Succession

Succession is the progressive transformation or origination of a biological community as new plant and animal species come into an area and alter the environmental conditions.

The complete process of succession may take hundreds or thousands of years and entails a number of intermediate communities, each called a seral community. The replacement of one seral community by another in most cases leads to the eventual formation of a climax community, a relatively stable community of plants and animals.

There are two types of succession: primary and secondary.

Primary succession takes place when plants and animals colonize a previously desolate area, such as a sand dune, new volcanic island, bare rock, or recently glaciated ground. In these situations, everything including the soil bacteria, insects, plants, and animals must come from nearby habitats.

Primary succession can be pictured in the following example, beginning with an abiotic (non-living) lake left behind by a retreating glacier. Sediment from tributaries and heavy rains slowly settles into the lake. Wind-borne spores of bacteria, algae, fungi, and protozoa drift into the water and germinate in the new sediment. These so-called pioneer species can survive under low-nutrient conditions and are quick to grow. As nutrients build up in the lake from these micro-organisms living and dying, water lilies, quillworts, and other plants root along the shoreline. Insects, frogs, and fish take up residence and augment the "soil" on the lake floor with their waste material and decaying bodies. As decaying organic matter collects on the bottom, the lake becomes shallower and reed mace and rushes appear. Their height eventually shades out the smaller aquatic plants. After a while vegetation grows throughout the lake which has become a marsh, another seral community. The marsh gradually fills in and willow trees crop up along the borders. Over time a birch forest may replace the willow community, followed by a pine forest, an oak forest, and finally a climax forest dominated by beech trees. Deer browse where fish once swam.

This succession process is self-propelled because each seral community changes the physical factors of the area so that the community cannot renew itself in its current form. As a pine forest, for example, grows its canopy shades the formerly sunny forest floor. New pine seedlings cannot germinate in this shade but shade-tolerant oak and hickory seedlings can, and they start to sprout.

Over the long term, climate change can also launch succession. Small variations in temperature or rainfall ranges can modify community structure for decades or centuries. Though still hotly debated, some scientists are concerned about the effects of climate change they fear will come as a result of global warming. This warming may be caused by the greenhouse effect, an increase in temperatures worldwide from pollution of the lower atmosphere by greenhouse gases, a natural phenomenon exacerbated by human actions.

Secondary succession occurs in areas where communities have become established before an event such as a fire, landslide, flood, hurricane, field-clearing, or tree-cutting disturbs them.

A field devastated by fire or cleared for industrial or agricultural use will recover its vegetation relatively quickly in the absence of erosion. In the first years of recovery, bare earth becomes grassland, populated by opportunistic species that can tolerate the bleak environmental conditions. Soon, shrubs and other more competitive plants intermingle and dominate. Tree seedlings crop up, and by the end of the first century a coniferous forest occupies what was an overgrazed or blackened stretch of earth. The shady forest forms a new environment in which, after another half-century, the seedlings of other kinds of trees may out-compete the initial residents. In this example, the climax community is dominated by deciduous trees. Ecological succession, driven by major environmental change, is an endless and recursive process.