Communities

An ecologic community is a group of organisms living together in a specific location at a particular time. This is the definition that many ecologists would accept; but the term community has been used in many other ways, and no real consensus has ever emerged. Some ecologists would use the definition with which I began, but would add that different species occur together because they share the same environmental requirements or limitations; and some would go further to say that some organisms occur together in such groups because of environmental constraints and because of interactions among the components. Others would stress the vagaries of recruitment and colonization. In the ecologic literature of the last twenty years or so, a community is most often considered to be either the organisms living in a defined space (the "geographic definition"—the ecologist specifies the space) or the local representatives of a higher taxonomic group (bird, lizard, or mammal communities—the "taxonomic definition").

Ecologists have spent a lot of time arguing about what communities are supposed to be and how they should be recognized: whether they are arbitrary units or natural associations, the nature of their boundaries, and the issues of stability and membership rules versus continuous variation and open membership. Some have even suggested dropping the term altogether, and using patch, association, or assemblage to refer to local co-occurrences of species. Others see communities as reflections of local ecosystems, consisting of population systems and at the same time forming the working parts ofregional ecosystems. It is at least safe to generalize that when ecologists talk about communities, they are stressing composition; when they talk about ecosystems, the emphasis is on processes. In this sense, description of community patterns provides a picture of the organization of organisms within local ecosystems—in other words, communities are the tangible frameworks of dynamic ecologic systems.

Community Composition

One can think about compositional properties either as parts of an idealistic community concept (the things that make communities what they are) or as attributes that can be measured quantitatively and used in statistical comparisons. One can arbitrarily define the extent of an association of interest, or analytic techniques can be employed to delineate the community. Listed and defined below are some of the most common attributes or properties used in community ecology.

• Species composition—A list of the different organisms co-occurring at a particular place or in a sample is the most fundamental characterization of a community. In most studies, a favorite taxonomic group is emphasized, or a functional group of unrelated organisms is the focus of attention. Few community inventories are exhaustive, because of limited taxonomic expertise, desire to study what are thought to be the most revealing parts of an assemblage, or time limitations on the study. The first thing a community ecologist does in a field survey or when processing a sample is to compile an accurate list of the species.

• Species richness—Species richness is simply the number of species in an area of interest or a sample unit. The basic measurement of species richness is the most straightforward description of biodiversity. For example, a tree canopy in a Louisiana swamp might contain a hundred species of invertebrates of various kinds, but one in a mature tropical forest in Ecuador might contain more than a thousand. Ecologists want to know what produces such striking differences in richness.

• Abundance—The number of individuals belonging to different species is another basic community attribute. When one gives the results of a census in terms of raw numbers of individual organisms, the pattern is one of absolute abundances; giving the results in the form of percentages is an expression of relative abundances; and ordering species from most to least abundant is the pattern of rank abundances. It is also possible to report abundance in terms of biomass, coverage of surfaces, productivity, or other means that relate more to function than simply to numbers of organisms.

• Species-abundance distributions—Relationships between species richness and abundance can be portrayed in several ways, and doing so is one of the most important representations of community composition. The basic method involves plotting the log of relative abundances against the rank order of species in a community. The resulting patterns are called dominance-diversity curves.

• Diversity and dominance—In ecology, diversity is a measure of the evenness in distribution of the individual organisms among the species present. Communities with few species—and most individuals concentrated in just one of them—are low-diversity assemblages; communities containing many species—with the individuals more evenly spread among the component species—are high-diversity assemblages. The diversity index used by most ecologists is some version of the following formula: Diversity = -X pi ln pi (In equations such as this, pi is the proportion [n/N] of each species in a sample; ln is the base of natural logarithms.) Dominance can be thought of as the inverse of diversity: communities with low diversity usually have low evenness and are often high-dominance assem-

Table 1

Diversity and Dominance Indices for Four Hypothetical Communities

Species Communities

Table 1

Diversity and Dominance Indices for Four Hypothetical Communities

Species Communities

I

II

III

IV

A

75

25

251

50

B

8

23

39

48

C

2

23

38

47

D

1

15

29

45

E

21

45

F

18

42

G

15

40

H

10

39

4

36

J

1

34

Number of individuals in

each community:

86

86

426

426

Species richness (number

of species present, S ):

4

4

10

10

Diversity ( -X p^ In p■):

0.48

1.37

1.47

2.30

Dominance ( X(pj )2 ):

0.77

0.26

0.37

0.10

Source: Based on Table 19-1 in MacNaughton, S. J. and Wolf, L. L. 1979. General Ecology, 2d ed. New York: Holt, Rinehart and Winston.

Source: Based on Table 19-1 in MacNaughton, S. J. and Wolf, L. L. 1979. General Ecology, 2d ed. New York: Holt, Rinehart and Winston.

Note: In both indices, pi is the proportional abundance of each individual species represented by a population in a community. The values computed obviously are sensitive to both pi and the species richness, S, and could be compared using simple statistical methods.

blages. One way to estimate this is as follows: Dominance = E (pi)2. Table 1 illustrates the application of these formulas in the analysis of different kinds of communities.

• Functional categories—Another way to express community composition is to list the species in functional categories (guilds). Imagine an intertidal sandbar containing several species of invertebrate animals positioned mostly at the surface (epibenthic); other species living 5 to 10 cm below the surface (shallow endobenthic); and a few others tunneling 30 to 40 cm below the surface of the loose sand (deep endobenthic). These animals belong to different species that divide the vertical space of the sandbar and collect food at different levels and in different ways. Mobility is greatest among the surface dwellers that produce no tunnels, moderate among shallow burrowers that build temporary domiciles, and essentially absent among the deep bur-rowers confined to thick-walled tubes. These categories are separate guilds or lifestyle divisions within the sandbar community. One could also simply divide the same community into feeding groups, including photosyn-thetic primary producers (algae, cyanobacte-ria), primary consumers (animals that directly exploit the primary production), predators and parasites, and scavengers. A simple class-frequency (histogram) or "pie" diagram could be used to illustrate the proportions of organisms in the various categories. Obviously, knowledge of the ways of living of the different kinds of organisms would be a prerequisite for recognizing these functional groups.

Organization of Communities

The attributes alone say little about the organization and development of communities and the ecosystems they represent. What are the factors responsible for structure and complexity of food webs in different kinds of communities, or in the same community at different stages of development? Are communities assembled so that they are capable of resisting disturbances, or is community organization entirely a reflection of constantly changing environmental factors or continuous variation along environmental gradients?

What controls the diversity of communities? Are such assemblages subject to intrinsic membership rules, such that only a select subset of the regional biota is ever represented? Or is membership essentially open, with internal organizing processes exerting minimal influ ence on organization? Are communities a reflection of a discrete functional entity (local ecosystems) consisting of many species interacting to produce the structure we see? Or do variations in colonization (recruitment) and coincidental adaptations of the component organisms account for the composition and structure? Determining the relationships between the parts of communities, how the components work, and how they originated sheds light on these basic questions in ecology. • Food webs—The exact relationships between the members of the various feeding groups are used to map out food webs, which reflect the trophic structure (the composition and organization of the energy-materials transfer system). The energy-importing organisms underwrite the requirements of the rest of the components of a community: they provide the fuel that sustains all the organisms connected in the food web. Most food webs are based on photoautotrophic organisms (such as blue-green bacteria, algal protists, and plants), which convert sunlight into biomass. In some communities, chemoautotrophic organisms provide the same service. In terms of absolute abundance and biomass, these primary producers dominate most communities. Many kinds of animals have evolved to take advantage of this resource. These primary consumers may be general-ists associated with many kinds of primary producers, or specialization may develop, involving one consumer organism intimately associated with only one producer organism. Secondary and tertiary consumers are the low- and high-level predators, respectively, that exploit the primary consumers. The farther removed from the primary source of energy, in terms of levels in the food pyramid, the fewer are the consumers, although some may be very large.

Scavengers and decomposers of various kinds form a recycling loop that returns nutrients and energy to lower levels in the pyramid. A diagram of these relationships, showing exactly which species occupy the different functional positions, is not only a picture of the flow of energy through an ecosystem but also a way to assess complexity in such a system.

• Other interactions—Other kinds of connections between component species giving structure to a community include antagonistic interactions (for example, competition for limited resources) and beneficial relationships (for example, com-mensalism, direct mutualism, facilitation, and certain forms of indirect interaction— as when a predator disrupts a competitive relationship between two consumers). Together with food web relationships involving predation and parasitism, these interactions make up both the internal framework of communities and the functional "wiring" of the local ecosystems they represent.

• Disturbance and stability—Environments rarely stay the same for very long. Some are characterized by high-amplitude, aperiodic changes in environmental factors, while others have a periodic or seasonal swing in the defining factors over time. Some environments experience rare disruption or have disturbances (which include the action of organisms) that are small-scale, localized events. Ecologists know that communities in stressful environments often have low species richness, low diversity, and simple organization; communities in more benign environments are richer in species, more diverse, and can be exceedingly complicated in terms of their internal organization. Communities that rarely experience any kind of disturbance, however, may come to be dominated by one or a few supercompetitors. Thus it appears that an intermediate level of disturbance is required to maintain the most diverse communities, such as those of coral reefs and tropical rain forests. The ability of communities or ecosystems to bounce back to a previous organizational state following a disturbance (resilience) or resist a change in structure in some other way (resistance, persistence) is a reflection of the stability of those assemblages or systems. Communities may recover along more or less repeatable pathways collectively known as secondary succession, in which internal interactions dominate the recovery process. In situations in which the waxing and waning of a community are paced overwhelmingly by outside processes, such as change in climate or nutrient availability, the resulting fluctuations in composition and structure are called community response. When assemblages are significantly changed owing to changes in the array of habitats, exceeding any ability to rebound to a previous organizational state, community replacement has taken place.

• Are communities natural associations or convenient fictions?—One of the longest running debates in community ecology is over the question of the natural reality of communities. Some ecologists have claimed that communities are real biologic entities (or reflect real entities), having a definable life history including a "birth" (establishment, termed primary succession), a history of disturbances and recoveries, and an eventual end (replacement); limited membership drawn from a regional pool of potential member species; and internal organization largely resulting from connection between component species (the Clementsian-Eltonian model). Others hold that communities are happenstance assem blages of organisms that are recruited to a site and coincidentally tolerate the conditions prevailing there (the Gleasonian model). All communities owe their composition and organization to the interplay of environmental factors defining the possible habitats (including spatial and temporal variation, or heterogeneity); the disturbance regime (local departures from the ordinary environmental characteristics—as during storms); recruitment from outside sources; resources, including nutrients/food and space; and interactions with other organisms, including incumbency (interference by established occupants) and facilitation (early components paving the way for later colonists). Most ecologists would be willing to recognize a spectrum of different assemblages, with some having the attributes of the tightly organized, durable, distinct units of organization dominated by internal connections among species; some appearing to be chance associations of species that share the same environmental requirements or were simply thrown together owing to probabilistic aspects of colonization and having few obligate connections; and some characterized by a mix of both kinds of properties.

—William Miller III

See also: Carbon Cycle; Coevolution; Conservation Biology; Coral Reefs; Ecological Niches; Ecosystems; Evolution; Extinction, Direct Causes of; Food Webs and Food Pyramids; Global Climate Change; Habitat Tracking; Hydrologic Cycle; Oceans; Paleontology; Succession and Successionlike Processes

Bibliography

Cody, Martin L., and Jared M. Diamond, eds. 1975. Ecology and Evolution of Communities. Cambridge: Harvard University Press; Diamond, Jared, and Ted J. Case. 1986. Community Ecology. New York: Harper and Row; Magurran, Anne E. 1988. Ecological Diversity and Its Measurement. Princeton: Princeton University Press; McNaughton, Samuel J., and Larry L. Wolf.

1979. General Ecology, 2d ed. New York: Holt, Rine-hart and Winston; Putman, Rory J. 1994. Community Ecology. London: Chapman and Hall; Real, Leslie A., and James H. Brown, eds. 1991. Foundations of Ecology: Classic Papers with Commentaries. Chicago: University of Chicago Press; Strong, Donald R., et al., eds. 1984. Ecological Communities: Conceptual Issues and the Evidence. Princeton: Princeton University Press.

Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Post a comment