Environmental filters shape alien plant community composition in fragmented native shrublands

Abstract Habitat fragmentation is a major contributor to alien plant invasions, yet little is known regarding the role of environmental filtering in determining the identity of species invading ecosystem fragments. We used Ellenberg values to identify why some alien plant species invade kānuka shrublands ( Kunzea ericoides , Myrtaceae), a highly fragmented and native ecosystem in New Zealand. Thirty kānuka fragments on Banks Peninsula were surveyed and alien plant species cover was recorded in 120 2 × 2 m plots placed along a transect in grassland, the edge and fragment interior. Community weighted Ellenberg indicator values (light, moisture, soil reaction, nutrient availability) were computed for each species and linear models and ordination were used to assess how environmental filters, grazing, and perimeter to area ratio influence alien plant community composition along the gradient. Alien plant community composition differed between edge and interior plots of kānuka fragments. Fragment edges shared similar alien plants to those from the grassland and included species associated with higher tolerance to acidic soils illustrating its high susceptibility to propagule pressures from adjacent anthropogenic grassland. In contrast, in the interior of kānuka fragments, there was greater representation of shade tolerant species and those adapted to higher nutrient availability. More targeted management strategies should be developed for fragmented ecosystems by accounting for the ecological niches of alien plant species regarding light, moisture, pH, and nutrients.