Succession

Disturbance is an episodic stress that causes substantial changes in the structure and function of ecosystems. Depending on its severity and extent, disturbance can influence individual organisms or entire stands, disrupting relationships among individuals within the community, and affecting ecological processes such as productivity, nutrient cycling, and decomposition.

Natural disturbances can be caused by intense events of physical disruption associated with, for example, a hurricane, tornado, oceanic tidal wave, earthquake, the blast of a volcanic eruption, or over geological timespans, the advance and retreat of glaciers. Natural disturbances are also associated with wildfire, and with biological events, such as an irruption of defoliating insects that can kill a mature forest. Human activities are also important sources of ecological disturbances, as is the case of agricultural practices such as plowing, the harvesting of trees from forests, construction activities, and explosions associated with military activities. There are numerous other examples of disturbances caused by human activities.

Note that all of these disturbances can operate at various spatial scales. In some cases, they can result in extensive disturbances over very large areas, for example, when a very severe wildfire consumes millions of hectares of forest. In other cases a disturbance may not cause a stand-level mortality, as when a lightning strike kills a single mature tree in a forest, creating a gap in the overstory, and initiating a microsuccession that culminates in occupation of the gap by another mature tree. Scale is a very important aspect of succession.

Once the intense physical, chemical, or biological stress associated with an event of disturbance is relaxed, succession begins, and this process may eventually restore an ecosystem similar to that present prior to the disturbance. However, depending on environmental circumstances, a rather different ecosystem may develop through post-disturbance succession. For example, if climate change has resulted in conditions that are no longer suitable for the establishment and growth of the same species of trees that dominated a particular stand of old-growth forest prior to its disturbance by a wildfire, then succession will result in the development of a forest of a different character.

Succession can also follow the alleviation of a condition of longer-term, chronic environmental stress. For example, if a local source of pollution by poisonous chemicals is cleaned up, succession will occur in response to the decreased exposure to toxic chemicals. This might occur, for example, if emissions of toxic sulfur dioxide and metals from a smelter are reduced or stopped. This abatement of longer-term environmental stress allows pollution-sensitive species to invade the vicinity of the source, so that succession could proceed. Similarly, removal of some or all of a population of cattle that are chronically overgrazing a pasture would allow the plant community to develop to a more advanced successional stage, characterized by greater biomass and possibly more biodiversity.

Succession can also be viewed as a process of much longer-term ecological change occurring, for example, as a lake basin gradually in-fills with sediment and organic debris, eventually developing into a terrestrial habitat such as a forest. This particular succession, which occurs through the development and replacement of a long series of community types, can take thousands of years to achieve its progression through the aquatic to terrestrial stages. The series of ecological changes is still, however, properly viewed as a successional sequence (or sere).