Results from Midterm I

A B C D F

Mean = 23.4Minimum score = 9

Maximum score = 37

How do individuals figure out wherein space they should be?

What should be the criterion of decision making? I.e. based on what value are habitats compared?

Does it matter what habitat decisions are made by other members of the population?

How do individuals in practice accomplish a reasonable use of space? How do they gather information? Do they need to “reason” things out?

Example 1: habitat selection cattle egrets

Cattle egret

Two horses grazed near an air strip in Costa Rica. For the egrets they represented two habitats of equal quality.

Egrets were fairly evenly distributed between the horses most of the time.

(Gerhardt and Taliaferro 2003)

Number of egrets

present at arrival of new egrets

No. choosing patch with fewer egrets

No. choosing patch with more egrets

1 (0,1)

2 (0,2)

3 (1,2)

4 (1,3)

5 (2,3)

6 (2,4)

7 (3,4)

8 (3,5)

9 (4,5)

Total for difference of 1

Total for difference of 2

5

5

6

4

3

3

0

1

1

15

13

5

0

4

0

4

0

3

0

0

16

0

Egret choice at the moment of arrival:

Egrets didn’t care about a difference of 1 bird per horse, but never chose the horse that had two more egrets.

90 cm

2 cm

Boiled spinach

24-48 tadpoles were introduced to middle of the tank, then

released.

Habitat A Habitat B

(Veeranagoudar et al. 2004)

Bronze frog

Example 2: habitat selection by tadpoles

No. of tadpoles after 30 min on side A

No. of tadpoles after 30 min on side B

Significance test

Test 1: no food on either side

11.94 0.28 12.06 0.28

Not significant

Test 2: equal amounts of food on both sides 11.93 .29 12.08 0.29

Not significant

Test 3: 2x more food in B than A

8.94 0.19 15.06 0.19

Significant difference

Test 4: 4x more food in B than A

11.40 0.60 36.60 0.60

Significant difference

(24 trials per test)

Results:

Blue Tit Crested TitGreat Tit

These species live in the same mixed forests and all eat insects.

Example 3: habitat selection in three European tit species

Quercus ilex

Highest insect abundance(high density & large size)

Pinus pinea

Intermediate insect abundance(low density & large size)

Juniperus oxycedrus

Low insect abundance(intermediate density &

small size)

Blue

tit

Great tit

Crested tit All three

Arthropod abundance

(g ha-1)

Breeding:

Quercus

Pinus

Juniperus

Post-breeding:

Quercus

Pinus

Juniperus

72

8

5

44

13

0

48

6

6

5

7

1

28

34

8

16

15

10

148 (69%)

48 (22%)

19 ( 9%)

65 (59%)

35 (32%)

11 (10%)

324 (69%)

128 (27%)

28 ( 6%)

71 (56%)

48 (38%)

7 ( 6%)

Results:

(Diaz et al. 1998)

The birds distributed in proportion to the food abundance in the trees, even as food abundance changed in the course of a year.

0

10

20

30

40

50

60

70

80

0 20 40 60 80

breeding

postbreeding

1:1 line

% birds on tree species

% arthropods on tree species

THE IDEAL FREE DISTRIBUTION(A general theory of habitat selection, first proposed by Fretwell and Lucas in 1970)

If habitats vary in the abundance of food items, individuals will distribute among them so that the individual rates of consumption are

approximately equal across all individuals.

• Habitats of higher quality will usually hold a higher density of individuals.

• Lower quality habitats will remain empty until competition in the higher quality habitats becomes so high that some individuals are better off in the lower quality habitats.

• Individual consumer choice establishes the distribution: each individual will strive to get either better or equal rewards than their competitors.

Predictions:

1. There is no cost associated with habitat switching.

2. Individuals have perfect knowledge of food availabilities and risks.

3. Resource levels are immediately reduced by increasing population densities.

4. No despotism: no individuals which monopolize better habitats and maintain superior fitness for themselves.

Assumptions for the IFD:

E.g. bears are “despots”: larger bears will hold larger and better habitats. Female bears that hold better habitats have more cubs.

General analysis of ideal-free distribution

Habitat A:

Habitat B:

Habitat C:

Habitat quality: A>B>C

Number of individuals in habitat

2 6 10 124 80

Rat

e of

foo

d in

take

per

ani

mal

How would 2 animals distribute across habitats?

X

X

Number of individuals in habitat

2 6 10 124 80

Rat

e of

foo

d in

take

per

ani

mal

2

xx

x

Number of individuals in habitat

6 10 124 80

Rat

e of

foo

d in

take

per

ani

mal

How would 6 animals distribute across habitats?

X

XX

Number of individuals in habitat

2 6 10 124 80

Rat

e of

foo

d in

take

per

ani

mal

How would 8 animals distribute across habitats?

How many animals does it take before the least preferred habitat is used by at least one animal?

Low density: only the best habitat is used.

Higher density: a second, lower quality habitat is used.

Very high density: all habitats are used. Local densities decrease with habitat quality, so that the fitness of all individuals is the same.

Number of individuals in habitat

2 6 10 124 80

Rat

e of

foo

d in

take

per

ani

mal

2 animals

6 animals

16 animals

The Negev Desert

A gerbil

Example 4: Foraging patterns in a gerbil

Two 2-ha exclosures, foraging activity measured as the total length of tracks left in the sand

Example 4: Foraging patterns in a gerbil

Two 2-ha exclosures, foraging activity measured as the total length of tracks left in the sand

Example 4: Foraging patterns in a gerbil

Control

Treatments:• Seed additions• Simulated moon light• Trained owl in flight

Two 2-ha exclosures, foraging activity measured as the total length of tracks left in the sand

Ger

bil

acti

vity

control

Seed addition on one plot G

erb

il ac

tivi

ty

control

Seed addition + light

Ger

bil

acti

vity

control

Seed addition + owl patrols

The gerbils were less active when they perceived risk.

The gerbils were moreactive where there wasmore food.

Gerbil seed collection saturated: they have a limit to how many seeds per night they can handle.

Gerbils that perceived risk collected fewer seeds when seeds were plentiful.

0 72 144 216 288 360 432 504 576

Seed added (g)

Gerbils that perceived risk collected 25% fewer seeds per time spent foraging (spending more time being vigilant).

Activity measure (length of tracks)

2) It is a remarkably good first approximation in many situations.

4) Local density should reflect the quality of the habitat: better habitats are more crowded.

5) Lower quality habitats are used only when regional density exceeds a threshold.

Summary for the IFD:

1) The theory of Ideal-Free- Distribution predicts the spatial distribution of organisms based on the principle of fitness equality among individuals.

3) Establishment of the IDF does not require cognitive abilities, it is the dispersal strategy that should evolve under natural selection.

1) When animals are at low density, they should always occupy only the best habitat.

2) When density goes up, animals should begin to accept lower quality habitats.

3) Density in the better habitat should always remain higher.

Summary for the IFD: