Faunal turnover of arthropod assemblages along a wide gradient of disturbance in Gabon

Abstract

We examined the results of a study in Gamba, Gabon, focusing on the impacts of disturbance on arthropods, including more than 400 000 individuals, from which 21 focal taxa were separated into 1534 morphospecies by parataxonomists. Replication included the understorey of three sites in each of four different stages of forest succession and land use (= ‘habitats’) after logging (old and young forests, savanna and gardens), surveyed over a whole year with three sampling methods. Generally, there was a good correspondence between the number of species sorted by taxonomists and the number of morphospecies sorted by parataxonomists. Despite higher taxonomic groups being present in most habitats, a large proportion of insect species was site- or habitat-specific. Anthropogenic modification of habitats did not result in a monotonic decline of abundance and diversity, as many herbivore pests and their associated predators and parasitoids invaded gardens, where plant productivity was kept artificially high year-round through watering and crop rotation. Because gardens were colonized mostly by invasive crop pests with little relation with the forest fauna, these results emphasize the concept of maintenance of ‘quality biodiversity’ and the value of considering other variables than species richness alone in conservation studies. Further, several lines of evidence indicated that savanna habitats at Gamba supported a species-poor arthropod fauna distinct from that in nearby habitats. It is therefore questionable whether in Africa insect assemblages of savanna represent a smaller subset of their neighboring forest fauna.