The Arch connects the Canadian Shield and the boreal forest to the forests of the Adirondack and Appalachian Mountains - a south to north/north to south migration route. The river valley is a route from the Great Lakes forest heartland of the continent to the forests of the Atlantic Coast. The Thousand Islands are at the very centre of that intersection. Here, the five great forest regions of the eastern continent meet and intermingle, with many species at range limits, and with many as remnant populations from forests altered by millennia of climate change and evolving landscapes. This region has, arguably, Canada's greatest diversity of plant and animal communities.