Impacts of exotic invertebrates on New Zealand's indigenous species and ecosystems

Biological invasions have significantly affected New Zealand's native species and ecosystems. Most prominent are the effects of exotic mammals and plants, whereas few invertebrate invasions are known to have major effects on native ecosystems. Exceptions are the well-known cases of Vespula wasps in Nothofagus forest ecosystems and Eriococcus scale insects in Leptospermunt shrublands. This limited impact is surprising because over 2000 exotic invertebrates have become established in New Zealand, among them many pests of exotic crop plants. The low impact of exotic invertebrates that invaded forests and other native ecosystems in New Zealand is in contrast to the situation in other parts of the world where many invertebrates have become important pests. We provide an overview of known invasions by exotic invertebrates in New Zealand, and explore in more detail several examples of invasive species, including herbivores, predators, parasitoids, decomposers and other groups in forests, grasslands, and other terrestrial ecosystems. Several hypotheses have been proposed to explain the comparative scarcity of such invasions that affect New Zealand's indigenous ecosystems. There is a common view that New Zealand's native species and ecosystems are inherently resistant to exotic invertebrate invaders, and there is some evidence to support this view. As a result of the high level of endemism in New Zealand's flora, many native plants are phylogenetically distant from the host plants of many plant-feeding invaders. This provides some protection. Less host-specific plant-feeding insects, generalist predators, parasitoids and decomposers are less affected by such constraints, and these groups are perhaps more represented among the successful invaders of natural ecosystems. However, the shortage of studies on invader impacts on native species and ecosystems, compared with studies on economically important crops and production ecosystems, means that an unbiased comparison is not possible at this time. Furthermore, many invaders go through extended lag phases where their impacts are not easily noticed until they become more abundant and create more damage. Likewise, indirect effects of invaders, through more complex interactions in food webs, as well as impacts on ecosystem functions such as decomposition and pollination, are more subtle and difficult to detect without careful study. There is clearly a need for more research to determine more accurately which exotic invertebrates are already present, what their direct and indirect impacts are, and what generalisations and predictions about threats to native species and ecosystems are possible.