1. Reminders I’ll be gone until 3/11... No e-mail access! Send HW to Dr. Inouye or Tashi. Updated study questions.

2. Buckyballs Helium and argon discovered Dated to Permian ME © Washington Post http://www.sciencemag.org/cgi/content/full/291/5508/1530

3. Review What causes populations to go extinct? –Deterministic Factors –Stochastic Intrinsic –demographic –genetic Extrinsic –environmental variation (good years, bad years) –catastrophes (really bad years)

4. The Allee Effect The Allee Effect is defined by a decrease in population growth rate at low density, or inverse DD.

5. The Allee Effect Growth Rate Density K 0 Threshold Density

6. Threshold Densities In some cases, the Allee Effect results in a threshold density, defined as the population density below which growth rate is negative (i.e. there are fewer births than deaths). Below the threshold density, populations WILL go extinct. Note that this is a deterministic, not stochastic process.

7. Mechanisms of an Allee Effect Difficulty in locating mates at low density. Social organisms, esp. cooperative breeders. Species interactions. Demographic stochasticity (? -Today) Loss of H and Inbreeding in small populations (Later)

8. Allee Effect and Fisheries Collapse

9. The Allee Effect in Cooperative Breeders

10. Allee Effect and Invasive Species

11. Allee Effect & Demographic Stochasticity? At each time step, 50% chance of birth, 50% chance of death 1 100 Time 1 0

12. Demographic Stochasticity Variability in population growth rates arising from random differences among individuals in survival and reproduction. –Random fluctuation in growth rate. –Random fluctuation in population structure (e.g., sex ratio, distribution of age classes)

13. The Heath Hen Nature Conservancy magazine July/August 1998 p. 8-9

14. Demographic Stochasticity: Sex Ratio 1.0 0 Probability of Extinction.5 20 2 8 Population Size Probability of population size 8 going extinct in 100 generations due to producing only male (or female) offspring is 50%

15. Allee Effect & Demographic Stochasticity At each time step, 50% chance of male, 50% chance of female Each breeding pair produces two offspring, then die. M,M M,F F,M F,F M,F What is mean lambda for an infinitely large population? (1) What is mean lambda for population size = 2? (.5)