An individual based approach to understanding prey regulation by spiders

In this thesis I investigate the processes by which spiders may regulate associated prey populations. The general hypothesis is that spiders, as generalist predators, should regulate prey if they exhibit a foraging phenotype that maximizes their relative fitness. Theoretical Treatment. Using an individual based, stochastic computer model I simulated the feeding of individual spiders on prey subpopulations in a patchy habitat. I examined the foraging success of spiders and prey numbers with respect to various environmental and behavioral treatments. Increased patchiness decreased prey numbers and spider foraging success, whereas a higher number of prey species increased spider success. The greater the number of spider species and the broader the range of prey sizes present in the system, the greater was the level of predation by spiders on the insects. The greater the range of body sizes of spiders present, however, the less was their predation effect. The prey selection strategy effect was significant: the superior strategy also captured the greatest biomass of prey in competitive phenotype simulations. Spider aggressiveness towards other spider in the simulation had no significant effect on results. Experimental Treatment. A series of experiments was devised to test model assumptions. First, it was established that hunger had no effect on the foraging activity of the spiders Tetragnatha laboriosa and Lycosa rabida nor on the predation of insects by Tetragnatha. In an experiment designed to test spider response to prey variability, the spider, Lycosa rabida, avoided prey variability treatments presenting the greatest probability of food deprivation. The test spiders generally switched positively between two prey species. Forecasting tests revealed that Lycosa preference for prey significantly exhibited a dependence on experience with past numbers of prey. The final experiment was a field validation of the model's qualitative predictions concerning the number of species of prey and spiders, and the number of patches. Increasing the number of species of spiders decreased prey biomass and increases in prey numbers with fewer patches occured only without spiders. A greater number of prey species increased the biomass of prey only in with spiders.