St. Joseph Island forms the northwesternmost point of the Mixedwood Plains ecozone that stretches through southern Ontario to the St. Lawrence shore to Quebec City. This makes the island relatively unique from the Ontario mainland across the North Channel as only it and Manitoulin Island are considered parts of this ecozone. The ecozone is characterized by a relatively mild and moist climate with rich, fertile soils capable of supporting mixed deciduous and evergreen forests and tolerant hardwood forests. This zone is home to fauna and flora that are among the most diverse in Canada.