Effects of habitat fragmentation on alien plant invasion in native shrublands

Abstract

Questions: Habitat fragmentation can promote alien plant invasions by increasing the exposure of the fragment edge to alien plants present in adjacent habitats. Yet, few studies have examined covariates that might exacerbate the effect of fragmentation on plant invasions. Here, we investigated the influence of grazing, fragment size and habitat structure on alien plant invasion of indigenous shrubland while considering plant growth form. Location: Banks Peninsula, Canterbury, New Zealand. Methods: We conducted a systematic vegetation survey in 30 fragments of kānuka shrubland (Kunzea ericoides, Myrtaceae). In each fragment, alien plant species richness and cover were recorded across four transects with 2 × 2 m plots positioned in the neighbouring grassland, the fragment edge and interior. We predicted alien species richness and cover from a combination of factors (grazing, fragment size, kānuka canopy cover and tree density) using generalised linear mixed models. Similar models were run investigating the proportion of alien grass species richness and cover. Results: The edge effect had a significant influence on alien plant invasion, with a higher alien richness and cover in the grassland and fragment edge than in the interior. Alien richness and cover declined with greater kānuka canopy cover. Grassland and edge plots showed greater cover of alien grass species, whereas interior plots showed higher richness and cover of other alien vascular species. Smaller kānuka fragments had a less dense kānuka canopy cover than larger fragments and a higher alien richness and cover. Conclusions: Most kānuka shrublands occur as small fragments within a matrix of anthropogenic grasslands emphasising the high vulnerability of these ecosystems to alien plant invasion. Alien grass species were filtered out along the grassland-edge-interior gradient whereas other alien vascular species such as shade-tolerant woody species were able to establish in the kānuka interior. Future restoration efforts should attempt to increase the size and intactness of existing fragments.