Predicting the Persistence of Amphibian Populations with the Help of a Spatial Model

Abstract

We have used a stochastic population model with immigration to calculate extinction rates for two amphibian species: the common toad, Bufo bufo, and the crested newt, Triturus cristatus. Population models for amphibians are difficult to parameterize. It was not possible to measure all the relevant parameters for the model. The other parameters were obtained from other studies or estimated on the basis of similar species. Contour maps for persistence were generated using the model. The persistence of populations associated with a pond is affected both by the pond's internal dynamics and its proximity to a source pond. On a 20-generation time scale, toad ponds that were initially occupied were relatively unaffected by the proximity of a source. Persistence was almost certain ( > 95%) when the average carrying capacity of a pond was greater than 30 adult females. Initially unoccupied toad-ponds were equally likely to persist if they lay within 4 km of a source and could support more than 50 adult females. Initially occupied newt ponds were likely to persist if they supported more than 40 females or lay within 0.5 km of a typical source pond. Initially unoccupied newt ponds were mainly dependent upon immigration rates. Very small ponds (N k > 10) only needed to be within 1.5 km of a source pond. This model allows us to use the data available, to make predictions about the criteria which must be met by a landscape to ensure the survival of amphibian populations. It offers the possibility of better predictions when the data is refined. The model also suggests directions for further research such as the statistical study of environmental variation, the nature of minimum viable populations for toads and the statistics of long-range dispersal.