mesic environments have extraordinarily low species richness (Wiser et al. 1998); the forest understory can consist of fewer than 12 species. It is unlikely that the native resident species collectively play all the roles (and to the same degree) of light and nutrient capture as are played by the scores of species in Northern Hemisphere forests in similar physical environments. The testable question that emerges is whether the low species richness of the native New Zealand forests has led to their being invaded more than, for example, forests with similar physical environments in North America or Europe. The many examples of nonindigenous species that have become invasive with little or no continued assistance by humans suggest that such opportunities do exist.
Disturbance can alter the availability and use of resources. In general, ecological communities modified by human or exogenous natural events, such as hurricanes and cyclones, to a condition that is not otherwise common are considered more susceptible to invasion. The almost ubiquitous occurrence of ruderal (usually nonindigenous) species along roadways, footpaths, and pastures—sites of routine disturbance—is forceful testimony to this principle. In addition, disturbances that remove biomass and recycle limited resources create opportunities for colonization and occupancy by nonindigenous species, such as ragwort Senecio jacobaea (McEvoy et al. 1993, McEvoy and Coombs 1999) in Oregon. So strong is the link between disturbance and the proliferation of agricultural invaders that it becomes necessary to separate these cases categorically from those in which disturbance is minor or even nil. The degree to which plant invasions in natural communities have been not only sparked but also sustained by continual disturbance deserves further investigation (Mack 1989).
Once established, some invaders can usurp resources in a new range, use them more efficiently than the natives, or even alter the resources themselves. In many invasive species, these competitive traits are revealed as the organism begins to proliferate. Evidence of interspecific competition among newly established insects or pathogens is scarce, but competition often plays a substantial role in plant invasions. In many plant communities, competition for light is so intense that a nonindigenous species’ ability to invade could be related directly to its ability to capture light that could otherwise be used by native species. So the “upmanship” or growth in stature that has arisen through natural selection since plants emerged on land is illustrated in some plant invasions. In its simplest terms, nonindigenous plants that can overtop their neighbors can replace them in the canopy, especially if they reduce light below the light-compensation point for any stage in the natives’ life cycle. In some cases, climbing nonindigenous vines