Effects of land use on local biodiversity on islands and mainlands worldwide

Abstract

Land-use change is a major driver of biodiversity change; while it increases many species’ extinction risk and often causes a loss of local diversity, it also promotes the establishment of novel alien and native species. All these processes change the species composition of assemblages. Global syntheses rarely compare the effects of land-use change across different ecological systems, limiting our ability to identify which systems are most affected. Islands and mainlands often face different human pressures and harbour very different species assemblages. Importantly, many islands harbour native species that are particularly sensitive to human pressures and they tend to be more vulnerable than mainlands to invasions by alien species. In this thesis, I model a global collation of site-level biodiversity data from sites facing different land uses and related pressures to answer three questions. Does the change in species composition caused by land-use change differ between islands and mainlands? Have land-use change and related pressures decreased biodiversity integrity more on islands than on mainlands? How do land use and related pressures affect the diversity of alien and native species on islands? My results highlight particular cases where land-use change causes a greater change in local species assemblages on islands than on mainlands. Based on the Biodiversity Intactness Index (BII), I also show that, on average, land-use change and related pressures have reduced biotic integrity more on islands than on mainlands, mainly because island species are more sensitive to human pressures. The chapters underpinning these findings represent the first global analyses to include a wide range of taxa while comparing biodiversity responses on islands and mainlands to land-use change. Additionally, through analyses focusing exclusively on islands, I show that land-use change reduces both local richness and total abundance of island native species but increases both the number and abundance of alien species. My results highlight the sensitivity of island natives to human pressures and attest to the establishment of alien species on islands, particularly in disturbed habitats and on more isolated islands. They support calls for stronger efforts to protect islands’ unique biodiversity. This thesis also contributes to the development of a new implementation of BII, improving on the statistical modelling approach used recently to estimate BII globally. In a final analysis integrating results from the different chapters, I show that this improved approach yields estimates that reflect the intactness of native biodiversity more accurately than previous estimates. Most of this improvement is attributable to the use of a more stringent definition for sites whose biota is assumed to have been minimally impacted by people, which was facilitated by a more efficient use of the underlying assemblage data.