Invasive species may be less of a threat when they’re abundant

Posted on March 23, 2015

Invasions of aquatic habitats by exotic species are often dramatic, involving rapid extensions of geographic range and increases in abundance by the invaders. Although such events encourage the belief that introduced species have strong negative competitive and predatory impacts on their native counterparts, this assumption may not always be true. The results of an experiment on interactions between an introduced fish (round goby, Neogobius melanostomus) and three native fish species (creek chub, white sucker, Johnny darter) in Wisconsin, U.S.A. suggest that a high abundance of invasives doesn’t necessarily generate a high impact on natives. In the study, 18 1.5 x 1.5 m enclosures were stocked with gobies and the native species. Gobies were kept at three different densities, while the densities of natives remained constant. At the end of the 52-day experiment, each enclosure was sampled for invertebrate prey. Comparisons of fish weights at the start and end of the experiment showed that the enclosures where gobies were stocked at low density supported the fastest growth rates of gobies, but the slowest growth rates of natives. It seemed that when goby densities were high, within-species competition limited the ability of gobies to exert a negative impact on the native species. Because invertebrate abundance and the amount of food in the guts of native species were unaffected by goby density, interference competition rather than resource competition was probably at work. Although more research is needed to unravel density-impact relationships, this study questions the common assumption that impacts always intensify with the density of the invading species.

Reference: Kornis, M.S. et al. 2014. Experimental evidence that ecological effects of an invasive fish are reduced at high densities. Oecologia 175, 325–334.