Kirsten A. Nelson, Scott F. Collins, Greg G. Sass and David H. Wahl
Biological invasions are an important cause of environmental change across the globe. Ecologists have long recognized that native and invasive species can greatly affect the structure of food webs. In some cases, one species can alter the food web in ways that have positive effects on another species, even those considered to be competitors. In this respect, native or invasive species can potentially facilitate one another.
Understanding the circumstances that impede or promote the success of an invasive species is of critical importance to ecologists. We conducted a series of experiments to examine how bluegill sunfish (Lepomis macrochirus), bighead carp (Hypophthalmichthys nobilis), and common carp (Cyprinus carpio) altered densities of aquatic invertebrates (zooplankton, rotifers, midge larvae), and in turn, how these changes influenced the positive or negative interactions between the three fish species. In the Mississippi River Basin of North America, bluegill are native, whereas common carp and bighead carp are invasive aliens. Carp and bluegill belong to families that are only distantly related.
In one experiment, we found that competition between bighead and common carp was not as strong as competition within species. These within-species interactions had a stronger effect on growth than the between-species interactions because prey resources were partitioned between the two invasive species. In another experiment, bighead carp strongly reduced the growth of native bluegill, whereas the reciprocal interaction resulted in enhanced growth of bighead carp. Interestingly, when facilitation occurred between these species, it was only at low densities, suggesting that enhanced growth was constrained by food productivity. The lack of enhanced growth of bighead carp at higher densities suggests that the mechanisms driving facilitation at low densities have a diminished effect as fish numbers increase. By enhancing the growth of low densities of invasive fishes, the duration of vulnerability of the invaders to larger predators should be shortened, which may increase the likelihood of survival in some settings. This would certainly be advantageous for exotic species in newly invaded ecosystems.
Image provided by authors.