Non-interactive effects of plant invasion and landscape modification on native communities

Aim Biological invasions and anthropogenic habitat modification are considered to be the leading drivers of global environmental change, yet their synergistic impacts on native communities remain poorly understood. We tested the effects of an invasive grass, Stenotaphrum secundatum, on an endangered coastal swamp forest community across an anthropogenic land use gradient. We also investigated the relative importance of landscape versus local community attributes in mediating the effects of S.secundatum invasion on the community. Location Four hundred kilometres of the coastline of south-eastern Australia. Methods We compared the diversity, composition and recruitment of resident vegetation in 32 invaded and 32 non-invaded forest stands across an anthropogenic land use gradient. Local disturbance and environmental attributes of the forest (e.g. fire severity, litter abundance, canopy openness, vegetation structure) and adjacent landscape matrix (e.g. cover of forest, urban and agricultural land) were measured in detail at each site. Results Invasion was associated with substantial local extinctions of native plant species (i.e. 83% fewer species in invaded sites), altered community compositions and an 85% reduction in rates of woody plant recruitment. Local disturbances and environmental attributes of forest stands and adjacent landscape matrix were similar between invaded and native sites. Invasion caused a twofold increase in litter biomass, which we hypothesize is the primary mechanism by which the invader excludes native species. Landscape modification had no effect on the diversity of adjacent swamp forest. There was no interactive effect of landscape modification and invasion on the native community; species losses in response to invasion were high regardless of the condition of the adjacent matrix. Main conclusions We show that impacts of non-native species are not dependent on landscape context and, unexpectedly, that invasion by an alien plant poses a greater threat to diversity of endangered swamp forests than modification of the adjacent matrix.