Two considerations with respect to the maintenance of genetic diversity.

1. inbreeding depression-but only requires small amount of immigration to prevent build-up of deleterious, lethal homozygotes.
2. drift – loss of genetic diversity due to random variance in frequencies of alleles

Leads to proposed 50/500 concept

The 50 refers to number of individuals in a population needed to dampen inbreeding effects.

For commerical breeders:

The inbreeding coefficient F increases by 1/2N_e per generation, where N_e = effective population size (i.e. all potentially breeding individuals).

So, if N_e = 50, F= ½(50) = 1/100 = 1 % increase/yr

If 2-3% increase (N_e = 25 and 15, respectively), possible reduction in fitness may have very negative effects on population.

This concern is potentially important if little or no breeding occurs between populations (e.g. prairie chicken) - however, for some species it seems not be a problem, e.g. European Bison, elephant seals, sea otters.

Templeton approach ® intense inbreeding, eliminates deleterious genes. Some evidence based on quantitative genetics studies we know that many genes are responsible for characters, even when inbreeding occurs traits remained linked – diversity may be maintained intrinsically via linkage and epistasis.

Theorem of natural selection states that the rate of evolution at a single locus via selection is limited by the amount of genetic variation available, e.g. ADH locus produces a fast and slow enzyme.

Estimated that an N_e=500 would be required to counteract loss of variation by drift in a single trait one locus model. At this level however loss of alleles by drift would be balanced by gain through mutation, assuming no selection. Or, of course, if out-crossing occurs between populations.

One solution to alleviate loss of alleles bydrift is to have populations fragmented into many infrequently linked subpopulations.

Concept – metapopulations (leading to cross breeding across between individual populations)

Heterozygosity levels may decrease in each population but with occasional migration between populations heterozygosity will increase. Assumes that infrequent migration among small populations is better than one large population of equal overall size.

Other advantages to model:

1. Stem disease, hedges against catastrophic events (i.e. risk spreading)
2. Stems drift
3. Locally adapted genotypes evolve that enhance overall genetic diversity.

Fragmentation will result from economic/political considerations.

More information needed on effectiveness of corridors, artificial insemination across refugia to maintain diversity.

Not a resolved issue, perhaps not needed if inbreeding can be overcome before population goes extinct.

Definition of stochasticity - random chance event, for life-history parameter which may be deterministic it can also vary by chance. Demographic stochasticity – l_(x), m_(x) or sex ratios usually refers to the effects of chance on influencing traits among members in the population. Environmental stochasticity – effect on the demographic picture of the entire population.

For example, environmental variation can change age-structure resulting in a shift in selection from low r to high r, reducing carrying capacity via loss of habitat, shifts selection for higher r to lower r. Studies have addressed the question of what factors determine the expected lifetime of a population. Important to conservation because we are often making discrete refugia for populations of endangered species. Understanding the critical parameters responsible for extinction could lead to

development of more sound management plans.

Even in a perfectly stable environment (i.e. no environmental stochasticity), small populations are at risk of extinction due to demographic stochasticity. In the real world, with environmental variation, large populations can become small populations. What are the traits in a small populations that might effect risk of extinction?

i.e. life-history traits: r, K, body size, longevity

In general,

These traits have to do with rate of recovery following disturbances and demographic considerations.

Extinction rates lower for species with:

1. high intrinsic rate of increase (r)
2. higher longevity of individuals
3. low temporal variability in density (assessed by CV=standard deviation/mean)

1. and 2) are usually inversely correlated since all are linked in life history theory. It turns out empirically that if you increase r by an order of magnitude (e.g. 1 ® 10) you decrease longevity by an order of magnitude (e.g. 10 ® 1)

Also, body size is negatively correlated with r and positively correlated with longevity.

@A - larger body size with higher longevity ® decrease rate of extinction.

@B – larger body size with lower r ® increases rate of extinction.

Based on models considering just extinction by demographic stochasticity –

Population carrying capacity (K) will influence species with high or low r.

In general, risk of extinction should be lower in large populations (high K) and will increase the more variable the population size is (i.e. greater the variance). If we compare larger and smaller-bodied species at the same average population size (N): large-bodied species should be at less risk of extinction when N is low but greater at high N relative to small-bodied species.

(see hand-out)

Study by Pimm et al. 1988 – On risk of extinction

Assessed theory using data for land-birds in Great Britain with 16 islands. (355 populations, 100 spp., 6 decades)

• number of breeding species all known and information on number of breeding pairs over consecutive years.
• noted local extinctions and founded populations.
• island of variable area, population of variable size, spp with variable life histories.

Found that affects of population size on local extinction could be predicted – some spp. constantly went extinct locally while others did not. Were able to obtain data on C.V. (i.e. variability).

Predicted: rate of extinction will be greater in:

1. small vs. large populations.
2. small-bodied, fast growing, short-lived spp @ low density went extinct more often than larger-bodied, slow growing, long-lived spp.
3. larger-bodied, slowing-growing, long-lived spp @ high density than small-bodied, fast growing, short-lived spp.
4. populations with high rather than low temporal CV.

Times to extinction in lifetime for populations with different K values due to demographic stochasticity only.

Calculated to years to extinction since this is a more accurate currency.

So for example,

r=0.1, K=5, 4x21 = 84

r=0.4, K=5, 1x72 =72

so, @ K=5, r(0.1) > r(0.4)

longer-lived sp will go extinct less quickly by chance

@ K>5, =10

r=0.1, K=10, 4x46=184

r=0.4,K=10, 1x2312=2312

r(0.4)>>r(0.1), shorter-lived sp will go extinct less quickly by chance.

Empirical data – results from Pimm et al.

Solid squares, small bodied; half-darkened circles large –bodied.

Summary

1. risk of extinction does decrease with an increase in N (where N~K).
2. Relative susceptibility to extinction of larger and smaller-bodied species reverses with increasing population size - above 7 pairs, large-bodied species are at greater risk of extinction.
3. Migratory species at greater risk than resident species.
4. Risk of extinction increases with high CV in population size (found no extinction in populations with > 18 pairs).

Implication for conservation efforts:

Strategies of focusing on rare species justified by Fig.3 but, moderately numerous populations of larger spp are at greater risk than moderately numerous populations of small-bodied species.

Since large-bodied species with few individuals are less prone to extinction it argues for a metapopulation approach.