Establishment And Spread

The majority of introduced insects, like most introduced species, do not survive, although only biological control introductions generate substantial data on failed introductions. For parasitoid species introduced to control insect pests, only about 30% establish populations, whereas for all insects introduced for plant control, the comparable figure is about 60%. Because biocontrol candidates are chosen and often tested for survival in the target environment, one might expect failure rates for inadvertently introduced introductions to be even higher. For most taxa, invasion biologists believe that 5 to 20% of introduced species establish populations, although many of these may remain for years or in perpetuity near the point of introduction, and a large fraction are restricted to anthropogenous habitats such as human habitations or agricultural fields.

An arriving propagule must be large enough to survive the initial threat of demographic stochasticity—that is, random elimination of so many individuals during the first few generations that the population fails. Data from the biological control literature show that probability of establishment increases with propagule size and number of attempts, but many very small propagules have established large, widespread populations. For example, a single fertilized female of the cochineal insect Dactylopius opuntiae from Sri Lanka initiated a large, ongoing population on Mauritius. In Puerto Rico, two females of a unisexual race of the encyrtid wasp Hambletonia pseudococcina were used to rear 7000 individuals, which were released, established, and quickly spread. For a parthenogenetic species, at least the difficulty of finding a mate is obviated, but demographic stochasticity has other components.

Even assuming an adequate propagule size, the environment, both physical and biotic, must be suitable for a species to survive and spread. Predators, parasites, competitors, and pathogens can eliminate an introduced species or restrict its ambit. For example, the Asian aphelinid wasp parasitoid Aphytis fisheri, introduced to California to control California red scale (Aonidiella aurantii), failed to establish because of competition from previously introduced A. melinus and A. lingnanensis. Conversely, the absence of natural enemies from its native range is often posited as the reason for the success of some invaders, such as the cynipid Andricus quercuscalicis, introduced into Great Britain. By contrast, the presence of another species, such as a food plant or a symbiont, might be necessary for an invader to survive. The monarch butterfly would not have survived in Australia but for the prior introduction of its host milkweeds. The physical environment is probably an even more frequent reason for introduced species to perish. A temperate climate does not augur well for a newly arrived tropical insect, but even subtler physical differences can be crucial to a species's survival. For instance, synanthropic species are unlikely to survive if they arrive in pristine natural habitats.

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