Habitat Fragmentation and Extinction Thresholds in Spatially Explicit Models

The incidence of habitat destruction on the survivorship of a single metapopulation is studied by means of a spatially explicit model. As the proportion of destroyed sites increases, the structural properties of the resulting landscape change in a non-linear way, showing the existence of critical thresholds and phase transitions. Such critical thresholds are identified by means of an order parameter, which discriminates a quantitative process, i.e. habitat loss, from a qualitative one, i.e. habitat fragmentation. This difference is only well understood using a spatially explicit framework. We introduce on such a fragmented landscape the dynamics of a metapopulation balanced by local colonization and extinction by means of the cellular automaton formalism. The existence of extinction thresholds when a given fraction of habitat is destroyed is reported. These thresholds are determined both by the critical behaviour of the landscape structural properties, and by the demographic properties of the metapopulation. Some differences between these results and those derived from the study of spatially implicit models are described and explained. In particular, the percentage of patch occupancy is lower for a given value of habitat destruction in the spatially explicit formulation. Extinction threshold also take place for a lower destruction value. Some implications for the management of natural landscapes are discussed.

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