plant invasion

Pathogen accumulation on an invasive plant species can occur over time, through co-invasion, or adaptation of native pathogen species. While accumulated pathogens can reduce the success and spread of an invasive species, they can also spill-over into native plant communities or valuable non-native populations. Transmission of pathogens may be density-dependent, with dense invasive populations creating better opportunities for pathogen spread than scattered individuals.

Climate change may exacerbate the impacts of plant invasions by providing opportunities for new naturalisations and for alien species to expand into regions where previously they could not survive and reproduce. Although climate change is not expected to favour invasive plants in every case, in Aotearoa-New Zealand a large pool of potential new weeds already exists and this country is predicted to be an ‘invasion hotspot’ under climate change.

We studied vegetation change on 142 permanently marked transects spread throughout tussock grasslands of Otago and Canterbury, in areas subject to both pastoral and conservation management. The transects were established between 1982 and 1986 and remeasured between 1993 and 1999, providing a record of vegetation change at each site over an interval varying from 10 to 15 years. Each transect consisted of 50 quadrats, each 0.25m(2), in which the presence of all vascular plant species had been recorded.

The effects of environment and management on the composition of short-tussock grasslands and the abundance of the invasive weed Hieracium pilosella were investigated in two small catchments. Species composition and site factors were recorded on a total of 182 plots and the management history of each catchment was reviewed. H. pilosella was present on >80% of all plots, but was at an early stage of invasion in one catchment (<5% cover) and dominant in the other (25% cover).

Species abundance, species richness, and ground cover were measured over 10 years on nine paired grazed and exclosure plots in short-tussock grassland in the early stages of invasion by Hieracium species. With and without grazing, H. pilosella and H. caespitosum increased markedly and H. lepidulum increased locally. In contrast, 50% of all other common species and species groups, and total, native, and exotic species richness declined significantly.