Issues in Ecology Number5 Spring 2000 common,but so far such lists are generally applicable for munities and some others are relatively impoverished only a small groupof.and xceptions abound. in numbers of native species and thus cannot provide Relati of in aders he biological resistance"to ne same genera, seem t o be obi however,many woul arsang land potential invaders.Many of the world's worst invasive would find no pollinators,symbionts.or other required plants belong to relatively few families and genera: associates among the native organisms,a factor that Asteraceae,Poaceae,Acacia,Mimosa,Cyperus.Both miaht provide island communities with a different form the starling and crow families have several invasive,or at of resistance to invasion.Yet actual demonstration least widely naturalized species but most biotic invad of vacant niches anywhere has proven difficult. ers have few,if any.similarly (wate constraints. hyacint ales as spores eggs,or some sive).This fact coud simply reflect a lack of opportuni other resting stage without their native associates ties for immigration rather than a lack of talent for inva- including their usual competitors,predators,grazers sion.But the circumstantial evidence suggests otherwise: and parasites.This"great escape"can translate into quilt by (taxonomic)association has proven imprecise at a powerful advantage for immiarants.All aspects of predicting invasive potential. performance such as growth,longevity,and fitness can be much g greater fo r species in new ranges A cording to this an invader ists As stated above,attempts to predict relative com proliferates not because it possesses a suite of ex munity vulnerability to invasions have also prompted gen- traordinary traits but rather because it has fortuitousl eralizations,including the following. arrived in a new range without virulent or at least Vacant niches.Some communities such as tropical debilitating associates.For example,the Australian oceanic islands appear to be particularly vulnerable brushtail possum has become an invader in new to invasions,although the evidence can be equivocal. Zealand since its introduction 150 vears ago.In New The vacant niche hypothesis suggests that island com Zealand it has fewer competitors for and she Figure 4-Many invasive grasses have greatly expanded their world-wide ranges at the expense of native grasslands and fo ests,ust ally facilitated by hun an-induced land-clearin recurring fire,and live HNalAenSetmgieamaniang到iomoihemAMiahsiepicahepaineeoieospwmopg Zing woodland (see remnant trees in background).It resprouts readily after its litter is burned:native plants are much less tolerant and are eventually eliminated from these sites. Issues in Ecology Number 5 Spring 2000 6 common, but so far such lists are generally applicable for only a small group of species, and exceptions abound. Relatives of invaders, particularly species in the same genera, seem to be obvious targets of suspicion as potential invaders. Many of the world’s worst invasive plants belong to relatively few families and genera: Asteraceae, Poaceae, Acacia, Mimosa, Cyperus. Both the starling and crow families have several invasive, or at least widely naturalized, species. But most biotic invad￾ers have few, if any, similarly aggressive relatives (water hyacinth, for instance, is the only Eichhornia that is inva￾sive). This fact could simply reflect a lack of opportuni￾ties for immigration rather than a lack of talent for inva￾sion. But the circumstantial evidence suggests otherwise: guilt by (taxonomic) association has proven imprecise at predicting invasive potential. Community Vulnerability to Invasion As stated above, attempts to predict relative com￾munity vulnerability to invasions have also prompted gen￾eralizations, including the following. • Vacant niches. Some communities such as tropical oceanic islands appear to be particularly vulnerable to invasions, although the evidence can be equivocal. The vacant niche hypothesis suggests that island com￾munities and some others are relatively impoverished in numbers of native species and thus cannot provide “biological resistance” to newcomers. In contrast, however, many would-be invaders arriving on islands would find no pollinators, symbionts, or other required associates among the native organisms, a factor that might provide island communities with a different form of resistance to invasion. Yet actual demonstration of vacant niches anywhere has proven difficult. • Escape from biotic constraints. Many immigrants arrive in new locales as seeds, spores, eggs, or some other resting stage without their native associates, including their usual competitors, predators, grazers and parasites. This “great escape” can translate into a powerful advantage for immigrants. All aspects of performance such as growth, longevity, and fitness can be much greater for species in new ranges. Ac￾cording to this hypothesis, an invader persists and proliferates not because it possesses a suite of ex￾traordinary traits but rather because it has fortuitously arrived in a new range without virulent or at least debilitating associates. For example, the Australian brushtail possum has become an invader in New Zealand since its introduction 150 years ago. In New Zealand it has fewer competitors for food and shel￾Figure 4 - Many invasive grasses have greatly expanded their world-wide ranges at the expense of native grasslands and forests, usually facilitated by human-induced land-clearing, recurring fire, and livestock grazing. On the island of Hawaii, Pennisetum setaceum (fountain grass) from northern Africa has replaced the native Metrosideros polymorpha woodland (see remnant trees in background). It resprouts readily after its litter is burned; native plants are much less tolerant and are eventually eliminated from these sites. Photo by Richard Mack