The ecological conditions that cause a given introduced species to become invasive are complex and rarely well understood. One of the problems in attempting to promote legislative barriers to the introduction of alien species, which continues at a high rate in the U.S., is that invasions seem to follow roughly what is known as the “Ten Percent Rule.” That is, about ten percent of introduced species become naturalized, and about ten percent of naturalized species become expensive and destructive pests.
When only one in a hundred species will be an economic problem, it is not easy to take political steps to prevent the importation of aliens in general, which most ecologists would prefer. If we knew which species were most likely to cause damage, we might have better success with a regulatory pathway, but unfortunately the problem remains that ecologists are not yet capable of predicting which particular species will get out of control (with a few exceptions – Burmese python in Florida was pretty much a no-brainer).
Our current policies are clearly not working to prevent new invasives, because the species importers (pet stores, commercial nurseries) currently get all the financial benefit of importing new species, without any of the societal cost for those that become invasive. Some argue sometimes that even if a species becomes a pest, the local ecosystem will just eventually adapt to its presence. It’s not really a great argument given that multiple native species affected by the invasive could go extinct over a much shorter time scale than evolution would normally act to curb it, but a recent paper could (unfortunately) give this idea some legs.
Some species become invasive because they have been released from their natural predators, making them much more prolific in the new habitat. Nils et al. (2009) found that cropping up are a few cases of native predators adapting to invasives, so there will be potential for control. There are several reasons why this might happen, but in most cases it is because predators are forced to use a less-preferred prey because the preferred one is decreasing in abundance (often due to negative interactions with the invasive).
Of course there is likely to be a lag in this process, but the authors find several hopeful examples, including turtles and Great Lakes whitefish eating zebra mussels; native snakes eating cane toads in Australia; and red rock crabs eating European green crab in North America.
The authors avoid the issue of “evolution taking care of aliens,” but instead comment on the intriguing notion that because some of the adaptive predators are harvested by humans, we have a new factor that we need to consider when defining a sustainable catch of those species. In fact, populations of alien species that have so far been kept in check by natives, without us even realizing it, could start to spread out of control if catch limits are set too high for that ecosystem function to be preserved. Up to this point, such food web considerations have not been made in setting harvest limits.
The trouble is, harvest limits usually have little basis in any ecological information. From the perspective of a resource economist that I know, this is probably partly because ecologists have not done a great job historically of quantifying ecological impacts (although partly due to invasive species impacts, economic measures of ecological issues have more commonly entered into ecological discussions). But when I asked if it would ever be realistic to take information on novel ecological interactions – such as presented in this paper – into account, he didn’t see any reason why not, if ecologists obtain reasonably quantified data that they could deliver to resource economists for use in their model.
It seems at this point (when it is almost too late anyway) that it will be a stretch for such a politicized issue as harvest limits to take into account the more subtle effects of food-web interactions, but community ecologists should certainly take note of Carlsson et al.’s findings, and start to attempt to quantify the ecosystem value of key predators (and the resulting economic value, when costs of damage and control of invasives are quantified) , starting with those that are preyed upon by humans. If they don’t, not only might native communities continue to collapse, but a slew of invasive species may become a whole lot more damaging than they already are. We shouldn’t be spending $10 on a crab that truly costs us thousands of dollars in the end.
Nils OL Carlsson, Orlando Sarnelle, David L Strayer, 2009. Native predators and exotic prey – an acquired taste? Frontiers in Ecology and the Environment 7(10):525-532