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A community of organisms does not spring into existence full-blown but develops gradually through a series of stages until it reaches maturity. The process of community development over time, which involves species in one stage being replaced by different species, is called succession. An area is initially colonized by certain organisms that are replaced over time by other organisms, which are themselves replaced, until a more or less stable community that is in equilibrium with existing environmental conditions develops. The relatively stable stage in a community's development is called a climax community or simply a climax. Climax communities represent the dominant vegetation of an area, but they are not permanent; they change as environmental conditions change.

Succession is usually described in terms of the changes in the species composition of the vegetation of an area, although each successional stage also has its own characteristic animal life. The time involved in ecological succession is on the order of hundreds or thousands of years, not the millions of years involved in the evolutionary time scale.

Primary Succession

Primary succession is the change in species composition over time in a habitat that has not previously been inhabited by organisms. No soil exists when primary succession begins. A bare rock surface, such as recently formed volcanic lava or rock scraped clean by glacial action, is a potential site for primary succession. Although the details vary from one site to another, one might first observe a community of lichens—dual organisms usually composed of a fungus and an alga .

Because they are the first organisms to colonize hare rock, lichens are called the pioneer community. Lichens secrete acids that help to break the rock apart, beginning the process of soil formation. Over time, the lichen community may be replaced by mosses and drought-resistant ferns, followed in turn by tough grasses and herbs. Once sufficient soil has accumulated, grasses and herbs may be replaced by low shrubs, which in aim are replaced by forest trees in several distinct stages. Primary succession from a pioneer community on hare rock to a climax forest community may take hundreds or thousands of years.

Lichens are able to live on the surfaces of many rocks and beneath the surface of porous rock in a sheltered and somewhat moister habitat. Lichens are very resistant to desiccation (drying our). They cease to grow when water is unavailable but quickly resume active growth when moisture returns; they can absorb their own weight in water within moments of moistening.

As generations of lichens pass on a rock surface, several important cumulative changes occur. First, the biomass (amount of living material) of the lichen community increases, so more and more solubilized minerals are stored in the living tissue of the community. Second, fine particles of rock he-come detached from the ruck¹-, surface or even within the rock itself. Third, as lichens die, their decomposing remains mix with the rock particles to form a rudimentary soil. Fourth, whenever water is available, it is absorbed by the lichens and retained in their tissues and in the new, thin soil layer for longer periods of time than ever before. As all of these changes occur, an increasing number of tiny animals move into the area and make their homes in the lichens and soil.

All of these changes—increased biomass, soil development, water retention, and an increased number of life forms—work together to moderate the harsh conditions under which the pioneer com

munity has lived, making it possible for mosses to grow there. In fact, because mosses can grow faster than lichens, they tend to replace any lichens that die. The higher productivity of mosses results in a greater accumulation of biomass and, ultimately, of soil. This leads to further habitat change, and ferns,

Primary Succession on Sand Dunes Lake and ocean shores often have extensive sand dunes that have been deposited by wind and water. These dunes arc not permanent; they move before the wind. The sand-dune environment is severe, with high temperatures during the day and low temperatures during the night; the sand may also be deficient in certain mineral nutrients needed by plants. As a result, few plants can tolerate the environmental conditions of a sand dune.

Grasses are a common pioneer plant on sand dimes. As the grasses extend over the surface of a dune, their roots help to hold the dune in place and stabilize it. At this point, mat-forming shrubs can invade the dune, further stabilizing it. Much later the shrubs are replaced by pines, which in turn are replaced by oaks. (Sometimes the pine stage is skipped.) Because the soil fertility remains low, oaks are rarely replaced by other forest trees; they are thus the climax community in primary succession of sand dunes.

Mini-Glossary of Succession

Climax community: A relatively stable community that is in equilibrium with current environmental conditions.

Primary succession: Ecological succession in a habitat that has not previously been inhabited.

Secondary succession: Ecological success has previously been inhabited.

Secondary Succession

Secondary succession is the change in species composition over time in a habitat already substantially modified by a pre-existing community; soil is already present. An area opened up by a forest fire and an abandoned field is common examples of sites where secondary succession occurs.

Secondary succession on abandoned farmland has been studied extensively. Although it takes

more than 100 years for secondary succession to occur at a given site, it is possible for a single researcher to study a case of old field succession in its entirety by observing multiple sites in the same area. The scientist examines court records to determine when each field was abandoned.

Abandoned farmland in North Carolina is colonized by a predictable succession of plant communities. The first year after cultivation ceases, the field is dominated by crabgrass. The following year the dominant species is horseweed. It does not dominate more than one year, however, more than 100 years for secondary succession to occur at a given site, it is possible for a single researcher to study a case of old field succession in its entirety by observing multiple sites in the same area. The scientist examines court records to determine when each field was abandoned.

Why Forests Need Squirrels

Until recently, the interdependency of plant and animal species was largely ignored by foresters, who endorsed clear cutting vast tracts of forest land as the only way to manage our timber resources. In doing so, of course, forest companies alter complex forest ecosystems forever by wiping out all tree species and habitats. In his 1990 book, The Redesigned forest, author and biologist Chris Maser demonstrated the close connection between its inhabitants in a description of the northern being squirrel. A native of the Public Northwest, the squirrel feeds on a certain fungus found in the forest floor. The fungus is digested by the squirrel and excreted in the form of droppings or "pellets." Each pellet, says Maser, is like a "symbiotic pill." It contains four components of great value to the forest: spores of the fungi, yeast, nitrogen-fixing bacteria, and the complete nutrient component for the nitrogen-fixing bacteria. Wherever the pellets fall on the ground, they create fertile conditions. More of the valuable fungi are propagated and the overall fertility of the forest is increased. In a clear-cut area, the squirrel disappears; without the squirrel, soil fertility is decreased. The northern flying squirrel illustrates only one of thousands of such relationships in any given forest.

Plants to decline topic is importance. Over time, pines give up their dominance to hardwoods such as oaks. This climax stage of secondary succession depends primarily on the environmental changes produced by the pines. The pine litter causes soil changes, such as an increase in water-holding capacity, that arc necessary in order for young oak seedlings to become established.

Animal Life in Secondary Succession As secondary succession proceeds, a progression of wildlife follows the changes in vegetation. Although a few species—the short-tailed shrew, for example—are found in all stages of abandoned farmland succession, most animals appear with certain stages and disappear with others. During the crabgrass and weed stages of secondary succession, the habitat is characterized by open fields that support grasshoppers, meadow mice, cottontail rabbits, and birds such as grasshopper sparrows and meadowlarks. As young pine seedlings become established, animals of open fields give way to animals common in mixed herbaceous and shrubby habitats. Now white-tailed deer, white-footed mice, ruffled grouse, robins, and song sparrows are common, whereas grasshoppers, meadow mice, grasshopper sparrows, and meadowlarks disappear. As the pine seedlings grow into trees, animals of the forest replace those common in mixed herbaceous and shrubby habitats. Cottontail rabbits give way to red squirrels, and ruffled grouse, robins, and song sparrows are replaced by warblers and varies. Thus, each stage of succession supports its own characteristic wildlife.