Distributional ecology and abundance of dung and carrion feeding beetles Scarabaeidae in tropical rain forests in Sarawak Borneo Malaysia

The distributional ecology of 66 spp. of Scarabaeidae was studied in 9 forest types in the Gunung Mulu National Park, Sarawak, a virgin forest area of 540 km2 in northern Borneo. Species richness was highest in the lowland forests, with the exception of the kerangas (tropical heath forest), where the density of scarabaeidae was low, apparently because of scarcity of resources. Species number began to decrease with increasing altitude at 200 m above sea level, and no species were found at the summit of Mt. Mulu (2376 m). The community in the lower montane rain forest was transitional between the lowland and upper montane communities, which were distinct in their species composition. Similar changes have been observed in plants and other animals, but a closer analysis of this material revealed 2 rather than 1 transitional zone, each with some habitat specialists. A tight species packing along the altitudinal gradient is suggested. Replacement by congeners with increasing altitude was frequent in necrophagous insects. Ten such replacements are described in detail, while the random expectation is 4.2 .+-. 1.5 (SD) in this material. The density of Scarabaeidae was high in the upper montane forest (UMF), in spite of low species richness, and density was exceptionally high in the limestone forest, where only 1 sp. was exceedingly abundant. These observations suggest density or excess density compensation in this community. Low species richness in the UMF is explicable by its small area and isolation, while the soil in the limestone forest is suggested to be unsuitable for the burrowing species of Scarabaeidae. Patterns of distribution in the 3 most species-rich genera Onthophagus (38 spp.), Phacosoma (8) and Phoeochroops (4) were different. In Phacosoma and Phaeochroops, habitat divergence was the rule. Exploitative competition is the probable cause of divergence in the large and abundant species of Phaeochroops. The closely related Phacosoma species did not overlap at all, unlike Phaeochroops, which suggests that their habitat selection is due to the lack of a premating reproductive barrier and low hybrid fitness. Onthophagus represents yet another pattern, most (36) of the 38 spp. coexisting in the lowland forests. The core-satellite species hypothesis of community structure finds support in the results on Onthophagus, but the large number of coexisting lowland species remains unexplained. Abundances were on average more evenly distributed among the species than predicted by Caswell's (1976) neutral model (logseries distribution). No difference was found in this respect between tropical and temperate communities. In 2 large guilds, Onthophagus (tropical) and Aphodius (temperate), evenness of the abundance distribution increased with species richness, and in Onthophagus there was some indication of increase in the degree of feeding specialization with increasing species richness. Lowland communities in Colombia are richer in species than comparable communities in Sarawak or Panama. The feeding habits of Scarabaeidae are similar in Sarawak and tropical America, but the African fauna is unique in its scarcity of especially large necrophagous species.