Title: Invasions: the trail behind, the path ahead, and a test of a disturbing idea
[Conservation Magazine 17 Jan 2012 by Daniel Strain]Jedi knights famously feel disturbances in the Force. Invasive species, on the other hand, react to disturbances to natural communities, often exploding following a destructive wildfire or flood. Or, at least, that’s the common thinking among ecologists. A new study challenges that maxim, however, showing that how the frequency of fires or storms changes over time may be the bigger driver of biological invasions. Biologists have long known that when a habitat gets shaken up–say a forest fire burns through a patch of pine trees–foreign species frequently pop up. Often, these disturbances open up space in a crowded ecosystem, giving invasive plants, such as California’s famous eucalyptus trees, access to sunlight. But researchers led by Angela Moles at the University of New South Whales in Sydney wondered if how fires or floods ebb and flow might be important, too. Plants used to lots of fire, for instance, might fare poorly if those conflagrations drop off, opening the door to invasives.
To test that theory, Moles and colleagues explored green plots across 200 locales from Argentina to Uganda. They carefully tallied the abundances of native and non-native plants in those squares. Then, the team turned to historical data to examine how disturbances, from fires to floods and even grazing pressures from herbivores, had shifted over time.
Change is key, the group reports in the Journal of Ecology. The present levels of forest fires or floods hitting a habitat did seem to encourage the spread of invasive species. But how the intensities or frequencies of fires or floods had fluctuated was nearly twice as important. Grazing, however, didn’t show the same effect.
Moles and colleagues then dug into eight sites across Australia and the United States in detail. Contrary to common thinking, in three of those sites, disasters actually slowed, not sped, the infiltration of foreign species.
Overall, disturbances may not be the biggest force controlling biological invasions, the researchers conclude. In fact, disturbance impacts (both current rates of disturbance and changes in the rate of disturbance) may be relatively paltry – accounting for just about 14% of the differences between habitats in domination by invasive species. Forest fires, then, may not be a tool of the Dark Side, after all. –
