Exotic species are the plants, animals, and microbes new to a continent such as North America. In one of my current research projects, several students and I are investigating invasions of exotic species into freshwater habitats and their impacts on the native communities. We would like to better understand the factors that facilitate dispersal and subsequent establishment of new populations. Specifically, we are examining the ecology of the cladoceran Daphnia Iumboltzi, a species native to Africa, Asia, and Australia, which has recently invaded North America. Following its discovery in 989, D. lumboltzi has quickly spread into numerous reservoirs throughout the southern United States.

My students and I are currently examining pattern through a detailed survey of zooplankton from 160 reservoirs in the Midwest and the population dynamics of algae and zooplankton in Stockton Lake, Missouri. Zooplankton are small animals, such as cladocerans and copepods, that eat algae and serve as food for fish. We have also begun some preliminary experiments to investigate the processes of competition and predation.

Our results to date indicate that D. lumboltzi has invaded 16% of the reservoirs, representing a wide range of water quality characteristics. We have found this species thriving in waters that are clear and turbid, soft and hard, and productive and nonproductive. The prevalence of this exotic in midwestern reservoirs has increased in each of the past three years and populations have persisted from year to year. In some sites, this exotic has become the dominant species in the zooplankton, with numbers greater than all other species combined. We are currently doing detailed studies to determine if the composition of other zooplankton has changed following invasion by D. lumboltzi.

We are also examining samples from Stockton Lake to determine the relationship between composition of the algae (corresponding to food quality), D. lumboltzi abundance, and birthrates of other zooplankton. Negative correlations between abundance and birthrates would be consistent with competition occurring between the species. A preliminary competition experiment, run in the laboratory using natural food (filtered reservoir water), suggested that D. lumboltzi populations could persist longer than the native Daphnia galeata. We plan to run additional experiments under more natural conditions, by suspending competition chambers at different depths in the lake.

In the near future, we also plan to investigate the diet of planktivorous fishes. Because it is very spiny, we believe that many of these fishes will avoid eating D. lumboltzi, and instead consume native species. If so, predation could facilitate the spread of this exotic in freshwater communities.