Dynamics of the Relationship between a Generalist Predator and Slugs over Five Years

Abstract

Although communities of generalist invertebrate predators are known to be capable of suppressing pests, little is known about the long-term population dynamics of individual species of generalist predators and single classes of prey in the field. We present evidence of a dynamic interaction between a generalist insect predator, the carabid beetle Pterostichus melanarius, and its slug prey. We analyzed the numbers of slugs and beetles in an arable field over a 5-yr period, during the main activity period of the beetles from June to September. The field contained 25 plots comprising five replicates of five cultural treatments. An index of the nutritional status of the 8497 beetles collected was obtained by weighing the crop of each beetle (a measure of total prey availability per predator). There was a strong relationship between the crop mass of the beetles and slug numbers in the soil, indicating that slugs were a major part of the diet of the beetles. The change in the beetle population from year to year was strongly related to both slug numbers in the soil and the crop mass of the beetles. This indicated that the slugs influenced the nutritional status, and hence the reproductive success, of the beetles. The predators had a significant effect on slug population growth between years but not between months within years. The temporal effect of these processes was a between-year coupling of beetle and slug population dynamics, buffered by feeding on other prey. The relationship appears to be similar to that between mammalian predators and limited numbers of prey species in the subarctic, where one prey species forms a substantial proportion of the total available food resources. Such relationships may be common in agroecosystems, where species diversity is low, and may be a significant factor driving periodic fluctuations in the abundance of both predators and pests.