invasion

Natural grasslands are among the most threatened biomes on Earth. They are under pressure from land cover change including afforestation, farming intensification, invasive species, altered fire regimes, and soil amendments, all of which impact native biodiversity and ecosystem functioning. In Aotearoa New Zealand, tussock-dominated native grasslands expanded due to increased fire activity during waves of human settlement. These areas have subsequently been maintained as modified grasslands by agricultural pastoral land management practices and effects of introduced feral mammals.

Invasive exotic tree and shrub species (woody weeds) form dense, monospecific stands in many areas of New Zealand. At some sites, the weed dies out naturally and is replaced by native species as succession proceeds, but at others the weed persists indefinitely. The ability to distinguish between these different trajectories is critical to effective weed management, but the conditions that determine successional outcomes remain poorly understood.

According to the most recent (2005) compendium on the history of the European hedgehog (Erinaceus europaeus) in New Zealand, this small insectivorous mammal was first brought from Europe to the South Island in the 19th century. This introduction has been presumed to be the source of hedgehogs that subsequently spread to the North Island. This view was informed by the absence of hedgehogs in the North Island throughout the 19th century and no evidence of direct shipments of hedgehogs from overseas to the North Island.

Cortaderia selloana (pampas grass) and C. jubata (purple pampas grass) are both invasive in New Zealand. Cortaderia selloana is found throughout most of the country, whereas C. jubata is restricted to the North Island and the northern South Island. We examine the genetic variation present in each of the species, and compare this to the findings of an earlier study that analysed the variation in invasive C. jubata plants from New Zealand.

Landscapes typically comprise various habitats that differ in their susceptibility to invasion by exotic species. Highly invasible habitats such as riparian corridors can act as a conduit for rapid movement across the landscape and as a propagule source to facilitate spread into adjacent, less invasible habitats.

Mid to late 20th century expansion of Dracophyllum scrub into tussock grassland on subantarctic Campbell Island has been attributed to the collective effects of global warming, cessation of farming in 1931, and continued grazing by feral sheep. To determine the importance of these, we dated the timing of scrub expansion by aging 241 Dracophyllum plants in 17 plots chosen to sample the range of environments this shrub/ small tree occupies on Campbell Island.

Changes in the vegetation of Flat Top Hill, a highly modified conservation area in semi;arid Central Otago, New Zealand, are described four years after the cessation of sheep and rabbit grazing. Unusually moist weather conditions coincide with the four-year period of change in response to the cessation of grazing. Between 1993 and 1997, the average richness and diversity (H') of species increased, and the average proportion of native species decreased significantly.

We examined the distribution and abundance of the invasive Hieracium species (hawkweeds) in the dry grasslands of the Upper Waitaki Basin (Canterbury) and Otago, using measures of Hieracium species frequency and hawkweed cover from 301 vegetation plots. Average hawkweed cover was significantly less in Otago than Canterbury. Hawkweed cover was also lower on drier sites, with hawkweeds having less cover at lower elevation, on more xeric sites and, in Canterbury, on soils with a lower moisture holding capacity.

Biological invasions are a widespread and significant component of human-caused global environmental change. The extent of invasions of oceanic islands, and their consequences for native biological diversity, have long been recognized. However, invasions of continental regions also are substantial. For example, more than 2,000 species of alien plants are established in the continental United States. These invasions represent a human-caused breakdown of the regional distinctiveness of Earth's flora and fauna—a substantial global change in and of itself.

New Zealand's protected natural areas are being increasingly threatened by weeds as the natural landscape is fragmented and surrounding land use intensifies. To assist in designing management to reduce the threat, we attempted to determine the most important reserve characteristics influencing the presence of problem weeds in forest and scrub reserves. Data on 15 reserve characteristics were derived from surveys of 234 reserves.