Movement of exotic plants into coastal native forests from gardens in northern New Zealand

The number and abundance of exotic weeds in native forest fragments are known to correlate with the distance to the nearest large town. This is of concern as land near lowland forest is increasingly being subdivided for housing throughout much of New Zealand. We quantified the relationship between settlements and exotic plants for the coastal forests in eastern Northland, New Zealand. Exotic plant species were sampled in 18 coastal forest areas of varying size, and related to attributes of nearby settlements (housing proximity, density, age, and the exotic plant species present). All settlement attributes were significantly related to the number of exotic species present in neighbouring forest fragments, unlike fragment size. The exotic species found in a particular forest area were significantly more likely to be present in the neighbouring settlement than in other settlements. The number of houses within 250 m of a forest area, alone, explained 66.8% of the variation in the number of exotic plant species in these forests. Our results suggest that proximity and size of settlements are currently the dominant factors controlling the number of exotic plant species in these forest areas, rather than ecological conditions within the forests. Properly managing the locations and densities of new subdivisions, as well as the species grown in gardens in existing and new subdivisions near forest reserves, will reduce the weed pressure and subsequent cost of weed control in these reserves.