Classics: Invasion Meltdown

26 10 2008

One for the Classics page…

melting_rat_by_xenatalhaoui-d71xr1yDaniel Simberloff is probably best known for his work on the implications of invasive (non-indigenous) species for biodiversity, although he has contributed to a wide range of conservation disciplines.

A seminal paper that he co-wrote with Betsy Von Holle is one I consider to be a conservation Classic: their 1999 paper in the inaugural issue of Biological Invasions entitled Positive interactions of nonindigenous species: Invasional meltdown?

The establishment of non-indigenous species can have severe negative impacts on ecosystems. Introduced species that become invasive (widespread and locally dominant) transform habitats, degrade ecosystem services, reduce biodiversity and are some of the greatest threats to ecosystems today (perhaps nearly as important as habitat loss and over-exploitation).

The so-called ‘invasion meltdown‘ describes the process by which the negative impacts induced on native ecosystems by one invading non-indigenous species are exacerbated by interactions with another exotic species.

Although there isn’t a lot of information on invasion meltdowns, one good example comes from Christmas Island in tropical Australia. The introduced yellow crazy ant (Anoplolepis gracilipes) exploded in numbers when another exotic species, a scale insect, was introduced about the same time that a native scale insect species also had a local outbreak.  Because ants protect scale insects from predators and parasites in return for scale honeydew, the crazy ant suddenly had a much more abundant food source, leading to the huge increase in the ant population. This large ant population devastated the population of native red crab (Gecarcoidea natalis) and resulted in massive increase in forest undergrowth due to reduced herbivory by crabs (see O’Dowd et al. 2003). The great decline in red crabs may also make the island more vulnerable to other plant invasions.

What did Simberloff & Van Holle’s idea and subsequent examples of invasion meltdowns teach us? I believe their paper really hit home the idea that invasive species were not only a threat to biodiversity, but the self-reinforcing mutualisms of invasive species could rival other forms of human-induced biodiversity decline. Indeed, many of the effects of invasive species will be reinforced by global climate change through increasing temperatures, rising sea levels and changing rainfall patterns that increase the potential range and spread of invading species, so the problem is only going to get worse. This is why the U.N. began the Global Invasive Species Programme (GISP), and world-wide, countries are attempting to restrict the flow of invasive species so that their negative effects are lessened. Identifying the extent of the problem has stimulated a lot of people to act accordingly.

CJA Bradshaw

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