Effects of fragmentation and degradation of an afrotropical rain forest on the diversity structure of leaf beetle communities (Coleoptera, Chrysomelidae)

Abstract

East African forests are particularly diverse and endangered but are still little-known ecosystems. With an increase of the human population in East Africa throughout the last decades, the pressure on biodiversity largely increased and the use of the forest ecosystems is far from being sustainable. At the beginning of the 20th century there were 240,000 ha of lowland rain forest in Kenya, where the present study was conducted. Recently there are only 23,000 ha left due to severe deforestation and fragmentation. One of these areas is the Kakamega Forest in western Kenya. The remaining forest is degraded by selective logging, charcoaling, cattle grazing, hunting and debarking of medical plants. Habitat fragmentation, reduction of fragment size, increasing isolation of the fragments and the distance of fragments to the main forest affect diversity, abundance and the risk of extinction of populations. For that reason, one of the most important questions of nature conservation is, to what extent are these fragments able to maintain the biodiversity of the continuously forested areas. <br /> At five different study sites in Kakamega Forest, the canopy dwelling arthropod fauna of two common tree species was collected, using the insecticide fogging method. The assemblages of arthropods were sorted to major groups and counted. Beetles were additionally allocated to morpho-types. The beetle fauna was analysed with respect to guild structure; the guilds are predacious beetles, mycetophagous beetles and phytophagous leaf beetles and weevils.<br /> There are differences of diversity and abundances of beetles between main forest and fragments. Results of all taxa investigated are fluctuating in each season without any visibly pattern. A decline in species richness in fragments is not visible. At two sites with adjacent open grassland, edge effects are visible, since the fauna included further “non-forest” species, which leads to increased species numbers by higher dynamics of disturbed habitats. The distribution of arthropods, especially of beetles, at different sites of Kakamega Forest is mainly based on stochastic events and predator-prey interactions, while fragmentation, forest degradation and interspecific competition have low influence.