Final Paper Outline
CHAPTER 18
Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.
18.1 Community Structure Changes through Time
Community structure varies in time as well as in space. Suppose that rather than moving across the landscape, as with the examples of zonation presented earlier (see Section 16.8, Figures 16.15– 16.19), we stand in one position and observe the area as time passes. For example, abandoned cropland and pastureland are common sights in agricultural regions in once forested areas of eastern North America (see Chapter 18, Ecological Issues & Applications). No longer tended, the land quickly grows up in grasses, goldenrod (Solidago spp.), and weedy herbaceous plants. In a few years, these same weedy fields are invaded by shrubby growth—blackberries (Rubus spp.), sumac (Rhus spp.), and hawthorn (Crataegus spp.). These shrubs eventually are replaced by pine trees (Pinus spp.), which with time form a closed canopy forest. As time passes, deciduous hardwood species develop in the understory. Many years later, this abandoned land supports a forest of maple (Acer spp.), oak (Quercus spp.), and other hardwood species (Figure 18.1). The process you would have observed, the gradual and seemingly directional change in community structure through time from field to forest, is called succession. Succession, in its most general definition, is the temporal change in community structure. Unlike zonation, which is the spatial change in community structure across the landscape, succession refers to changes in community structure at a given location on the landscape through time.
The sequence of communities from grass to shrub to forest historically has been called a sere (from the word series), and each of the changes is a seral stage. Although each seral stage is a point on a continuum of vegetation through time, it is often recognizable as a distinct community. Each stage has its characteristic structure and species composition. A seral stage may last only one or two years, or it may last several decades. Some stages may be missed completely or may appear only in abbreviated or altered form. For example, when forest trees immediately colonize an abandoned field (as in Figure 18.1), the shrub stage appears to be bypassed; however, structurally, the role of shrubs is replaced by the incoming young trees.
Like zonation, the process of succession is generally common to all environments, both terrestrial and aquatic. The ecologist Wayne Sousa of the University of California–Berkeley carried out a series of experiments designed to examine the process of succession in a rocky intertidal algal community in southern California. A major form of natural disturbance in these communities is the overturning of rocks by the action of waves. Algal populations then recolonize these cleared surfaces. To examine this process, Sousa placed concrete blocks in the water to provide new surfaces for colonization. Over time, the study results show a pattern of colonization and replacement, with other species displacing populations that initially colonized the concrete blocks (Figure 18.2). This is the process of succession. The initial, or early successional species (often referred to as pioneer species), are usually characterized by high growth rates, smaller size, high degree of dispersal, and high rates of per capita population growth. In contrast, late successional species generally have lower rates of dispersal and colonization, slower per capita growth rates, and are larger and longer-lived. As the terms early and late succession imply, the patterns of species replacement with time are not random. In fact, if Sousa’s experiment were to be repeated tomorrow, we would expect the resulting patterns of colonization and local extinction (the successional sequence) to be similar to those presented in Figure 18.2.
A similar pattern of succession occurs in terrestrial plant communities. Figure 18.3 depicts the patterns of woody plant species replacement after forest clearing (clear-cutting) at the Hubbard Brook Experimental Forest in New Hampshire. Before forest clearing in the late 1960s, seedlings and saplings of beech (Fagus grandifolia) and sugar maple (Acer saccharum) dominated the understory. Large individuals of these two tree species dominated the canopy, and the seedlings represent successful reproduction of the parent trees. After the larger trees were removed by timber harvest, the numbers of beech and maple seedlings declined and were soon replaced by herbaceous species (ferns, sedges, and grasses), raspberry thickets, and seedlings of sun-adapted (shade-intolerant), fast-growing, early successional tree species such as pin cherry (Prunus pennsylvanica) and yellow birch (Betula alleghaniensis). Over the next 20 years these early successional species came to dominate the site, and now after half a century these species are currently being replaced by the late successional species of beech and sugar maple that previously dominated the site.
Interpreting Ecological Data
Q1. At what time does the species Gigartina canaliculata first appear in the experiments (month and year)? At what period during the experiment does this species of algae dominate the community?
Q2. Which algal species dominates the community during the first year of succession? Which species dominates during the last year of the experiment?
Q3. Which algal species never dominate the community (greatest relative abundance)?
Q4. During which period of the observed succession (early, mid, or late) is overall species diversity the highest?
The two studies just presented point out the similar nature of successional dynamics in two different environments. They also present examples of two different types of succession: primary and secondary. Primary succession occurs on a site previously unoccupied by a community—a newly exposed surface such as the cement blocks in a rocky intertidal environment. The study at Hubbard Brook after forest clearing is an example of secondary succession. Unlike primary succession, secondary succession occurs on previously occupied sites (previously existing communities) after disturbance. In these cases, disturbance is defined as any process that results in the removal (either partial or complete) of the existing community. As seen in the Hubbard Brook example, the disturbance does not always result in the removal of all individuals. In these cases, the amount (density and biomass) and composition of the surviving community will have a major influence on the proceeding successional dynamics. Additional discussion of disturbance and its role in structuring communities is presented later (Chapter 19).
18.2 Primary Succession Occurs on Newly Exposed Substrates
Primary succession begins on sites that have never supported a community, such as rock outcrops and cliffs, lava fields, sand dunes, and newly exposed glacial till. For example, consider primary succession on an inhospitable site: a sand dune. Sand, a product of weathered rock, is deposited by wind and water. Where deposits are extensive, as along the shores of lakes and oceans and on inland sand barrens, sand particles may be piled in long, windward slopes that form dunes (Figure 18.4a). Under the forces of wind and water, the dunes can shift, often covering existing vegetation or buildings. The establishment and growth of plant cover acts to stabilize the dunes. The late plant ecologist H. C. Cowles of the University of Chicago first described colonization of sand dunes and the progressive development of vegetation in his pioneering classic study (published in 1899) of plant succession on the dunes of Lake Michigan. Later work by the ecologist John Lichter of the University of Minnesota would quantify the patterns first described by Cowles by examining a chronosequence of dunes (determined by radiocarbon dating) on the northern border of Lake Michigan (Figure 18.4b). A chronosequence (or chronosere) is a series of sites within an area that are at different stages of succession (seral stages). Because it is not always possible to monitor a site for the decades or centuries over which the process of succession occurs, it is often necessary to identify sites of different ages that represent the various stages of succession. In effect, the use of a chronosequence substitutes space for time.
In the process of primary succession on the newly formed dunes (Figure 18.4b), grasses, especially beach grass (Ammophila breviligulata), are the most successful pioneering plants and function to stabilize the dunes with their extensive system of rhizomes (see Section 8.1). Once the dunes are stabilized, mat-forming shrubs invade the area. Subsequently, the vegetation shifts to dominance by trees—first pines and then oak. Because of low moisture reserves in the sand, oak is rarely replaced by more moisture-demanding (mesophytic) trees. Only on the more favorable leeward slopes and in depressions, where microclimate is more moderate and where moisture can accumulate, does succession proceed to more mesophytic trees such as sugar maple, basswood, and red oak. Because these trees shade the soil and accumulate litter on the soil surface, they act to improve nutrients and moisture conditions. On such sites, a mesophytic forest may become established without going through the oak and pine stages. This example emphasizes one aspect of primary succession: the colonizing species ameliorate the environment, paving the way for invasion of other species.
Newly deposited alluvial soil on a floodplain represents another example of primary succession. Over the past 200 years, the glacier that once covered the entire region of Glacier Bay National Park, Alaska, has been retreating (melting; Figures 18.5. As the glacier retreats, a variety of species such as alder (Alnus spp.) and cottonwood (Populus spp.) initially colonize the newly exposed landscape. Eventually, the later successional tree species of spruce (Picea spp.) and hemlock (Tsuga spp.) replace these early successional species, and the resulting forest (Figure 18.5c) resembles the forest communities in the surrounding landscape
18.3 Secondary Succession Occurs after Disturbances
A classic example of secondary succession in terrestrial environments is the study of old-field succession in the Piedmont region of North Carolina by the eminent plant ecologist Dwight Billings (Duke University) in the late 1930s (see Figure 18.1). During the first year after a crop field has been abandoned, the ground is claimed by annual crabgrass (Digitaria sanguinalis); its seeds, lying dormant in the soil, respond to light and moisture and germinate. However, the crabgrass’s claim to the ground is short-lived. In late summer, the seeds of horseweed (Lactuca canadensis), a winter annual, ripen. Carried by the wind, the seeds settle on the old field, germinate, and by early winter have produced rosettes. The following spring, horseweed, off to a head start over crabgrass, quickly claims the field. During the second summer, other plants invade the field: white aster (Aster ericoides) and ragweed (Ambrosia artemisiifolia).
By the third summer, broomsedge (Andropogon virginicus), a perennial bunchgrass, colonizes the field. Abundant organic matter and the ability to exploit soil moisture efficiently permits broomsedge to dominate the field. About this time, pine seedlings, finding room to grow in open places among the clumps of broomsedge, invade the field. Within 5 to 10 years, the pines are tall enough to shade the broomsedge. Eventually, hardwood species such as oaks and ash grow up through the pines, and as the pines die, they take over the field (Figure 18.6). Development of the hardwood forest continues as shade-tolerant trees and shrubs—dogwood, redbud, sourwood, hydrangea, and others—fill the understory.
Similarly, studies of physical disturbance in marine environments have demonstrated secondary succession in seaweed, salt marsh, mangrove, seagrass, and coral reef communities. Ecologist David Duggins of the University of Washington examined the process of secondary succession after disturbance in the subtidal kelp forests of Torch Bay, Alaska (Figure 18.7). The dominant herbivore in the subtidal communities of the north Pacific is the sea urchin (Strongylocentrotus spp.). In the absence of their predators, the sea otter (Enhydra lutris), sea urchins overgraze the kelp (macroalgae), removing virtually all algal biomass (see previous example in Section 16.4, Figure 16.6). In a series of studies, Duggins examined the recovery of the kelp forests following the removal of sea urchins. In the first year following the removal of the urchins, both annual and perennial kelps colonized the plots. A mixed canopy of annual kelp species dominated by Nereocystis luetkeana formed, and an understory of Alaria fistulosa and Costaria costata developed. By the second and third year, however, all annual species declined in abundance and a continuous stand of the perennial species Laminaria groenlandica developed. As a result of the dense canopy formed by Laminaria, shading and abrasion of the substrate suppressed the further recruitment and growth of annual species. A similar pattern has been observed in the subtidal kelp forests off the California coast.
Secondary succession in seagrass communities has been described for a variety of locations, including the shallow tropical waters of Australia and the Caribbean. Wave action associated with storms or heavy grazing by sea turtles and urchins creates openings in the grass cover, exposing the underlying sediments. Erosion on the down-current side of these openings results in localized disturbances called blowouts (Figure 18.8). Ecologist Susan Williams of the University of Washington has described secondary succession in detail in the seagrass communities of the Caribbean. Williams examined the recovery of the seagrass community (St. Croix, United States Virgin Islands) on a number of experimental plots following the removal of vegetation.
Rhizophytic macroalgae, comprised mostly of species of Halimeda and Penicillus (Figure 18.9), initially colonized the disturbed sites. These algae have some sediment-binding capability, but their ability to stabilize the sediments is minimal, and their major function in the early successional stage seems to be the contribution of sedimentary particles as they die and decompose. After the first year, algal densities begin to decline. There was no evidence that rhizophytic algae inhibited recolonization of the seagrasses, which invaded the plots during the first few months following disturbance. The density of the early successional species of seagrass, manatee grass (Syringodium filiforme), increased linearly during the first 15 months, eventually declining as the slower-growing, later-successional species, turtle grass (Thalassia testudinum) colonized the plots. The leaves and extensive rhizome and root systems of the sea grasses effectively trap and retain particles, increasing the organic matter of the sediment, and the once-disturbed area again resembles the surrounding seagrass community.
18.4 The Study of Succession Has a Rich History
The study of succession has been a focus of ecological research for more than a century. Early in the 20th century, botanists E. Warming in Denmark and Henry Cowles in the United States largely developed the concept of ecological succession. The intervening years have seen a variety of hypotheses attempting to address the processes that drive succession, that is, the seemingly consistent directional change in species composition through time.
Frederic Clements (1916, 1936) developed a theory of plant succession and community dynamics known as the monoclimax hypothesis. The community is viewed as a highly integrated superorganism and the process of succession represents the gradual and progressive development of the community to the ultimate, or climax stage (see Section 16.10 for further discussion). The process was seen as analogous to the development of an individual organism.
In 1954, Frank Egler proposed a hypothesis he termed initial floristic composition. In Egler’s view, the process of succession at any site is dependent on which species get there first. Species replacement is not an orderly process because some species suppress or exclude others from colonizing the site. No species is competitively superior to another. The colonizing species that arrive first inhibit any further establishment of newcomers. Once the original colonizers eventually die, the site then becomes accessible to other species. Succession is therefore individualistic and dependent on the particular species that colonize the site and the order in which they arrive.
In 1977, ecologists Joseph Connell of University of California–Santa Barbara and Ralph Slatyer of Australian National University proposed a generalized framework for viewing succession that considers a range of species interactions and responses through succession. They offered three models.
The facilitation model states that early successional species modify the environment so that it becomes more suitable for later successional species to invade and grow to maturity. In effect, early-stage species prepare the way for late-stage species, facilitating their success (see Chapter 15 for discussion of facilitation).
The inhibition model involves strong competitive interactions. No one species is completely superior to another. The first species to arrive holds the site against all invaders. It makes the site less suitable for both early and late successional species. As long as it lives and reproduces, the first species maintains its position. The species relinquishes it only when it is damaged or dies, releasing space to another species. Gradually, however, species composition shifts as short-lived species give way to long-lived ones.
The tolerance model holds that later successional species are neither inhibited nor aided by species of previous stages. Later-stage species can invade a newly exposed site, establish themselves, and grow to maturity independently of the species that precede or follow them. They can do so because they tolerate a lower level of some resources. Such interactions lead to communities composed of those species most efficient in exploiting available resources. An example might be a highly shade-tolerant species that could invade, persist, and grow beneath the canopy because it is able to exist at a lower level of one resource: light. Ultimately, through time, one species would prevail.
Since the work of Connell and Slatyer, the search for a general model of plant succession has continued among ecologists. The life history classification of plants put forward by ecologist J. Phillip Grime of the University of Sheffield is based on three primary plant strategies (see Section 10.13, Figures 10.25 and 10.26). Species exhibiting the R, or ruderal, strategy rapidly colonize disturbed sites but are small in stature and short-lived. Allocation of resources is primarily to reproduction, with characteristics allowing for a wide dispersal of propagules to newly disturbed sites. Predictable habitats with abundant resources favor species that allocate resources to growth, favoring resource acquisition and competitive ability (C strategy). Habitats in which resources are limited favor stress-tolerant species (S strategy) that allocate resources to maintenance. Grime’s theory views succession as a shift in the dominance of these three plant strategies in response to changing environmental conditions (habitats). Following the disturbance that initiates secondary succession, essential resources (light, water, and nutrients) are abundant, selecting for ruderal (R) species that can quickly colonize the site. As time progresses and plant biomass increases, competition for resources occurs, selecting for competitive (C) species. As resources become depleted as a result of high demand by growing plant populations, the (C) species will eventually be replaced by the stress-tolerant (S) species that are able to persist under low resource conditions. The pattern of changing dominance in plant strategies in response to changing environmental conditions is shown in Figure 18.10.
Plant ecologist Fakhri Bazzaz of Harvard University approached developing an understanding of plant succession by examining the nature of successional environments and the eco-physiological characteristics of different functional groups of plants involved in the process of colonization and replacement during the process of succession. He focused on characteristics of seed dispersal, storage, germination, and species photosynthetic and growth response to resource gradients of light and water availability. Bazzaz concluded that early and late successional plants have contrasting physiological characteristics that enable them to flourish in the contrasting environmental conditions presented by early and late successional habitats (Table 18.1).
Michael Huston of Texas State University and Thomas Smith of the University of Virginia proposed a model of community dynamics based on plant adaptations to environmental gradients. Their model is based on the cost-benefit concept that plant adaptations for the simultaneous use of two or more resources are limited by physiological and life history constraints. Their model focuses on the resources of light and water. The plants themselves largely influence variations in light levels within the community, whereas the availability of water is largely a function of climate and soils. Succession is interpreted as a temporal shift in species dominance, primarily in response to changes in light availability. There is an inverse relationship between the ability to survive and grow under low light conditions and the ability to photosynthesize and grow at high rates when the availability of light is high (see Section 6.8, Figure 6.8). The pattern of species dominance shifts from fast-growing, shade-intolerant species in early succession to slower-growing, shade-tolerant species later in succession.
The ecologist David Tilman of the University of Minnesota proposed a model of succession based on a trade-off in characteristics that enable plants to compete for the essential resources of nitrogen and light, referred to as the resource-ratio hypothesis. The ability to effectively compete for light is associated with the allocation of carbon to the production of aboveground tissues (leaves and stems). Conversely, the ability to effectively compete for nitrogen is associated with the production of root tissue. This pattern of changing allocation of carbon under varying nutrient and light resources is discussed in Chapter 6. Succession comes about as the relative availability of nitrogen and light change through time. In Tilman’s model, the availability of these two essential resources is inversely related. Environmental conditions range from habitats with soils poor in nitrogen but with a high availability of light at the soil surface to habitats with nitrogen-rich soils and low availability of light. Community composition changes along this gradient as the ratio of nitrogen to light changes (Figure 18.11).
Although many hypotheses have been put forward to explain the general patterns of species colonization and replacement during succession, despite the differences among the various hypotheses, a general trend in thinking has emerged. The current focus is on how the adaptations and life history traits of individual species influence species interactions and ultimately species distribution and abundance under changing environmental conditions.
18.5 Succession Is Associated with Autogenic Changes in Environmental Conditions
The changes in environmental conditions that bring about shifts in the physical and biological structures of communities across the landscape are varied. They can, however, be grouped into two general classes: autogenic and allogenic. Autogenic environmental change is a direct result of the presence and activities of organisms within the community. For example, the vertical profile of light in a forest is a direct result of the interception and reflection of solar radiation by the trees (see Section 4.2 and Chapter 4, Quantifying Ecology 4.1). In contrast, allogenic environmental change is a feature of the physical environment; it is governed by physical rather than biological processes. Examples are the decline in average temperature with elevation in mountainous regions, the decrease in temperature with depth in a lake or ocean, and the changes in salinity and water depth in coastal environments (see Sections 2.8 and 3.4).
Previously, we defined succession as change in community structure though time, specifically, change in species dominance (Section 16.3). One group of species initially colonizes an area, but as time progresses, it declines and is replaced by another group of species. We observe this general pattern of changing species dominance as time progresses in most natural environments, suggesting a common underlying mechanism.
One feature common to all plant succession is autogenic environmental change. In both primary and secondary succession, colonization alters environmental conditions. One clear example is the alteration of the light environment. Leaves reflecting and intercepting solar radiation create a vertical profile of light within a plant community. In moving from the canopy to ground level, less light is available to drive the processes of photosynthesis (see Chapter 4, Quantifying Ecology 4.1). During the initial period of colonization, few if any plants are present. In the case of primary succession, the newly exposed site has never been occupied. In the case of secondary succession, plants have been killed or removed by some disturbance. Under these circumstances, the availability of light at the ground level is high, and seedlings are able to establish themselves. As plants grow, their leaves intercept sunlight, reducing the availability of light to shorter stature plants (Figure 18.12). This reduction in available light decreases rates of photosynthesis, slowing the growth of the shaded individuals. Assuming that not all plant species can photosynthesize and grow at the same rate, plant species that can grow tall the fastest have greater access to the light resource. They subsequently reduce the availability of light to the slower-growing species, enabling the fast-growing species to outcompete the other species and dominate the site. However, in changing the availability of light below the canopy, the dominant species create an environment that is more suitable for the species that will later displace them as dominants.
Recall that not all plant species respond in the same way to variation in available light. There is a fundamental physiological trade-off between the adaptations that enable high rates of growth under high light conditions and the ability to continue growth and survival under shaded conditions (Section 6.8, Figure 6.8). In the early stages of plant succession, shade-intolerant species can dominate because of their high growth rates. Shade-intolerant species grow above and shade the slower-growing, shade-tolerant species. As time progresses and light levels decline below the canopy, however, seedlings of the shade-intolerant species cannot grow and survive in the shaded conditions (see Section 6.8, Figure 6.10). At that time, although shade-intolerant species dominate the canopy, no new individuals are recruited into their populations. In contrast, shade-tolerant species are able to germinate and grow under the canopy. As the shade-intolerant plants that form the canopy die, shade-tolerant species in the understory replace them.
Figure 18.13 shows this pattern of changing population recruitment, mortality, and species composition through time in the forest community in the Piedmont region of North Carolina presented in Figure 18.6. Fast-growing, shade-intolerant pine species dominate in early succession. Over time, the number of new pine seedlings declines as the light decreases at the forest floor. Shade-tolerant oak and hickory species, however, are able to establish seedlings in the shaded conditions of the understory. As the pine trees in the canopy die, the community shifts from a forest dominated by pine species to one dominated by oaks and hickories.
In this example, succession results from changes in the relative environmental tolerances and competitive abilities of the species under autogenically changing environmental conditions (light availability). Shade-intolerant species are able to dominate the early stages of succession because of their ability to grow quickly in the high light environment. However, as autogenic changes in the light environment occur, the ability to tolerate and grow under shaded conditions enables shade-tolerant species to rise to dominance.
Light is not the only environmental factor that changes during the course of succession, however. Other autogenic changes in environmental conditions can influence patterns of succession. The seeds of some plant species cannot germinate on the surface of mineral soil; these seeds require the buildup of organic matter on the soil surface before they can germinate and become established. In the examples of secondary succession on sand dunes (Figure 18.4) and in seagrass communities (Figure 18.9), early colonizing species function to stabilize the sediments and add organic matter, allowing for later colonization by other plant species.
Consider the example of primary succession on newly deposited glacial sediments (see Figure 18.5). Because of the absence of a well-developed soil, little nitrogen is present in these newly exposed surfaces, thus restricting the establishment, growth, and survival of most plant species. However, those terrestrial plant species that have the mutualistic association with nitrogen-fixing Rhizobium bacteria are able to grow and dominate the site (see Section 15.11, Figure 15.11). These plants provide a source of carbon (food) to the bacteria that inhabit their root systems. In return, the plants have access to the atmospheric nitrogen fixed by the bacteria. Alder, which colonizes the newly exposed glacial sediments in Glacier Bay, is one such plant species (see Figure 18.5c).
As individual alder shrubs shed their leaves or die, the nitrogen they contain is released to the soil through the processes of decomposition and mineralization (Figure 18.14; see also Chapter 21). Now other plant species can colonize the site. As nitrogen becomes increasingly available in the soil, species that do not have the added cost of mutualistic association and that exhibit faster rates of growth and recruitment come to dominate the site. As in the Piedmont forest example, succession is a result of autogenic change in the environment and the relative competitive abilities of the species colonizing the site.
The exact nature of succession varies from one community to another, and it involves a variety of species interactions and responses that include facilitation, competition, inhibition, and differences in environmental tolerances. However, in all cases, the role of temporal, autogenic changes in environmental conditions and the differential response of species to those changes are key features of community dynamics.
18.6 Species Diversity Changes during Succession
In addition to shifts in species dominance, patterns of plant species diversity change over the course of succession. Studies of secondary succession in old-field communities have shown that plant species diversity typically increases with site age (that is, time since abandonment). The late plant ecologist Robert Whittaker of Cornell University, however, observed a different temporal pattern of species diversity for sites in New York (Figure 18.15). Species diversity increases into the late herbaceous stages and then decreases into shrub stages. Species diversity then increases again in young forest, only to decrease as the forest ages.
The processes of species colonization and replacement drive succession. To understand the changing patterns of species richness and diversity during succession, we need to understand how these two processes vary in time. Colonization by new species increases local species richness. Species replacement typically results from competition or an inability of a species to tolerate changing environmental conditions. Species replacement over time acts to decrease species richness.
During the early phases of succession, diversity increases as new species colonize the site. However, as time progresses, species become displaced, replaced as dominants by slower-growing, more shade-tolerant species. The peak in diversity during the middle stages of succession corresponds to the transition period, after the arrival of later successional species but before the decline (replacement) of early successional species. The two peaks in diversity seen in Figure 18.15 correspond to the transition between the herbaceous- and shrub-dominated phases, when both groups of plants are present, and the transition between early and later stages of woody plant succession. Species diversity declines as shade-intolerant tree species displace the earlier successional trees and shrubs.
The rate of displacement is influenced by the growth rates of species involved in the succession. If growth rates are slow, the displacement process moves slowly; if growth rates are fast, displacement occurs more quickly. This observation led Michael Huston, an ecologist at Texas State University, to conclude that patterns of diversity through succession vary with environmental conditions (particularly resource availability) that directly influence the rates of plant growth. By slowing the population growth rate of competitors that eventually displace earlier successional species, the period of coexistence is extended, and species diversity can remain high (Figure 18.16). This hypothesis predicts the highest diversity at low to intermediate levels of resource availability by extending the period of coexistence.
(Data from Huston, Michael A., Biological Diversity: the Coexistence of Species on Changing Landscapes [Cambridge University Press, 1994].)
Disturbance can have an effect similar to that of reduced growth rates by extending the period during which species coexist. In the simplest sense, disturbance acts to reset the clock in succession (Figure 18.17). By reducing or eliminating plant populations, the site is once again colonized by early successional species, and the process of colonization and species replacement begins again. If the frequency of disturbance (defined by the time interval between disturbances) is high, then later successional species will never have the opportunity to colonize the site. Under this scenario, diversity remains low. In the absence of disturbance, later successional species displace earlier ones and species diversity declines. At an intermediate frequency of disturbance, colonization can occur, but competitive displacement is held to a minimum. The pattern of high diversity at intermediate frequencies of disturbance was proposed independently by Michael Huston and by Joseph Connell of the University of California–Santa Barbara and is referred to as the intermediate disturbance hypothesis.
18.7 Succession Involves Heterotrophic Species
Although our discussion and examples of succession have thus far focused on temporal changes in the autotrophic component of the community (plant succession), associated changes in the heterotrophic component also occur. As plant succession advances, changes in the structure and composition of the vegetation result in changes in the animal life that depends on the vegetation as habitat. Each successional stage has its own distinctive fauna. Because animal life is often influenced more by structural characteristics than by species composition, successional stages of animal life may not correspond to the stages identified by plant ecologists.
During plant succession, animals can quickly lose their habitat as species composition and structure of the vegetation changes. For example, we can return to the patterns of secondary succession that occurs in abandoned agricultural lands in eastern North America presented in Figure 18.1 (Figure 18.18). In the early stages of succession following abandonment, grasslands and old fields support meadowlarks, meadow mice, and grasshoppers. When woody plants—both young trees and shrubs—invade, a new structural element appears. Grassland animals soon disappear, and shrubland animals take over. Towhees, catbirds, and goldfinches claim the thickets, and meadow mice give way to white-footed mice. As woody plant growth proceeds and the canopy closes, species of the shrubland decline and are replaced by birds and insects of the forest canopy. As succession proceeds, the vertical structure becomes more complex. New species appear, such as tree squirrels, woodpeckers, and birds of the forest understory, including hooded warblers and ovenbirds.
Ecologists Davis Johnson and Eugene Odum of the University of Georgia examined changes in the breeding bird community along a secondary successional gradient in the coastal region of Georgia. The researchers carried out a census of the breeding bird community on 10 sites representing a chronosequence ranging from 1 to more than 150 years since abandonment following agriculture. They found that the sites could be classified into four broad successional stages or seres dominated by four distinct plant life forms that succeed one another: herbs (grass forbs), shrubs, pines, and hardwoods. The occurrence of most bird species is limited to a given seral stage, but some species persist through many stages. As a result, each seral stage was characterized by a unique bird community. The researchers found that both species richness and diversity (Simpson’s index; see Section 16.2) increased with successional age through the first 60 to 100 years (forest stages; Figure 18.19). The increase in bird species diversity is a function of the increasing vertical structure of the vegetation during succession (see relationship between bird species diversity and foliage height diversity, Section 17.6, Figure 17.17).
Similar changes in the diversity of the small mammal community during plant succession have been observed in old-field communities. Nancy Huntley and Richard Inouye of the University of Minnesota examined the small mammal communities of 18 successional old fields in Minnesota ranging in age from 2 to 57 years since agricultural abandonment. The species composition, biomass, and cover of the vegetation in the old fields changed along the chronosequence, and the species richness of the small mammal community likewise increased with field age (Figure 18.20). The researchers found that increase in both abundance and diversity of small mammals was related to increases in plant cover and nitrogen content (increased nitrogen content of food plants).
Maria Barberaena-Arias and T. Mitchell Aide of the University of Puerto Rico examined species composition of insects inhabiting the forest floor during secondary succession of tropical forests in Puerto Rico. The researchers documented chronosequences of secondary forest succession at four locations (different regions) on the island. At each of the four locations, sites representing approximately 5, 30, and 60 years following agricultural abandonment were sampled throughout the year to determine the species composition of the insect community. At each of the four locations, the researchers observed associated changes in the forest floor insect community during the process of plant succession. Species diversity of the insect community increased with forest age (Figure 18.21a). The increase in species diversity of insects on the forest floor was associated with the increased accumulation of leaf litter (dead and decomposing leaves on the forest floor), providing a more complex array of habitats and resources (Figures 18.21b and 18.21b).
In each of the examples presented, the changes in species composition and diversity of the heterotrophic communities over time are a product of changes in the associated vegetation during the process of plant succession. As illustrated in Figure 18.18, changes in the composition and structure of the vegetation through time alter the availability of habitats and resources available, shifting the array of animal species that can survive, grow, and reproduce within the community. In other cases, however, heterotrophic succession can be a product of autogenic changes in the environmental conditions brought about by the heterotrophic organisms themselves. A well-studied example of this type of succession is provided by the observed changes in the heterotrophic communities involved in decomposition. Dead plant tissues, animal carcasses, and droppings form substrates on which communities of organisms involved in decomposition exist. Within these communities, groups of fungi and animals succeed one another in a process of colonization and replacement that relates to changes in the physical and chemical properties of the substrate through time. These changes in the substrate are a direct function of the feeding activities of the decomposer organisms. We will examine the process of decomposition and associated changes in the decomposer (heterotrophic) community in detail in Chapter 21.
18.8 Systematic Changes in Community Structure Are a Result of Allogenic Environmental Change at a Variety of Timescales
The focus on succession thus far has been on shifting patterns of community structure in response to autogenic changes in environmental conditions. Such changes occur at timescales relating to the establishment and growth of the organisms that make up the community. However, purely abiotic environmental (allogenic) change can produce patterns of succession over timescales ranging from days to millennia or longer. Environmental fluctuations that occur repeatedly during an organism’s lifetime are unlikely to influence patterns of succession among species with that general life span. For example, annual fluctuations in temperature and precipitation influence the relative growth responses of different species in a forest or grassland community, but they have little influence on the general patterns of secondary succession outlined in Figures 18.1 and 18.3. In contrast, shifts in environmental conditions that occur at periods as long as or longer than the organisms’ life span are likely to result in successional shifts in species dominance. For example, seasonal changes in temperature, photoperiod, and light intensity produce a well-known succession of dominant phytoplankton in freshwater lakes that is repeated with very little variation each year. Seasonal succession of phytoplankton in Lawrence Lake, a small temperate lake in Michigan, is presented in Figure 18.22. Periods of dominance are correlated with species’ optimal temperature, nutrient, and light requirements, all of which systematically change over the growing season. Competition and seasonal patterns of predation by herbivorous zooplankton also interact to influence the temporal patterns of species composition.
Over a much longer timescale of decades to centuries, patterns of sediment deposition can have a major influence on the successional dynamics of coastal and freshwater communities. Ponds and small lakes act as a settling basin for inputs of sediment from the surrounding watershed (Figure 18.23). These sediments form an oozy layer that provides a substrate for rooted aquatics such as the branching green algae, Chara, and pondweeds. These plants bind the loose matrix of bottom sediments and add materially to the accumulation of organic matter. Rapid addition of organic matter and sediments reduces water depth and increases the colonization of the basin by submerged and emergent vegetation. That, in turn, enriches the water with nutrients and organic matter. This enrichment further stimulates plant growth and sedimentation and expands the surface area available for colonization by larger species of plants that root in the sediments. Eventually, the substrate, supporting emergent vegetation such as sedges and cattails, develops into a marsh. As drainage improves and the land builds higher, emergent plants disappear. Meadow grasses invade to form a marsh meadow in forested regions and wet prairie in grass country. Depending on the region, the area may pass into grassland, swamp woodland of hardwoods or conifers, or peat bog.
Over an even longer timescale, changes in regional climate directly influence the temporal dynamics of communities. The shifting distribution of tree species and forest communities during the 18000 years that followed the last glacial maximum in eastern North America is an example of how long-term allogenic changes in the environment can directly influence patterns of both succession and zonation at local, regional, and even global scales.
18.9 Community Structure Changes over Geologic Time
Since its inception some 4.6 billion years ago, Earth has changed profoundly. Landmasses emerged and broke into continents. Mountains formed and eroded, seas rose and fell, and ice sheets advanced to cover large expanses of the Northern and Southern Hemispheres and then retreated. All these changes affected the climate and other environmental conditions from one region of Earth to another. Many species of plants and animals evolved, disappeared, and were replaced by others. As environmental conditions changed, so did the distribution and abundance of plant and animal species.
Records of plants and animals composing past communities lie buried as fossils: bones, insect exoskeletons, plant parts, and pollen grains. The study of the distribution and abundance of ancient organisms and their relationship to the environment is paleoecology. The key to explaining present-day distributions of animals and plants can often be found in paleoecological studies. For example, paleoecologists have reconstructed the distribution of plants in eastern North America after the last glacial maximum of the Pleistocene.
The Pleistocene (approximately 2.6 million to 11,700 b.p.) was an epoch of great climatic fluctuations throughout the world. Some 20 glacial cycles occurred during which ice sheets advanced and retreated. At maximum glacial extent, up to 30 percent of Earth’s surface was covered by ice. The last great ice sheet, the Laurentian, reached its maximum advance about 20,000 to 18,000 b.p. during the Wisconsin glaciation stage in North America (Figure 18.24). Canada was under ice. A narrow belt of tundra about 60 to 100 km wide bordered the edge of the ice sheet and probably extended southward into the high Appalachians. Boreal forest, dominated by spruce and jack pine (Pinus banksiana), covered most of the eastern and central United States as far as western Kansas.
As the climate warmed and the ice sheet retreated northward, plant species invaded the glaciated areas. The maps in Figure 18.25 reflect the advances of four major tree genera in eastern North America after the retreat of the ice sheet. Margaret Davis of the University of Minnesota developed these maps from patterns of pollen deposition in sediment cores taken from lakes in eastern North America. By examining the presence and quantity of pollen deposited in sediment layers and radiocarbon dating the sediments, she was able to obtain a picture of the spatial and temporal dynamics of tree communities over the past 18,000 years.
These analyses identify plants at the level of genus rather than species because, in many cases, we cannot identify species from pollen grains. Note that different genera, and associated species, expanded their distribution northward with the retreat of the glacier at markedly different rates. Differences in the rates of range expansion are most likely the result of the differences in temperature responses of the species, distances and rates at which seeds can disperse, and interactions among species. The implication is that, during the past 18,000 years, the distribution and abundance of species and the subsequent structure of forest communities in eastern North America have changed dramatically (Figure 18.26).
18.10 The Concept of Community Revisited
Our initial discussion of the processes influencing community structure and dynamics contrasted two views of the community (see Section 16.10). Through his organismal concept, Frederic Clements viewed the community as a quasi-organism made up of interdependent species. By contrast, in his individualistic or continuum concept, H. A. Gleason saw the community as an arbitrary concept and stated that each species responds independently to the underlying features of the environment. Research reveals that, as with most polarized debates, the reality lies somewhere in the middle, and our viewpoints are often colored by our perspective. The organismal community is a spatial concept. As we stand in the forest, we see a variety of plant and animal species interacting and influencing the overall structure of the forest. The continuum view is a population concept, focusing on the response of the component species to the underlying features of the environment.
A simple example of the continuum concept is presented in Figure 18.27, which represents a transect up a mountain in an area with four plant species present. The distributions of the four plant species are presented in two ways. In the first view, the species distributions are plotted as a function of altitude or elevation. Note that the four species exhibit a continuum of species regularly replacing one another in a sequence of A, B, C, and D with increasing altitude—similar to the individualistic view of communities. The second view of species distributions is their spatial location along the transect. As we move up the mountainside, the distributions of the four species are not continuous. As a result, we might recognize several species associations as we walk along the transect. These associations are identified in Figure 18.27 by different symbols representing the combination of species. Communities composed of coexisting species are a consequence of the spatial pattern of the landscape.
The two views are quite different yet consistent. Each species has a continuous response to the environmental variable of elevation. Yet it is the spatial distribution of that environmental variable across the landscape that determines the overlapping patterns of species distributions, that is, the composition of the community.
This same approach can be applied to the patterns of forest communities in Great Smoky Mountains National Park (Figure 18.28a). Different elevations and slope positions are characterized by unique tree communities, identified by and named for the dominant tree species (Figure 18.28b). When presented in this fashion, the distributions of plant communities appear to support the organismal model of communities put forward by Clements. Yet if we plot the distributions of major tree species along a direct environmental gradient, such as soil moisture availability (Figure 18.28c), the species appear to be distributed independently of one another, thus supporting Gleason’s view of the community.
The simple example in Figure 18.27 examines only one feature of the environment (elevation), yet the structure of communities is the product of a complex interaction of pattern and process. Species respond to a wide array of environmental factors that vary spatially and temporally across the landscape, and the interactions among organisms influence the nature of those responses. The product is a dynamic mosaic of communities that occupy the larger landscape.
Ecological Issues & Applications Community Dynamics in Eastern North America over the Past Two Centuries Are a Result of Changing Patterns of Land Use
Old-field communities, such as the one shown in Figure 18.1, are a common sight in the eastern portion of the United States. These fields represent the early stages in the process of secondary succession, a process that began with the abandonment of agricultural lands (cropland or pasture) and that will eventually lead to forest (see Figure 18.1). Although more than 50 percent of the United States land area east of the Mississippi River is currently covered by forest, the vast majority of these forest communities are less than 100 years old, the product of a continental-scale shift in land use that has occurred over the past 200 years.
When colonists first arrived on the eastern shores of North America in the 17th century, the landscape was dominated by forest. American Indians historically used fire to clear areas for planting crops or to create habitat for game species. However, their impact on the landscape was minor compared to what was to come as a result of the westward expansion of European colonization. The clearing of forest was driven by the need for agricultural lands and forest products, and as the population of colonists grew and expanded westward, so did the clearing of land (Figure 18.29). By the 19th century, most of the forests in eastern North America had been felled for agriculture. But by the early part of the 20th century, this trend was reversed.
The Dust Bowl period in the 1930s saw the beginning of the decline in small family farms in the agricultural regions west of the Mississippi (see discussion of Dust Bowl in Chapter 4, Ecological Issues & Applications). With the mechanization of agriculture and the large-scale production of chemical fertilizers by the late 1940s (see Chapter 21, Ecological Issues & Applications), agriculture in the West underwent a major transition, moving from small, family-owned farms to large commercial farms. The rise of large-scale commercial agriculture hastened the decline in agriculture east of the Mississippi River—a decline that began in the 1800s with the end of the large plantation farms in the Southern states. By the 1930s, the amount of agricultural land in the east had peaked, and it has been declining ever since (Figure 18.30). Since 1972 alone, more than 4500 square miles of farmland in the eastern United States has been abandoned and is currently reverting to forest.
The result of these trends is a regional-scale pattern of secondary succession in the eastern United States that has seen a shift from agricultural lands to old-field communities and the eventual reestablishment of forests. As we have seen in the various examples presented throughout the chapter, this transition has major implications for the species composition and patterns of species diversity. The reforestation of eastern North America enhanced the population and diversity of plant and animal species that depend on forested habitats (Figure 18.31) but have led to the decline of species dependent on the grassland habitats that were maintained by agricultural land-use practices. There are many examples of population decline in grassland birds in the eastern United States, most notably the extinction of the heath hen from the northeast. Likewise, over the 25-year period from 1966 to 1991, New England upland sandpiper and eastern meadowlark populations declined by 84 and 97 percent, respectively. The greater prairie-chicken has experienced an average annual rate of decline of more than 10 percent during this same 25-year period.
Despite the long-term trend of reforestation in the eastern regions of North America over the past century, recent studies indicate that this trend may be starting to reverse. Mark Drummond and Thomas Loveland examined land-use changes in the eastern United States from 1973 to 2000 as part of the United States Geological Survey’s (USGS) Land Cover Trends project, using satellite data, survey data, and ground photographs. Over the observation period (1973–2000) the researchers document a 4.1 percent decline in total forest area, a net loss equivalent to more than 3.7 million hectares. The researchers found considerable regional variation, with net loss being particularly marked in the coastal plains of southeast. The major sources of forest decline are forestry activities, including both the harvesting of current forested lands and their conversion to pine plantations and development from urban expansion (Figure 18.32).
The future dynamics of the landscapes of eastern North America will certainly depend on the changing patterns of land use and the growing demand for land for urban development. However, the potential for future changes to the global climate system as a result of human activities (see Chapter 2, Ecological Issues & Applications) may well be the primary determinate of the future of the forests in the United States, a topic we will explore in detail in Chapter 27.
Summary
Succession 18.1
With time, natural communities change. This gradual sequential change in the relative abundance of species in a community is succession. Opportunistic, early successional species yield to late successional species. Succession occurs in all environments. The similarity of successional patterns in different environments suggests a common set of processes.
Primary Succession 18.2
Primary succession begins on sites devoid of or unchanged by organisms. Examples include newly formed sand dunes, lava flows, or newly exposed glacial sediments.
Secondary Succession 18.3
Secondary succession begins after disturbance on sites where organisms are already present. Terrestrial examples include abandoned agricultural lands or the reestablishment of vegetation after logging or fire. In aquatic ecosystems, disturbances caused by storms, wave action, or herbivory can initiate the process of secondary succession.
History 18.4
The study of succession has been a focus of ecological research for more than a century. The intervening years have seen a variety of hypotheses attempting to address the processes that drive succession. These hypotheses include a variety of processes related to colonization, facilitation, competition, inhibition, and differences in environmental tolerances.
Autogenic Environmental Change 18.5
Environmental changes can be autogenic or allogenic. Autogenic changes are a direct result of the activities of organisms in the community. Changes in environmental conditions independent of organisms are allogenic. Succession is the progressive change in community composition through time in response to autogenic changes in environmental conditions. One example is the changing light environment and the shift in dominance from fast-growing, shade-intolerant plants to slow-growing, shade-tolerant plants observed in terrestrial plant succession. Autogenic changes in nutrient availability, soil organic matter, and stabilization of sediments can likewise have a major influence on succession.
Species Diversity and Succession 18.6
Patterns of species diversity change during the course of succession. Species colonization increases species richness, whereas species replacement acts to decrease the number of species present. Species diversity increases during the initial stages of succession as the site is colonized by new species. As early successional species are displaced by later arrivals, species diversity tends to decline. Peaks in diversity tend to occur during succession stages that correspond to the transition period, after the arrival of later successional species but before the decline of early successional species. Patterns of diversity during succession are influenced by resource availability and disturbance.
Heterotrophic Species 18.7
Changes in the heterotrophic component of the community also occur during succession. Successional changes in vegetation affect the nature and diversity of animal life. Certain sets of species are associated with the structure of vegetation found during each successional stage.
Allogenic Environmental Change 18.8
Fluctuations in the environment that occur repeatedly during an organism’s lifetime are unlikely to influence patterns of succession among species with that general life span. Allogenic, abiotic environmental changes that occur over timescales greater than the longevity of the dominant organisms can produce patterns of succession over timescales ranging from days to millennia or longer.
Long-Term Changes 18.9
The current pattern of vegetational distribution reflects the glacial events of the Pleistocene. Plants retreated and advanced with the movements of the ice sheets. The rates and distances of their advances are reflected in the present-day ranges of species and the distribution of plant communities.
Community Revisited 18.10
The community is a spatial concept; the individual continuum is a population concept. Each species has a continuous response to an environmental gradient, such as elevation or moisture. Yet the spatial distribution of that environmental variable across the landscape determines the overlapping patterns of distribution, that is, the composition of the community.
Reforestation Ecological Issues & Applications
Although more than 50 percent of the U.S. land area east of the Mississippi River is currently covered by forest, the vast majority of these forest communities are less than 100 years old, the product of a continental-scale shift in land use that has occurred over the past 200 years. In recent decades, the trend of net forest gain has begun to reverse as demands for urban development and forest products have increased.
