Mechanisms structuring a guild of raptorial spiders

Resource utilization patterns of a guild composed of four syntopic species of raptorial ambush spiders were investigated over a three year period. Coefficient of similarity values were calculated for six species pairs based on exploitation of four niche dimensions (phenology, microhabitat, prey taxa and prey size). Differences in seasonal patterns were evident for all guild members. Microhabitat distribution indicates that when the space resource (flowerheads) is not in limited supply, each spider species utilizes a separate component of the habitat. There was a large amount of overlap in prey size among all four species but only moderate overlap in prey taxa taken. Overall results show that species overlap in resource use varies from year to year with no obvious pattern in the magnitude or direction of change.

To test for interspecific competition controlled field experiments were conducted wherein the dominant species, Peucetia viridans, was removed over a two year period. Removal of P. viridans did not produce any of the expected effects on the species remaining (Misumenops lepidus, M. californicus, M. deserti). There was no significant increase in fecundity, altered pattern of food resource use or habitat utilization, and the population density of the remaining species did not increase. Thus, interspecific exploitation competitive interactions appear to be minimal among the guild members. However, the available evidence suggests that interference competition in the form of cannibalism and intraguild predation is important in structuring the guild when resources are in limited supply.