Simulating viability of a spadefoot toad Pelobates fuscus metapopulation in a landscape fragmented by a road

Authors

  • Tove Hels,

  • Gösta Nachman


T. Hels (the@fsl.dk), National Environmental Research Inst., Dept of Landscape Ecology, Grenåvej 12, DK-8410 Rønde, Denmark. – G. Nachmann, Dept of Population Ecology, Univ. of Copenhagen, Universitetsparken 15, DK-2100 København K, Denmark. Present address of T.H.: Danish Centre for Forest, Landscape and Planning, Hørsholm Kongevej 11, DK-2970 Hørsholm, Denmark).

Abstract

A stochastic simulation model allowing for demographic and environmental stochasticity was developed in order to predict the dynamics of a Pelobates fuscus metapopulation. The metapopulation (consisting of ca 1000 adult individuals) was intensively studied in the field for a period of four years and the simulation model was parameterised (sex ratio, age-specific survival rates, fecundity and dispersal between subpopulations) using field data. A sensitivity analysis revealed that a change in the juvenile yearly survival rate has a relatively larger effect on subpopulation persistence than adult survival rate and fecundity rate have. The probability of subpopulation persistence for one hundred years increased from 0 to 0.6 with a change in juvenile yearly survival rate from 0.35 to 0.40. Varying dispersal rate (from 0 to 1% of the individuals in a subpopulation moving to another subpopulation within a year) showed that four of the five subpopulations are dependent on the last one for persistence, indicating a source-sink structure of the metapopulation. The subpopulation with the highest estimated juvenile survival has a far higher persistence probability than the others; they in turn would go extinct were it not for the occasional input of individuals from the source subpopulation. The source-sink structure was also apparent when simulating the isolation effect of the road: persistence of the subpopulation isolated by the road decreases markedly with only a 20% decrease in the number of individuals dispersing to this pond. Environmental stochasticity decreases persistence time of the source subpopulation, but increases persistence time of the unstable ones. This is probably due to the presence of overcompensating density-dependent factors affecting the subpopulations and to the more general effect of stochasticity: it may temporarily change reproduction and mortality rates, increase time to extinction for unstable subpopulations through a rescue effect; and decrease time to extinction for the more stable subpopulations.

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