Distribution of native European spiders follows the prey attraction pattern of introduced carnivorous pitcher plants

Abstract

Carnivorous plants and spiders are known to compete for resources. In North America, spiders of the genus Agelenopsis are known to build funnel-webs, using Sarracenia purpurea pitchers as a base, retreat and storage room. They also very likely profit from the insect attraction of S. purpurea. In a fen in Europe , S. purpurea was introduced ~65 years ago and co-occurs with native insect predators. Despite the absence of common evolutionary history, we observed native funnels-spiders (genus Agelena) building funnel webs on top of S. purpurea in similar ways as Agelelopsis. Furthermore, we observed specimen of the raft-spider (Dolmedes fimbriatus) and the pygmy-shrew (Sorex minutus) stealing prey-items out of the pitchers. We conducted an observational study, comparing plots with and without S. purpurea, to test if Agelena were attracted by S. purpurea, and found that their presence indeed increases Agelena abundance. Additionally, we tested if this facilitation was due to the structure provided for building webs or enhanced prey availability. Since the number of webs matched the temporal pattern of insect attraction by the plant, we conclude that the gain in food is likely the key factor for web installation. Our results provide an interesting case of facilitation by an introduced plant for a local predator, which has developed in a very short time scale.