Population growth - exponential/geometric Stochasticity and population growth Population growth - logistic All

1. Outline for this week Population growth - exponential/geometric Stochasticity and population growth Population growth - logistic Allee effects and population growth

2. Last lecture you should be able to explain what environmental and demographic stochasticity are explain why stochasticity reduces expected population growth rates discuss the implications of stochastic processes for small populations

3. The ecology of small populations Habitat loss Pollution Overexploitation Exotic spp Small fragmented isolated popn’s Env variation Allee effects Demographic stochasticity Reduced N Demographic processes Catastrophes Stochastic processes

4. Outline of this lecture • 1 Recap logistic growth • Describe allee effects Graphically Empirical examples 3. Discuss conservation implications

5. Exponential growth 0 Logistic growth Growth rate r = r0(1-N/K) Continuous dN = r0(1-N/K).N dt Discrete Nt+1 = Nt + Ntr0 1-Nt K

6. Biological interpretation r0 can be interpreted as population growth in the absence of intra-specific competition r0 is the maximum per capita rate of growth K is the upper limit of population growth K is the carrying capacity ie the maximum pop’n size that can be supported by the available resources 1-N/K - unused portion of the carrying capacity

7. Seychelles warblers - endemic species to Seychelles Cousin Island Coconut plantations ---> massive habitat destruction 1959 1 pop’n n=26 1968 Island bought by ICBP Managed as nature reserve Coconut removed Forest regenerated

8. Logistic population growth What happens to the per capita growth rate as N increases? Per capita growth rate dN/dt Pop’n size (N) K

9. The allee effect Inverse density dependence at low densities Warder Clyde Allee 1931,1949 Per capita growth rate dN/dt 0 Pop’n size (N) K Pop’n growth -ve +ve -ve

10. Allee effects What causes them and how important are they? Inbreeding and loss of heterozygosity Demographic stochasticity A shortage of interactions among conspecifics at low densities

11. Allee effects EG 1 - reduced opportunities to reproduce Caribbean Queen Conch Shells >30 cm Heavily harvested Fisheries closed Bermuda 1978 Florida 1986 Illegal fishing continues Fishing allowed in Caribbean Stocks have not recovered Appeldoorn 1988 - could this be due to an allee effect? http://www.wildlifeextra.com/conch-imports129.html

12. Stoner and Ray-Culp 2000 Bahamas - 2 areas 54 sites density 2-812 conch ha-1 - assessed reproductive activity Conc’n?

13. Allee effects What causes them and how important are they? Eg 2- a reduced ability to reproduce - indirect Pink ribbons Clarkia concinna Seed set (%) Pollen grains Isolation distance of patch (m) In large patches pollen grain number or seed set does not decline with isolation. Q. What could be happening?

14. Allee effects What causes them and how important are they? Eg 3 cooperation among conspecifics increases fitness African wild dogs IUCN Endangered ca 3000 left Packs 6-20 adults 1 breeding pair Litters 2-20 pups

15. Eg 3 cooperation among conspecifics increases fitness Foraging success increases with pack size

16. Eg 3 cooperation among conspecifics increases fitness Number of extra hunts needed Number of pups Pack size

17. Allee effects What causes them and how important are they? EG 4 cooperation among conspecifics increases fitness Lesser mole rat Naked mole rat

18. Allee effects What causes them and how important are they? EG 4 cooperation among conspecifics increases fitness Mole-rats are extremely social Food is cached and shared among colony members The ability to find food increases with colony size Small colonies are more likely to fail Jarvis et al. 1998

19. Allee effects What causes them and how important are they? cooperation among conspecifics increases fitness Sentinel systems Schooling behaviour Passive protection Antipredator tactics are more efficient in large groups

20. Allee effects What causes them and how important are they? EG 5 modification of the environment is density dependent At low densities, hemlock is less able to acidify soil and sequester water in the upper soil horizons. Production thus increases with density

21. Allee effects Mechanisms leading to +ve relationship between population size and individual fitness Ability to locate a mate increases Fertilization rates/seed set increases Foraging success increases with group size Antipredator strategies are more effective Ability to modify environment improves

22. The allee effect Logistic growth Weak allee effect Strong allee effect Per capita growth rate dN/dt K Pop’n size (N) Unstable equilibrium stable equilibrium Conservation implications Where the allee effect is strong models predict a lower unstable equilibrium population size Below the lower equilibrium populations will decline to extinction

23. Are allee effects strong enough to impact population dynamics? Glanville fritillary - found in dry meadow - 300-500 of possible 1500 suitable patches occupied Evidence for Allee effect Males and females are more likely to emigrate if N is small See Fig Kuussaarri et al 1998 Oikos

24. Are allee effects strong enough to impact population dynamics? What influences whether a pop’n grows? Discuss

25. Are allee effects strong enough to impact population dynamics? Taylor et al 2005 Animal Conservation 8: 415-20 Does founder size of Saddleback and New Zealand robin influence the success of reintroductions? Data 31 reintroductions to offshore islands Numbers released ranged from 5- 188

26. Are allee effects strong enough to impact population dynamics? Taylor et al 2005 Animal Conservation 8: 415-20 Does founder size of Saddleback and New Zealand robin influence the success of reintroductions? NO

27. SUMMARY Evidence for allee effects exists for diverse taxa Allee effects reduce population growth at low densities Allee effects can influence population dynamics and may increase extinction risk Q What spp are most likely to be affected?