The invasion ecology of mammals: a global perspective

Top predators often have powerful direct effects on prey populations, but whether these direct effects propagate to the base of terrestrial food webs is debated. There are few examples of trophic cascades strong enough to alter the abundance and composition of entire plant communities. We show that the introduction of arctic foxes (Alopex lagopus) to the Aleutian archipelago induced strong shifts in plant productivity and community structure via a previously unknown pathway. By preying on seabirds, foxes reduced nutrient transport from ocean to land, affecting soil fertility and transforming grasslands to dwarf shrub/forb-dominated ecosystems.

Cases in which introduced species facilitate one another's establishment, spread, and impacts are increasingly noted, and several experimental studies have provided strong evidence of a population-level impact. However, a full 'invasional meltdown', in which interspecific facilitation leads to an accelerating increase in the number of introduced species and their impact, has yet to be conclusively demonstrated. The great majority of suggested instances of 'invasional meltdown' remain simply plausible scenarios of long-term consequences based on short-term observations of facilitatory interactions between individuals of two species. There is a particular dearth of proven instances in which two invasive species each enhance the impact and/or probability of establishment and spread of the other. By contrast, in many authenticated cases, at least one partner is aided. The metaphor of meltdown focused attention on facilitation in invasion and has probably helped inspire recent studies. As have other metaphors from invasion biology and other sciences, 'meltdown' has struck a responsive chord with writers for the lay public; some have stretched it well beyond its meaning as understood by invasion biologists. There is no evidence that this hyperbole has impeded scientific understanding or caused loss of scientific credibility.