Week 7: Habitat selection Flashcards
Habitat
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.
Animals have 4 basic habitat needs
- water
- weather and climate - food
- evaluate habitat on basis of food requirement - cover
- thermal: shade in summer, shelter from cold in winter
- hiding: protection from predators - space
- breeding and nesting
Niche
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
Home range
an area that an animal occupies but does not defend
term related to individual
Territory
an area that an animal defends against intruders
term related to individual
Habitat selection
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.
Ideal free distribution
C = Q(i)/n(i) or n(i) = Q(i)/C
The number of competitors in each patch is proportional to patch quality
Assumptions of IFD
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
IFD with two habitats of different quality
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.
Aphids
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.
Habitat preference for gall-forming aphids
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
Cost-benefit analysis of territoriality
Costs:
1. energetic costs
- time
- energy
- physiological costs
2. risk costs
- injury or exhaustion
3. opportunity costs
Benefits:
- exclusive access to mates or food
Dispersal
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.
Sex-biased dispersal in mammals
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)
Belding’s ground squirrel
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.
Sex-biased dispersal in ground squirrels
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.
Why sex-biased dispersal? Inbreeding avoidance hypothesis:
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.
Why sex-biased dispersal? mate competition hypothesis:
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
Migration
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.
Cost of migration
- 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.
Red-eyed vireos
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
Benefits of migration
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
Monarch butterfly migrate to avoid killing freezes
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.
Logging of oyamel trees
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.
