The Experimental Analysis of Competition between Two Syntopic Species of Orb‐Web Spiders (Araneae: Araneidae)

Nonexperimental studies by several investigators have provided indirect evidence of interspecific competition between Argiope aurantia and A. trifasciata, two syntopic orb—weaving spiders common in old fields. We performed replicated field experiments in 1979 and 1980 to test directly for both inter— or intraspecific competition among these spiders. Densities of one or both species were altered within open 12 x 12 m plots in an old field in Maryland, USA. Evidence for competition was based upon the effects of the density manipulations upon web location, prey capture, growth, and survival. The experiments yielded no evidence of interspecific competition between A. aurantia and A. trifasciata. The manipulations did provide statistically significant evidence of intraspecific competition during 1979. However, no treatment effects were evident in 1980, though densities of both species were higher that year and prey appeared to have been less available than in 1979. Increasing the density of conspecifics caused an increase in the mean web height of both species during the early census period of 1979. The growth rate of A. aurantia was slightly less at the higher density in 1979, and survival (net effect of mortality, immigration, and emigration) of A. trifasciata was negatively density dependent that year. Responses to the density manipulations were variable between experimental plots and years. Some of the responses were correlated with seasonal changes in the weather and with variation in the composition and structure of the vegetation. Thus our observations provide indirect evidence of the importance of physical factors and possibly vegetation in the distribution and abundance of A. aurantia and A. trifasciata. Interspecific competition appears to have no, or only minor, influence on resource utilization, growth, and survival of these species.