ECOLOGICAL SUCCESSION

Even when the climate of a given area remains stable year after year, ecosystems have a tendency to change from simple to complex in a process known as succession. This process is familiar to anyone who has seen a vacant lot or cleared woods slowly but surely become occupied with larger and larger plants and more and more different kinds of them, or a pond become filled with vegetation that encroaches from the sides and gradually turns it into dry land
Succession is continuous and worldwide in scope. If a wooded area is cleared and the clearing is left alone, plants will slowly reclaim the area. Eventually, the traces of the clearing will disappear and the whole area will again be woods. This kind of suc­cession, which occurs in areas that have been disturbed and that were originally occu­pied by living organisms, is called secondary succession. Humans are often respon­sible for initiating secondary succession throughout portions of the world that they inhabit. Secondary succession may also take place after fire has burned off an area, for example, or after the eruption of a volcano.
Primary succession, in contrast to secondary succession, occurs on some bare, lifeless substrate, such as rocks, or in open water, where organisms gradually occupy the area and change its nature. Thus primary succession occurs in lakes left behind after the retreat of glaciers; another would be the processes that occur on a volcanic island that rises above the sea. Primary succession that occurs on places such as dry rocks is called xerarch succession, to distinguish it from the hydrarch primary suc­cession that occurs in open water. On bare rocks, lichens may grow first, forming small pockets of soil and breaking down the stone. Acidic secretions from the lichens and from the plants that grow on the rocks later may help to break down the sub­strate and add to the accumulation of soil. Mosses may then colonize these pockets of soil, eventually followed by ferns and the seedlings of flowering plants. Over many thousands of years, or even longer, the rocks may be completely broken down, and the vegetation over an area where there was once a rock outcrop may be just like that of the surrounding grassland or forest.
In a similar example involving hydrarch succession, an oligotrophic lake-one poor in nutrients-may gradually, by the accumulation of organic matter, become eutrophic-rich in nutrients. Plants standing along the edges of the lake, such as cattails and rushes, and those growing submerged, such as pondweeds, together with other organisms, may contribute to the formation of a rich or­ganic soil. As this process continues, the pond may increasingly be filled in with ter­restrial vegetation. Eventually, the area where the pond once stood, like the rock out­crop we just described, may become an indistinguishable part of the surrounding veg­etation.
Oligotrophic ponds and bare rocks in a given region may over the very long term come to feature the same kind of vegetation as one another-the vegetation charac­teristic of the region as a whole. This relationship led the American ecologist F.E. Clements, at about the turn of the century, to propose the concept of climax vegeta­tion (and the related term climax community). However, with an in­creasing realization that the climate keeps changing, the process of succession is often very slow, and the nature of a region's vegetation is being determined to a greater extent by human activities, ecologists do not consider the concept of "climax vegetation" to be as useful as they once did.
The characteristics that we will now outline are general ones that appear to hold for succession of all sorts. As ecosystems mature, there is an increase in total biomass but a decrease in net productivity. The earlier successional stages are more productive than the later ones. Agricultural systems are examples of early successional stages in which the process is intentionally not allowed to go to completion and the net produc­tivity is high. There are many more species in mature ecosystems than in immature ones, and the number of heterotrophic species increases even more rapidly than the number of autotrophic species. This progression is related to the decreasing net productivity of increasingly mature ecosystems and to the fact that mature ecosystems have a greater ability to regulate the cycling of nutrients than do disturbed and immature ones. It appears that the plants and animals that appear in the later stages of succession may be more specialized, in general, than those that ex­ist in the earlier stages. The late-successional species appear to fit together into more complex communities and to have much narrower ecological requirements, or niches.
In many communities, there is a constant progression in areas where trees have fallen or other local disturbances have occurred. The ways in which various kinds of organisms in the community refill the gaps, which gradually come to be occupied by mature forest, are of central importance in understanding community dynamics. Tor­nadoes, landslides, killing of trees over large areas by pests, and other similar natural disturbances bring about a local renewal of the communities in which they occur. Fires may release nutrients into the soil, accelerating the progress of colonization.
Some species are fugitive species, which occur at the earlier successional stages and disappear from the area-as succession proceeds. Such species often have high re­productive rates and efficient means of dispersal. Foxglove (Digitalis purpurea), for ex­ample, is a fugitive species that temporarily becomes abundant in forest clearings and then disappears as the canopy cover of the trees becomes more complete. Other op­portunities for fugitive species are afforded by fires; certain species may be seen only growing on burned areas. Certain fugitive species may occur only in newly formed ponds or islands and then apparently disappear from the area. Many weeds, also, are fugitive species, disappearing soon from the communities where they appear.