Modelling the extinction risk of isolated populations of natterjack toad iBufo calamita/i

摘要

Many local populations of thenatterjack toad Bufo calamita in Germany are endangered. Due to the fragmentation and destruction ofnatural habitats by man, toads have often been forced to switch to secondaryhabitats. The permanent existence of these habitats is uncertain. Habitat andarea requirements have been investigated in various parts of Germany in recentyears. Our study uses field observations from sites in Halle (Saxony-Anhalt)and List (Schleswig-Holstein) as a basis for analyses of the populationdynamics under different environmental conditions. Deterministic trends ofthese populations are calculated with the help of a Leslie matrix consisting ofaverage parameters for mortality and reproduction. For a more thorough analysiswe use a stochastic simulation model in order to assess survival probabilitiesof local toad populations. This model also takes into account environmentalfluctuations affecting mortality and reproduction. Using scenarios fromdifferent locations, a sensitivity analysis of the parameters indicates whichmanagement options are the most promising to preserve a population. Our resultsindicate that the mortality rates of juveniles and the availability of spawningground have the greatest influence on a population’s survival.Consequently, habitat management should focus on these aspects. In addition, weinvestigate the risk of extinction for different reproductive strategies.Natterjack toad populations observed in the field actually follow a strategywith three breeding periods. We find that this strategy supports the survivalof the population better than strategies with less periods, which are morelikely to result in a complete breeding failure during one season.