Small, isolated patches are predicted to have reduced species diversity relative to larger, more continuous expanses of habitat because they tend to support viable populations of fewer species, to have less resource heterogeneity and fewer available niches, to be more degraded due to increased external pressures, and as a simple consequence of passive sampling. Indeed, the species-area relationship is considered one of the few genuine, general patterns in ecology. Taxa present in smaller, isolated patches are also expected to be nested subsets of those found in larger habitat fragments. Nested distributional patterns result from ordered extinctions from and colonization of habitat islands because of differential vulnerabilities and vagilities of different species. Here, I report results of two separate year-long surveys of fruit-feeding butterfly communities in relict sacred forest groves and forest reserves in the moist semi deciduous forest zone of Ghana, West Africa. Forest destruction has been pervasive in Ghana. The surveyed habitat remnants, which ranged in size from 6 to 5000 hectares, were once part of continuous forest cover. They now exist as isolated relicts of climax forest embedded in a converted, agropastoral landscape, presenting prime opportunity to test spatial predictions about community diversity and assembly across fragmented landscapes.
Results/Conclusions
Sites were sampled from 27-58 times, resulting in 15,000+ specimens from more than 120 species. Despite predictions of a positive species-area relationship, there was minimal evidence of an association between forest patch size and species diversity. Although rarefied species richness tended to be significantly higher in larger fragments, this relationship was only weakly supported. In all but one case, the small patches retained 84-93% of the species diversity found in the much larger remnants even though they constituted less than 1-10% their area. Neither total estimated richness nor Simpson’s diversity were related to fragment size. Importantly, the limited decline in species richness documented in the sacred groves was not due to species replacements, whereby common, broadly distributed, generalists supplanted more vulnerable species in these communities. Even with this limited loss of biodiversity from these small relict forests, their species assemblages were nonetheless found to be nested subsets of those in larger fragments, which indicates that species lost from these small patches has been non random. Analysis of species’ abundance patterns across both surveys and the seven fragments is used to identify winners and losers, in terms of species persistence versus extinction in fragmented landscapes, and their characteristic traits.