Defining an invasive species- not as straightforward as you would think.

We've all heard, at one point or another, that "invasive species are bad for the environment." In fact, many scientists cite invasive species and habitat degradation as the two greatest threats to global biodiversity. But how do we define an invasive species? Why are many non-native species ignored while others are treated like the plague?

It turns out that the definition of invasive is not as straightforward as you would think. Definitions vary broadly, and should be separated from "alien", "non-indigenous",“non-native”, "introduced", or “exotic” labels, as not all introduced species are considered invasive.

The federal government defines an invasive species as a non-native species that causes harm to the economy, to the environment, or to human health (ISAC 2006). Many ecologists have adapted a broader definition and consider a non-native species invasive if the population is reproducing and expanding geographically (Lockwood et al. 2013). These definitions have different implications, and this philosophical dichotomy can complicate things.

Take blue catfish, for example. The federal definition requires evidence of harm, either to the environment, to the economy, or to human health. Moreover, for a species to be considered invasive, the negative impacts associated with it must outweigh any benefit that it provides (ISAC 2006). Our diet study demonstrates that predation of native fish is rare, and only a small percentage of the population is actually big enough to eat other fish. Most of the population is eating vegetation (hydrilla, Brazilian waterweed, and common waterweed) and Asiatic clams. Blue catfish are eating blue crabs in brackish areas, but blue crabs are still found in fewer than 15% of stomachs (James River) and less than 5% of stomachs in the other rivers. On the other hand, blue catfish support a world-renowned trophy fishery in the James River near Richmond, a fishery that has produced several blue catfish over 100 lbs. This fishery supports several full time guides, fishing clubs, tournaments, and local tackle shops. In addition, commercial harvest of this species has increased steadily in recent years, and blue catfish now support a multi-million dollar fishery in Virginia. While the total economic value of the species has not been quantified, it appears to be substantial. It seems unlikely that the cost of blue catfish (blue crab and other native losses) outweighs the benefits that it provides, but we can't really be sure of this without credible estimates of population size. If this is the case, blue catfish would not be considered invasive under the federal definition.

Many ecologists; however, label a species as invasive if it is reproducing and expanding beyond its initial area(s) of introduction. Under this definition, blue catfish are undeniably invasive. Blue catfish were initially stocked in the James, York, and Rappahannock drainages, but they have since expanded to every major tributary of the Chesapeake Bay. They are reproducing, and populations exploded in the early 2000s. I researched blue catfish in their native range while completing my master's degree at Oklahoma State. Blue catfish were densely populated there, and we would catch up to 700 fish per hour using traditional methods (low frequency electrofishing). In Virginia, capture rates using the same gear can be as high as 6,000 fish/hr (Greenlee and Lim 2011). So blue catfish have reproduced and expanded at an alarming rate, making them invasive under the more liberal definition (Lockwood et al. 2013).

It's important to remember that freshwater portions of our tidal rivers are loaded with non-native species, and have been for a long time. In fact, most of Virginia’s freshwater gamefish are non-indigenous, including largemouth bass, smallmouth bass, rainbow trout, brown trout, muskellunge, and channel catfish (all of which are non-native throughout all or most of Virginia; Jenkins and Burkhead 1994). So why do blue catfish receive an “invasive” label? What separates them from all the other non-native fish that continue to be regulated and stocked in Virginia's waters? I think that the real reason for the hype is that they've expanded downriver into brackish areas, areas that have historically had fewer exotic fish species. The numeric density of the species, coupled with their ability to grow to large sizes, has worried fisheries managers, and rightfully so. The good news is that blue catfish populations peaked in the early 2000s, and growth rates and population sizes are declining. Furthermore, very few blue catfish actually get big enough to eat other fish, and their diet mostly consists of either invasive or highly abundant organisms.

While some feel that all non-natives should be "considered guilty until proven innocent" (Simberloff 2007) others insist that conclusions about non-native species be based on "quantifiable empirical evidence" and not speculation (Gozland 2008). As a scientist I agree with the latter. Jumping to conclusions without evidence to support your claims is incredibly irresponsible, and should be avoided. Unfortunately this has not been the case with blue catfish in the Chesapeake Bay. Numerous claims have been made about this species, yet a review of the scientific literature quickly reveals that many of these claims are unsubstantiated. We will continue to fill these knowledge gaps with our research.


Gozlan, R. E. 2008. Introduction of non‐native freshwater fish: is it all bad? Fish and Fisheries 9(1):106-115.

Greenlee, R. S., and C. N. Lim. 2011. Searching for equilibrium: population parameters and variable recruitment in introduced Blue Catfish populations in four Virginia tidal river systems. Pages 349-367 in P. H. Michaletz and V. H. Travnichek, editors. Conservation, Ecology, and Management of Catfish, the second international symposium. American Fisheries Society, Symposium 77, Bethesda, Maryland.

Jenkins, R. E., and N. M. Burkhead. 1994. Freshwater Fishes of Virginia. American Fisheries Society, Bethesda, Maryland.

(ISAC) Invasive Species Advisory Committee. 2006. Invasive species definition clarification and guidance white paper. National Invasive Species Council. US Department of Agriculture, National Agricultural Library. Washington, DC. Available at (Oct 2016).

Lockwood, J. L., Hoopes, M. F., and M. P. Marchetti. 2013. Invasion Ecology. John Wiley & Sons, Hoboken, New Jersey.

Moyle, P. B., and T. Light. 1996. Biological invasions of fresh water: empirical rules and assembly theory. Biological conservation, 78(1-2): 149-161.

Simberloff, D. 2007. Given the stakes, our modus operandi in dealing with invasive species should be “guilty until proven innocent”. Conservation Magazine 8:18–19.