birth, death, and movement), genetic effects (including hybridization), population dynamic effects (mean and variance in abundance, population growth rates, and so on), community effects (species richness, diversity, and trophic structure), and effects on ecosystem processes (primary or secondary productivity, hydrology, nutrient cycling, soil development, and disturbance frequency). From a sample of the literature on the impacts of invaders, Parker et al. (1999) recorded the number of published studies that produced quantitative data at five biological levels on five guilds of invading species: freshwater fish, freshwater invertebrates, marine invertebrates, algae and vascular plants, and insects and other terrestrial invertebrates. Population-level effects were the most frequently studied impacts. Studies of community-level impacts were common only for plants. The least-studied impacts, except for freshwater fishes, were genetic changes and long-term evolutionary effects.

Individual Effects

Invaders can have a variety of effects on the performance of individuals. For example, invasive plants can compete with native plants and reduce their growth (Gentle and Duggin 1997) and change their structure, such as rooting depth (D’Antonio and Mahall 1991). Invasive insects and pathogens can decrease hosts’ rates of growth, development, survival, reproduction, and movement. Such changes in individual growth and life cycles can translate into changes in population size and fate. Population models built around a simple description of the life cycle can be used to link the individual and the population (Caswell 2000).

Genetic Effects

Nonindigenous species sometimes invade areas inhabited by closely related native species. If nonindigenous and native species interbreed, genetic exchange between the two species can alter the genetic makeup of native populations. Such hybridization between an invader and a native is common and can have several consequences, including the spawning of new invaders (Rhymer and Simberloff 1966). Repeated hybridization of North American cordgrass (Spartina alterniflora) with British native cordgrass (S. maritima) eventually yielded a new, highly invasive species, S. anglica (Thompson 1991). Production of hybrid swarms and widespread introgression can lead to virtual extinction of native taxa through a swamping of their original genomic makeup through recombination with genes of the invader (Rhymer and Simberloff 1996), especially if the invader becomes much more common than the native species; this seemingly hypothetical threat is both real and serious: three species federally listed as endangered in the United States have gone extinct since enactment of the Endangered Species Act because of hybridization with nonindigenous species (McMillan and Wilcove 1994). Finally, hybridization among plants may spawn invasion by plant pathogens.

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