Q1 ecological succession refers to

Ecosystem And Community Structure

The final paper will combine the ecological concepts from the course under one analysis: ecosystem and community ecology. The overall objective is to understand how the biotic and abiotic factors influence the physical and biological structures of ecosystems and the communities within them. This knowledge is fundamental for developing effective policy, and managing wildlife and habitats.

To begin the assignment, select one of the following ecosystems. Then, address the questions below. Analyzing ecological communities can be accomplished by taking apart all of the physical and biological pieces that make up an ecological community, and then relating them back to one another in order to understand how they interact to form a functioning system. Conclude the paper with an assessment of human impacts on your selected ecosystem and how this will affect the ecology and future adaptation capabilities of the species in your community. Reference your textbook and a minimum of six scholarly sources (i.e., academic journal research articles) to provide examples and evidence that support your main points.

Before beginning the assignment, be sure to read chapters 16 through 18 on community ecology carefully and to learn more about specific ecosystems, review chapters 23 through 25 in the course text, Smith, T.M. & Smith, R.L. (2015). Elements of Ecology. 9th edition. MA: Pearson.

Step 1: Choose one ecosystem to analyze.

Lakes and ponds

Step 2: Address the following points in your paper.

Describe the physical structure of the selected ecosystem and explain how structural factors come together to create the ecosystem. The physical structure refers to the abiotic (non-living) components—such as temperature, precipitation, bedrock, soil type, riverbed or ocean floor, boulders, and topography—that make up your ecosystem.
Describe the biological structure of your selected ecosystem. Explain how life forms come together to shape your ecosystem. The biological structure is how the ecosystem is shaped by plants and animals, and it includes forest levels and coral reef structures. It also includes the overall level of diversity, species composition, and dominant plant and animal species with details about niche, height, size, density, and/or function.
Analyze the function of the selected ecosystem by indicating how the physical and biological structures are related to the ecosystem’s productivity (i.e., the amount of biomass produced), complexity of food webs, nutrient availability, and nutrient cycling.
Explain the process of succession in your selected ecosystem.
Describe one mutualistic interaction in your selected ecosystem. Explain the role that each species has in the interaction and the resulting mutual benefits to each species population. In other words, what does each species give and what does each one get in return.
Describe one predator-prey (or plant-herbivore) interaction. Explain how this interaction impacts the ecological community as a whole. Evaluate the magnitude of human impact on your ecosystem. This includes how humans are affecting the physical structure and species in your ecosystem. Predict whether the species and their interactions presented in your paper will be able to adapt to these changes. For example, what would happen to pollinators in a plant-pollinator mutualism, if plants are destroyed due to urbanization?

Must be 8 to 12 double-spaced pages in length (not including title and references pages) and formatted according to APA style

Include an introduction stating your selected aquatic and terrestrial ecosystem followed by an overview of topics that will be discussed.
Include a summary of the main points discussed in the paper, with particular attention to ecosystem function and the future of your selected ecosystem.
Must use at least six scholarly sources (i.e., academic journal articles) in addition to the course text.
ATTCHED ARE THE CHAPTERS NEEDED. ATTACHED IS THE OUTLINE ALREADY ESTABLISHED. ATTACHED IS THE GRADING RUBICS( READ IT SO YOU KNOW WHAT NEEDS TO BE DONE) ATTACHED IS THE CORRECTIONS FOR THE OUTLINE ( READ THEM SO YOU KNOW WHAT NEEDS TO BE FIXED). MUST USE SIX (6) SCHOLORLY RESOURCES PLUS THE ATTACHED TEXT FOR A TOTAL OF 7 RESOURCES. MUST BE DONE ON TIME AND DONE RIGHT THE FIRST TIME PLEASE!!!

CHAPTER 16

Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.

16.1 Biological Structure of Community Defined by Species Composition

The biological structure of a community is defined by its species composition, that is, the set of species present and their relative abundances. For example, Table 16.1 contains samples representing tree species composition of two forest communities in northern West Virginia. For each forest, the first column provides the number of individuals of each species, and the second column provides a measure of their relative abundance expressed as the proportion each species contributes to the total number of individuals of all species within the community. Relative abundance in Table 16.1 is calculated as:

pi=ni/Npi=ni/N

where pi is the proportion of individuals in the community belonging to species i, ni is the number of individuals belonging to species i, and N is the total number of individuals of all species in the community. These values have then been converted to percentages.

The sample from the first forest community consists of 256 individuals representing 24 species. Two species—yellow poplar and white oak—make up nearly 44 percent of the total number of individuals. The four next most abundant trees—black oak, sugar maple, red maple, and American beech—each make up a little more than 5 percent of the total. Nine species range from 1.2 percent to 4.7 percent, and the 9 remaining species as a group represent about 0.4 percent each. The second forest presents a somewhat different picture. This community consists of 274 individuals representing 10 species, of which two species—yellow poplar and sassafras—make up almost 84 percent of the total number of individuals in the tree community.

A common method for examining the patterns of relative abundance within communities involves plotting the relative abundance of each species against rank, where rank is defined by the order of species from the most to the least abundant (the tree species in Table 16.1 are presented in order from the most to the least abundant). Thus, the most abundant species is plotted first along the x-axis, with the corresponding value on the y-axis being its value of relative abundance. This process is continued until all species are plotted. The resulting graph is called a rank-abundance diagram . Figure 16.1 depicts the rank-abundance curves for the two forest communities presented in Tables 16.1.

The rank-abundance diagram illustrates two features of community structure: species richness and species evenness. Species richness is a count of the number of species occurring within the community, and is typically denoted by the symbol S. Species evenness refers to the equitability in the distribution of individuals among the species. The maximum species evenness would occur if each species in the community was equally abundant.

Interpreting Ecological Data

1. Q1. How does the slope of the rank-abundance curve vary with increasing species evenness? Why?

2. Q2. What would the rank-abundance curve look like for a forest community consisting of 10 species, in which all of the 10 tree species are equally abundant?

The rank-abundance curves presented in Figure 16.1 shows that the two forest communities from Table 16.1 differ in both species richness and how individuals are apportioned among the species (evenness). The first forest community has greater species richness and a more equitable distribution of individuals among the species. The greater species richness is reflected by the greater length of the rank-abundance curve (24 species compared to 10 in the second community). The more equitable distribution of individuals among the species (species evenness) is indicated by the more gradual slope of the rank-abundance curve. If each species was equally abundant, the rank-abundance curve would be a straight line parallel to the x-axis at the value 1.0/S on the y-axis (where S is species richness, the number of species in the community).

16.2 Species Diversity Is Defined by Species Richness and Evenness

Although the graphical procedure of rank-abundance diagrams can be used to visually assess (interpret) differences in the biological structure of communities, these diagrams offer no means of quantifying the observed differences. The simplest quantitative measure of community structure is the index of species richness (S). However, species richness does not account for differences in the relative abundance of species within the community. For example, two communities may both be inhabited by the same number of species and therefore have the same value of species richness; yet in one community the vast majority of individuals may be of a single species, whereas in the other community the individuals may be more equally distributed among the various species (greater evenness; Figure 16.2). Ecologists have addressed this shortcoming by developing mathematical indices of species diversity , which consider both the number and relative abundance of species within the community.

One of the simplest and most widely used indices of species diversity is the Simpson’s index. The term Simpson’s diversity index can actually refer to any one of three closely related indexes.

Simpson’s index (D) measures the probability that two individuals randomly selected from a sample will belong to the same species (category):

D=∑pi2D=∑pi2

Where pi is the proportion of the total individuals in the community represented by species i (relative abundance). The value of D ranges between 0 and 1. In the absence of diversity, where only one species is present, the value of D is 1. As both species richness and evenness increase, the value approaches 0.

Because the greater the value of D, the lower the diversity, D is often subtracted from 1 to give:

Simpson's index of diversity=1−DSimpson's index of diversity=1−D

The value of this index also ranges between 0 and 1, but now the value increases with species diversity. In this case, the index represents the probability that two individuals randomly selected from a sample will belong to different species.

The most common way to use the Simpson’s index is to take the reciprocal of D:

Simpson's reciprocal index=1/DSimpson's reciprocal index=1/D

The lowest possible value of this index is 1, representing a community containing only one species. The higher the value, the greater is the species diversity. The maximum value of the reciprocal index is the number of species in the community, the value of species richness (S). For example, there are 10 tree species in the second forest community presented in Table 16.1, so the maximum possible value of the index is 10. Because S is the maximum value of the index, a measure of evenness (ED) can be calculated as the ratio of the reciprocal index (1/D) divided by S:

ED=(1/D)SED=(1/D)S

Values of evenness (ED) range from 0 to 1, with a value of 1 representing complete evenness (all species equally abundant).

Because the Simpson’s index actually refers to three related but different indexes, it is important to identify which is being used and reported.

Another widely used index of diversity that also considers both species richness and evenness is the Shannon index (also called the Shannon–Weiner index).

The Shannon index (H) is then computed as:

H=−∑(pi)(lnpi)H=−∑(pi)(lnpi)

Where pi is the proportion of the total individuals in the community represented by species i, and ln is the natural logarithm.

In the absence of diversity, where only one species is present, the value of H is 0. The maximum value of the index, which occurs when all species are present in equal numbers, is Hmax = ln S, where S is the total number of species (species richness). As with the Simpson’s index of diversity, the maximum value of the Shannon index (Hmax) can be used to calculate an index of species evenness (EH):

EH=H/HmaxEH=H/Hmax

As with the Simpson’s index, values of evenness (EH) range from 0 to 1, with a value of 1 representing complete evenness (all species equally abundant).

16.3 Dominance Can Be Defined by a Number of Criteria

Although the numbers of tree species occurring in the two forest communities (species richness) presented in Tables 16.1 differ more than twofold, the two communities share a common feature. Both communities are composed of a few common tree species with high population density, whereas the remaining tree species are relatively rare and at low population density. This is a characteristic of most communities (Figure 16.3). When a single or few species predominate within a community, those species are referred to as dominants .

Dominance is the converse of diversity. In fact, the basic Simpson index, D, is often used as a measure of dominance. Recall that values of D range from 0 to 1, where 1 represents complete dominance; that is, only one species is present in the community.

Dominant species are usually defined separately for different taxonomic or functional groups of organisms within the community. For example, yellow poplar is a dominant tree species in both of the forest communities just discussed, but we could likewise identify the dominant herbaceous plant species within the forest or the dominant species of bird or small mammal.

Dominance typically is assumed to mean the greatest in number. But in populations or among species in which individuals vary widely in size, abundance alone is not always a sufficient indicator of dominance. In a forest, for example, the small or understory trees can be numerically superior, yet a few large trees that overshadow the smaller ones will account for most of the biomass (living tissue). For example, the species composition of trees in a forest community in central Virginia is presented in Table 16.2. When the structure of the forest is quantified in terms of relative abundance (percentage of total individuals in community) two species—red maple and dogwood—account for approximately 60 percent of individuals in the forest. When the structure of the community is quantified in terms of relative biomass (percentage of total biomass in community), however, the picture of dominance that emerges is quite different. Now white oak, which accounts for less than 9 percent of the individuals, accounts for approximately 60 percent of the total biomass, and the two numerically dominant tree species (red maple and dogwood) account for slightly more than 10 percent. This discrepancy between relative abundance and relative biomass occurs because a few large white oak trees that make up the forest canopy account for the majority of the biomass, and the much larger number of smaller red maple and dogwood occupy the understory (see Figure 16.12 for description and graphic of vertical structure of forest). In such a situation, we may wish to define dominance based on some combination of characteristics that include both the number and size of individuals.

Because dominant species typically achieve their status at the expense of other species in the community, they are often the dominant competitors under the prevailing environmental conditions. For example, the American chestnut tree (Castanea dentata) was a dominant component of oak–chestnut forests in eastern North America until the early 20th century. At that time, the chestnut blight introduced from Asia decimated chestnut tree populations. Since then a variety of species—including oaks, hickories, and yellow poplar—have taken over the chestnut’s position in the forest. As we shall see, however, processes other than competition can also be important in determining dominance within communities (Chapter 17).

16.4 Keystone Species Influence Community Structure Disproportionately to Their Numbers

Relative abundance is just one measure, based only on numerical supremacy, of a species’ contribution to the community. Other, less-abundant species, however, may play a crucial role in the function of the community. A species that has a disproportionate impact on the community relative to its abundance is referred to as a keystone species .

Keystone species function in a unique and significant manner, and their effect on the community is disproportionate to their numerical abundance. Their removal initiates changes in community structure and often results in a significant loss of diversity. Their role in the community may be to create or modify habitats or to influence the interactions among other species. One organism that functions as a keystone species by creating habitat is the coral Oculina arbuscula, which occurs along the eastern coast of the United States as far north as the coastal waters of North Carolina. It is the only coral in this region with a structurally complex, branching morphology that provides shelter for a species-rich epifauna (organisms that live on and among the coral). More than 300 species of invertebrates are known to live among the branches of Oculina colonies, and many more are reported to complete much of their life cycle within the coral (see Chapter 15, Field Studies John J. Stachowicz).

In other cases, keystone herbivores may modify the local community through their feeding activities. An excellent example is the role of the African elephant in the savanna communities of southern Africa. This herbivore feeds primarily on a diet of woody plants (browse). Elephants are destructive feeders that often uproot, break, and destroy the shrubs and trees they feed on (Figure 16.4a). Reduced density of trees and shrubs favors the growth and production of grasses. This change in the composition of the plant community is to the elephant’s disadvantage, but other herbivores that feed on the grasses benefit from it. In a study of the influence of tree cover on grass productivity and local densities of large herbivore populations in the savanna communities of East Africa (Kenya), Corrina Riginos of the University of California–Davis found that both grass productivity and large herbivore density increase with decreasing tree cover (Figures 16.4b and 16.4c). In addition to benefiting grazing herbivores, the destruction of trees creates a variety of habitats for smaller vertebrate species. Ecologist Robert Pringle of Stanford University found that by damaging trees and increasing their structural complexity, browsing elephants create refuges used by arboreal lizards. In a study conducted at the Mpala Research Center in central Kenya, Pringle found that lizard density increased with the density of trees damaged by elephants. Daniel Parker of Rhodes University in South Africa found a similar influence of elephant feeding on community diversity. In a comparison of paired sites with and without elephants in savanna grassland communities of the Eastern Cape region of South Africa, Parker found an increase in both grass and bird species diversity in those sites inhabited by elephants (Figure 16.5). In addition, Parker found that insect and small mammal communities also appeared to benefit from elephant foraging through the modification of habitats.

Predators often function as keystone species within communities (see Section 17.4 for further discussion of keystone predators). For example, sea otters (Enhydra lutris) are a keystone predator in the kelp bed communities found in the coastal waters of the Pacific Northwest. Sea otters eat urchins, which feed on kelp. The kelp beds provide habitat to a wide diversity of other species. Since the 1970s, however, there has been a dramatic decline in sea otter populations. In a study of sea otter populations in the Aleutian Island of Alaska, James Estes and colleagues at the United States Geological Survey and the University of California–Santa Cruz found that sea otter populations are declining as a result of increased predation by killer whales (Orcinus orca). With the decline of sea otters, the sea urchin population has increased dramatically (Figure 16.6). The result is overgrazing of the kelp beds and a loss of habitat for the many species inhabiting these communities.

16.5 Food Webs Describe Species Interactions

Perhaps the most fundamental process in nature is that of acquiring the energy and nutrients required for assimilation. The species interactions discussed earlier—predation, parasitism, competition, and mutualism—are all involved in acquiring these essential resources (Part Four). For this reason, ecologists studying the structure of communities often focus on the feeding relationships among the component species, or how species interact in the process of acquiring the resources necessary for metabolism, growth, and reproduction.

An abstract representation of feeding relationships within a community is the food chain . A food chain is a descriptive diagram—a series of arrows, each pointing from one species to another, representing the flow of food energy from prey (the consumed) to predator (the consumer). For example, grasshoppers eat grass, clay-colored sparrows eat grasshoppers, and marsh hawks prey on the sparrows. We write this relationship as follows:

grass→grasshopper→sparrow→hawkgrass→grasshopper→sparrow→hawk

Feeding relationships in nature, however, are not simple, straight-line food chains. Rather, they involve many food chains meshed into a complex food web with links leading from primary producers through an array of consumers (Figure 16.7). Such food webs are highly interwoven, with linkages representing the complex interactions of predator and prey.

A simple hypothetical food web is presented in Figure 16.8 to illustrate the basic terminology used to describe the structure of food webs. Each circle represents a species, and the arrows from the consumed to the consumer are termed links . The species in the webs are distinguished by whether they are basal species, intermediate species, or top predators. Basal species feed on no other species but are fed on by others. Intermediate species feed on other species and they are prey of other species. Top predators are not subject to predators; they prey on intermediate and basal species. These terms refer to the structure of the web rather than to strict biological reality.

Food webs can provide a useful tool for analyzing the structure of communities and a number of measures have been developed to quantify food web structure. As stated previously, each arrow linking predator (consumer) and prey (consumed) is referred to as a link or linkage. The maximum number of links in a food web is a direct function of the species richness, S. For a food web consisting of S species—assuming that each species may link to every other species including itself—the maximum number of links is S2. The actual number of observed links in a food web (L) expressed as a proportion of the maximum possible number of links (S2) provides a measure of food web connectance (C) :

C=L/S2C=L/S2

An alternative measure of food web connectance considers only the number of possible unidirectional links (the link between any two species flows in only one direction). In this case, the maximum number of links is: S(S – 1)/2. It is important to note which approach is being used when reporting results.

Linkage density (LD) is a measure of the average number of links per species in the food web. It is calculated as the total number of observed links in the food web (L) divided by the total number of species (S):

LD=L/SLD=L/S

The length of any given food chain within the food web is measured as the number of links between a top predator (see Figure 16.8) and the base of the web (basal species). The mean chain length (ChLen) is the arithmetic average of the lengths of all chains in a food web. Examples of each of these measures (connectance, linkage density, and mean chain length) using a hypothetical food web is presented in Figure 16.9a.

It is apparent from the measures of food web structure presented that the number of possible species interactions (links) in a community increases with species richness (S), but how does species richness actually influence the complexity of food webs? Jennifer Dunne of the Santa Fe Institute (New Mexico) and colleagues examine the food web structure of a wide variety of terrestrial, freshwater, and marine ecosystems. Results of the analysis indicate that connectance decreases with species richness, whereas both linkage density and mean chain length increase as the number of species in the community increases (Figure 16.10).

As species richness increases, the structure of food webs become more complex and often food webs become compartmentalized. Species within the same compartment (group of species) interact frequently among themselves but show fewer interactions with species from other compartments (Figure 16.9b). For example, Enrico Rezende of the Universitat Autònoma de Barcelona (Spain) and colleagues analyzed a Caribbean marine food web depicting a total of 3313 trophic interactions between 249 species (Figure 16.11a). Their analyses indicated a division of the food web into five distinct compartments (Figure 16.11b). The researchers determined that the compartments were associated with differences in body size, the range of prey sizes selected, use of shore versus off-shore habitats, and their associated predators.

Although any two species are linked by only a single arrow representing the relationship between predator (the consumer) and prey (the consumed), the dynamics of communities cannot be understood solely in terms of direct interactions between species. For example, a predator may reduce competition between two prey species by controlling their population sizes below their respective carrying capacities. An analysis of the mechanisms controlling community structure must include these “indirect” effects represented by the structure of the food web; we will explore this topic in more detail later (Chapter 17).

The simple designation of feeding relationships using the graphical approach of food webs can become incredibly complex in communities of even moderate diversity. For this reason, ecologists often simplify the representation of food webs by lumping species into broader categories that represent general feeding groups based on the source from which they derive energy. Earlier, we defined organisms that derive energy from sunlight as autotrophs, or primary producers (Part Two). Organisms that derive energy from consuming plant and animal tissue are called heterotrophs, or secondary producers and are further subdivided into herbivores, carnivores, and omnivores based on their consumption of plant tissues, animal tissues, or both. These feeding groups are referred to as trophic levels , after the Greek word trophikos, meaning “nourishment.”

16.6 Species within a Community Can Be Classified into Functional Groups

The grouping of species into trophic levels is a functional classification; it defines groups of species that derive their energy (food) in a similar manner. Another approach is to subdivide each trophic level into groups of species that exploit a common resource in a similar fashion; these groups are termed guilds . The concept of guilds was first introduced by the ecologist Richard Root of Cornell University to describe groups of functionally similar species in a community. For example, hummingbirds and other nectar-feeding birds form a guild of species that exploits the common resource of flowering plants in a similar fashion. Likewise, seed-eating birds could be grouped into another feeding guild within the broader community. Because species within a guild draw on a shared resource, there is potential for strong interactions, particularly interspecific competition, between the members, but weaker interactions with the remainder of their community.

Classifying species into guilds can simplify the study of communities, which allows researchers to focus on more manageable subsets of the community. Yet by classifying species into guilds based on their functional similarity, ecologists can also explore questions about the very organization of communities. Just as we can use the framework of guilds to explore the interactions of the component species within a guild, we can also use this framework to pose questions about the interactions between the various guilds that compose the larger community. At one level, a community can be a complex assembly of component guilds interacting with each other and producing the structure and dynamics that we observe.

In recent years, ecologists have expanded the concept of guilds to develop a more broadly defined approach of classifying species based on function rather than taxonomy. The term functional type is now commonly used to define a group of species based on their common response to the environment, life history characteristics, or role within the community. For example, plants may be classified into functional types based on their photosynthetic pathway (C3, C4, and CAM), which, as we have seen earlier, relates to their ability to photosynthesize and grow under different thermal and moisture environments (Chapter 6). Similarly, plant ecologists use the functional classification of shade-tolerant and shade-intolerant to reflect basic differences in the physiology and morphology of plant species in response to the light environment (Section 6.8). Grouping plants or animals into the categories of iteroparous and semelparous also represents a functional classification based on the timing of reproductive effort (Chapter 10, Section 10.8).

As with the organization and classification of species into guilds, using functional groups allows ecologists to simplify the structure of communities into manageable units for study and to ask basic questions about the factors that structure communities, as we shall see later in the discussion of community dynamics (Chapter 18).

16.7 Communities Have a Characteristic Physical Structure

Communities are characterized not only by the mix of species and by the interactions among them—their biological structure—but also by their physical features. The physical structure of the community reflects abiotic factors, such as the depth and flow of water in aquatic environments. It also reflects biotic factors, such as the spatial arrangement of the resident organisms. For example, the size and height of the trees and the density and spatial distribution of their populations help define the physical attributes of the forest community.

The forms and structures of terrestrial communities are defined primarily in terms of their vegetation. Plants may be tall or short, evergreen or deciduous, herbaceous or woody. Such characteristics can describe growth forms. Thus, we might speak of shrubs, trees, and herbs and further subdivide the categories into needle-leaf evergreens, broadleaf evergreens, broadleaf deciduous trees, thorn trees and shrubs, dwarf shrubs, ferns, grasses, forbs, mosses, and lichens. Ecologists often classify and name terrestrial communities based on the dominant plant growth forms and their associated physical structure: forests, woodlands, shrublands, or grassland communities (see Chapter 23).

In aquatic environments, communities are also classified and named in terms of the dominant organisms. Kelp forests, seagrass meadows, and coral reefs are examples of such dominant species. However, the physical structure of aquatic communities is more often defined by features of the abiotic environment, such as water depth, flow rate, or salinity (see Chapter 24).

Every community has an associated vertical structure (Figure 16.12), a stratification of often distinct vertical layers. On land, the growth form of the plants largely determines this vertical structure—their size, branching, and leaves—and this vertical structure in turn influences, and is influenced by, the vertical gradient of light (see Section 4.2). A well-developed forest ecosystem (Figure 16.12a), for example, has multiple layers of vegetation. From top to bottom, they are the canopy, the understory, the shrub layer, the herb or ground layer, and the forest floor.

The upper layer, the canopy , is the primary site of energy fixation through photosynthesis. The canopy structure has a major influence on the rest of the forest. If the canopy is fairly open, considerable sunlight will reach the lower layers. If ample water and nutrients are available, a well-developed understory and shrub strata will form. If the canopy is dense and closed, light levels are low, and the understory and shrub layers will be poorly developed.

In the forests of the eastern United States, the understory consists of tall shrubs such as witch hobble (Viburnum alnifolium), understory trees such as dogwood (Cornus spp.) and hornbeam (Carpinus caroliniana), and younger trees, some of which are the same species as those in the canopy. The nature of the herb layer depends on the soil moisture and nutrient conditions, slope position, density of the canopy and understory, and exposure of the slope, all of which vary from place to place throughout the forest. The final layer, the forest floor , is where the important process of decomposition takes place and where microbial organisms feeding on decaying organic matter release mineral nutrients for reuse by the forest plants (see Chapter 21).

In the savanna communities found in the semi-arid regions of Africa, the vertical structure of the vegetation is largely defined by two distinct layers: an herbaceous layer typically dominated by grasses and a woody plant layer dominated by shrubs or trees of varying stature and density dependent on rainfall (Figure 16.12b; also see Chapter 23, Section 23.3).

The strata of aquatic ecosystems such as lakes and oceans are determined largely by the physical characteristics of the water column. As we discussed in Chapter 3, open bodies of water (lakes and oceans) have distinctive profiles of temperature and oxygen (see Sections 3.4 and 3.6). In the summer, well-stratified lakes have a surface layer of warm, well-mixed water high in oxygen, the epilimnion; a second layer, the metalimnion, which is characterized by a thermocline (a steep and rapid decline in temperature relative to the waters above and below); and the hypolimnion, a deep, cold layer of dense water at about 4oC (39oF), often low in oxygen (see Figures 3.8, 3.9, 3.12, and 3.13). Two distinct vertical layers are also recognized based on light penetration through the water column (Figure 16.12c; also see Section 3.3, Figure 3.7): an upper layer, the photic layer , where the availability of light supports photosynthesis, and a deeper layer of waters, the aphotic layer , an area without light. The bottom layer of sediments, where decomposition is most active, is referred to as the benthic layer .

Characteristic organisms inhabit each available vertical layer, or stratum, in a community. In addition to the vertical distribution of plant life already described, various types of consumers and decomposers occupy all levels of the community (although decomposers are typically found in greater abundance in the forest floor [soil surface] and sediment [benthic] layers). Considerable interchange takes place among the vertical strata, but many highly mobile animals are confined to only a few layers (Figure 16.13). Which species occupies a given vertical layer may change during the day or season. Such changes reflect daily and seasonal variations in the physical environment such as humidity, temperature, light, and oxygen concentrations in the water, shifts in the abundance of essential resources such as food, or different requirements of organisms for the completion of their life cycles. For example, zooplankton migrate vertically in the water column during the course of the day in response to varying light and predation (Figure 16.14).

16.8 Zonation Is Spatial Change in Community Structure

As we move across the landscape, the biological and physical structure of the community changes. Often these changes are small, subtle ones in the species composition or height of the vegetation. However, as we travel farther, these changes often become more pronounced. For example, in a study of the vegetation of the Siskiyou Mountains of northwestern California and southwestern Oregon, the eminent plant ecologist Robert Whittaker of Cornell University provides a description of changes in the structure of the forest communities along an elevation gradient from the base of the mountains to the summit. A description of changes in the forest community along part of this elevation gradient is presented in Figure 16.15. At mid-elevations (1370–1680 meters [m]) the forest community is dominated by white fir (Abies concolor) with a diverse array of conifer and deciduous species. As you move up in elevation (1680–1920 m), white fir remains a dominant component of the community; however, a second species, noble fir (Abies nobilis), a minor species at lower elevations, emerges as co-dominant. In addition to the shift in species composition, there is a decline in species richness from 17 tree species to 9 species. As you move further up the slope (1920–2140 m), there is once again both a shift in species composition and a further decline in species richness. At this elevation the community is effectively limited to only two species: Noble fir and mountain hemlock (Tsuga mertensiana). What Whittaker observed was a gradual change in the species composition and decline in species diversity in the forest community as one moves up in elevation. Besides changes in the vegetation, the animal species—insects, birds, and small mammals—that occupy the forest also change. These changes in the physical and biological structures of communities as one moves across the landscape are referred to as zonation .

Patterns of spatial variation in community structure or zonation are common to all environments—aquatic and terrestrial. Figure 16.16 provides an example of zonation in a salt marsh along the northeastern coastline of North America. In moving from the shore and through the marsh to the upland, notice the variations in the physical and biological structures of the communities. The dominant plant growth forms in the marsh are grasses and sedges. These growth forms give way to shrubs and trees as we move to dry land and the depth of the water table increases. In the zone dominated by grasses and sedges, the dominant species change as we move back from the tidal areas. These differences result from various environmental changes across a spatial gradient, including microtopography, water depth, sediment oxygenation, and salinity. The changes are marked by distinct plant communities that are defined by changes in dominant plants as well as in structural features such as height, density, and spatial distribution of individuals.

The intertidal zone of a sandy beach provides an example in which the zonation is dominated by heterotrophic organisms rather than autotrophs (Figure 16.17). Patterns of species distribution relate to the tides. Sandy beaches can be divided into supratidal (above the high-tide line), intertidal (between the high- and low-tide lines), and subtidal (below the low-tide line; continuously inundated) zones; each are home to a unique group of animal organisms. Pale, sand-colored ghost crabs (Ocypode quadrata) and beach fleas (Talorchestia and Orchestia spp.) occupy the upper beach, or supratidal zone. The intertidal beach is the zone where true marine life begins. An array of animal species adapted to the regular periods of inundation and exposure to the air are found within this zone. Many of these species, such as the mole crab (Emerita talpoida), lugworm (Arenicola cristata), and hard-shelled clam (Mercenaria mercenaria), are burrowing animals, protected from the extreme temperature fluctuations that can occur between periods of inundation and exposure. In contrast, the subtidal zone is home to a variety of vertebrate and invertebrate species that migrate into and out of the intertidal zone with the changing tides.

16.9 Defining Boundaries between Communities Is Often Difficult

As previously noted, the community is a spatial concept involving the species that occupy a given area. Ecologists typically distinguish between adjacent communities or community types based on observable differences in their physical and biological structures: the different species assemblages characteristic of different physical environments. How different must two adjacent areas be before we call them separate communities? This is not a simple question. Consider the elevation gradient of vegetation in the Siskiyou Mountains illustrated in Figure 16.15. Given the difference in species composition that occurs with changes in elevation, most ecologists would define these three elevation zones as different vegetation communities. As we hike up the mountainside, however, the distinction may not seem so straightforward. If the transition between the two communities is abrupt, it may not be hard to define community boundaries. But if the species composition and patterns of dominance shift gradually, the boundary is not as clear.

Ecologists use various sampling and statistical techniques to delineate and classify communities. Generally, all employ some measure of community similarity or difference (see Quantifying Ecology 16.1). Although it is easy to describe the similarities and differences between two areas in terms of species composition and structure, actually classifying areas into distinct groups of communities involves a degree of subjectivity that often depends on the study objectives and the spatial scale at which vegetation is being described.

The example of forest zonation presented in Figure 16.15 occurs over a relatively short distance moving up the mountainside. As we consider ever-larger areas, differences in community structure—both physical and biological—increase. An example is the pattern of forest zonation in Great Smoky Mountains National Park (Figure 16.18). The zonation is a complex pattern related to elevation, slope position, and exposure. Note that the description of the forest communities in the park contains few species names. Names like hemlock forest are not meant to suggest a lack of species diversity; they are just a shorthand method of naming communities for the dominant tree species. Each community could be described by a complete list of species, their population sizes, and their contributions to the total biomass (as with the communities in Table 16.1 or Figure 16.15). However, such lengthy descriptions are unnecessary to communicate the major changes in the structure of communities across the landscape. In fact, as we expand the area of interest to include the entire eastern United States, the nomenclature for classifying forest communities becomes even broader. In Figure 16.19, which is a broad-scale description of forest zonation in the eastern United States developed by E. Lucy Braun, all of Great Smoky Mountains National Park shown in Figure 16.18 (located in southeastern Tennessee and northwestern North Carolina) is described as a single forest community type: oak-chestnut, a type that extends from New York to Georgia.

Quantifying Ecology 16.1 Community Similarity

When we say that a community’s structure changes as we move across the landscape, we imply that the set of species that define the community differ from one place to another. But how do we quantify this change? How do ecologists determine where one community ends and another begins? Distinguishing between communities based on differences in species composition is important in understanding the processes that control community structure as well as in conservation efforts to preserve natural communities.

Various indexes have been developed that measure the similarity between two areas or sample plots based on species composition. Perhaps the most widely used is Sorensen’s coefficient of community (CC). The index is based on species presence or absence. Using a list of species compiled for the two sites or sample plots that are to be compared, the index is calculated as:

As an example of this index, we can use the two forest communities presented in Table 16.1:

s1=24speciess2=10speciesc=9speciesCC=(2×9)(24+10)=1824=0.529s1=24 speciess2=10 species c=9 speciesCC=(2×9)(24+10)=1824=0.529

The value of the index ranges from 0, when the two communities share no species in common, to 1, in which the species composition of the two communities is identical (all species in common).

The CC does not consider the relative abundance of species. It is most useful when the intended focus is the presence or absence of species. Another index of community similarity that is based on the relative abundance of species within the communities being compared is the percent similarity (PS).

To calculate PS, first tabulate species abundance in each community as a percentage (as was done for the two communities in Table 16.1). Then add the lowest percentage for each species that the communities have in common. For the two forest communities, 16 species are exclusive to one community or the other. The lowest percentage of those 16 species is 0, so they need not be included in the summation. For the remaining nine species, the index is calculated as follows:

PS = 29.7 + 4.7 + 4.3+ 0.8 + 3.6 + 2.9+ 0.4 + 0.4 + 0.4 = 47.2PS = 29.7 + 4.7 + 4.3+ 0.8 + 3.6 + 2.9+ 0.4 + 0.4 + 0.4 = 47.2

This index ranges from 0, when the two communities have no species in common, to 100, when the relative abundance of the species in the two communities is identical. When comparing more than two communities, a matrix of values can be calculated that represents all pairwise comparisons of the communities; this is referred to as a similarity matrix.

1. Calculate both Sorensen’s and percent similarity indexes using the data presented in Figure 16.15 for the forests along the elevation transect in the Siskiyou Mountains.

2. Are these two forest communities more or less similar than the two sites in West Virginia?

These large-scale examples of zonation make an important point that we return to when examining the processes responsible for spatial changes in community structure: our very definition of community is a spatial concept. Like the biological definition of population, the definition of community refers to a spatial unit that occupies a given area (see Chapter 8). In a sense, the distinction among communities is arbitrary, based on the criteria for classification. As we shall see, the methods used in delineating communities as discrete spatial units have led to problems in understanding the processes responsible for patterns of zonation (see Chapter 17).

16.10 Two Contrasting Views of the Community

At the beginning of this chapter, we defined the community as the group of species (populations) that occupy a given area, interacting either directly or indirectly. Interactions can have both positive and negative influences on species populations. How important are these interactions in determining community structure? In the first half of the 20th century, this question led to a major debate in ecology that still influences our views of the community.

When we walk through most forests, we see a variety of plant and animal species—a community. If we walk far enough, the dominant plant and animal species change (see Figure 16.15). As we move from hilltop to valley, the structure of the community differs. But what if we continue our walk over the next hilltop and into the adjacent valley? We would most likely notice that although the communities on the hilltop and valley are quite distinct, the communities on the two hilltops or valleys are quite similar. As a botanist might put it, they exhibit relatively consistent floristic composition. At the International Botanical Congress of 1910, botanists adopted the term association to describe this phenomenon. An association is a type of community with (1) relatively consistent species composition, (2) a uniform, general appearance (physiognomy), and (3) a distribution that is characteristic of a particular habitat, such as the hilltop or valley. Whenever the particular habitat or set of environmental conditions repeats itself in a given region, the same group of species occurs.

Some scientists of the early 20th century thought that association implied processes that might be responsible for structuring communities. The logic was that the existence of clusters or groups of species that repeatedly associate was indirect evidence for either positive or neutral interactions among them. Such evidence favors a view of communities as integrated units. A leading proponent of this thinking was the Nebraskan botanist Frederic Clements. Clements developed what has become known as the organismic concept of communities . Clements likened associations to organisms, with each species representing an interacting, integrated component of the whole. Development of the community through time (a process termed succession) was viewed as development of the organism (see Chapter 18).

As depicted in Figure 16.20a, the species in an association have similar distributional limits along the environmental gradient in Clements’s view, and many of them rise to maximum abundance at the same point. Transitions between adjacent communities (or associations) are narrow, with few species in common. This view of the community suggests a common evolutionary history and similar fundamental responses and tolerances for the component species (see Chapter 5 and Section 12.6). Mutualism and coevolution play an important role in the evolution of species that make up the association. The community has evolved as an integrated whole; species interactions are the “glue” holding it together.

In contrast to Clements’s organismal view of communities was botanist H. A. Gleason’s view of community. Gleason stressed the individualistic nature of species distribution. His view became known as the individualistic , or continuum concept . The continuum concept states that the relationship among coexisting species (species within a community) is a result of similarities in their requirements and tolerances, not to strong interactions or common evolutionary history. In fact, Gleason concluded that changes in species abundance along environmental gradients occur so gradually that it is not practical to divide the vegetation (species) into associations. Unlike Clements, Gleason asserted that species distributions along environmental gradients do not form clusters but rather represent the independent responses of species. Transitions are gradual and difficult to identify (Figure 16.20b). What we refer to as the community is merely the group of species found to coexist under any particular set of environmental conditions. The major difference between these two views is the importance of interactions—evolutionary and current—in the structuring of communities. It is tempting to choose between these views, but as we will see, current thinking involves elements of both perspectives.

Ecological Issues & Applications Restoration Ecology Requires an Understanding of the Processes Influencing the Structure and Dynamics of Communities

As we have discussed in previous chapters, human activities have led to population declines and even extinction of a growing number of plant and animal species. Land-use changes associated with the expansion of agriculture (Chapter 9, Ecological Issues & Applications) and urbanization (Chapter 12, Ecological Issues & Applications) have resulted in dramatic declines in biological diversity associated with the loss of essential habitats. Likewise, dams have removed sections of turbulent river and created standing bodies of water (lakes and reservoirs), affecting flow rates, temperature and oxygen levels, and sediment transport. These changes have impacted not only the species that depend on flowing water habitats (see Figure 9.15) but also coastal wetlands and estuarine environments that depend on the continuous input of waters from river courses (see Chapter 25).

In recent years, considerable efforts have been under way to restore natural communities affected by these human activities. This work has stimulated a new approach to human intervention that is termed restoration ecology . The goal of restoration ecology is to return a community or ecosystem to a close approximation of its condition before disturbance by applying ecological principles. Restoration ecology involves a continuum of approaches ranging from reintroducing species and restoring habitats to attempting to reestablish whole communities.

The least intensive restoration effort involves the rejuvenation of existing communities by eliminating invasive species (Chapter 8, Ecological Issues & Applications), replanting native species, and reintroducing natural disturbances such as short-term periodic fires in grasslands and low-intensity ground fires in pine forests. Lake restoration involves reducing inputs of nutrients, especially phosphorus, from the surrounding land that stimulate growth of algae, restoring aquatic plants, and reintroducing fish species native to the lake. Wetland restoration may involve reestablishing the hydrological conditions, so that the wetland is flooded at the appropriate time of year, and the replanting of aquatic plants (Figure 16.21).

More intensive restoration involves recreating the community from scratch. This kind of restoration involves preparing the site, introducing an array of appropriate native species over time, and employing appropriate management to maintain the community, especially against the invasion of nonnative species from adjacent surrounding areas. A classic example of this type of restoration is the ongoing effort to reestablish the tall-grass prairie communities of North America.

When European settlers to North America first explored the region west of the Mississippi River, they encountered a landscape on a scale unlike any they had known in Europe. The forested landscape of the east gave way to a vast expanse of grass and wildflowers. The prairies of North America once covered a large portion of the continent, ranging from Illinois and Indiana in the east into the Rocky Mountains of the west and extending from Canada in the north to Texas in the south (see Section 23.4, Figures 23.14 and 23.15). Today less than 1 percent of the prairie remains and mostly in small isolated patches, which is the result of a continental-scale transformation of this region to agriculture (see Figure 9.17). For example, in the state of Illinois, tallgrass prairie once covered more than 90,000 km2, whereas today estimates are that only 8 km2 of the original prairie grassland still exists.

To reverse the loss of prairie communities, efforts were begun as early as the 1930s in areas of the Midwest, such as Illinois, Minnesota, and Wisconsin, to reestablish native plant species on degraded areas of pastureland and abandoned croplands. One of the earliest efforts was the re-creation of a prairie community on a 60-acre field near Madison, Wisconsin, that began in the early to mid-1930s by a group of scientists, including the pioneering conservationist Aldo Leopold. The previous prairie had been plowed, grazed, and overgrown. The restoration process involved destroying occupying weeds and brush, reseeding and replanting native prairie species, and burning the site once every two to three years to approximate a natural fire regime (Figure 16.22). After nearly 80 years, the plant community now resembles the original native prairie (Figure 16.23).

These early efforts were in effect an attempt to reconstruct native prairie communities—the set of plant and animal species that once occupied these areas. But how does one start to rebuild an ecological community? Can a community be constructed by merely bringing together a collection of species in one place?

Many early reconstruction efforts met with failure. They involved planting whatever native plant species might be available in the form of seeds, often on small plots surrounded by agricultural lands. The native plant species grew, but their populations often declined over time. Early efforts failed to appreciate the role of natural disturbances in maintaining these communities. Fire has historically been an important feature of the prairie, and many of the species were adapted to periodic burning. In the absence of fire, native species were quickly displaced by nonnative plant species from adjacent pastures.

Prairie communities are characterized by a diverse array of plant species that differ in the timing of germination, growth, and reproduction over the course of the growing season. The result is a shifting pattern of plant populations through time that provides a consistent resource base for the array of animal species throughout the year. Attempts at restoration that do not include this full complement of plant species typically cannot attract and support the animal species that characterize native prairie communities.

The size of restoration projects was often a key factor in their failure. Small, isolated fragments tend to support species at low population levels and are thus prone to local extinction. These isolated patches were too distant from other patches of native grassland for the natural dispersal of other species, both plant and animal. Isolated patches of prairie often lacked the appropriate pollinator species required for successful plant reproduction.

Much has been learned from early attempts at restoring natural communities, and many restoration efforts have since succeeded. Restored prairie sites at Fermi National Accelerator Laboratory in northern Illinois are the product of more than 40 years of effort and now contain approximately 1000 acres; it is currently the largest restored prairie habitat in the world.

Attempts at reconstructing communities raise countless questions about the structure and dynamics of ecological communities, questions that in one form or another had been central to the study of ecological communities for more than a century. What controls the relative abundance of species within the community? Are all species equally important to the functioning and persistence of the community? How do the component species interact with each other? Do these interactions restrict or enhance the presence of other species? How do communities change through time? How does the community’s size influence the number of species it can support? How do different communities on the larger landscape interact?

As we shall see in the chapters that follow, ecological communities are more than an assemblage of species whose geographic distributions overlap. Ecological communities represent a complex web of interactions whose nature changes as environmental conditions vary in space and time.

Summary

Biological Structure 16.1

A community is the group of species (populations) that occupy a given area and interact either directly or indirectly. The biological structure of a community is defined by its species composition, that is, the set of species present and their relative abundances.

Diversity 16.2

The number of species in the community defines species richness. Species diversity involves two components: species richness and species evenness, which reflect how individuals are apportioned among the species (relative abundances).

Dominance 16.3

When a single or a few species predominate within a community, they are referred to as dominants. The dominants are often defined as the most numerically abundant; however, in populations or among species in which individuals can vary widely in size, abundance alone is not always a sufficient indicator of dominance.

Keystone Species 16.4

Keystone species are species that function in a unique and significant manner, and their effect on the community is disproportionate to their numerical abundance. Their removal initiates changes in community structure and often results in a significant loss of diversity. Their role in the community may be to create or modify habitats or to influence the interactions among other species.

Food Webs 16.5

Feeding relationships can be graphically represented as a food chain: a series of arrows, each pointing from one species to another that is a source of food. Within a community, many food chains mesh into a complex food web with links leading from primary producers to an array of consumers. Species that are fed on but that do not feed on others are termed basal species. Species that feed on others but are not prey for other species are termed top predators. Species that are both predators and prey are termed intermediate species.

Functional Groups 16.6

Groups of species that exploit a common resource in a similar fashion are termed guilds. Functional group or functional type is a more general term used to define a group of species based on their common response to the environment, life history characteristics, or role within the community.

Physical Structure 16.7

Communities are characterized by physical structure. In terrestrial communities, structure is largely defined by the vegetation. Vertical structure on land reflects the life-forms of plants. In aquatic environments, communities are largely defined by physical features such as light, temperature, and oxygen profiles. All communities have an autotrophic and a heterotrophic layer. The autotrophic layer carries out photosynthesis. The heterotrophic layer uses carbon stored by the autotrophs as a food source. Vertical layering provides the physical structure in which many forms of animal life live.

Zonation 16.8

Changes in the physical structure and biological communities across a landscape result in zonation. Zonation is common to all environments, both aquatic and terrestrial. Zonation is most pronounced where sharp changes occur in the physical environment, as in aquatic communities.

Community Boundaries 16.9

In most cases, transitions between communities are gradual, and defining the boundary between communities is difficult. The way we classify a community depends on the scale we use.

Concept of the Community 16.10

Historically, there have been two contrasting concepts of the community. The organismal concept views the community as a unit, an association of species, in which each species is a component of the integrated whole. The individualistic concept views the co-occurrence of species as a result of similarities in requirements and tolerances.

Restoration Ecology Ecological Issues & Applications

The goal of restoration ecology is to return a community or ecosystem to a close approximation of its condition before disturbance by applying ecological principles. Restoration ecology requires an understanding of the basic processes influencing the structure and dynamics of ecological communities.