but few experimental studies supporting each view. Obviously, a pathogen or arthropod faced with a monoculture of compatible host plants is in a good position to thrive, but many more nonindigenous pathogens or arthropods will have a better chance of finding hosts in a diverse community. It may also be more useful to focus on the concept of functional groups within a community, regardless of the taxonomic groups involved, or on resource availability and how variation in limiting resources might foster or hinder the invasion process.

Some clues to the ability of nonindigenous plants to become invasive are suggested by the traits by which some compete with native species or alter the new range to their advantage. Those traits include light-sequestering abilities (as in climbing vines), deep or dense root systems that capture water, abundant fruit or nectar that attracts pollinators, nitrogen-fixing capacity that alters soil composition, and fire-facilitating and fire-resisting attributes that alter fire cycles. Expression of any of these traits can greatly diminish the role of native species.

Genetic differentiation might be important in the long-term success of an established nonindigenous population in a new environment. The tempo of differentiation varies enormously among species, depending on how the organism uses sexual and asexual reproduction and on life-history characteristics. Multiple introductions and hybridization with native species are sources of genetic variation that can favor evolutionary diversification. As a result, novel genotypes that are locally adapted to conditions in the new range potentially can evolve. Modern agricultural practices can create selection pressures that shape plant pests that are specialized to the local crop environments. Selection pressures encountered during the invasion of natural habitats are even more complicated.

EVALUATING THE IMPACT OF INVASIONS

Predicting the consequences of invasions is challenging in invasion biology, in part because investigators disagree on how to measure the consequences. Invasions can be considered as having effects at five levels of biological organization: individual (including rates of growth, development, birth, death, and movement), genetic (including hybridization), population (mean and variance in abundance, population growth rates, and so on), community (species richness, diversity, and trophic structure), and ecosystem (primary or secondary productivity, hydrological characteristics, nutrient cycling, soil development, and disturbance frequency).

Furthermore, the influence of an invader can drastically affect the aesthetic value of an area. The loss in aesthetic value incurred when a biotic invasion occurs in a national park or national monument undercuts much of the rationale for the land’s protection in the first place. There can also be cumulative and indirect effects of invasion by more than one nonindigenous species. The chief reason that the impacts of invasions are so difficult to evaluate stems from the lack of sufficiently detailed data on the species composition, structure, and



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