Trends in Ecology & Evolution
ReviewPopulation viability analyses in plants: challenges and opportunities
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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