Results from Midterm I

1. Results from Midterm I A B C D F Mean = 23.4 Minimum score = 9 Maximum score = 37

2. How do individuals figure out where in 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?

3. Example 1: habitat selection cattle egrets Cattle egret

4. 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)

5. 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.

6. Example 2: habitat selection by tadpoles 24-48 tadpoles were introduced to middle of the tank, then released. Bronze frog 90 cm 2 cm Habitat A Habitat B Boiled spinach (Veeranagoudar et al. 2004)

7. Results: (24 trials per test)

8. Blue Tit Great Tit Crested Tit Example 3: habitat selection in three European tit species These species live in the same mixed forests and all eat insects.

9. Pinus pinea Juniperus oxycedrus Quercus ilex Highest insect abundance (high density & large size) Intermediate insect abundance (low density & large size) Low insect abundance (intermediate density & small size)

10. Results: (Diaz et al. 1998)

11. % arthropods on tree species % birds on tree species The birds distributed in proportion to the food abundance in the trees, even as food abundance changed in the course of a year.

12. 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. Predictions: • 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.

13. Assumptions for the IFD: • There is no cost associated with habitat switching. • Individuals have perfect knowledge of food availabilities and risks. • Resource levels are immediately reduced by increasing population densities. • No despotism: no individuals which monopolize better habitats and maintain superior fitness for themselves.

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

15. 2 6 10 12 4 8 0 General analysis of ideal-free distribution Habitat A: Habitat quality: A>B>C Habitat B: Rate of food intake per animal Habitat C: Number of individuals in habitat

16. 2 6 10 12 4 8 0 How would 2 animals distribute across habitats? X X Rate of food intake per animal Number of individuals in habitat

17. x x x 2 6 10 12 4 8 0 How would 6 animals distribute across habitats? Rate of food intake per animal Number of individuals in habitat

18. X X X 2 6 10 12 4 8 0 How would 8 animals distribute across habitats? Rate of food intake per animal Number of individuals in habitat

19. 2 6 10 12 4 8 0 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. 2 animals Higher density: a second, lower quality habitat is used. 6 animals Rate of food intake per animal Very high density: all habitats are used. Local densities decrease with habitat quality, so that the fitness of all individuals is the same. 16 animals Number of individuals in habitat

20. The Negev Desert A gerbil

21. Example 4: Foraging patterns in a gerbil Two 2-ha exclosures, foraging activity measured as the total length of tracks left in the sand

22. Example 4: Foraging patterns in a gerbil Two 2-ha exclosures, foraging activity measured as the total length of tracks left in the sand

23. 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

24. Seed addition on one plot Seed addition + light Gerbil activity Gerbil activity control control Seed addition + owl patrols Gerbil activity control The gerbils were more active where there was more food. The gerbils were less active when they perceived risk.

25. 0 72 144 216 288 360 432 504 576 Seed added (g) 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.

26. Activity measure (length of tracks) Gerbils that perceived risk collected 25% fewer seeds per time spent foraging (spending more time being vigilant).

27. 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. 2) It is a remarkably good first approximation in many situations. 3) Establishment of the IDF does not require cognitive abilities, it is the dispersal strategy that should evolve under natural selection. 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.

28. Summary for the IFD: 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.