Likely the second biggest threat to the Don is non-native invasive species, especially plants and trees. These are aggressive species that literally take over an area where nothing else grows. They love to colonize disturbed sites which is pretty much the state of the entire lower Don. Species that are a big threat are Japanese Knotweed, Dog-strangling Vine, Garlic Mustard, and Canada Thistle (actually a native of Europe).
I have just found a weblog devoted to invasive species. Entitled Invasive Species Weblog, it gathers articles from all over the world about invading plants, fish, crustaceans, and other animals. The blog is written by Dr. Jennifer Forman Orth. Each entry is chock-a-block full of information and links to other related websites. If your interested in this type of information then be sure to bookmark this blog because it is top-notch stuff!
Recent posts have included an article on how native Lake Erie water snakes are making a comeback by feasting on invasive Round Gobys; an article from New Zealand about an invasive algae from North America called Rock Snot (Didymosphenia geminata); and recently an article from South Africa about an initiative to make coffins out of the wood from invasive trees.
P.S. I said the 2nd biggest threat. So what's the biggest threat? It has to be stormwater runoff, or is it dealing with the city bureaucracy? Perhaps a list of the top five threats to the Don will be the subject of a future posting...