Prey scarcity as a density-independent limiting factor

Accumulating evidence supports the hypothesis that a relative shortage of prey affects growth and fecundity of web-building spiders by acting as a density-independent limiting factor. Thus, though food supply is often limiting, exploitative intraspecific competition is absent or weak. Consequently, exploitative competition for prey between different species of spiders is even less prevalent, given that two species will differ in resource requirements because they are different gene pools. Below I present experimental evidence that interspecific competition among web-building spiders is infrequent because intraspecific competition for prey is a minor interaction.

Old-field orb weavers

Manipulating Argiope densities (Horton & Wise 1983) failed to uncover evidence of substantial intraspecific competition. Some negative intraspecific density effects appeared during the first year of the study (1979). Increasing the density of conspecifics increased the mean web height of both species, but the differences between treatments (33% for A. trifasciata and 74% for A. aurantia) were statistically significant only for the census immediately following the density manipulations. A. aurantia, but not A. trifasciata, had a significantly higher growth rate in plots in which intraspecific density was lower (proportional changes in length over the season were 1.7 and 1.3, respectively). Survival, defined as the net effects of mortality and migration, was significantly density dependent for A. trifasciata when calculated to the last pre-reproductive census (p < 0.05), but was non-significant when calculated to the census 2.5 weeks earlier (F of ANOVA = 0.44).