However, if a site can be kept clear of gulls until the terns are ready to nest, there is no evidence that interference from gulls reduces their reproductive success. Flooding, vegetational succession and earlier nesting by gulls have likely affected Common and Caspian terns throughout their evolutionary history. They will move considerable distances between nesting attempts, and as long as other nesting areas were available, colony extinctions would have a minor effect on the population. In July 1994 the governments of Canada and Ontario signed a major agreement detailing their cooperative activities to restore, protect and conserve the Great Lakes basin ecosystem. The focus of the first plan is the coastal wetlands of the lower Great Lakes.
Habitat loss is perhaps an even more important issue for marsh-nesting terns.
Development pressures have resulted in the loss of 80 per cent of the marshes that once existed along the shores of Lakes Ontario and Erie. Artificial control of Great Lakes water levels for shipping and to prevent shoreline erosion has caused a reduction in the size of the remaining marshes, which gradually fill in with vegetation when water levels are not allowed to fluctuate. Many marshes have been invaded by dense stands of a single species such as cattails or purple loosestrife, changing the mix of open water, live emergent vegetation and floating dead vegetation these birds need. Artificial control of lake levels has been shown by many researchers to contribute to this process. Lake levels have been relatively high over the past decade and this can cause additional problems for marsh-nesting terns.