Home and away: biogeographical comparison of species diversity in Thymus vulgaris communities

We examined the effects of Thymus vulgaris (common thyme) on associated vegetation in both its native and introduced range. We compared local (within-community) and landscape (among-community) species richness and community composition between thyme-dominated communities in France (native range) and New Zealand (introduced range). From 7 native sites (France) and 10 introduced sites (New Zealand), all plant species present in 20 (New Zealand, 25 in France) randomly placed 100 cm × 50 cm quadrats were recorded. Local species richness was determined by calculating mean species richness/quadrat inside and outside thyme-dominated plant communities and tested for significance with the factors of range and across sites. Landscape scale differences were determined by comparing total species richness inside and outside thyme communities across all sites from both ranges. Species differences between native and introduced thyme communities were analysed using similarity percentages. We found native range microenvironments with thyme harbour more species than microenvironments without thyme and this pattern was reversed in the introduced range with thyme decreasing local species richness. A higher percentage of shared species occurred both with and without thyme in the native range compared to the introduced range. In both ranges and across all sites (except for one) species composition of thyme-associated plant communities differed from communities without thyme. Native plant communities with thyme were more similar in species composition than plant communities without thyme, but in the introduced range species composition was most similar in plant communities without thyme. These results suggest thyme’s ecosystem engineering consequences are context-dependent. Thyme may filter out competitive species that could negatively impact local species richness in its native range, but when introduced to a disturbed landscape in a novel biogeographic region, thyme reduces local species richness.