Emergent ecological diseases are relatively recent phenomena involving extensive damage being caused to natural communities and ecosystems. In some cases, the specific causes of the ecological damage are known, but in others they are not yet understood.

Examples of relatively well-understood ecological diseases mostly involve cases in which introduced, non-native pathogens are causing extensive damage. There are, unfortunately, many examples of this kind of damage caused by invasive organisms. One case involves the introduced chestnut blight fungus (Endothia parasitica), which has virtually eliminated the once extremely abundant American chestnut (Castanea dentata) from the hardwood forests of eastern North America. A similar ongoing pandemic involves the Dutch elm disease fungus (Ceratocystis ulmi), which is removing white elm (Ulmus americana) and other native elms from North America. Some introduced insects are also causing important forest damage, including the effects of the balsam wooly adelgid (Adelges picea...