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Ecology is the study of the patterns and processes governing the abundance and distribution of organisms and their relationships to their environment.The environment includes abiotic factors – such as the soils, geology, sunlight, climate, and other physical and chemical factors as well asbiotic factors,such as other organisms within thesameor neighboring habitats. The term ecology derivesfrom oekologie, first coined in 1866 by the biologist Ernst Haeckel, joining the Greek oikos, or household, and logos, orstudy.The field of ecology combines diverse scientific traditions from natural history, experimentation, field study and mathematical modeling toadvanceour understanding of the processesand patterns maintaining and altering biodiversity. Asa positive science, ecology does not make a priori value judgments; nevertheless, it is strongly associated with the normative goals of modern environmentalism that ascribe a fundamental intrinsic andutilitarian value to nature. As such, ecological research and study is a key component of conservation biology, concerned with understanding and protecting biological diversity at multiple scales.

Ecology is a broad field that encompasses several thematic, areal, hierarchical, systematic, and methodological foci and traditions. For instance, distinct thematic/areal traditions are reflected intropical ecology, desert ecology, freshwater ecology, marine ecology, and so on. Distinct hierarchical scales of biodiversity correspond to behavioral ecology (individual adaptations), autecology (populations of onespecies), synecology (communities ofmultiple species), and landscape ecology (structure, composition and function of landscapes). Disciplinary and methodological approaches define chemical ecology, genetic ecology, mathematical/theoretical ecology, statistical ecology, spatial ecology, and evolutionary ecology. These distinct traditions are not mutually exclusive,but often overlap in significant ways and have evolved over time.

History of Ecological Thought

The history of ecological thought, like that of the field of conservation biology, traces back several centuries.The earliest formal practice ofecological research on the relationship between organisms and their environment dates to the botanist Alexander von Humboldt, who in the early 19th century, described the relationship between plant distributions and regional climates. His work was followedby the publication of Charles Darwin’sThe Origin of Species in the mid 1800s, postulating an evolutionary, mechanistic perspective for ecology that departed from its earlier, descriptive focus. As developments in ecology continued over the next several decades, advances were made in the understanding of global bio-geochemical cycles (e.g., the nitrogen cycle); and the term biosphere cametobecoined in1875 by geologist Eduard Suess, to refer to that global sphere where the biota interacts with the lithosphere, atmosphere and hydrosphere. Such dynamic interactions were the focus of ecologists such asHenryCowlesand Frederic Clements toward the dawn of the 20th century, who established a tradition known as “dynamic ecology.” I n the coastal dunes of the Great Lakes and the western prairies of the United States, respectively, Cowles and Clement examinedthe process of ecological succession, the sequence of ecological changes following a disturbance. Succession is the process by which an ecological community progresses over time from an initial, simple state to a latter, complex state as the systemapproacheda stable equilibrium (sometimes called homeostasis). The change in the overall ecological community over time reflects, among other things, theloss and gain of individual species. A disturbance; such as wind damage, openingupof aforestgap by treefall, plowing of a field, creation of a patch by waves in an inter-tidal zone, or a rainfall event that creates an ephemeral pool, creates new localized habitats for different species to colonize and exploit. According to successional theory, early colonizers or invaders tend to be those that are best adapted to reproduce rapidly and quickly colonize the new habitat, and typically have high reproductive rates and small life spans. Such species are often referred to as r-strategists. During later stages in the successional sequence, r-strategists are gradually replaced by species that are slower to exploit the initial post-disturbance conditions,but are better adapted to continuing a viable population in the long term at or near the system’s carrying capacity. Such slow-growing species are often referred to as K-strategists. According to strict Clementsian interpretation, most successional patches (seres) in a given locality will tend eventually toward a particular assemblage of “climax” species (i.e.,a “monoclimax”) at the conclusion of the successional sequence, even whenthoseseres reflect different stages in the successional series. The role ofhumansin ecological processes was viewedina negative light, as interfering with the processes of natural succession.

The ecologist Arthur Tansley often acknowledged the significance of the work of Cowles and Clements, and yet, disagreed with the Clementsian notion of a monoclimax. He argued instead that environmental gradients and varying disturbance regimes within aclimaticzone meant that later successional stages would support not a single but multiple climax communities. He took further issue with the uniformly negative portrayal of human agency in ecological dynamics, suggesting instead that human-nature interactions gave rise to anthropogenic climax communities(suchas agro-ecosystems). In 1935, Tansley introduced the term ecosystem to refer to the interacting system formed by biota with its environment. Hismodels were strongly influenced by emerging ideas about systems and fields in physicsatthattime, and suggested that organisms couldbestbestudied as interacting components of (bio)physical systems. This method of ecological study, one that focuses on calibrating and understanding the behavior of the system’s component to understand systembehavior, is sometimes referred to as a tactical approach. The ecosystem concept was subsequently adopted and elaborated by Eugene Odum, often referred to as the father of modern ecology.

In 1953, Eugene Odum and his brother, Howard Odum, jointly authored the first definitive textbook on modern ecology, established ecology as a bona fide academic discipline and educated the first generations of ecosystem ecologists in North America. Eugene Odum was also an early developer of the strategic ecosystem approach to studying ecological communities, maintaining that in order to understand system functioning, it is most expedient to focus on the essence or key aspects of thesystem’s overall behavior rather than its components in all their detail. Odum applied the strategic ecosystem approach to ecological communities in their successional paths, theorizing that older, more advanced communities should contribute to overall ecosystem stability, or homeostasis, securing protection against environmental disturbances.

The Rise of the Ecosystem Concept

Other developments preceded and paralleled the riseoftheecosystem concept in ecology. Charles Eltonexpanded on the ecological form (structure) and function that exists at any given time in a successional sequence, rather than the process of change over time. In 1927, Elton proposed a set of principles in his text Animal Ecology that aimed to explain an organization of ecological communities focused upon the food chain and laid the foundation forpresent thinking on trophic interactions. Elton’s food chain consisted of the photosynthetic conversion of solar energy as the first link with herbivores and predators making up the remaining two or three links, and ascribed distinct roles to plant and animal speciesas producers, consumers, decomposers, etc. Elton also proposed the pyramidal structure of the food chain in considering how the size and populations of a species (as food or consumer) relate to its position in the pyramid (e.g., smaller populations of slowly reproducing, large predators such as whales depend upon larger numbers of rapidly reproducing,tiny zooplankton). Finally, Eltonproposed the idea of the ecological niche as the function of a species in a community, maintaining that no two species in a community could occupy the exact same niche because of competitive exclusion.

Elton, Tansley, the Odums and other ecologists influenced by developments in physics turned to the second law of thermodynamics to focus on the flow of materials (e.g., food/nutrients and water) and energy through ecosystems, and further unified the consideration of biotic and abiotic components in ecology. There were attempts to merge lessons from trophic structureand ecosystem function. For instance, Raymond Lindeman and others studied the productivity of each trophic level and the efficiency of the transfer of energy from one level to the next in order to understand the functioning of entire ecosystems. Ecosystem function began to be quantified and measured in energy units. For instance, the net primary productivity (NPP) of diverse natural and human-modified ecosystems is calculated and compared to assess aspects of ecosystem function, such as carbon sequestration (therate at that carbon dioxide is photosynthetically removed from the atmosphere).

Several systems ecologists now include humans aspartofan expanded ecosystem, the ecological–economic system. The Odums’s unified theory of ecosystems as applied to ecological succession also postulated, similar to Clements, a stable, homeostatic system that expended less energy on production (therefore maintaining steady biomass) and more on ensuring stability in the face of environmental fluctuations. Unlike Clements, the new homeostasis entailed a dynamic and open ecosystem that could theoretically allow periodic flows of organisms, materials and energy across its boundaries. The Odums and subsequent ecosystem ecologists became the foremost proponents of ecosystem-focused science in the 1960s and 1970s, engaging in large-scale research projects in diverse biomessuch as grasslands, deciduous forests, tropical forests, etc. to advance theoretical understanding. From a policy perspective, many ecosystem ecologists advanced the preservation of nature untouched, as far aspossible,bythe human hand.

Communication and Population

Although systems ecology provides a holistic framework for the consideration of ecosystems,community and population ecology continue to be important approaches to understanding species diversity, distribution and turnover. Proponents of population ecologysuch as Robert MacArthur argued that unified ecosystem theory failed to generate testable hypotheses, producing instead abstractions thatwere notveryuseful for disciplinary advancement. Aut ecology and synecology play critical roles in the field ofconservation biology, which is concernedwith the conservation of species and other higher levels of biotic diversity. Population ecology (autecology) focuses on demographic patterns and changes, geographic distribution of species abundances and the processes that influence such patterns.

Among such processes are competition, predation, dispersal and extinctions. MacArthur andhis research colleague Edward 0. Wilson conducted studies of species diversity on islands in the Caribbean and the results of their work formed the basis for their theory of island biogeography published in 1967. According to island biogeography theory, the equilibrium number of species on an island is a function of the island’s size and its distance from the mainland. An island’s size has a well-established relationship to the numbers of species it can support: species are more likely to undergo extinctions on smaller islands; larger islands therefore typically retain higher numbers of species. An island’s distance fromthe mainland influences the rate of immigration of species from the mainland. With increasing island size or decreasing distance from the mainland – or both – the rate of species increase drops off after some point, and species richnessreaches an equilibrium. The theory of island biogeography is explicitly linked to the metapopulation concept in ecology in drawing attention not only to populations in individual patches, but also toward how those patches and their populations are connected in space and time to form a metapopulation. Island biogeography theory has been applied extensively in the field of conservation biology and reserve design (wherein reserves may be viewed as islands supporting species richness) and has inspired much debate about the relative biodiversity merits of singlelarge or several small (SL0SS) reserves.

Population ecology examines the geographic range of populations (individuals of a species within alocalarea) as influenced by that of suitablehabitat, and focuses on population dispersion (e.g., clumped, evenlyspaced or random spacing of individuals), dispersal and mortality as functions of spatial variation inhabitat quality and quantity, as well as of biotic interactions. The structure of a population includes the density and spatial distribution of its individuals, proportions in various age classes, and the changein eachofthose variables over time.

The Poisson distribution is often usedtoanalyze spatial patterns in population data and reveal the density of populations in a given area. Local populations may interact with one another, forming metapopulations residing in a network of source and sink populations. Certain habitats may be resource rich,enabling higher reproduction ratesthancan be maintained in the area, forming a source population that may emigrate to lower-quality habitats that house sink populations. Processes ofemigration and immigration are captured mathematically in dispersal models. Population models differ based onwhetherthey assume seasonal or continuous reproduction, and whether or not generations may overlap. According to many equilibrium models, population increase may be regulated by factors that are density-dependent (e.g., food availability, predation, disease) or density-independent (e.g., temperature, rainfall). Prevailing theories diverge from equilibrium assumptions, focusing instead on demographic stochasticity (random variation in birth and death rates) and environmental stochasticity (random environmental variability). Developments in metapopulation theory afford some room for the integration of equilibrium-based population dynamics models with demographic andenvironmental stochasticity.

Genetic Structure: DNA

The genetic structure of a population is studied using moderntechniques of DNA analysis. Small populations are particularly vulnerable to the loss of genetic variation through inbreeding and genetic drift – often referredto as a population bottleneck – such as that experienced by the small, genetically uniform populations of cheetahs in Africa. Concepts such as effective population size and minimum viable populationsize derive from population ecology and are of particular interest in conservation biology; they relate to how largeapopulation has to be to avoid the loss of genetic diversity and survive for a specified time.

Species interactions in ecology are generally of four kinds: competitive, predator-prey (or consumer-resource), detritivore-detritus, and mutualism interactions. Competitive relations in particular have long been a strong focus in evolutionary ecological theory. Competition can occur over resources such as space, nutrients and water, and through physical orchemical means. Charles Darwin’s theory of natural selection focuses on intra-specific competition, wherein those organisms with traits that result in a competitive (and therefore reproductive) advantage are those that are selected for and prevail. Intraspecific competition is thus related to population regulation and evolutionary change. At the interspecific level, species that are thebestperformers inan intense competition for limited resources tend to survive, while those that are poor competitors adapt or perish. Such competition may therefore affect community structure and composition. Experiments by Tansley and others mustered support for the importance of inter-specific competition in determining the presence of absence of a species, although the results were mediated by environmental conditions. Garrett Hardin’s principle of competitive exclusion predicts that two competing species cannot coexist on a single limiting resource. The ornithologist David Lack observed, however, that several species with similar ecological needs did,in factcoexistin natural settings, and hypothesized that species may evolve to co-exist within the same habitat by diverging in their ecological needs and therebyreducing competition. Coexistence among competitors is also enabled by disturbance regimes that effectively maintain fluctuating environmental conditions (i.e., a nonequilibrium system) and prevent competitive exclusion.

Coevolution is said to occur when two species not only coexist, but evolve in a reciprocal manner inresponseto each other’s characteristics, such as yucca plants and their insect pollinators. However, coevolutionis usually investigated amongst local populations of interacting species. Localvariation in environmental conditions means that species mayinteractin different ways in different populations, and therefore a coevolutionary response may bespecifictotwo particular populations of the interacting species, rather than the two species in general across their entire ranges. The choice of spatial scale, therefore, is critical to the study of coevolutionary interactions.

Predator-prey relationships in populationecologyare perhaps best summarized in the work of Alfred Lotka, Vito Volterra, Georgii Gause and others,whotriedto describe though experimentation and mathematical equations how populations of interacting species – such as a predator and a prey – reached a stable equilibrium. Predators may drive prey populations extinct, or in the presence of spatially distributed prey refuges and/or additional (source) populations of predators, result in alternative outcomes. Predator-prey relations areoften considered density-dependent: increases in prey density can positively affect predator populations by improving their growth and/or immigration rates; however, the greatest number of predatorsis supported at an intermediate prey density, at which the prey population reaches its maximum recruitment rate. Similar mathematical formulations have been derived for other types of interactions, such as parasitoid-host interactions.

The Role of Spatial Pattern

The role of spatial pattern is critical in population and metapopulation dynamics. The patchiness of resources,habitats and populations of predator, prey, parasite and/or host populations strongly structures processesof interaction within a localpatchand across its surrounding regional context or landscape. The field of landscape ecology is distinctive forits explicit focus on spatial pattern and its implications for ecological processes. In other words, landscape ecology deals with how the structure and composition of the landscape drives the ecological patterns andprocesses at various hierarchical scales (e.g., organisms, populations, species, communities and ecosystems). Strongly influenced by applied fieldssuch asforestry, landscape architecture and agriculture, landscape ecology has straddled the divide between basicand applied ecological research since its consolidation in the early 1980s. In its initial phases, landscape ecology focused on developing techniques for the quantification and scaling of spatial pattern. Theseefforts produced a vast array of metricsto describethe spatial arrangement of habitatpatches, suchas fragmentation, fractal dimension, connectivity, and contagion, as well as techniques to determine the appropriate scale at which pattern-process relationships of interest could be analyzed and correlated. With the maturation of the discipline came a more concerted effort to conduct pattern-process experiments at the landscape scale, and develop insights bridging with other long-standing ecological traditions and theories.

Because ecological questions are posed and analyzed within a broad scale, landscape ecology offers an interesting opportunity to synthesize insights from multiple ecological traditions, including theoretical and behavioral ecology, community and metapopulations, genetics and evolutionary ecology and ecosystems research. In addition, landscape ecology affords a ready interface for collaboration with other (non-ecological) disciplines and applied traditions, particularly; geography, environmental science, regional and land-use planning, photogrammetry, remote sensing and geographic information science, restoration ecology, conservation biology andwildlife management, watershed management, forestry and landscape architecture, and global environmental change–including climate change as well as land use/cover change.

Landscape Heterogeneity

The basic components of landscape heterogeneity include the patch, boundary/edge/ecotone and mosaic,while relevant processes may be those that define or affect disturbance, fragmentation, and connectivity. Patches are landscape units that may be considered relatively homogenous for purposes of study and analysis, and can change in area, shape, and quality over time. Boundaries or edges refer to the area of transition between two dissimilar environments (or between a patch and its surroundings, sometimes referred to as the matrix). An ecotone is typically an edge area as well, but is used to denote the varying gradient of environmental conditionsin the transitional zone.

Ecotones can be sharp or gradual.Thenumbers of different types of patches, their relative size, shapeand abundance, and their spatialarrangement (e.g., average distance between patches) together define the structure and composition of the landscape; a landscape mosaic in particular refers toacollectionof patches. Landscape functionrefers tothe interaction of landscape componentsandthe flow of organisms, materials, andenergythrough the landscape.

The issue of hierarchy, scale (extent, map scale, spatial resolution or minimum mapping unit, and temporal scale), and scalar dynamics comprise important concerns in landscape ecology. Different sets of ecological criteria matter at distinct hierarchical scales. Kotliar and Wiens demonstrated, for example, that insects used different sets of criteriatoselecta leaf vs. a tree or patch. Studies have found that relationships between spatial pattern and processat onescaleof analysis are typically not generalizable to other scales (e.g., the ecological fallacy). Landscapescale simulation modeling experiments and percolation theory suggest critical thresholds at which particular ecological processes, such as colonization by an invasive species, or a disturbance such as fire, will spread across the landscape. Theories from population ecology, such as island biogeography theory or mathematical models of metapopulation theory have long focused on spatial heterogeneity in patchy environments, and are particularly relevant for landscape ecology. Perhaps the most interesting examples of theoretical development in landscape ecology derived from its engagement with social science theories of land use and landscape change.

For instance, research in the Human Dimensions of Global Environmental Change has conducted landscape ecological studies integrating geographic, sociological and anthropological theories of human decision-making strategies, explaining and predicting deforestation and other land use transformations in tropical forests and other environments. It is precisely owing to its analytical focus on spatial heterogeneity and disturbance, and its broad synthetic scope including human roles in ecological systems that landscape ecology has particular relevance for conservation biology and land use planning. Insights into how disturbance maintains or alters landscape structure and function, biodiversity and ecosystem stabilityand resilience are relevant for conservation planning and reserve design. It is far more ecologically and economically feasible to manage disturbance regimes rather than restore landscapes or ecosystems after dramatic degradation.

Most ecological studies employ theoretical, mechanistic or empirical/statistical modelsinorder to explicate the pattern-process relationships of interest in a given region or ecosystem. Models can range from simple abstractions that capture only essential elements of systems, to complex models with detailed specifications and multiple parameters that aim to accurately replicate and predict system interactions. Mechanistic models are built on a causal or process-based understanding of a system, particularly useful for scenario testing and impact assessments, but prone to problems of calibration and validation as well as to oversimplifying reality. Statistical models, on the other hand, are based on empirical data, but may capture only correlations rather than causal relationships. Most landscape ecological models today are spatially explicit, meaning that they use spatially referenced datasets, such as those derived from satellite imagery and/or maps and geographic information systems. Aside from the relative strengths and weaknesses of the modeling approach itself, models are also limited by the quality of the data. For instance, seasonal changesin highly local land uses in a tropical forest-agriculture mosaic may be difficult to derive in sufficient detail from satellite imagery, sinceitis often difficult to obtain cloud-free scenes in such areas. More frequent imagery, such as that provided by the Moderate Resolution Imaging SpectroRadiometer (MODIS) or the Advanced Very High Resolution Radiometer (AV HRR), may notofferdataat afine enough pixel resolution for detecting activity. Other limitations may include insufficient or inappropriate thematic resolution, spatial resolution, accuracy and uncertainty, and mismatch between social and ecological spatial variables.

The traditions in ecology constitute complementary and sometimes contradictory approaches to understanding the patterns of distribution of biological diversity and the processes that explain that distribution. They contribute to fundamental ecological concepts regarding ecosystems; the structure, composition and functioning of ecological systems; the bioticandabiotic determinants of change, stability, resilience and productivity; concepts of equilibria versus nonequilibria; and the effects of spatial heterogeneity on ecological processes. These and other insights from ecology are brought to bear upon the contemporary problem of global biodiversity loss, altered biogeochemical cycles and transforming climate regimes–all aspects of global environmental change.

Ecology Movements

Ecology’s general identification with the studyand valuationof nature, moreover, makes itacommon ifsometimes unwilling ally in the modern environmental movement since the 1960s. The social movement, as distinct from the scientific ecological tradition, has been influenced by conservation ideas and philosophies dating back at least two centuries, and is fueled by a publicly perceived global crisis of environmental contamination and species extinctions. The publication of Rachel Carson’s Silent Spring in the 1960s inaugurated the environmental movement in the West. The focus on pesticides, and other environmental contamination in the 1960s was succeeded in the 1970s, 1980s, and 1990s by concerns about the threat of nuclear disasters, acid rain,ozone depletion and its effects onhumanand ecological health, rising rates and extents of tropical deforestation and biodiversity loss, and climate change (including global warming). While ecology has engaged with the environmental movement through these crises, albeit in ways that lacked a unified approach or a consistent and clear set of recommendations, a strong dissonance between the scientific ecology and environmentalists revolved around the Gaia hypothesis. Certain elements of the environmental movement draw on the concept of Gaia proposed by the atmospheric scientist James Lovelockinthe 1960s. According to this hypothesis, the biosphere is a system that self-regulates through feedback relationships andfunctionsasa single organism. While ecologists overwhelmingly acknowledge the interactions between bioticand abiotic components of the biosphere, as well the ability of biota to alter its physical environment, they debate the concept of homeostasis implied in the Gaia hypothesis and criticize the hypothesis itself for being overly teleological.

Bibliography:

B. Kotliar andJ.A.Wiens,”Multiple Scales of Patchiness and Patch Structure: A Hierarchical Framework for the Study of Heterogeneity,” Oikos (v.59, 1990);
H. MacArthur and E.O.Wilson, The Theory of Island Biogeography (Princeton University Press, 1967);
E. Ricklefs and G.L. Miller, Ecology, 4^th ed. (W.H. Freeman and Company, 2000);
G. Turner, “Landscape Ecology: The Effect of Pattern on Process,” Annual Review of Ecology and Systematics (v.20, 1989);
L. Turner, W. Clark, R.W. Kates, J.F. Richards, J.T. Mathews, and W.B. Meyer, eds., The Earth as Transformed by Human Action: Global and Regional Changes in the Biosphere Over the Past 300 Years (Cambridge University Press, 1990);
P.M. Vitousek, “Beyond Global Warming: Ecology and Global Change,” Ecology (v.75, 1994);
Worster, Nature’s Economy: A History of Ecological Ideas (Cambridge University Press, 1994).