Factors influencing cannibalism in the wolf spider Pardosa agrestis (Araneae, Lycosidae)

Cannibalistic tendencies are well known in spiders and may be a significant factor influencing population size. The wolf spider, Pardosa agrestis, is the dominant non-web-building spider in a wide range of central European ag ricultural habitats. Preliminary field observations indicated an extended r eproductive period, which results in a very wide size distribution of juven ile instars. We hypothesised that if cannibalism is enhanced by differences in size, especially during periods when prey is scarce, these populations might be susceptible to cannibalism in an ecologically significant way. Lab oratory studies were conducted on juvenile P. agrestis in arenas. We analys ed the following specific aspects of cannibalism: (1) the effect of the wei ght ratio between the opponents; (2) the effect of weight per se, and (3) t he role of hunger level in determining cannibalistic tendencies of spiders. The role of weight and hunger were analysed in separate experiments, in bo th cases by controlling for the other variable. The results showed that can nibalism was strongly positively correlated with both weight ratio and hung er, but absolute size/age of an individual could not predict the occurrence of a cannibalistic event. These experiments generated the plausible hypoth esis that cannibalism might be an important phenomenon in the regulation of real populations, which should be tested specifically in future field expe riments.