Review
Population viability analyses in plants: challenges and opportunities

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Abstract

This review of 95 plant population viability analyses (PVAs) reveals that most studies consider one species, only a few populations and are based on data collected for less than five years. Only five studies referred to themselves as PVAs. Plants offer numerous challenges, such as seed banks and periodic recruitment, but these can be answered with suitable data collection and modeling. New approaches, such as metapopulation models, inclusion of disturbance cycles, and integration of genetics and demography, are producing more realistic PVAs. Although exact solutions are fraught with limitations, plant PVAs can be useful in comparing management regimes, populations and microhabitats, and in using these results to guide conservation and management.

Section snippets

Review of plant PVAs

Recent reviews of PVAs have included few plant studies (e.g. for 2, 3, 4, two of 136, zero of 166, and three of 58 records, respectively). Is it really true that plant PVAs are rare? The answer, of course, depends on the definition of a PVA. In this review, I use a broad definition for a PVA, considering a study a PVA if it includes empirical data on the entire life cycle of a wild population and if it uses quantitative modeling to project future populations [e.g. the finite rate of increase

Challenges to plant PVAs

Many aspects of plant life history can present obstacles when obtaining data for PVAs. These include plant and seed dormancy, periodic recruitment and clonal growth.

Environmental stochasticity and matrix element correlation

Environmental stochasticity creates variation in demographic parameters over time and tends to decrease projected growth rates and increase extinction risk19. There are a myriad of ways to model environmental stochasticity. Stochastic analyses often assume (owing to lack of data) that elements within matrices have no correlation with each other, and that there is no autocorrelation over time. However, demographic parameters are often positively correlated across environments12, thus creating

Using comparative PVAs to assess management

Plant PVAs will continue the trend towards providing more detailed results under a range of scenarios and assumptions13. Beissinger and Westphal2 recommend that PVAs examine relative, rather than absolute, rates of extinction. This is an especially useful approach when contrasting alternative management strategies. For example, in a PVA of royal catchfly, λs and extinction probabilities were compared among three groups of populations with contrasting management regimes24. Comparisons among

Prospects

This review has documented many of the limitations of the data that underpin plant PVAs; specifically the short duration of most studies, the small number of populations monitored, and the inherent difficulties in generating numbers for difficult parts of the plant’s life cycle, such as dormancy and the occurrence of seed banks. De-emphasizing the exact values of λ and of extinction probabilities avoids some of the problems of uncertainty in demographic parameters. They provide the basis to

Uncited references

44, 45

Acknowledgements

This article was improved by the comments of Daniel Gagnon, Ed Guerrant, Samara Hamzé, Christine Hawkes, Pedro Quintana-Ascencio and four anonymous reviewers. Thanks also to Per Sjögren-Gulve, Isabelle Olivieri and the participants in the 1997 Swedish workshop: ‘The use of population viability analyses in conservation planning’, for encouraging this review.

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