"Management and control of [non-native] species is perhaps the biggest challenge that conservation biologists will face in the next few decades," says Fred Allendorf of the University of Montana in Missoula, who edited a six-paper special section called "Population Biology of Invasive Species" in the February issue of Conservation Biology.
Key points in the special section include:
- Arguments that species invasions are natural and so acceptable are false. While it is true that invasions have occurred throughout evolutionary history, people have greatly accelerated the rate of introductions so that there are far more invasions today than there were only a few hundred years ago. Similarly, arguments that new species will evolve to replace those lost to invasions are also flawed. Even if this is true – and that's a big if -- it would take millions of years. This work is by David Lodge and Kristin Shrader-Frechette of the University of Notre Dame in Indiana.
- It's hard to get rid of invasive species that benefit some people economically. For instance, the non-native brown trout has had huge effects on the freshwater ecosystems of New Zealand's South Island. The widespread brown trout eat five times as many mayfly larvae and other stream invertebrates, increasing the algae level six times and decreasing the nutrient levels in the water. Moreover, streams with brown trout often do not have native fish. But there is little support for eradicating the brown trout because the sport fishery is so popular, generating more than $300 million per year. On the positive side, the story of the brown trout helped the New Zealand government decide that the risks of importing the channel catfish for aquaculture were too great. This work is by Colin Townsend of the University of Otago in New Zealand.
- Invasions often owe their success to fundamental biological differences between introduced populations and the original populations in their native range. A prime example is Argentine ants, which are now found on six continents. In their native range Argentine ant colonies attack outsiders, which are distinguished via chemical cues based on genetic differences. But the introduced colonies generally don't attack each other because they can't distinguish each other -- they are quite similar genetically because they descended from a small number of introduced ants. The result is that the introduced colonies cooperate with each other, forming "supercolonies" that outnumber and wipe out native ants. "A single supercolony occupies virtually all [of the Argentine ant's] California range," say Neil Tsutsui of the University of California at Davis and Andrew Suarez of the University of California at Berkeley.
- While only a small percentage of introduced species actually become invasive, it's hard to predict which ones will. One of the reasons is that some species rapidly adjust to or evolve in new environments. "With the introduction of non-native species, evolution is our largest source of uncertainty and therefore our largest source of anxiety," say Ingrid Parker, Joseph Rodriguez and Michael Loik of the University of California at Santa Cruz. "We lose the ability to predict where an introduced species will end up and what sort of impact it will have."
- Biocontrol should be a last resort because it can do more harm than good. The problem is that the natural enemies introduced to control invasive non-native species can also harm native species. And more often than not, biocontrol doesn't even work. One reason is that the tests to predict the effectiveness and specificity of proposed biocontrol agents are often inadequate, which can lead to surprises when they are released in the field. For instance, a European seed-eating weevil was predicted to prefer the invasive Canada thistle but ended up preferring the native Tracy's thistle in Colorado. There are few, if any, options for addressing biocontrol gone amok. "'Recall' of problematic species from ecosystems where they are damaging native species is either impossible or prohibitively expensive," say Svata Louda and three colleagues at the University of Nebraska in Lincoln.
- From a conservation standpoint, the best approach to invasive species is a combination of a no-tolerance policy for non-native species and immediate eradication of any that are found. "Shoot first, ask questions later," says Daniel Simberloff of the University of Tennessee at Knoxville. All too often, studies to determine the best way of controlling an invasive species take such a long time that the species spreads significantly, making it much harder and more expensive to control. Simberloff argues for using whatever methods are currently available, adopting any new methods as they are found. This "brute force" approach has controlled plenty of invasions effectively: for instance, rats, goats, and other introduced mammals were eradicated from Mexican islands with traps, hunting dogs and rifles; and witchweed, an African root parasite, is controlled in the Carolinas with herbicides and a rigorous quarantine on anything that could carry soil out of infested areas.
Fred Allendorf (406-243-5503, firstname.lastname@example.org)
David Lodge (email@example.com)
Colin Townsend (firstname.lastname@example.org)
Neil Tsutsui (email@example.com)
Ingrid Parker (firstname.lastname@example.org)
Svata Louda (email@example.com)
Daniel Simberloff (firstname.lastname@example.org)
PDFs of these papers are available to the media; contact Robin Meadows email@example.com