Competition between two spider species: experimental field study

To test for interspecific competition between two common orb-weaving spiders, Metepeira grinnelli and Cyclosa turbinata, I conducted a controlled competitor-removal experiment in the field. Metepeira prey-consumption rate and fecundity were higher in plots where Cyclosa were removed than in control plots. Cyclosa density and vertical distribution were higher in plots where Metepeira were removed than in control plots, but Cyclosa prey-consumption rate was not higher in the removals. A separate food-supplement experiment, conducted while the competitor-removal experiment was in progress, showed that food was a limited resource for both species. During this experiment, Metepeira often displaced and sometimes killed Cyclosa, but Cyclosa were never observed displacing Metepeira. The experiments suggest that exploitation was probably the predominant mechanism responsible for the effects of Cyclosa on Metepeira, whereas interference was probably the major mechanism responsible for the effects on Meteperia on Cyclosa. During this study, interspecific competition between spiders was significant and appeared to play an important role in structuring their community. However, other experimental studies have found no evidence of interspecific competition between spiders. Predators or abiotic sources of mortality may have been reducing the spider populations during these other studies, reducing competition. I suggest that the abundance of predators of spiders was low at this study site because of physical constraints.