Two's company? Invasional meltdown as a template for research and management

Background/Question/Methods

Since the term “invasional meltdown” was proposed in 1999 and defined as “the process by which a group of nonindigenous species facilitate one another’s invasion in various ways, increasing the likelihood of survival and/or of ecological impact, and possibly the magnitude of impact,” it has been used hundreds of times, and the phenomenon is viewed as a leading threat to native biodiversity and major potential impact of climate change. However, many references simply use the term as an exploded metaphor for a situation with many introduced invasives or at least one such species that has ecosystem-wide impacts.  Others have redefined the term in strict analogy with the concept of nuclear meltdown,” in which an initial event triggers series of subsequent events, which trigger yet further events, etc.  This welter of uses of the term confounds an assessment of the importance of the phenomenon.

Results/Conclusions

As in many other areas of ecology, in invasion biology research on facilitation long took a back seat to study of negative interactions, such as competition and predation.  However, even in the early days of modern invasion biology, certain facilitative interactions were observed, such as those involving introduced plants and introduced mycorrhizal fungi, pollinators, or seed dispersers. An increasing number of such pairwise interactions have been recorded, although in most cases the population and/or community consequences have not been proven (as is also true for other interactions, such as competition and predation).  Other, subtle but nonetheless consequential facilitations are increasingly reported, such as the influence of introduced nitrogen-fixing plants on subsequent invasion by other species; similar effects are reported for introduced plants that concentrate phosphorus.  All of these examples accord with the original definition, although quantification of impact and confirmation that increased survival constitutes a population response often is still lacking.  An important focus for future research is indeed quantification of population- and community-level impacts of facilitation (again, just as with negative interactions such as predation).  Management implications of certain documented facilitations are already evident, such as the added impetus for quick action on plants such as nitrogen-fixers that are often implicated in facilitating other invasions or exacerbating their impacts.