Population Viability Analysis

Seeks relationship between population size and probability of extinction– does not need to calculate MVP– concerned more with determination of

probability that a population will persist for some arbitrary time (Boyce 1992)

– combine all sources of stochasticity and deterministic population growth into one model (usually requires computer simulation).

PVA is More than Modeling

It is the process of synthesizing information about a species or population and developing the best possible model for the dynamics of population size (Boyce 1992)– Learn what you do not know– Sensitivity analysis indicates what parameters may be

especially influential– Suggests management starting points that should be

monitored and adjusted through adaptive management

Steps in PVA (Boyce 1992)

Project demographics of populations through time– basically simulate life tables if age-structure is

needed

Forecast viability through time– affected by model of demographics and

incorporation of stochasticity• errors get amplified as projections span longer periods

Importance of Genetics versus Demographics in Modeling Emphasis should be on ecological,

environmental, and demographic factors not genetic ones– little evidence that genetic factors lead to extinction– allee effect, demographic stochasticity, disruption of

dispersal by habitat fragmentation are important (Lande 1988)

• allee effect: disruption of critical social behaviors when populations decline too far leads to rapid extinction for nongenetic reasons

Dangers of Using Too Few Variables

Early PVAs and MVP calculations were quick to use genetic considerations only– Led to 50, 500 rule (Franklin 1980, Soule and Wilcox 1980)– Ne >50 needed for short-term survival (avoid inbreeding)

– Ne>500 needed for long-term survival (ability to evolve in changing environments)

Spotted Owl and Red-Cockaded woodpecker conservation strategies were designed to maintain 500 breeders– Lande (1988) argues this would lead to extinction because

dispersal ability in fragmented habitat was not accounted for

PVA Shortcomings Simple assumptions as already stated

– but models are supposed to simplify!• So don’t rely on them for absolute management advice

Ignore ecology and focus on stochasticity, esp. genetics as mentioned– need to take into account habitat and spatial structuring of

populations– Effects of other species (biotic interactions)

Need to account for interactions among various vortices

Does More Data Help?

Yes, you can then estimate variation in demographics– Variance tends to increase as sample increases

up to a point. (Boyce 1992)• Insects >8 years

• birds and mammals may take 30-40 years

Ongoing Improvement in PVA (Mann and Plummer 1999)

Workshop last year in Berkeley Stretching PVAs to cover more types of life

forms– application to plants will remain difficult

• seed banks hard to model

Should genetic factors be modeled or are demographic ones sufficient?

Include human population growth and expected changes in landcover in models

Example of PVA for Alala

Double clutchingand predator controlmay be effective

Catastrophes Really Limit Ability to Recover Population

Managing the Captive and Wild Populations Together Increases Chance of Recovery

Two Populationsare better than oneeven with catastrophes

References

Dennis, B. Munholland, PL, and Scott, JM. 1991. Estimation of growth and extinction parameters for endangered species. Ecol. Monogr. 61:115-143.

Shaffer, ML. 1981. Minimum population sizes for species conservation. Bioscience 31:131-134.

Boyce, MS. 1992. Population viability analysis. Annual Review of Ecology and Systematics. 23:481-506.

Leigh, E. G. Jr. 1981. The average lifetime of a population in a varying environment. J. Theor. Biol. 90:213-239.

Lande, R. 1988. Genetics and demography in biological conservation. Science 241:1455-1460.

Simberloff, D. 1988. The contribution of population and community biology to conservation science. Ann. Rev. Ecol. Syst. 19:473-511

More References

Thompson, G.G. 1991. Determining minimum viable populations under the endangered species act. NOAA Technical Memorandum NMFS F/NWC-198.

Mann, C. C., and M. L. Plummer. 1999. A species’ fate, by the numbers. Science 284:36-37.

Soule, M. E. 1987. Where do we go from here? In M. E. Soule (editor), Viable populations for conservation. P. 175-183. Cambridge University Press, Cambridge.