I actually did a study that looked at the main threats to rare species, and I think that study is a microcosm of what's actually impacting the ecosystem as a whole. Habitat degradation, I think, would be considered the greatest threat, which would include things like loss of wetlands, modification of shorelines, and so on. The second threat in our study was invasive species. They have various effects, from impacting native species through to having socio-economic impacts. We've all heard of children getting their feet cut on zebra mussel shells, for example.
It really drops off after that because those are really the two main risks. In the past it would have been overfishing, which at this point is not a major issue in the Great Lakes and hasn't been for quite a while. A century ago, some of the greatest losses in the Great Lakes were related to overfishing, but it's no longer a major issue. It's the same with contaminants. They're no longer a major issue.