Effects of Environmental Fluctuations and Habitat Fragmentation on Community Structures

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We have considered two systems of prey and predator, one with a predator and a single prey, and the other with a predator and two prey species, in a heterogeneous environment composed of a source and sinks.We have found that both in these two models, the predator species is vulnerable to extinction, if productivity in the source is insufficient to rescue demographically deficient populations in sink patches. On the other hand, if the source is too rich, one of the two prey species may be driven into extinction by apparent competition, because the predator can maintain a large population by virtue of an alternative prey. Increasing the rate of predator movement has the opposite effects on persistence of prey and predator. High emigration rate exposes the predator population to danger of extinction, reducing individuals that breed and produce offsprings in the source habitat. Then, this may promote coexistence of prey by relaxing predation pressure and apparent competition between two prey species. Spatial arrangement of patches, or connectivity between patches, also influences persistence of species. A linear pattern with the source at one end is profitable for predator, and a centrifugal pattern in which the source is surrounded by sinks is advantageous to prey. It is also reveald that the spatial atrusture is particularly important compared with the number of sinks, when the dispersal rate is low. Therefore, productivity in patches and patterns of connectivity between patches influence persistence of populations differently depending on the trophic level to which the species belong.