Invasive plant species may serve as a biological corridor for the invertebrate fauna of naturally isolated hosts

Abstract

The negative effects of alien invasive plants on habitats have been well-documented. However, the exchange of organisms between these and native taxa has been far less researched. Here we assess the exchanges of arthropod associates of a native (Virgilia divaricata) and an invasive (Acacia mearnsii) legume tree within the ecotone between forest and fynbos vegetation within the Cape Floristic Region of South Africa. Arthropod species richness, abundance, species assemblage composition and measures of beta-diversity were assessed between these two legume species where they grow sympatrically. Except for spiders and ants, arthropod species richness did not differ significantly between the two tree taxa. The overall abundance of arthropods was, however, significantly higher on the native tree species. This pattern was strongly driven by herbivores, as is consistent with predictions of the Enemy Release Hypothesis. When excluding rare taxa, over 75 % of all arthropod species collected in this study were associated with both host trees. However, arthropod community composition differed significantly between the two host plant taxa, largely due to differences between their herbivore communities. Arthropod beta diversity was high on the native host, with arthropod communities on the invasive host being much more homogenous across the sampling range. These results indicate that there are numerous exchanges of arthropods between these native and invasive plants. The invasive plant may provide arthropods with a pathway to other habitats between previously isolated native populations. This will have significant implications for biodiversity conservation at the habitat, species and population level.