Define “beneficial”: Even good invasive species have downsides

When we discuss invasive Asian carp, we’re usually just talking about a few specific species of carp, the silver, black, and the bighead. This is with good reason—in some parts of the Mississippi and Illinois Rivers those fish now make up 97 percent of the total biomass, with no predators around to hold them back. The grass carp, meanwhile, though found in all of the Great Lakes and plenty of other places, was actually introduced—theoretically, this was done quite carefully—and has not been considered a nuisance species at all. But new research suggests that even those invasives we think of as beneficial, often aren’t.

According to work published in the Canadian Journal of Fisheries and Aquatic Sciences, the grass carp was introduced to U.S. waters in the 1960s “as a biocontrol control agent for nuisance aquatic macrophytes.” So: a weed a killer. Federal and state agencies got on board, and by the 1970s the grass carp was in lakes and ponds in 16 states. They even tried to ensure that no breeding populations of the fish could establish themselves, by introducing “monosex” and triploid versions that were theoretically incapable of reproducing.

So how did it go? Well, the grass carp do indeed seem to help out with those macrophytes, but a meta-analysis of a number of studies showed that there were also some negative effects on birds like the hooded merganser and ruddy duck, as well as on some populations of largemouth bass, bluegill, and other fishes. To be clear, there were also some positive effects seen in bird and fish species, but that’s sort of the point here: when you introduce a non-native animal to do one thing, it will end up doing others. It is never clear if those other things will always be positive or negative, but of course if you’re changing an ecosystem at all from its natural state one could argue all effects are negative.

Also, the researchers found instances of diploid grass carp, meaning fish that actually could reproduce. It isn’t quite female dinosaurs spontaneously changing into males, but Jurassic Park’sLife finds a way” edict seems appropriate. The researchers, led by Marion Wittmann of the University of Notre Dame in Indiana, suggest several routes that this particular bit of life might have found to avoid our imposed sterility, such as simple contamination of the original introduced samples with diploid as well as triploid fish.

Grass carp are now present in all of the Great Lakes, and given the presence of diploid individuals it is now feasible in terms of environmental suitability that populations could spring up all over the region. What exactly this would do to other species of plant and animal is becoming mildly more clear with studies like this one, but again, the magnitude and direction of impacts is never clear until they happen. In spite of all our increasing knowledge on the dangers of actively putting an animal where it shouldn’t be, the authors write that “freshwater fish introductions continue to increase globally. Thus, the need remains to iteratively evaluate and update ecological risk assessments to maximize the benefits of introduced species while reducing unwanted occurrences and impacts.” In other words: even if your first look suggests good things would happen, you should probably check again, and again, and again, when it comes to invasive species. – Dave Levitan | July 1 2014

Source: Wittman ME, Jerde CL, Howeth JG, et al (2014). Grass carp in the Great Lakes region: establishment potential, expert perceptions, and re-evaluation of experimental evidence of ecological impact, Canadian Journal of Fisheries and Aquatic Sciences, 71 (7) 992-999. DOI: 10.1139/cjfas-2013-0537

Image:, grafvision