Thousand Islands – Frontenac Arch is one of four ecoregions of the Mixedwood Plains. The Thousand Islands in the Saint Lawrence River have a distinctive flora and fauna and are a part of the biological corridor. The bedrock is mostly gneiss, granite and marble, more than a billion years old. The Frontenac terrain is the youngest of the Ontario portions of the arch, dated at 1.2 billion years old. These rocks were once part of a large mountain chain of the ancient supercontinent of Rodinia. There is also a meteor crater near Holleford, where the shield meets the limestone plains of southeastern Ontario. The area
North American river otter component of their diets. They are less likely to be prey for the North American river otters since they are fast-swimming and can find good escape cover. However, river otters will prey on trout, pike, walleye ("Sander vitreus vitreus"), salmon ("Oncorhynchus" spp.), and other game fish during spawning. Adult North American river otters are capable of consuming of fish per day. A study conducted on captive otters revealed they preferred larger fish, ranging from , more than smaller fish, ranging from , and they had difficulty catching fish species less than or larger than . Otters are known to take