Habitat loss, fragmentation and predator impact: spatial implications for prey conservation.
Abstract
Because predators threaten the survival of endangered prey in many places, predator management is a widespread conservation tool. At the same time, the effects of predators on their prey are greatly influenced by landscape structure. Therefore, the management of landscapes could be an alternative to predator regulation. A spatially explicit presence/absence model (a stochastic one-layer cellular automaton) was used to investigate 2 different predator-prey systems that were subject to changes in the number and size of habitat patches in a model landscape. The 1st scenario included grey-sided voles Clethrionomys rufocanus, Norwegian lemmings Lemmus lemmus and small mustelids (stoats, Mustela erminea and weasels, M. nivalis) interacting in a tundra landscape. In the 2nd scenario, the effect of habitat perforation by human settlements with subsidized predators (house cats, Felis silvestris catus) on the dynamics of lemmings (as surrogate for endangered prey) was studied. Both the total area of lemming habitat and the degree of fragmentation were important determinants of the population size and persistence of lemmings. A qualitative change in the effect of fragmentation was observed when the area of lemming habitat decreased from 70% (positive effect) to 50% (negative effect). When lemming habitat covered 50% or less of the landscape, fragmentation had a negative effect on lemming population size and viability, even though habitat area did not decrease. The spatial configuration of settlements as predator sources was important. A few evenly spaced predator sources had less negative effect on lemming populations than the same proportion of predator habitat that was randomly distributed, which in turn had less effect than many evenly spaced patches. Including predator management in the model did not decrease the predators' negative impact on the population size and persistence of the endangered prey when settlements occurred in many small patches. It is concluded that predator management is not a viable strategy to combat the threat to the survival of endangered prey, but that careful planning of landscape pattern could compensate for negative predation effects. The location and size of patches of predator habitat should be optimized in order to minimize the negative effects of predators visiting adjacent areas of natural habitat.