Ecological succession

Ecological succession, a fundamental concept in ecology, is the process by which a natural community moves from a simpler level of organisation to a more complex community. Succession is a natural process that occurs after some form or perturbation which simplifies the system. Ecological succession is usually differentiated into primary succession, which proceeds on a site in which no living material has survived the disturbance (e.g., on a lava flow or after a severe landslide), and secondary succession which occurs after some disturbance has disrupted the community, but has not eliminated all living components (e.g., after a fire or logging).

During succession, a change in species composition and organisation of an ecological community is observed over time. Within the community some species may become less abundant, or may entirely disappear, while other species may become more abundant or just enter the community from adjacent ecosystems.

The very early stage of an ecological succession may be a bare rock slightly covered by moss, or a mud flow slowly being colonized by a grass. Usually, this stage shows very low diversity. A mature ecosystem will often show high species diversity, many mutualistic interactions, and a high degree of nutrient recycling.

Ecological succession was formerly seen as having an end-stage called the climax, which was thought to represent the ultimate vegetation (called the climax community) in an equilibrium that depends on the local climate. This idea has been largely abandoned by modern ecologists as it is out of step with non-equilibrium ideas of how ecosystems function. Almost all natural ecosystems experience disturbance at a rate the makes a "climax" community unattainable, and thus, unrealistic. In fact, many species are specialized to live in disturbed ecosystems. One example is trees such as Yellow birch and Black cherry which are adapted to living in forest gaps. These trees are intolerant of shade and would eventually be succeeded by other species in any particular gap, but disturbance generally reopens gaps or provides new ones that they disperse into. These are known as gap-phase species.

Clementsian view of successional trajectories

A sere is a plant community resulting from a single stage of succession. The Clementsian view of succession saw it as an orderly, predictable sequence:

Primary Sere → Secondary Sere → ... → nth Sere → Climax

A community develops over time to a climax community.

Primary Sere → Factor that stops development → Subclimax

The community may not reach climax, instead stopping at a subclimax community.

Primary Sere → Arresting factor → Disclimax

The community may be diverted to a disclimax community by an arresting factor. The arresting factor could be, for example, a feral animal or feral weed. A different arresting factor may be introduced, which could lead to a return to the climax community, however.

History of the idea

Historically, there have been two contrasting views of succession: Clementsian and Gleasonian. The Clementsian model argues for a predictable, orderly process which culminates in a stable climatic climax. The Gleasonian model is more complex, invoking interactions between the physical environment, population-level interactions between species and disturbance regimes in determining the composition and spatial distribution of species.

The obvious extension of the Gleasonian view is the view that continuous change in vegetation is the norm (similar to Henry Chandler Cowles view of succession) and that multiple steady states exist in ecosystem dynamics. This challenges the popular concept of reference (or 'original') conditions in most ecosystems.

At the turn of the 20th century, Henry Chandler Cowles was one of the prime movers in the emerging study of "dynamic ecology", through his study of the Indiana Dunes, sand dunes at the southern end of Lake Michigan. Cowles found that he could relate the vegetation at any point in the dunes to several variables: the distance from the lake shore, the estimated age of the dune, and the type of soil that had developed. In 1899 his classic paper "The ecological relations of the vegetation of the sand dunes of Lake Michigan" appeared in the Botanical Gazette.

The development of some ecosystem attributes, such as pedogenesis and nutrient cycles, are critical factors which influence community development but take considerable time. Coupled with the stochastic nature of disturbance events and other long-term (e.g., climatic) changes, it is doubtful whether a true 'climax' exists in any ecosystem, except as a purely theoretical construct. In modern times, less stress has been placed on the idea of a single, natural "climax forest" or other climax vegetation, and more study has gone into the roles of contingency in the actual development of several possibilities at any one site.