Temporal spacing, competition and aggregation in Aphodius-dominated dung beetle communities
This paper deals with temporal spacing, competition and aggregation of adult beetles in dominated communities inhabiting cattle dung pats. Data from an Italian Alpine area are re-analyzed to point out interspecific temporal spacing. Evidence for the presence or absence of competition is reviewed from north-temperate and Alpine communities. Beetles were collected from pastures at Palanfrè in the Valle Grande (Cuneo Province, northwestern Italy) at about 1400 m a. s. 1. from June to September in 1994. The entire material collected comprises 9000 adult beetles of 25 different species. The dendrogram produced by Cluster Analysis clearly shows that species can be classified into three groups: 1) species arriving early in the season, occurring in large numbers in June-July (A. foetens, A. haemorrhoidalis, A. fossor), 2) late-arriving species, concentrated in September (A. borealis, A. uliginosus, A. sticticus, A. corvinus, A.contaminatus), 3) species occurring throughout the period (A. rufipes,A. erraticus,A. rufus,A. depressus). O. fracticornis was classified as a late-arriving species, but at a rather high Euclidean distance from the other species of the same group. Anoplotrupes stercorosus was clustered apart from all the other species. The ordination of species obtained by Principal Component Analysis placed A. erraticus well apart from the other species, hence emphasizing the dominance of this species, which accounted for about half the individuals collected.
The present data together with those from other north-temperate and Alpine communities give no convincing evidence that competition is a major organizing force in Aphodius-dominated adult dung beetle communities. In many areas, like in the Alps, the low number of adults per dung pat would maintain intra- and interspecific coexistence at a non-competitive level. Occasional situations of local abundance (also observed in the Alps) are probably not important for competition because adults use only small amounts of dung for feeding or reproducing and, at any rate, they can emigrate to less crowded droppings. Given this picture, aggregation of adults might be interpreted as a mechanism, whose cause is still unknown, that works independently of competition. Because of the overall low relevance of competition, there is no reason to interpret dung beetle aggregation on the basis of the Aggregation model.