question 22

Monarch butterfly migrate to avoid killing freezes

Answer 22

Fall migration: from USA and Canada to central Mexico

Spring migration: from Mexico to USA and Canada

The stands of Oyamel fir (trees) used by the monarchs provide a uniquely favorable microclimate that promotes winter survival

Monarch butterflies are vulnerable to freezes.

As many as 2 million monarchs can die in a single night of subfreezing temperature.

Overwintering in the Mexican mountain refugia
- at about 3,000 meters elevation
- temperatures rarely drop below 4 °C
- create cool and dry conditions ideal for overwintering Monarch butterflies

How about low-elevation locations?
- warmer and drier areas
- Monarchs would quickly use up their water and energy reserves.

Question 23

Logging of oyamel trees

Answer 23

Logging of Oyamel trees opened up the forest canopy

increased the chance that the butterflies become wet

increased the risk that they will freeze

Protection from freezing depends on a dense tree canopy that reduces wetting of the butterflies by rain or snow and their exposure to open sky.