Ecology is usually defined as the study of interactions between organisms and their envi

Composting garden waste. Ecology is associated with a political movement advocating government, business, and citizen involvement in taking responsibility for the ways that humans interact with life on earth. (Ecosystems/Corbis)

ronments, which nearly always includes other organisms. This definition covers a lot of territory: the function and adaptation of individual organisms of all kinds—from bacteria to gray whales—are included, as are the properties of population systems consisting of the local representatives of a single species, local ecosystems consisting of many different pop ulation systems, and larger, more inclusive systems at the regional up to the global scale. Some ecologists use an even broader definition: ecology is the study of interactions, distribution patterns, and abundance dynamics of organisms at varied scales of resolution. Ecology also is associated with a political movement that pushes for governments, institutions, and

Figure 1

Hierarchical Structure of Ecology

Figure 1

Source: Delcourt, Hazel, R., and Paul A. Delcourt. 1991. Quaternary Ecology: A Paleoecological Perspective. Figure 1.6, p. 18. London: Chapman and Hall. (Reprinted with kind permission of Kluwer Academic Publishers).

Note: A popular view of hierarchy in ecology, in terms of different disturbance regimes, forms of biotic accommodation and turnover, and traditional units used to describe terrestrial vegetation patterns. Other representations have been proposed, but this one is popular among terrestrial ecologists involved in developmental or historical studies of communities, ecosystems, and landscapes.

Source: Delcourt, Hazel, R., and Paul A. Delcourt. 1991. Quaternary Ecology: A Paleoecological Perspective. Figure 1.6, p. 18. London: Chapman and Hall. (Reprinted with kind permission of Kluwer Academic Publishers).

Note: A popular view of hierarchy in ecology, in terms of different disturbance regimes, forms of biotic accommodation and turnover, and traditional units used to describe terrestrial vegetation patterns. Other representations have been proposed, but this one is popular among terrestrial ecologists involved in developmental or historical studies of communities, ecosystems, and landscapes.

businesses, as well as individual citizens, to take responsibility for protection of endangered species, conservation of habitats, and, in general, the ways that humans interact with life on earth—to ensure the protection of biodiversity.

Scientific ecology is about different levels of interrelational organization within and among organisms, the past history of ecologic systems, the present and future composition and behavior of those systems, and the quest to identify generalizations about how life works. Or, one could simply use a definition intro duced by the nineteenth-century biologist Ernst Haeckel: ecology is about the economy of nature. Although some ecologists are interested in purely scientific questions, most are now involved with the worsening problems created by the activities, by-products, and population expansions of humans, including habitat degradation and elimination, over-harvesting of economically important species, intentional or unintentional introduction and spread of invasive species, and the associated collapse of modern biodiversity. Ecology can be seen as one of the two major divisions of biology, the other complementary and re-enforcing division being evolutionary biology. In sum, ecology is the study of the way that organisms fit into the world.

Ecology, like all large scientific endeavors, can be divided into branches or subdisciplines that correspond to the different approaches used to study the economy of life. The branches of ecology correspond in most instances to the different organizational levels that can be studied. Although the grandest generalizations about how life works should apply to all levels of organization, sensitivity to scaling is one of the major concerns of modern ecology, because specific processes and patterns are associated with different kinds of ecologic units having characteristic sizes, time-related properties, and hierarchical positions. (The most important aspects of the eco-logic hierarchy have to do with the fact that the systems of interest [individual organisms, populations, ecosystems] are divisible into component systems and at the same time are the components of more inclusive systems; interactions take place both within the different levels and between levels of organization.) The following list of subdisciplines illustrates both the diversity of approaches in ecology and the varieties of ecologic systems available for study.

• Chemical and physical ecology—The description and interpretation of specific chemical reactions and physical processes occurring on a moment-to-moment basis and contributing to the survival and reproductive success of organisms of all kinds is called chemical and physical ecology. This is the most fundamental level of ecology. Chemical ecologists, for example, study the various chemical reaction pathways in photo-synthetic plants having different ways of producing stable carbohydrates from the radiant energy of the sun. A physical ecol-

ogist would be interested in the physics of bird flight and might use computer models borrowed from mechanical engineering to help understand the processes involved.

• Physiologic ecology—Organization, function, and development of individual organisms and the chemical cycles and physical interactions of specific adaptations are studied at this level. Physiologic ecologists focus on the economy of individuals: the processing of food and respiration; fluctuations in functional properties during intervals of stress and ensuing relaxation; tradeoffs between moment-to-moment processes that support survival versus processes involved in production of offspring; individual costs or benefits from interactions with other individuals; and changes in physiologic properties caused by fluctuations in environmental factors such as temperature, salinity, or fluid pressure.

• Behavioral ecology—In the broadest sense, behavior is what an individual organism does; it is the way that organisms react to each other and manipulate their surroundings (including other organisms) to ensure survival and the leveraging of their genes into subsequent generations. Some organisms have rather minimal impact on their environments, while others appear to have the ability to re-engineer their surroundings to new specifications. Some make up social groups with complex internal behavioral processes. This branch of ecology is also called ethology.

• Population ecology—Localized groups of organisms belonging to the same species are populations. Populations are the natural divisions of local communities (from the point of view of patterns) or the working parts of ecosystems (from the perspective of processes), and they are sometimes referred to as avatars (local manifestations of the same species). Colonizations, abundance changes, and local extinctions of these units are the things that interest population ecologists. Recent developments in this core discipline of modern ecology include the establishment of connections to genetics, the use of population studies to test evolution theories, and the widespread use of mathematical modeling techniques. The maturation of ecology as a rigorous scientific discipline resulted largely from the development of hypothesis-testing, laboratory and field experiments, a productive connection to evolutionary biology, and attempts to discover generalizations applicable to all kinds of organisms in this branch of ecology, beginning in the 1950s and 1960s.

Community ecology—One of the fundamental observations of both ecology and of everyday experience is that different environments contain different groups of populations belonging to different species. Localized groupings may consist of organisms that co-occur for unrelated reasons, or that exist together in time and space because of shared environmental requirements or possibly obligate connections to each other. Describing and interpreting these groups of organisms is what community ecolo-gists do. Some approaches to community study emphasize one important group of organisms (for example, a bird community, a deposit-feeder community), while other studies incorporate different groups (such as seasonal assemblages, or food chains). The basic method of study involves producing a list of species and estimating their relative abundances in a particular area or sample. Interestingly, few community studies have ever documented the entire biota of a specific area, because of time limitations on research projects and because no ecologist is an expert on all groups of organisms.

Ecosystem ecology—Picture a local grouping of different organisms as consisting of living populations acting as the dynamic components that process energy and materials, changing through time on account of internal as well as external forces, influencing each other's abundance through interactions, and forming close connections with their surrounding habitats. This picture of

Figure 2

Energy Flow through an Idealized Ecosystem

Figure 2

Source: Odum, Howard T. 1957. "Trophic Structure and Productivity of Silver Springs, Florida." Ecologocial Monographs 27(1): 55-112. (Reprinted with permission by The Ecological Society of America)

Note: A typical representation of energy flow through a well-documented ecosystem (Silver Springs, Florida). Trophic (feeding) levels in the structure of the system are shown with boxes; amount of energy flowing between levels is represented by the width of the connections. P symbolizes gross primary production and R system respiration. Many ecosystems have energy flow networks that are more complicated than this one and have significant connections to adjacent systems.

connected, dynamic populations is a sketch of a local ecosystem. Ecosystem ecologists are interested in the function and development of local multispecies assemblies, and they often approach the description of such systems by identifying and quantifying the pathways of energy flow and nutrient cycling. In some studies, component populations are viewed as compartments or processors, and interactions between populations are the connecting pathways that give the system its structure. Whereas a list of species and the interpretation of how they connect to one another at a particular place are sufficient to draw an outline of a community, something like a circuit diagram is needed to document the organization and function of a local ecosystem.

• Landscape ecology and macroecology—The description and interpretation of systems of connected communities (metacommunities), networks of local population systems (metapopulations), regional ecosystems, and the large-scale patterns of species ranges, abundances, and body sizes all go into these branches of ecology. Viewing ecologic properties at the scale of regions is new and has grown out of a realization that the properties of local populations and communities have as much to do with regional processes as with local factors.

• Global ecology—The most ambitious attempts to generalize about the economy of nature come from very large scale studies that sometimes encompass the entire earth. Characterizations of major chemical cycles that involve organisms (for example, the carbon cycle), reciprocal interactions between life and global-scale environmental processes (such as the Gaia hypothesis), and large-scale pictures of productivity or the impact of humans on the biosphere fit into this branch of ecology.

• Exoecology—Interest in developing ways to detect life on other planets has grown rapidly in recent years because of the possibility of sampling the surface of Mars and because new planetary systems are being discovered by astronomers all the time—some of which may harbor life forms. Some of the same approaches used in global ecology would apply to the remote sensing of possible life on other worlds.

• Conservation ecology—This is the most active division of ecology, and it involves most of the approaches mentioned above in one way or another. Conservation ecologists study the impacts of human activities (including very large-scale problems of climate change, regional problems such as the spread of harmful invasive species, and more localized problems such as chemical pollution in a specific area) to understand the steps that need to be taken to preserve species diversity, habitat quality, and the integrity of ecologic systems. They are also actively involved in public education and the political activity needed to control or reverse these impacts, and they use eco-logic methods and theory to propose remediation and preservation plans.

• Paleoecology—Paleoecology is the daunting task of doing ecology with fossils. Traditionally, paleoecologists tried to identify and interpret small-scale units such as populations and communities preserved in sedimentary rock formations. It is now appreciated that, owing to the way in which fossils accumulate in sediments, the pale-oecologic record is not so much a document of short-term, small-area processes and patterns as it is a robust record of large, long-lived ecologic systems. Because some major taxonomic groups (such as bivalve and gastropod mollusks, articulate brachiopods, and corals with hard skeletons) are readily preserved as fossils, the historical record of ancient metapopulations and regional ecosystems can be reconstructed in great detail. Recent developments include using paleoecologic patterns as baseline data in conservation ecology, using reconstructions of terrestrial plant assemblages to study climate change during the Quaternary Period, and the increasing awareness that ecology is not the backdrop but the driving force of evolution in many cases. In general, paleo-ecology is about large-scale, durable, inclusive units of organization; patterns in the history of life for which there is a fossil record but no modern counterpart to study; and the largest generalizations about eco-logic processes and patterns. • Evolutionary ecology—During the modern development of ecology, the connections between evolution and ecology were often ignored. Other than consideration of particular adaptations and niches of individual organisms or local populations, surprisingly little attention was devoted to the possible linkages between ecologic processes and evolutionary patterns. This is now a very vigorous branch of ecology that attempts to understand, among other things, the controls on adaptive radiations, the selectivity of extinctions, the nature of large-scale originations (for example, the "Cambrian explosion" of animal life) and replacements (such as mammals replacing dinosaurs after the end-Cretaceous extinctions), the perennial problem of latitudinal diversity gradients, and the reason that evolutionary rates at the level of populations observed in modern environments can be very fast but the evolutionary pattern of most species detected in the fossil record is one of morphologic stasis.

More effort than ever before will have to be expended in the early twenty-first century by new generations of ecologists to address the worsening biodiversity crisis, and it will seem increasingly difficult to justify purely docu-mentational or theoretical work in ecology as the crisis unfolds. We must never forget, however, that progress in applied ecology depends on understanding fundamental properties of ecologic systems, proposal of new theories, ecologic interpretation of the fossil record, and on a thorough understanding of the natural history of organisms and the regions in which they live. It is also clear that the vigor of conservation ecology has both bolstered the importance and the workforce of ecology, and has quickened the pace and improved tremendously the quality of research on ecologic systems of all kinds. Ecology must continue to be a diverse endeavor, always ready with critical applications but continuing to illuminate the economy of nature in all of its forms.

—William Miller III

See also: Adaptation; Biogeography; Climatology; Communities; Conservation Biology; Ecological Niches; Ecosystems; Food Webs and Food Pyramids; Global Climate Change; Land Use; Nutrient/Energy Cycling; Succession and Successionlike Processes Bibliography

Delcourt, Hazel, R., and Paul A. Delcourt. 1991. Quaternary Ecology: A Paleoecological Perspective. London: Chapman and Hall; Dodson, Stanley I., et al. 1998. Ecology. Oxford: Oxford University Press; Elton, Charles. 2001 [1927]. Animal Ecology. Chicago: University of Chicago Press; Gaston, Kevin J., and Tim M. Blackburn. 2000. Pattern and Process in Macro-ecology. Oxford: Blackwell Science; Kingsland, Sharon E. 1995. Modeling Nature: Episodes in the History of Population Ecology, 2d ed. Chicago: University of Chicago Press; Mcintosh, Robert P. 1985. The Background of Ecology: Concept and Theory. Cambridge: Cambridge University Press; McNaughton, Samuel J., and Larry L. Wolf. 1979. General Ecology, 2d ed. New York: Holt, Rinehart and Winston; Odum, Howard T. 1957. "Trophic Structure and Productivity of Silver Springs, Florida." Ecological Monographs 27(1): 55-112; Rambler, Mitchell B., Lynn Margulis, and René Fester, eds. 1989. Global Ecology: Towards a Science of the Biosphere.

Boston: Academic; Real, Leslie A., and James H. Brown, eds. 1991. Foundations of Ecology: Classic Papers with Commentaries. Chicago: University of Chicago Press; Ricklefs, Robert E., and Gary L. Miller. 2000. Ecology, 4th ed. New York: W. H. Freeman; Valentine, James W. 1973. Evolutionary Paleoecology of the Marine Biosphere. Englewood Cliffs, NJ: Prentice-Hall.

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