On reduced size in spiders from marginal habitats

Four heathland spider species, Agroeca proxima, Centromerita concinna, Theridion simile, and Philodromus histrio, were studied in the Kalmthout National Nature Reserve in northern Belgium. They were caught by pitfall trapping or sweeping in different Calluna heath plots where the vegetation was in varying stages of development after being burnt, mowed, or grazed. Each species proved to thrive in a different plot, its numbers being greater and its size larger (cephalothorax width) in that plot. For the first two species positive correlations were found, for males as well as for females, between average size and numbers caught during a complete year cycle in a particular habitat.In contrast to non-predators, which are smaller when their populations are denser, spiders show a functional response that explains their increase in size with increasing density. It is suggested that in this respect a fundamental difference exists between predators and non-predators.