Thyme and space invasion: Thymus vulgaris decreases exotic plant species diversity in Central Otago, New Zealand

Abstract

Understanding the influence of spatial scale on ecological processes that occur with exotic plant invasion can help us identify underlying mechanisms of successful invasion. Here we studied spatial associations between Thymus vulgaris invasion and plant community species richness, composition and abundance at multiple spatial scales. These scales included among-sites, differences between aspects at the same site, and differences that occur between the inside, edge and outside of thyme stands at the same site. We examined how aspect (north or south) and site location influence spatial distribution patterns of thyme (cover and canopy height). We also quantified the relationship between variability in plant community composition inside, at the edge and outside of thyme-invaded communities and soil physical and chemical properties, cover of animal faeces, bare ground and schist, altitude and slope. We found lower species richness inside thyme-dominated communities, and this change was often greater on the north-facing slopes than the south-facing slopes. Plant species and plant functional groups had individualistic abundance patterns associated with thyme’s invasion. Exotic grasses, exotic forbs and native shrubs showed significant decreases in cover inside thyme stands, but native forb and native grass abundances were not significantly affected by thyme’s presence. Thyme density varied with aspect at some sites, generally being less dense on the south-facing slopes compared to the north-facing slopes. A subset of 15 environmental variables explained 54 % of plant assemblage variation inside, at the edge and outside of thyme-invaded communities, highlighting the important influence of site differences on invaded plant community assemblages.