Illinois Natural History Survey - University of Illinois

Great Lakes Invasion

Why so many invaders in the Great Lakes? About 140 plant and animal invaders have become established in the Great Lakes since the mid-1800s. Why have so many species successfully invaded these lakes? Biological invasions are not predictable but we have learned about what conditions make it more likely that invasive species may become established in ecosystems. These conditions include historical isolation of native species, high levels of human disturbance, easy invasion routes, and disruption of the native community. The Great Lakes were isolated from many source populations during the last major glaciation about 10,000 years ago and so do not contain a diverse set of native species. Human disturbance has increased since European settlement. It has included clear cutting and farming practices that increased sedimentation and turbidity, industrial pollution, urbanization, intensive commercial fishing, and the intentional introduction of certain species. The Great Lakes also provide easy man-made access for invaders through canal systems linking the lakes with both the Atlantic and Mississippi drainages and through extensive worldwide shipping traffic. Native communities have been disrupted almost continuously since the mid-1900s by invading sea lamprey, alewife, and zebra mussel, resulting in the extirpation or extinction of many native species. This disruption has led to additional invasions that may not yet have run their course. As a result, the Great Lakes is one of the world's most easily invaded ecosystems, a status unlikely to change in the near future.

There are so many introduced species in the Great Lakes that some biologists argue that it is now a man-made aquaculture system. For example, sport fisheries in the Great Lakes depend on salmon and trout (Family Salmonidae) that are not native to the lakes but are artificially propagated in hatcheries (coho, Oncorhynchus kisutch; chinook, Oncorhynchus tshawytscha; rainbow or steelhead, Oncorhynchus mykiss; brown trout, Salmo trutta). The salmonids were introduced originally to control alewives (Alosa pseudoharengus), which entered the Great Lakes after the Welland Canal was completed in 1829. The canal circumvented Niagara Falls, a natural barrier to migration of fishes from the St. Lawrence River and Lake Ontario into the other four Great Lakes. The alewives did not become a problem until the 1960s, when populations exploded, then died in huge numbers, fouling swimming beaches and clogging water intakes. Previously, alewives apparently were kept in check by a predator native to the Great Lakes, the lake trout (Salvelinus namaycush), and perhaps by nine species of native whitefishes (genus Coregonus) that competed with alewives for food (zooplankton).


Unfortunately the lake trout and the whitefishes were decimated a decade earlier (1950s) by yet another invading species, the predatory sea lamprey (Petromyzon marinus), which attaches to large fishes with a sucker mouth armed with teeth and then consumes flesh and fluids from its prey. The sea lampreys do not attack small fishes, such as alewives, which initially were freed from predation and competition. Although the lampreys attack the introduced trout and salmon, the parasites are now kept in check by poisoning their larvae in the streams and rivers that run into the Great Lakes, or by using electric barriers to keep adults out of their spawning streams. The larvicide is highly toxic to the sea lamprey and less toxic to other aquatic species and its use is limited to the reaches of streams where lampreys are known to spawn. The research and control program is costly (estimated at $10 million annually by the President's Office of Technology Assessment in 1992), but is considered necessary because the direct and indirect effects of losing the fisheries could exceed $500 million annually.

The story of invasive species in the Great Lakes illustrates several important principles. First, invasive species can change self-maintaining ecosystems into ecosystems that have to be maintained by humans. In the Great Lakes, the supply of species highly valued by humans (sport and commercial fishes) previously provided free by the ecosystem now has to be maintained by hatcheries and lamprey control programs. Other goods, such as water for industrial or municipal use, can become more costly because intakes and pipes have to be treated to remove fouling organisms, such as zebra mussels. Second, there are uncertainties and risks associated with control programs. For example, species introduced to control other introduced species (e.g., chinook salmon to control alewives) can indirectly cause other problems. In this case, chinook salmon are affected by bacterial kidney disease (BKD), which sometimes reaches levels that kill these important stocked predators. Concern is also growing that chinook salmon may have increased the prevalence of BKD to the point that it could affect native species of the genus Coregonus. Third, the increasing number of invaders can have progressive impacts on native species. Native yellow perch (Perca fulvescens) persisted in the face of competition and predation from alewife but have declined dramatically since 1988 and may not have been able to withstand the additional pressure placed on their food resources by two recent invaders, the zebra mussel and spiny water flea (Bythotrephes cederstroemi). These principles demonstrate that costs of invasive species are real both in ecological and economic terms and demonstrate the importance of research conducted by Survey scientists to understand how we can prevent additional invasions, control invaders already here, and prevent their spread.

John Dettmers, Center for Aquatic Ecology

Illinois Natural History Survey

1816 South Oak Street, MC 652
Champaign, IL 61820

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