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

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

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

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

Exponential growth Logistic growth Growth rate r = r0(1-N/K) 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

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

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 1988 29 birds to Aride Island 1990 29 birds to Cousine Island

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

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

What causes them and how important are they? 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

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? Stoner and Ray-Culp (2000) Evidence for Allee effects in an over-harvested marine gastropod: density dependent mating and egg production Mar Ecol Prog Series 202: 297-302 http://www.wildlifeextra.com/conch-imports129.html

Conc’n? Stoner and Ray-Culp 2000 Bahamas - 2 areas 54 sites density 2-812 conch ha-1 - assessed reproductive activity Stoner and Ray-Culp (2000) Evidence for Allee effects in an over-harvested marine gastropod: density dependent mating and egg production Mar Ecol Prog Series 202: 297-302 Conc’n?

What causes them and how important are they? Allee effects What causes them and how important are they? Eg 2- a reduced ability to reproduce - indirect Pink ribbons Clarkia concinna Pollen grains Seed set (%) Groom 1998 Isolation distance of patch (m) In large patches pollen grain number or seed set does not decline with isolation. Q. What could be happening?

What causes them and how important are they? 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

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

Eg 3 cooperation among conspecifics increases fitness Number of extra hunts needed Courchamp et al (2002) Small pack size imposes a trade-off between hunting and pup guarding in the painted hunting dog Behav Ecol 13: 20-27 Number of pups Pack size

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

What causes them and how important are they? 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

What causes them and how important are they? 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

What causes them and how important are they? 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

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

The allee effect K Unstable equilibrium stable equilibrium 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

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

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

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? Taylor, SS et al. (2005). Successful island reintroductions of New Zealand robins and saddlebacks with small numbers of founders ANIMAL CONSERVATION 8:415-420 Data 31 reintroductions to offshore islands Numbers released ranged from 5- 188

NO 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? Taylor, SS et al. (2005). Successful island reintroductions of New Zealand robins and saddlebacks with small numbers of founders ANIMAL CONSERVATION 8:415-420 NO

Evidence for allee effects exists for diverse taxa 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?