A model for the spatial dynamics of linyphiid spiders in farmland.
Abstract
Linyphiid spiders are important predators of pests in agricultural ecosystems. Their ability to disperse aeronautically is a pre-adaptation to ephemeral agricultural habitats and is believed to play an important role in their ability to survive agricultural practices, including pesticide use in cultivated farmland. A simulation model for spider dispersal in a one-dimensional agricultural landscape composed of different field types is presented. The model parameters were set using 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 persistence of spider populations exposed to different levels of insecticide use in different landscape structures. The model was used to demonstrate the importance of landscape heterogeneity for survival and abundance. The inclusion of small amounts of grassland in large areas of intensive cereal production dramatically increased the population of spiders in the landscape. Moreover, the inclusion of non-habitat areas may have a beneficial effect by providing temporary refugia in landscapes with heavy pesticide use. Crop rotation reduced average population size. Field-size up to 4 km2 was found to have little effect on population density because of the large dispersal distances achieved by spiders. If different dispersal strategies are compared, an optimum level was observed for particular landscape structures in terms of the maximum population that a landscape can sustain. For a wide variety of landscape, fitness was maximized when the proportion of dispersers in the population was approximately 50%.