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Predicting Invasions of Nonindigenous Plants and Plant Pests
An Allee effect occurs when low-density populations sustain a zero or even negative rate of increase because of reduced reproduction or survival when con-specifics are insufficiently abundant. Eventually, “undercrowding”—inverse density dependence at low density—drives the population below a critical threshold and extinction occurs (Allee 1931, Courchamp et al. 1999). The dynamics of Allee effects, therefore, can exert a substantial influence on whether a colony of a newly arrived organism is able to persist and become established. One frequent cause of Allee effects is a scarcity of reproductive opportunities at low densities. For example, in some insect populations, difficulty in locating conspecific mates will reduce the likelihood that individuals in the next generation will produce offspring, and in the case of arrhenotokous insects (a population in which unmated females produce only males), will result in a population with a male-biased sex ratio. Ultimately, this form of demographic stochasticity leads to collapse of the population. Other factors may also generate inverse density dependence in low-density populations, including a reduction in the ability of individuals to find or use suitable host plants (Way and Banks 1967), decreased ability to cooperate in defense against predators (Turchin and Kareiva 1993), and genetic inbreeding that leads to decreased fitness (Courchamp et al. 1999, Lamont et al. 1993).
The strength of an Allee effect on the persistence of a population depends on the processes influenced by inverse density dependence. Species in which fitness is enhanced in some way by conspecific facilitation or cooperation may be subject to Allee effects only at very low densities. In contrast, species with obligate sexual reproduction may be more strongly affected by an Allee effect and at a wider range of densities (Courchamp et al. 1999). Species that are subject to a strong Allee effect may also be more vulnerable to extinction due to environmental stochasticity because the population size below which they cannot recover from an unfavorable weather event will be larger than for other species.
Influence of Allee effects on establishment has been addressed by examining the dynamics of populations of nonindigenous species released in biological control programs. Hopper and Roush (1993) found that parasitoid wasp and fly species released for biological control of leaf-feeding insects were subject to an Allee effect when dispersal from low-density populations led to low mating success, which resulted in male-biased sex ratios. Modeling showed that the Allee effect could drive populations to extinction and that it limited establishment of introduced parasitoid species more than the limits imposed by stochastic environmental variation or lack of genetic variation. Grevstad (1999a,b) used simulation modeling to show that if the net reproductive rate of a newly introduced species was even slightly greater than 1.0, demographic stochasticity was unlikely to limit population persistence. Environmental stochasticity, however, interacted with an Allee effect. When an Allee effect was present and environmental conditions were relatively constant, establishment of a species was most likely to be