Biology in the News Explained

The food on your table might create more invasive species

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.

Reference

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

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6 Responses to “The food on your table might create more invasive species”

  1. Nice post – thanks. I’d think that well-managed populations would be near their maximum levels anyway, so there’d be little change in management just from the hope that the species would adapt to a new food source. Maybe it’s more of a matter of catch management for recovering populations that gets problematic.

    So do you think that climate change will open up new niches for damaging invasion by non-natives? Seems like a non-native already adapted to a different environment could better exploit a new niche than natives might.

  2. biotunes says:

    Are there any “well managed” populations of predators that humans like to eat? Really we’re talking about marine species here, crabs, fish, etc. I agree that no one would consider affecting management for species that aren’t known to eat invasives, but a fair number of harvested species are known to now, and we should be taking it into account.

    Absolutely climate change is an issue. I think you hit the nail right on the head, that invasives are by their very nature more likely to come out on top when habitats change due to climate change.

    This is happening now in several well documented cases. A big one in my part of the country is mountain pine beetle, which is native and historically had occasionally large outbreaks, which used to only last 1-2 years. Now the warmer winters are not killing them off, and their spread is like a slow wildfire. I did a post on them:
    http://bioblog.biotunes.org/bioblog/2008/02/02/climate-change-fire-suppression-ecological-disaster/

    Cheers, and thanks for reading!

  3. Georg Felis says:

    Simply looking at nature in the long term would show that invasive species eventually reach a peak, and go back to being just “species”. Otherwise, the entire earth would have huge sections of monocultures of critters as some odd thousand years ago one single preggers female wandered into an area it had never been seen before.

    Personally I kind of like a few select invasive species here in Kansas.
    http://tcslacerta.tripod.com/tcsphotos/id22.html

    Italian Wall Lizards, one of only 2 known colonies in the States. Don’t know how this winter will affect them, its very cold here.

  4. biotunes says:

    Georg, this is not quite right, because the key difference between natural dispersal and human-mediated dispersal is the scale at which it happens. It is especially clear in regions with unique biotas such as Hawaii, California, and Florida that the native species in these regions are being completely overwhelmed by invaders and many extinctions of endemic species are a result. It is also clear that these unique biotas would not have ever existed in the first place if what you say is true. There is a huge difference to an ecosystem whether or not new species are appearing on a “human” timescale or a geologic timescale. Without humans, a new species might reach Hawaii at most on the scale of years or decades. Right now, there are new ones coming in every week, transforming the ecosystems almost as we watch. This is why some scientists believe that the current mass extinction (we are very much in the middle of one) has the potential to dwarf previous ones.

    “Alien” doesn’t necessarily mean “invasive,” and no one ever said a species can’t be beautiful to the human eye just because it has been moved to a new place. This is where the “ten percent” rule comes in. I’m a bit of a purist in the sense that I believe that aliens can be affecting ecosystems deeply in ways we just have not studied yet, and so all establishments are negative to at least some degree. I look at your lizards from the point of view of an entomologist – how are they affecting local insect diversity? We likely have no idea, but just because we haven’t looked doesn’t mean it’s not happening. We aren’t even doing that much to prevent or deal with the invasions that are obviously destructive. My point here is just that we should start thinking about these issues more, even if all we care about is the human-centric economy. Zebra mussels, for example, are a massive drain on the economy.

  5. I read that when some genius introduced 100 starlings to NY Central Park, they weren’t very successful for the first six years, and then expanded rapidly to today’s 200 million.

    Maybe any single isolated colony of a non-native species is just a genetic mutation or two away from becoming a significant menace. The wild parrots here in the San Francisco area are pretty cool, but I’d capture them if it were up to me. The African clawed frogs, not cool at all.

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