Even without human activity, natural communities do not stay the same forever. Change in the species composition of communities over time is a perpetual process in nature. In this module we will look at how both terrestrial and aquatic communities change over time.

Learning Objectives

After reading this module you should be able to

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Virtually every community experiences ecological succession, which is the predictable replacement of one group of species by another group of species over time. Depending on the community type, ecological succession can occur over time spans varying from decades to centuries.

Some terrestrial communities begin with bare rock and no soil. For example, a community may begin forming on newly exposed rock left behind after a glacial retreat, newly cooled lava after a volcanic eruption, or an abandoned parking lot. When succession begins with bare rock and no soil, we call it primary succession.

FIGURE 21.1 shows the process of primary succession in a temperate forest biome in New England. The bare rock can be colonized by organisms such as algae, lichens, and mosses—organisms that can survive with little or no soil. As these early-successional species grow, they excrete acids that allow them to take up nutrients directly from the rock. The resulting chemical alteration of the rock also makes it more susceptible to erosion. When the algae, lichens, and mosses die, they become the organic matter that mixes with minerals eroded from the rock to create new soil.

Over time, soil develops on the bare surface and it becomes a hospitable environment for plants with deep root systems. Mid-successional plants such as grasses and wildflowers are easily dispersed to such areas. These species are typically well adapted to exploiting open, sunny areas and are able to survive in the young, nutrient-poor soil. The lives and deaths of these mid-successional species gradually improve the quality of the soil by increasing its ability to retain nutrients and water. As a result, new species colonize the area and outcompete the mid-successional species.

The type of community that eventually develops is determined by the temperature and rainfall of the region. In the United States, succession produces forest communities in the East, grassland communities in the Midwest, and shrubland communities in the Southwest. In some areas, the number of species increases as succession proceeds. In others, late-successional communities have fewer species than early-successional communities.

Secondary succession occurs in areas that have been disturbed but have not lost their soil. Secondary succession follows an event, such as a forest fire or hurricane, that removes vegetation but leaves the soil mostly intact. Secondary succession also occurs on abandoned agricultural fields, such as the New England farms we discussed at the beginning of the chapter.

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FIGURE 21.2 shows the process of secondary succession, again for a typical forest in New England. It usually begins with rapid colonization by plants that can easily disperse to the disturbed area. The first to arrive are typically such plants as grasses and wildflowers that have light, wind-borne seeds. As in primary succession, these species are eventually replaced by species that are better competitors for sunlight, water, and soil nutrients. In regions that receive sufficient rainfall, trees replace grasses and flowers. The first tree species to colonize the area are those that can disperse easily and grow rapidly. For example, the seeds of aspen trees are carried on the wind, and cherry tree seeds are carried by birds that consume the fruit and excrete the seeds on the ground. Trees such as aspen and cherry are often called pioneer species because of their ability to colonize new areas rapidly and grow well in full sunshine. As the pioneer trees increase in number and grow larger, however, they cause an increased amount of shade on the ground. Because pioneer trees need full sunshine to grow, new seedlings of these trees cannot persist. In contrast, species that are more shade tolerant, including many species of beech and maple, survive and grow well in the shade of the pioneer tree species. These shade-tolerant species grow up through the pioneer canopy and eventually outcompete the pioneer trees and dominate the forest community.

The process of secondary succession happens on many spatial scales, from small-scale disturbances such as a single treefall to huge areas cleared by natural disasters such as hurricanes or wildfires. The process is similar in both cases. For example, if a large tree falls and creates an opening for light to reach the forest floor, the seedlings of trees that grow rapidly in full sunlight outcompete other seedlings, and secondary succession begins again.

Historically, ecologists have described succession as ending with a climax stage. In forests, for example, ecologists considered the oldest forests to be climax forests. However, it is now recognized that, because natural disturbances such as fire, wind, and outbreaks of insect herbivores are a regular part of most communities, a late-successional stage is typically not final because at any moment it can be reset to an earlier stage.

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Disturbances also create opportunities for succession in aquatic environments. For example, the rocky intertidal zone along the Pacific coast of North America is exposed to the air during low tide and lies under water during high tide. From time to time, major storms turn over rocks or clear their surfaces of living things. These bare rocks can then be colonized through the process of primary succession. Diatoms and short-lived species of red and green algae are usually the first to arrive. Not long afterward, the rocks are colonized by barnacles and several species of long-lived red algae. This mid-successional stage develops rather quickly—in about 15 months. Finally, if the rock is not disturbed again for 3 years or more, the area may become dominated by several species of attached barnacles and mussels.

FIGURE 21.3 shows the pattern of succession in freshwater lakes. A glacier may carve out a lake basin, scouring it of sediments and vegetation. Over time, algae and aquatic plants colonize the lake. The growth of plants and algae and the erosion of surrounding rock and soil slowly fill the basin with sediment and organic matter, making it increasingly shallow. This process, which may take hundreds or even thousands of years, will eventually fill in the lake completely and make it a terrestrial habitat.

As we have seen, species are not distributed evenly on Earth. They are organized into biomes by global climate patterns and into communities whose composition changes regularly as species interact. In a given region within a biome, the number and types of species present are determined by three basic processes: colonization of the area by new species, speciation within the area, and losses from the area by extinction. The relative importance of these processes varies from region to region and is influenced by four factors: latitude, time, habitat size, and distance from other communities.

Latitude

As we move from the equator toward the North or South Pole, the number of species declines. For example, the southern latitudes of the United States support more than 12,000 species of plants, whereas a similar-sized area in northern Canada supports only about 1,700 species. This latitudinal pattern is also observed among birds, reptiles, amphibians, and insects. For more than a century, scientists have sought to understand the reasons for this pattern, yet those reasons remain unclear.

Time

Patterns of species richness are also regulated by time. The longer a habitat exists, the more colonization, speciation, and extinction can occur in that habitat. For example, Lake Baikal in Siberia—more than 25 million years old—is one of the oldest lakes in the world. Its benthic zone is home to over 580 species of invertebrates. By contrast, only 4 species of invertebrates inhabit the benthic zone of the Great Slave Lake in northern Canada—a lake that is similar in size and latitude to Lake Baikal, but is only a few tens of thousands of years old. This difference suggests that older communities have had more opportunities for speciation.

Habitat size and distance from a source of species

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The final two factors that influence species richness are the size of the habitat and the distance of that habitat from a source of colonizing species. These factors are the basis for the theory of island biogeography, which demonstrates the dual importance of habitat size and distance from a mainland in determining species richness.

Larger habitats typically contain more species. In FIGURE 21.4, for example, we can see that larger islands of reed habitat in Hungary contain a greater number of bird species than smaller islands. There are three reasons for this pattern. First, dispersing species are more likely to find larger habitats than smaller habitats, particularly when those habitats are islands. Second, at any given latitude, larger habitats can support more species than smaller habitats. Larger habitats are capable of supporting larger populations of any given species, and larger populations are less prone to extinction. Third, larger habitats often contain a wider range of environmental conditions, which in turn provide more niches that support a larger number of species. A wider range of environmental conditions also provides greater opportunities for speciation over time.

The distance between a habitat and a source of colonizing species is the second factor that affects the species richness of communities. For example, oceanic islands that are more distant from continents generally have fewer species than islands that are closer to continents. Distance matters because, while many species can disperse short distances, only a few can disperse long distances. In other words, if two islands are the same size and contain the same resources, the nearer island should accumulate more species than the farther island because it has a higher rate of immigration by new species.

Conservation and island biogeography

The effects of colonization, speciation, and extinction on the species richness of communities have important implications for conservation. The theory of island biogeography was originally applied to oceanic islands, but it has since been applied to “habitat islands” within a continent, such as the “islands” of protected habitat represented by national parks. These habitat islands are often surrounded by less hospitable habitats that have been dramatically altered by human activities. If we wish to set aside natural habitat for a given species or a group of species, we need to consider both the size of the protected area and the distance between the protected area and other areas that could provide colonists.