understand retrospectively the circumstances that led to an invasion than to predict the outcome of any particular case–a conclusion with which the committee concurs and sees as applicable to the whole issue of establishment.
Nonindigenous pathogens can also face “enemies”. Populations of native leaf epiphytes or endophytes or natural communities of soil microorganisms that suppress native pathogens might also serve as antagonists to the establishment and spread of nonindigenous pathogens (Cook 1993). These microbial antagonists range from bacteria to mycoparasites. A unique group of hypoviruses has enabled substantial biological control of chestnut blight in Europe and Michigan (MacDonald and Fulbright, 1991). When virulent strains of Cryphonectria parasitica are infected with the hypoviruses, their ability to infect chestnut and reproduce is typically reduced; they become hypovirulent. The hypoviruses appear to have their origin in eastern Asia with their fungal host (Peever et al., 1998). Hypoviruses or their genetically modified variants eventually may prove useful as biological control agents for chestnut blight within chestnut’s native North American range (Dawe and Nuss 2001). Similarly, mycoviruses (called “d-factor”) that correlate with the presence of multiple dsRNA segments in the Dutch elm disease fungus, Ophiostoma novo-ulmi may attenuate the pathogenicity of the fungus. However, their overall effect on the dynamics of Dutch elm disease is unclear, as is their potential use as biological control agents (Brasier 1990).
Predators of seed and vegetative tissue cause immediate death of a plant immigrant. Seed predators can be particularly effective because plant immigrants are most likely to arrive as seeds, and of course seed production is likely to be crucial to the immigrants’ persistence in the new range. Ants, for example, have apparently blocked the establishment of some nonindigenous tree species in the Caribbean region (Little and Wadsworth 1964). But the list of seed predators is by no means restricted to ants or even insects; rodents can be such voracious seed predators of Cakile maritima that it has been prevented from expanding its range locally in California. Predation of nonindigenous plants, apart from seeds, has commonly involved seedlings. Again, the plants are characteristically small and unable to withstand even a single attack (Mack 1996a).
The action of grazers–organisms that remove plant material in one or more nonlethal events–can be cumulative to the point at which the plant dies outright or dies from the infections that grazing can facilitate. Some termites, which as a group characteristically attack only dead wood, can attack living wood. Their attacks have been so severe for some nonindigenous plants as to thwart their establishment. Eucalypts, which are native to Australia and New Guinea, have been prevented from establishing in some locales in Brazil and West Africa by chronic termite grazing. And grazing can contribute indirectly to extirpation by reducing the ability of immigrants to survive competition or parasitism (Mack 1996a).