nonindigenous enemies. That an immigrant species will eventually acquire new biotic constraints seems inevitable, although the constraints might not arrive until long after the nonindigenous species has become persistent or even invasive (Strong et al. 1984).

A multitude of biotic factors can prevent establishment of immigrant insects and pathogens in a new range. The immigrants must be able to locate suitable host plants or come into contact with a suitable host through the deposition of spores and eggs or other dispersal units, survive interactions with newly encountered enemies, and compete with native species. A new nonindigenous insect or pathogen is often not detected for years or even decades (Carey 1996), and the difficulty of early detection makes it difficult to identify factors that were conducive to establishment. General patterns can be recognized, however, and might be useful in efforts to predict future invasions.

The establishment of nonindigenous pathogens and arthropods is subject to the availability of a suitable host. Some plant pathogens, such as the cereal rust fungi, are extreme specialists; they infect only specific plant varieties. Others— for example, such root pathogens as Pythium species, will infect a wide taxonomic range of hosts. Although the wide host compatibility of some pathogens increases the probability of their finding a suitable host, many pathogens with narrow host ranges (such as Sporisorium sorghi, the sorghum smut fungus) have become established and even invasive in the United States. For example, plant pathogens that have invaded the temperate forests of North America cause epidemics on a few closely related species. Generalist pathogens are rare in forest ecosystems; one example is Erwinia amylovara, the fire blight pathogen (Vanneste 2000).

Not surprisingly, North American forests have been invaded by pathogens from areas with similar hosts and climates. But the long-term isolation of the North American forest flora from floras that are the source of pathogens might predispose the forests to invasion because susceptible genotypes could have evolved in the absence of pathogens that would have constrained their survival (von Broembsen, 1989).

Pathogens of minor significance in one location can cause epidemics when transferred elsewhere. For example, races of the stem rust pathogen Puccinia graminis were blown from moderately resistant wheat varieties in Australia to highly susceptible ones in New Zealand (Watson 1970). A most noteworthy example in the United States is the recent identification of the fungus Phytophthora ramorum, which has been associated with the disease, sudden oak death. Its hosts in California include oak and numerous other plant species. Although the origin of the fungus is not known, it was probably introduced and is known to cause a leaf spot and dieback of rhododendron in Europe (California Oak Mortality Task Force 2001).