Intra- and interspecific aggregation among dung beetles (Coleoptera: Scarabaeoidea) in an Alpine pasture

Abstract

Spatial distribution of scarabeoid beetles on dung pats was studied in the Argentera Natural Park (south-western Italian Alps) from June to September in 1994 and 1995. Artificial dung pats were distributed evenly within the study area (about 1400 m a.s.l.) and sampled at suitable dung ages (1–13 days). Two experimental tests were carried out. In the first test, experimental pats were covered with metallic nets which prevented the largest species from reaching the dung, whilst in the second test a variable number of individuals of large or relatively large species was added to the experimental pats.

The community was dominated by Aphodius species (19 out of 25 species), which usually were also the most abundant in the pats. The densities of most species were relatively low most of the time with average abundances lower than two individuals per pat. Even though some interspecific differences were observed, ecological overlap between species was rather high with regard both to seasonality and succession in the dropping. Manipulative tests suggested that small beetles were not excluded by larger beetles. Patterns of spatial distribution showed some degree of intra- and interspecific aggregation. Proximate mechanisms of aggregation are probably non-specific. The ultimate cause of aggregation might be mate-finding. Alternatively, individuals might aggregate because the very same presence of beetles within a dung pat would modify its chemical status improving its quality. Intraspecific aggregation (measured by means of the J index) was much higher than interspecific aggregation (C index) which suggests, according to the Aggregation Model, that interspecific coexistence might be facilitated. However, the data gave no evidence for intra- and interspecific competition between adult beetles.