Simple count-based PVA

Authors

Gareth J. Russell, Department of Ecology, Evolution and Environmental biology, Columbia University, New York, NY, USA.

William F. Morris, Department of Biology, Duke University, Durham, NC, USA.

Daniel F. Doak, Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, CA USA.

Introduction

This Eco-Tool performs a simple count-based population viability analysis as described in Chapter 3 of Quantitative Conservation Biology by William Morris and Daniel Doak (Morris and Doak 2002). It is strongly recommended that the Eco-Tool be used in conjunction with a copy of the book. In short, this is the simplest possible form of PVA, based only on a timeseries of censuses of the population in question. The method of estimating population parameters is the one described in Dennis et al. 1991.

To conduct a PVA with such limited data, one must make a number of assumptions, which are listed below. Please consider carefully whether your data meet these assumptions, and refer to the book for a more detailed discussion.

Assumptions

From Morris and Doak (2002), Table 3.3 et. seq. Italics indicate how to test whether the assumption is appropriate.

  1. No change in mean and variance of growth rate λ over time. This means:
    1. No temporal environmental trends. See trend analysis in output.
    2. No demographic stochasticity. As demographic stochasticity is much more important when the population size is small, this assumption argues that one should set the quasi-extinction threshold above the level at which demographic stochasticity is expected to become important. See pp. 22–25 of Morris and Doak.
    3. No density dependence. See plot of Nt+1 vs. Nt in output.
  2. No autocorrelation in environmental conditions. See autocorrelogram of λ values in output.
  3. Little or moderate environmental variation. See residual analysis in output.
  4. No observation error in the population counts: counts represent true population size or a constant fraction thereof. Cannot be tested from count data; requires knowledge of censusing scheme. (Note: even if counts are unbiased, error in estimating population size can inflate the estimate of σ2.

References

Dennis, B., P. L. Munholland and J. M. Scott (1991) Estimation of growth and extinction parameters for endangered species. Ecological Monographs 61: 115–143.

Morris, W. F. and D. F. Doak (2002) Quantitative Conservation Biology: Theory and Practice of Population Viability Analysis. Sinauer, MA.

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