Differential impacts of invasion on plant communities of two types of savannas in India

Abstract

Biological invasions pose a threat to biodiversity in tropical savannas. Invasive plants can alter savanna communities in complex ways, where impacts can vary with the intensity of invasion, the spatial scales examined, and by climate. However, our understanding of such impacts on Asian tropical savannas is limited. To address this knowledge gap, we examined how plant invasion impacted plant communities across increasing levels of invasion, at different spatial scales, in two climatically determined savannas in tropical India. We sampled vegetation at three sites each in drier fine-leaf savannas (FLS) and wetter broadleaf savannas (BLS). At each site, we sampled replicate locations with plots assigned to uninvaded savannas, invaded savannas at three invasion levels, and old-growth savannas that served as a benchmark for comparison. We quantified alpha and beta plant diversity, and differences in community composition that were partitioned into components representative of species loss and replacement. We found that while richness and diversity decreased linearly, evenness increased linearly with invasion. The loss of species was much greater in BLS (60%) than in FLS (30%). Differences in composition with invasion were more pronounced in BLS and driven by species turnover in both savannas. We did not find evidence for homogenization at larger spatial scales, with no differences in beta diversity in FLS and an increase in BLS with invasion. These results highlight the utility of examining multiple measures of diversity, as they provide insights into how relationships between richness and evenness can translate into diversity. The linear nature of responses of the community to increasing invasion suggests that management efforts may be equally effective at different stages of invasion in these savannas. Further, the observed increase in beta diversity in invaded BLS communities adds to the growing reports of spatial scale-dependent effects and reinforces the need to examine invasion at multiple spatial scales. Overall, the greater loss of species and pronounced community changes in BLS, which is the dominant savanna type in the Indian subcontinent and home to many endemic species, suggest that it should be prioritized for conservation, management, and future restoration.