Succession

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Succession

Succession is the gradual transformation or creation of a biological community as new species move into an area and modify local environmental conditions. Primary succession occurs when plant and animal species colonize a previously barren area, such as a new volcanic island, a sand dune, or recently glaciated ground. In these cases every living thing, from soil bacteria and fungi to larger plants and animals, must arrive from some adjacent habitat . Secondary succession is the development of communities in an area that has been disturbed by fire, hurricanes, field clearing, tree felling, or some other process that removes most plants and animals. Intermediate successional communities are known as "seral stages" or "seres."

As early successional species become established, they alter their environment and make it more habitable for later seral stages. Usually a disturbed or barren area has low soil nutrient levels, intense sunlight, and no protection from violent weather. Because precipitation quickly runs off the bare ground, little moisture is available for plant growth. Species that can survive under such harsh conditions have little competition , and they spread quickly. As they grow and thicken, these plants add organic matter to the soil, which aids moisture retention and helps soil bacteria to grow. As soil nutrients and moisture increase, larger shrubs and perennial plants can take root. The shade of these larger species weakens and eventually eliminates the original pioneering species, but it cools the local environment, further improving moisture availability and allowing species that demand relative environmental stability , such as woodland species, to begin moving in. Eventually, shade-tolerant species of plants will come to dominate the area that sun-loving plants had first colonized.

One of the most well-documented examples of primary succession occurred in 1883 when the volcano on the Indonesian island of Krakatoa erupted, destroying most of the island and its life forms, and leaving a new island of bare volcanic rock and ash. Within a few years several species of grasses, ferns, and flowering shrubs had managed to arrive, carried by wind, water, or passing birds from islands 25 mi (40 km) or more distant. Just 40 years later, more than 300 plant species were growing, and in some areas over 12 in (30 cm) of soil had developed: soil bacteria, insects, and decomposers had managed to reach the island and were turning fallen organic detritus into soil. Over the decades, the total number of species gradually stabilized, but the character of the community continued to change as incoming species replaced earlier arrivals. On Krakatoa succession happened with unusual speed because the climate is warm and humid and because the fresh volcanic ash made a nutrient-rich soil.

Early successional species, those most able to quickly establish a foothold, are known as pioneer species. Usually pioneer species are opportunists, able to find nourishment and survive under a great variety of conditions, quick to grow, and able to produce a great number of seeds or offspring at once. Pioneer species have very effective means of dispersing seeds or young, and their seeds can often remain dormant in soil for some time, sprouting only after conditions become suitable. Dandelions are well-known pioneers because they can quickly produce a seed head with hundreds of seeds. Each tiny seed has a lightweight structure that allows the wind to carry it long distances. Dandelions are quick to invade a lawn because of this effective seed-dispersal tactic, because they can survive under sunny or shady conditions, and because they grow and reproduce quickly. In open sunshine, they are competitive enough to establish themselves despite the presence of a thick mat of turf grass.

Later successional species tend to be more shade tolerant, slower to grow and reproduce, and longer lived than pioneer species. Their larger seeds do not disperse as easily (compare the size of an acorn or walnut with that of a dandelion seed), and their seeds cannot remain dormant for very long before they lose viability. Seedlings of some late successional species, such as the Pacific Northwest hemlock, require shade to survive, and most require considerable moisture and soil nutrients.

We usually think of succession occurring after a catastrophic environmental disturbance, but in some cases the gradual environmental changes of succession proceed in the absence of disturbance. Two outstanding examples of this are bog succession and the invasion of prairies by shrubs and trees.

In cool, moist, temperate climates, plant succession often turns ponds into forest through bog succession. Water-loving plants, mainly rush-like sedges and sphagnum moss, gradually creep out from the pond's edges. Often these plants form a floating mat of living and dead vegetation. Other plants—cranberries, Labrador tea, bog rosemary—become established on top of the mat. Organic detritus accumulates below the mat. Eventually the bog becomes firm enough to support black spruce, tamarack, and other tree species. As it fills in and dries, the former pond slowly becomes indistinguishable from the surrounding forest.

Many grasslands persist only in the presence of occasional wildfires. During wet decades or when human activity prevents fires, woody species tend to creep in from the edges of a prairie . If sufficient moisture is available and if fires do not return, open grassland can give way to forest. Fire suppression has aided the advance of forests in this way across much of the United States, Canada, and Mexico. Grazing or browsing animals can also be important forces in maintaining biological communities. Adding or removing grazers can initiate successional processes.

Longer, slower disturbances than fire or field clearing can also initiate succession. Over the course of centuries, climate change can cause significant alteration in the character of biomes. Minor variations in rainfall or temperature ranges can alter community structure for decades or centuries. The end of the last glacial period about 10,000 years ago allowed tundra , then grasslands, then temperate forests to advance northward across North America. Even geologic activity, such as mountain building or changes in sea level, has caused succession. More recently, human introductions of species from one continent to another have caused significant restructuring of some biological communities.

Turnover from one community to another may occur in just a few years or decades, as it did on Krakatoa. Stages in bog succession may take a hundred years or more. Often, succession continues for centuries or millennia before a stable and relatively constant biological community emerges. This is especially likely in areas whose climate is moist and mild enough to support the high species diversity of temperate or tropical rain forests . However, simpler ecosystems such as arctic tundra may take centuries to recover from disturbance because low temperatures and short summers make growth rates almost universally slow.

Generally, successional processes are understood to lead ultimately to the emergence of a climax community, a group of species perfectly suited to a region's climate, soil types, and other environmental conditions. Climax communities are said to exist in equilibrium in their environments; where early successional species groups tend to facilitate the development of later stages, climax communities, sometimes called mature communities, tend to have self-perpetuating characteristics or mechanisms that help maintain local environmental conditions and help the community to persist. In the 1930s, F. E. Clements, one of the best-known plant ecologists in the United States, identified a list of just 14 climax communities that he claimed were the final result of succession for all the various sets of environmental conditions in the country.

More recent evaluations of climax communities, however, have concluded that climax forests are often a patchwork of stable and unstable areas. In an old-growth forest in the Pacific Northwest, hemlock and spruce may compose the local climax community, but every time a large tree falls, it creates a sunny opening that makes way for other seral species to grow for a time. Occasional fires disturb larger patches of forest, and succession starts again from the beginning in disturbed areas. The entire biome is more a shifting and changing mosaic than a uniform and unchanging forest. Often, slight gradations in soil types, slope, exposure, and moisture cause a variety of stable communities to blend in an area. The term "polyclimax community" was developed to recognize the complexity of such assemblages.

In some cases, climax communities may be difficult to distinguish at all. For instance, in a biome regularly disturbed by fire, "climax" species usually require fire to aid seed germination and eliminate competitors. Definitions here become somewhat blurred: Is a climax community one that exists without disturbance? Is fire a disturbance or a maintenance factor? Even if a very old forest community is considered, on a time scale of thousands of years, it may be difficult to determine whether what is observed today is really a permanent climax community or just a very longstanding seral stage. Despite these questions, the concept of an ultimate climax community is central to the idea of succession.

Usually climax communities are considered more diverse than intermediate communities. Because climax communities can remain stable for a long time, species can diversify and develop specialized niches. This specialization can lead to the coexistence of a great number of species, as is the case in tropical rain forests, where thousands of species may share just a few acres of forest. In some cases, however, intermediate seral stages actually have greater diversity because they contain elements of multiple communities at once.

[Linda Rehkopf ]