the ability to propagate asexually and self-fertilize when the population is small; plants that outbreed when the population grows larger gain the benefits of genetic diversity in the long run. Traits associated with establishment include long flowering and fruiting periods that increase the chance of pollination and seed dispersal, a short juvenile period (the time from seed germination to the onset of flowering), high seed production, a capacity for seed dormancy and germination cuing, and the ability to use light efficiently.

In some plant groups, a combination of those traits has been shown to have predictive power in identifying invasive species, and these results should be useful in assessing new plant introductions in the taxonomic groups in question. It has not yet been possible to generalize the results across a broad taxonomic spectrum among vascular plants; moreover, there are exceptions (some invasive plants apparently have none of the traits, and some plants with the traits have not been shown to establish in a new range).

A key shortcoming of most studies that consider abiotic and biotic forces, as well as organisms’ traits, is the lack of experimentally derived, explicit, quantitative data that would allow systematic analysis of the relative importance of factors.

TRANSITION TO INVASIVENESS

Experimental data related to a population’s ability or inability to grow and spread once established are scant. It is possible to recognize an invading population and to speculate retrospectively on factors related to its success in the new range, but it has proved difficult to predict the success of a species in a given environment.

Many of the same species’ traits and abiotic and biotic forces that affect a population’s establishment continue to be important in its numerical and geographic expansion. Having reached a threshold size, an established population is much less subject to stochastic forces that could drive it to extinction, but the size and spread of the population are affected by the availability of and competition for resources and by mechanisms that facilitate dispersal.

Following in detail the movement of propagules (seeds, adult insects, eggs, spores, and so on) could provide useful information on whether some categories of potential plant pests might be more likely to become invasive than others. The spatial distribution of established populations that result from dispersal also probably plays an important role in their ability to become invasive. It is known that wind dispersal is more common in arid treeless ecosystems and that bird dispersal is more common in forest systems. Birds have often played a critical role in the spread of species with fleshy fruits. Consequently, nonindigenous species with fleshy fruits (and the pathogens associated with them) could as a group carry a high threat of spread and invasion.

There is no consensus as to whether a biologically diverse community is more or less likely to be invaded than one that is less diverse; there are theories



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