CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

16. Community Interactions

16.6. Succession

Biomes consist of communities that are relatively stable over long periods of time. A relatively stable, long-lasting community is called a climax community. The word climax implies the final step in a series of events. That is just what the word means in this context, because communities can go through a series of predictable, temporary stages, which eventually result in a long-lasting, stable community. The process of changing from one type of community to another is called succession, and each intermediate stage leading to the climax community is known as a successional stage or successional community.

Scientists recognize two kinds of succession. Primary succession occurs when a community of plants and animals develops where none existed previously. Secondary succession occurs when a community of organisms is disturbed or destroyed by a natural or human-related event (e.g., a hurricane, volcano, fire, forest harvest, farming) and is returned to an earlier stage in the process of succession.

Primary Succession

Primary succession is much more difficult to observe than secondary succession because there are relatively few places on Earth that lack communities of organisms. The tops of mountains, newly formed volcanic rock, and rock newly exposed by erosion or glaciers can be said to lack life. However, bacteria, algae, fungi, and lichens quickly begin to grow on the bare rock surface, beginning the process of succession. The first organisms to colonize an area are often referred to as pioneer organisms, and the community is called a pioneer community.

Terrestrial Primary Succession

Lichens are frequently important in pioneer communities. They are unusual organisms consisting of a combination of algae cells and fungi cells—a very hardy combination that is able to grow on the surface of bare rock (figure 16.30). Because algae cells are present, the lichen is capable of photosynthesis and can form new organic matter. Furthermore, many tiny consumer organisms can use the lichen as a food source and a sheltered place to live. The lichen’s action also tends to break down the rock surface on which it grows. This fragmentation of rock by lichens is aided by the physical weathering processes of freezing and thawing, dissolution by water, and wind erosion. Lichens also trap dust particles, small rock particles, and the dead remains of lichens and other organisms that live in and on lichens. These processes of breaking down rock and trapping particles result in the formation of a thin layer of soil.

FIGURE 16.30. Pioneer Organisms

The orange or black lichens growing on rock begin the process of soil formation that is necessary for the development of later successional stages. They carry on photosynthesis, trap organic matter, and break down the surface of the rock.

As the soil layer becomes thicker, small plants, such as mosses, may become established, increasing the rate at which energy is trapped and adding more organic matter to the soil. Eventually, the soil may be able to support larger plants that are even more efficient at trapping sunlight, and the soilbuilding process continues at a more rapid pace. Associated with the producers in each successional stage are a variety of small animals, fungi, and bacteria. Each change in the community makes it more difficult for the previous group of organisms to maintain itself. Tall plants shade the smaller ones they replace; consequently, the smaller plants become less common, and some disappear entirely. Only shade-tolerant species are able to grow and compete successfully in the shade of the taller plants. As this takes place, one stage succeeds the other. Figure 16.31 summarizes these changes.

FIGURE 16.31. Primary Succession

The formation of soil is a major step in primary succession. Until soil is formed, the area is unable to support large amounts of vegetation. The vegetation modifies the harsh environment and increases the amount of organic matter that can build up in the area. As the kinds of plants change, so do the animals. As taller plants become established, the shorter plants that were part of earlier successional stages are eliminated. If given enough time, a climax community may develop.

Depending on the physical environment and the availability of new colonizing species, succession from this point can lead to different kinds of climax communities. If the area is dry, it might stop at a grassland stage. If it is cold and wet, a coniferous forest might be the climax community. If it is warm and wet, it may become a tropical rainforest. The rate at which succession takes place is also variable. In some warm, moist, fertile areas, the entire process might take place in less than 100 years. In harsh environments, such as mountaintops and very dry areas, it may take thousands of years.

Aquatic Primary Succession

Primary succession also occurs in the progression from an aquatic community to a terrestrial community. Lakes, ponds, and slow-moving parts of rivers accumulate organic matter. Where the water is shallow, this organic matter supports the development of rooted plants. In deeper water, only plants with floating leaves, such as water lilies, send their roots down to the mucky bottom. In shallower water, upright, rooted plants, such as cattails and rushes, develop. As the plants contribute more organic matter to the bottom, the water level becomes shallower. Eventually, a mat of mosses, grasses, and even small trees may develop on the surface along the edge of the water. If this continues for perhaps 100 to 200 years, an entire pond or lake will become filled in. More organic matter accumulates because of the large number of producers and because the depression that was originally filled with water becomes drier. This usually results in a wet grassland, which in many areas is replaced by the climax forest community typical of the area (figure 16.32).

FIGURE 16.32. Succession from a Pond to a Wet Meadow

A shallow pond will slowly fill with organic matter from producers in the pond. Eventually, a floating mat will form over the pond and grasses will become established. In many areas, this will be succeeded by a climax forest.

Secondary Succession

Secondary succession occurs when a climax community is altered or destroyed by natural events or human activity. For example, when land is converted to agriculture, the original forest or grassland community is destroyed and replaced by crops. However, when agricultural land is abandoned, it returns to something like the original climax community. One obvious difference between primary succession and secondary succession is that, in secondary succession, there is no need to develop a soil layer. Another difference is that there is likely to be a reservoir of seeds from plants that were part of the original climax community. The seeds can survive in the soil for years in a dormant state, or they might be transported to the disturbed site from undisturbed sites nearby.

If we begin with bare soil the first year, it is likely to be invaded by a pioneer community of annual weed species. Within a year or two, perennial plants, such as grasses, become established. Because most of the weed species need bare soil for seed germination, they are replaced by the perennial grasses and other plants that live in association with grasses. The more permanent grassland community is able to support more insects, small mammals, and birds than the weed community could. In regions where rainfall is low, succession is likely to stop at this grassland stage. In regions with adequate rainfall, several species of shrubs and fast-growing trees that require lots of sunlight (e.g., birch, aspen, juniper, hawthorn, sumac, pine, spruce, and dogwood) become common. As the trees become larger, the grasses fail to get sufficient sunlight and die out. Eventually, shade-tolerant species of trees (e.g., beech, maple, hickory, oak, hemlock, and cedar) replace the shade-intolerant species, and a climax community results (figure 16.33).

FIGURE 16.33. Secondary Succession on Land

When agricultural land is abandoned, it goes through a series of changes. The general pattern is for annual weeds to become established in the first year or two following abandonment. These weeds are replaced by grasses and other perennial herbs, which are replaced by shrubs, which are replaced by trees. As the plant species change, so do the animal species.

Modern Concepts of Succession and Climax

The discussion of the nature of succession and climax communities in the Succession section is an oversimplification of the true nature of the process. Some historical perspective will help to clarify how ecologists have altered their concept of successional change. When European explorers traveled across the North American continent, they saw huge expanses of land dominated by specific types of communities: hardwood forests in the east, evergreen forests in the north, grasslands in central North America, and deserts in the southwest. These regional communities came to be considered the steady-state or normal situation for those parts of the world. When ecologists began to explore the way in which ecosystems developed over time, they began to think of these ecosystems as the end point or climax of a long journey, beginning with the formation of soil and its colonization by a variety of plants and other organisms.

As settlers removed the original forests or grasslands and converted the land to farming, the original “climax” community was destroyed. Eventually, as poor farming practices destroyed the soil, many farms were abandoned and the land was allowed to return to its “original” condition. This secondary succession often resulted in forests that resembled those that had been destroyed. However, in most cases, these successional forests contained fewer species and in some cases were entirely different kinds of communities from the originals. These new stable communities were also called climax communities, but they were not the same as the original climax communities.

In addition, the introduction of species from Europe and other parts of the world changed the mix of organisms that might colonize an area. Many grasses and herbs that were introduced either on purpose or accidentally have become well established. Today, some communities are dominated by these introduced species. Even diseases have altered the nature of climax communities. Chestnut blight and Dutch elm disease have removed tree species that were at one time dominant species in certain plant communities.

Ecologists began to recognize that there was no fixed, predetermined community for each part of the world, and they began to modify the way they looked at the concept of climax communities. The concept today is a more plastic one. It is still used to talk about a stable stage following a period of change, but ecologists no longer feel that land will eventually return to a “preordained” climax condition. They have also recognized in recent years that the type of climax community that develops depends on many factors other than simply climate. One of these is the availability of seeds to colonize new areas. Some seeds may lie dormant in the soil for a decade or more, while others may be carried to an area by wind, water, or animals. Two areas with very similar climate and soil characteristics may develop very different successional and “climax” communities because of the seeds that were present in the area when the lands were released from agriculture.

Furthermore, we need to recognize that the only thing that differentiates a “climax” community from a successional one is the time scale over which change occurs. “Climax” communities do not change as rapidly as successional ones. However, all communities are eventually replaced, as were the swamps that produced coal deposits, the preglacial forests of Europe and North America, and the pine forests of the northeastern United States.

Many human activities alter the nature of the successional process. Agricultural practices obviously modify the original community to allow for the raising of crops. However, several other management practices have also significantly altered communities. Regular logging returns a forest to an earlier stage of succession. The suppression of fire in many forests has also changed the mix of organisms present. When fire is suppressed, those plants that are killed by regular fires become more common and those that are able to resist fire become less common. Changing the amount of water present will also change the kind of community. Draining an area makes it less suitable for the original inhabitants and more suitable for those that live in drier settings. Similarly, irrigation and flooding increase the amount of water present and change the kinds of organisms that can live in an area.

So what should we do with these concepts of succession and climax communities? Although the climax concept embraces a false notion that there is a specific end point to succession, it is still important to recognize that there is an identifiable, predictable pattern of change during succession and that later stages in succession are more stable and longer lasting than early stages. Whether we call a specific community of organisms a climax community is not really important.

Succession and Human Activity

Most agricultural use of ecosystems involves replacing the natural climax community with an artificial early successional stage. Therefore, it requires considerable effort on our part to prevent succession back to something like the original climax community. This is certainly true if remnants of the original natural community are still locally available to colonize agricultural land. Small woodlots in agricultural areas of the eastern United States serve this purpose. Much of the work and expense of farming (e.g., tilling, herbicides) is necessary to prevent succession to the natural climax community. It takes a lot of energy to fight nature.

Many human-constructed lakes and farm ponds have weed problems, because they are shallow and provide ideal conditions for the normal successional processes that lead to their being filled in. Often, humans do not recognize what a powerful force succession is.

16.6. CONCEPT REVIEW

15. How does primary succession differ from secondary succession?

16. How does a climax community differ from a successional community?

17. Describe the steps in the secondary succession of abandoned farmland to a climax community.

18. Describe the steps in the succession of a pond to a climax community.

19. Describe three important factors that determine the kind of climax community that will develop in an area.