Lesson 18: Habitat Selection

Question 1

how do animals choose a place to live?

Answer 1

probably need to have:

1.) sufficient resources (food, shelter, etc)
2.) not too much competition
3.) access to mates
4.) some combination thereof that maximizes fitness

Question 2

ideal free distribution model

Answer 2

• this model predicts that resource availability and the presence of others influences how animals DISTRIBUTE themselve avross their possible habitats
• IDEAL means that the animals have the ability to act in an ideal manner (that is, they know what they should do to maximize fitness)
• FREE implies that the animals are free to travel wherever

Question 3

ideal free distribution model assumes

Answer 3

• individuals attempt to maximize fittness
• individuals are equal competitors
• individuals are free to move at no cost
• habitat locations differ in resources
• fitness is negatively density-dependent (meaning it is bad to be too crowded)

Question 4

consider 2 habitats A and B

Answer 4

• A is the better habitat because at any population density, the fitness of individual is higher (on y-axis)
• now suppose that the 2 habitats start out empty, as individuals arrive, they sould choose habitat A for maximizing fittness
• but note how fitness goes down as population density goes up
• for example, as habitat A starts to fill in, the attainable fitness in that area will decrease (the blue line slopes down)
• at some point habitat B will suddenly provide the same fittness as A - then individuals should be equally likely to use either habitat
• so the animals distribute themselves based on habitat quality and the competition for resources

Question 5

you can have the same fitness in habitats of different quality as long as population densities are appropriate

Answer 5

• individuals in habitats 1 and 2 have the same fitness but are in populations with different densities

Question 6

the model also implies that as the density of habitat 1 increases, so will the density of habitat 2

Answer 6

• as habitat 1 fills up, the animals will move into habitat 2 and a linear relationship will be seen as an isodar on the graph

Question 7

the ideal free distribution model and the pike of lake windermere

Answer 7

key info: pike are large predatory lake dwelling fish
good habitat consists of water that is:
1.) sufficiently cold
2.) sufficiently deep
3.) has plenty of smaller fish as prey

key info: lake windermere is a sort of natural experiment in habitat selection
– the 2 basins are of different depths and temperatures and habitat quality for the pike
- habitat locations differ in resources

Question 8

more on pike

Answer 8

• used 50 years of density data from pike fishery
• used results of previous “mark-recpature” studies in which animals were marked and then caught repeatedly over the years
  RESULT: when pike abundance was lower, survivorship was higher
  – fitness is negatively density-dependant

estimated the fecundity of the pike based on the growth of the population
(fecundity = reproductive output)

RESULTS
- a plot of relative abundance (density) versus individual fecundity indicates that fitness is negatively dependent on density
- or to put it another way:: if the lake is too crowded the repriductive abilities of the pike go down (an assumption of the ideal free distribution)
- the assumptions of the model are met in this situation
- a plot of relative abundance (density) in the north basin vs. south basin shows a linear relationship or isodar and indicates the pike are behaving as the IDF predicts

Question 9

allee effect in desert clicker grasshoppers

Answer 9

key info: desert clickers are a kind of grasshopper in which males call to attract females
- preliminary observations suggest that males tend to cluster together when calling - perhaps for signal amplification .. this would fit the idea that fitness INCREASES with density

METHODS: gave males a choice

Experiment 1:habitat choice in response to a playback - asking if males will actively go to areas with other males already there — heterospecific call played back
RESULT
17 out of 20 males chose the bush with the conspeficic call instead of the bush with no sound
16 out of 20 males chose conspecific call vs. heterospecific call

Experiment 2: Habitat choice in response to real grasshoppers
METHODS
- placed a male in one bush and waited to see if other (wild) males were attracted to the bush
RESULTS
males recruited to the occupied bush over the empty budh

Question 10

allee effect

Answer 10

positive density dependence

Question 11

conspecific cueing

Answer 11

individual use the presence of members of their own species as a signal that the territory quality is good

Question 12

how does conspecific cueing differ from the allee effect

Answer 12

individuals are not seeking to increase density for conspecific cueing

Question 13

conspecific cueing in american redstarts

Answer 13

key info: American redstarts are songbirds that migrate south in winter
- in the spring, males fly north and set up territories and then sing to attract and then sing to attract mates
- females then fly in and select the territories they like

Q: do male redstarts use the presence of other male redstarts to signal that an area is a good place to set up a territory
Methods
- in previous year, had baneded all males in an area, so could identify males as they arrived to set up territories

Determined whether the males were
1.) returning males that had nested there before
2.) immigrant males who had nesteed elsewhere the previous year (a second-year adult or older with no leg bands)
3.) first-year males who had not nested before

Methods
- set up plots with speakers playng the redstart song
- control plots had no song
- recorded the age and status of males as they arrived to set up territories

RESULTS:
- new adult immigrant preferred playback plits (these are individuals who had nested elsewhere before)
- returning adults and yearlings showed no preference