Effects of newly planted hedges on ground-beetle diversity (Coleoptera, Carabidae) in an agricultural landscape

The diversity of ground-beetle communities (Coleoptera, Carabidae) was studied to assess the short-term impact of new hedges planted in an intensive agro-ecosystem. Hedges represent a new and undisturbed habitat for those polyphagous predators and may provide increased prey availability. Carabids were sampled with pitfall traps in a hedge and an adjacent cultivated field during the whole activity period of these insects to assess the effect of distance from the hedge on species diversity. Fenced pitfall traps were also used to estimate absolute population densities. Several diversity indices were calculated at various sampling levels (total area, distance treatment, trap). Kendall's coefficient of rank correlation between communities at adjacent distances showed that the greatest changes in species relative abundances occurred close to the hedge. Indices of species richness, dominance concentration, equitability and Fisher's a all indicated a significant decrease in species diversity with increasing distance from the hedge. These trends are explained by two complementary factors. First, the number of species decreased significantly with distance from the centre of the hedge. Second, the various species had different spatial patterns of total capture and absolute density. Four groups of species can thus be distinguished: species restricted to the hedge, species preferring the hedge, species preferring the crop, and species unaffected by the hedge. The respective roles of small-scale abiotic changes in habitat structure and differences in prey availability are discussed.