A MODEL FOR THE SPATIAL DYNAMICS OF LINYPHIID SPIDERS IN FARMLAND

1. Linyphiid spiders are important predators of pests in agricultural ecosystems. Their ability to disperse aeronautically is a pre-adaptati on to ephemeral agricultural habitats and is believed to play an impor tant role in their ability to survive agricultural practices, includin g pesticide use in cultivated farmland. 2. We present a simulation mod el for spider dispersal in a one-dimensional agricultural landscape co mposed of different field types. This model has been parameterized by estimates from published laboratory studies and field observations of dispersal, and the effects of various agricultural practices on local populations, Model output is presented to show the abundance and persi stence of spider populations exposed to different levels of insecticid e use in different landscape structures. 3. We use the model to demons trate the importance of landscape heterogeneity for survival and abund ance. The inclusion of small amounts of grassland in large areas of in tensive cereal production dramatically increases the population of spi ders in the landscape. Moreover, the inclusion of non-habitat areas ma y have a beneficial effect by providing temporary refugia in landscape s with heavy pesticide use. Crop rotation reduces average population s ize. 4. Field-size up to 4 km(2) is found to have little effect on pop ulation density because of the large dispersal distances achieved by s piders. 5. If different dispersal strategies are compared, an optimum level is observed for particular landscape structures in terms of the maximum population that a landscape can sustain. For a wide variety of landscapes, fitness is maximized when the proportion of dispersers in the population is approximately 50%.