Mechanisms of coexistence between two species of cliff-dwelling spiders, Achaearanea tepidariorum and Coelotes montanus

Results from removal, addition, and enclosure experiments conducted with two spider species inhabiting sandstone cliffs indicate that an interplay of inter and intraspecific competitive effects shape their coexistence. Reciprocal removal experiments of Achaearanea tepidariorum and Coelotes montanus on similar, noncontiguous cliffs showed that Coelotes populations experienced a competitive release in the absence of Achaearanea. Adult and immature Coelotes increased in number, adults reproduced and weighed more, and the immatures ate more and shifted their use of prey types and microhabitats where Achaearanea had been removed. Addition and enclosure experiments further established that interspecific territoriality within Achaearanea influences their population levels.

Coexistence of these two species appears to be mediated in part by Achaearanea limiting the number of Coelotes on the cliff through predation and exploitation of food and space. The territorial behavior of Achaearanea may also influence the coexistence of Achaearanea and Coelotes by maintaining spaces on the cliffs which are excluded to use by other Achaearanea. This probably provides areas which are then available to inhabitation by Coelotes. Here is an example of intra specific territoriality within one species of spider facilitating the coexistence of another.