Land-use intensity and relatedness to native plants promote exotic plant invasion in a tropical biodiversity hotspot.
Abstract
Exotic plant invasions threaten biodiversity and are costly to farmers. Land use is a major pathway promoting the spread of exotic plant species; however, little is known about the processes underlying the success of exotic plants in tropical agricultural landscapes. Focussing on the heterogeneous smallholder landscapes of north-eastern Madagascar, we studied exotic plants of understorey communities across a land-use intensity gradient from unburned lands (old-growth forests, forest fragments and forest-derived vanilla agroforests) to burned ones (fallow-derived vanilla agroforests, woody fallows and herbaceous fallows). We quantified the absolute species richness, abundance and cover of exotic plants across land-use types and their proportional contribution to community richness, abundance and cover as indicators of exotic plant invasion. We tested for the effects of land-use parameters, namely land-use history, canopy closure and landscape-level forest cover, on exotic plants. Additionally, we tested whether the phylogenetic relatedness between exotic and native species in the same plot affected invasion success, testing Darwin's naturalisation and pre-adaptation hypotheses. All indicators of exotic plant invasion were lowest in old-growth forests and forest fragments and highest in fallow-derived vanilla agroforests, woody fallows and herbaceous fallows. Absolute and proportional exotic richness were negatively affected by canopy closure, and landscapes with high forest cover had lower proportions of exotic plant richness. High phylogenetic relatedness between exotics and natives was associated with lower proportional richness but higher proportions of exotics in abundance and cover. However, individual exotic species showed contrasting responses to land-use parameters and relatedness to natives. Synthesis and applications: Our results indicate that maintaining unburned lands, land-use types with dense canopies and landscapes with high forest cover prevents the spread of exotic plants within the agricultural landscapes of north-eastern Madagascar. Supporting Darwin's pre-adaptation hypothesis, exotic plants that are phylogenetically closely related to native plants are more likely to become successful invaders in terms of abundance and cover. Nevertheless, individual species show different responses to land-use changes and phylogenetic relatedness. Therefore, land-use decisions and management choices can be tailored to limit the spread of exotic species and to preserve native plants in this global biodiversity hotspot.