Emergent ecological diseases

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 ) on Fraser fir (Abies fraseri ) in the Appalachian Mountains.

Other cases of ecological diseases involve widespread damages that are well-documented, but the causes of which are not yet understood. One of them affects native forests of Hawaii dominated by the tree ohia (Metrosideros polymorpha ). For some unknown reason, stands of ohia decline and then die when they reach maturity. This may be caused by the synchronous senescence of a cohort of trees that established following a stand-replacing disturbance, such as a lava flow, and then reached maximum longevity at about the same time. Other causal factors have, however, also been suggested, including nutrient dysfunction and pathogens.

Another case is known as birch decline, which occurred over great regions of the northeastern United States and eastern Canada from the 1930s to the 1950s. The disease affected yellow birch (Betula alleghaniensis ), paper birch (B. papyrifera ), and grey birch (B. populifolia ), which suffered mortality over a huge area. The specific cause of this extensive forest damage was never determined, but it could have involved the effects of freezing ground conditions during winters with little snow cover.

Rather similar forest declines and diebacks have affected red spruce (Picea rubens ) and sugar maple (Acer saccharum ) in the same broad region of eastern North America during the 1970s to 1990s. Although the causes of these forest damages are not yet fully understood, it is thought that air pollution or acidifying atmospheric deposition may have played a key role. In western Europe, extensive declines of Norway spruce (Picea abies ) and beech (Fagus sylvatica ) are also thought to somehow be related to exposure to air pollution and acidification . In comparison, the damage caused by ozone to forests dominated by ponderosa pine (Pinus ponderosa )in California is a relatively well-understood kind of emergent ecological disease.

In the marine realm, widespread damage to diverse species of corals has been documented in far-flung regions of the world. The phenomenon is known as coral "bleaching," and it involves the corals expelling their symbiotic algae (known as zooxanthellae), often resulting in death of the coral. Coral bleaching is thought to possibly be related to climate warming, although it can be caused by both unusually high or low water temperatures, changes in salinity , and other environmental stresses.

Another unexplained case of an ecological disease appears to be afflicting species of amphibians in many parts of the world. The amphibian declines involve severe population collapses, and have even caused the extinction of some species. The specific causes are not yet known, but they likely involve introduced microbial pathogens , or possibly increased exposure to solar ultraviolet radiation , climate change, or some other factor.

[Bill Freedman Ph.D. ]

RESOURCES

BOOKS

Freedman, B. Environmental Ecology. San Diego, CA: Academic Press, 1995.