Population Viability Analysis

1
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).

2
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

3
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

4
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

5
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 gt50 needed for short-term survival (avoid
  inbreeding)
• Negt500 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

6
PVA Shortcomings

• Simple assumptions as already stated
• but models are supposed to simplify!
• So dont 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

7
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 gt8 years
• birds and mammals may take 30-40 years

8
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

9
Example of PVA for Alala
Double clutching and predator control may be
effective
10
Catastrophes Really Limit Ability to Recover
Population
11
Managing the Captive and Wild Populations
Together Increases Chance of Recovery
Two Populations are better than one even with
catastrophes
12
References

• Dennis, B. Munholland, PL, and Scott, JM. 1991.
  Estimation of growth and extinction parameters
  for endangered species. Ecol. Monogr. 61115-143.
• Shaffer, ML. 1981. Minimum population sizes for
  species conservation. Bioscience 31131-134.
• Boyce, MS. 1992. Population viability analysis.
  Annual Review of Ecology and Systematics.
  23481-506.
• Leigh, E. G. Jr. 1981. The average lifetime of a
  population in a varying environment. J. Theor.
  Biol. 90213-239.
• Lande, R. 1988. Genetics and demography in
  biological conservation. Science 2411455-1460.
• Simberloff, D. 1988. The contribution of
  population and community biology to conservation
  science. Ann. Rev. Ecol. Syst. 19473-511

13
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 28436-37.
• 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.