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| Earth on Aug 30, 2012

Are we experiencing an invasional meltdown?

The ‘invasional meltdown’ theory says the establishment of one invasive species in a new environment makes it easier for other non-native species to invade.

For a long time, scientists having been trying to predict why some invasive species survive and thrive in their new environments while others die out. In a new study published on August 22, 2012 in the journal NeoBiota, scientists examined six popular hypotheses regarding biological invasions and found that the concept of invasional meltdown has held up well during experimental tests conducted across different taxonomic groups of exotic species and habitats.

The term ‘invasional meltdown’ was first proposed by Daniel Simberloff and Betsy Von Holle in 1999 (pdf) to describe the process whereby the establishment of one type of invasive species in a new environment can facilitate the invasion of other non-native species.

For example, when zebra mussels (Dreissena polymorpha) invaded the Great Lakes during the mid-1980s, their voracious appetites for phytoplankton improved water clarity and the penetration of sunlight into deeper waters of the lakes. The extra sunlight, in turn, helped to facilitate the invasion of the Great Lakes by exotic Eurasian watermilfoil plants.

Boat propeller with zebra mussels. Photo credit: TownePost Network

Another example of invasional meltdown occurred in the western United States when livestock such as cattle and sheep were introduced into the region. Grazing and trampling of native grasses by the livestock is thought to have helped to facilitate the invasion of the area by exotic cheatgrass (Bromus tectorum).

In a new study published on August 22, 2012 in NeoBiota, scientists examined the scientific literature to determine if the invasional meltdown hypothesis and other popular hypotheses about biological invasions were supported or refuted by experimental tests carried out scientists. They found that the invasional meltdown hypothesis had the highest level of support among the six hypothesis examined.

Eurasian watermilfoil. Image Credit: Washington State Department of Ecology.

The scientists uncovered 30 studies that explicitly tested the concept of invasional meltdown, and 77% of those experimental tests found evidence in support of the hypothesis. High levels of experimental support amounting to 54% were also found for the enemy release hypothesis – the idea that invasive species thrive in new environments because those environments do not contain enemies such as predators and parasites that can keep population levels of the invasive species in check. The novel weapons hypothesis – the idea that invasive species carry novel traits into their new environments that give them a competitive advantage – was supported by 74% of experimental studies.

Low levels of experimental support were found for hypotheses that state that ecosystems with high biodiversity are more resistant to biological invasions then ecosystems with low biodiversity.

Jonathan Jeschke, lead author of the new paper, is a German evolutionary ecologist. His co-authors included Lorena Gómez Aparicio, Sylvia Haider, Tina Heger, Christopher Lortie, Petr Pyšek and David Strayer. Their research was inspired in part by discussions during a March 2010 workshop titled “Tackling the emerging crisis of invasion biology: How can ecological theory, experiments, and field studies be combined to achieve major progress?”

Bottom line: Scientists examined six popular hypotheses regarding biological invasions in a paper published on August 22, 2012 in the journal NeoBiota. They found that the concept of invasional meltdown – the process whereby the establishment of one type of invasive species in a new environment can facilitate the invasion of other non-native species – has held up well during experimental tests conducted across different taxonomic groups of exotic species and habitats. Low levels of experimental support were found for hypotheses that state that ecosystems with low biodiversity are more susceptible to biological invasions

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