Understanding Ecological Succession

Early successional habitat in Pennsylvania
Golden-winged warbler habitat in Pennsylvania. Justin Fritscher/NRCS

Ecological succession is the progressive change, in an ecosystem, of species composition over time. With the change in species composition comes a series of modifications in community structure and function.

A classic example of succession involves the series of changes observed in an abandoned field in what is normally a forested area. Once the field is no longer grazed or mowed, the seeds of shrubs and trees will sprout and rapidly start growing. Before long, shrubs and tree saplings will be the dominant vegetation form. The tree species will then grow to the point of shading out the shrubs, eventually forming a complete canopy. The species composition in that young forest will continue to turn over until it is dominated by a stable, self-maintaining group of species called a climax community.

Primary vs. Secondary Succession

Ecological succession where there was no vegetation prior is called primary succession. We can observe primary succession on bulldozed sites, after an intense fire, or following a volcanic eruption, for example. The first plant species to show up have the ability to very quickly colonize and grow in these bare areas. Depending on the region, these pioneer species can be grasses, broadleaf plantain, Queen Anne’s lace, or trees like aspen, alder, or black locust. The pioneers set up the stage for the next phase of succession, improving soil chemistry and adding organic matter which provides nutrients, better soil structure, and greater water-holding capacity.

Secondary succession occurs when a new set of organisms appears where there was an ecological set-back (for example a clear-cut logging operation) but where a cover of living plants was left behind. The abandoned agricultural field described above is a perfect example of secondary succession. Common plants during this stage are raspberries, asters, goldenrods, cherry trees, and paper birch.

Climax Communities and Disturbance

The last stage of succession is the climax community. In a forest, climax species are those that can grow in the shade of taller trees – hence the name shade-tolerant species. The composition of climax communities varies geographically. In parts of the eastern United States, a climax forest will be made of sugar maples, eastern hemlock, and American beech. In Washington State’s Olympic National Park, the climax community may be dominated by western hemlock, Pacific silver fir, and western redcedar.

A common misconception is that climax communities are permanent and frozen in time. In reality, the oldest trees eventually die and are replaced by other trees waiting under the canopy. This makes climax canopy part of a dynamic equilibrium, always changing but overall looking the same. Significant changes will occasionally be brought about by disturbances. Disturbances can be wind damage from a hurricane, a wildfire, an insect attack, or even logging. The type, size, and frequency of disturbances vary by region – some coastal, wet locations experience fires on average once every few thousand years, while eastern boreal forests may be subject to spruce budworm kills every few decades. These disturbances knock back the community into an earlier successional stage, restarting the process of ecological succession.

The Value of Late Successional Habitat

The dark shade and tall canopies of climax forests provide shelter for a number of specialized birds, mammals, and other organisms. The cerulean warbler, wood thrush, and red-cockaded woodpecker are dwellers of old forests. The threatened spotted owl and Humboldt fisher both require large stands of late successional redwood and Douglas-fir forests. Many small flowering plants and ferns rely on the shady forest floor beneath old trees.

The Value of Early Successional Habitat

There is also considerable value in early successional habitat. These shrubland and young forests rely on recurring disturbances that set succession back. Unfortunately, in many places, these disturbances often turn forests into housing developments and other land uses that short-cut the ecological succession process. As a result, shrublands and young forests can become quite rare on the landscape. Many birds rely on early successional habitats, including the brown thrasher, golden-winged warbler, and prairie warbler. There are also mammals that need shrubby habitat, perhaps most notably the New England cottontail.