Kinds of Ecosystems

The simplest classification of local ecosystems involves associating such systems with unique environments—a biogeographic classification. An ecosystem inhabiting the floor of a submarine trench—receiving most of its resources from an adjacent landmass as phytodetritus and experiencing disturbances only from burrowing organisms bulldozing other organisms aside and from extremely rare turbidity currents— is strikingly different from a temperate forest system whose energy requirements are supplied entirely by photoautotrophic plants and that is subject to both seasonal fluctuations in climate and frequent storms. In other words, different environments support different kinds of ecosystems. This is one of the oldest observations of ecology.

A more general classification is based on whether function and development are shaped by extrinsic forces, or are mostly the result of internal dynamics—a classification emphasizing controls.

Ecosystems that are environmentally controlled are paced from the outside and are typical of rigorous, stressful settings. Such sys tems vary in structure and function with either aperiodic or seasonal change in environmental factors. These systems exist essentially at the whim of the surrounding environment. Adaptations have to do mostly with physical and chemical factors that define the environment. Biologically controlled ecosystems have structural and functional properties that result more from internal interaction of the population systems, resulting in habitat modifications, recuperation of resources, coevolutionary adjustments, and developmental self-regulation. Adaptations in this kind of system would have as much to do with biologic interaction as with the external environment, and could involve instances of incorporation (that is, exploitation of formerly deleterious aspects of the environment—for example, adaptation to fire in some terrestrial plant assemblages) or habitat engineering (that is, restructuring proximal environments—for example, beaver dam-lodge-pond complexes). The ecosystem of a mountain spring, which must endure dramatic changes in water availability and quality, a harsh climate, and must depend on the irregular recruitment of organisms from distant sources, would be environmentally controlled. A tropical forest, having an elaborate network of mutually compensating and regulating interactions among thousands of species, many of which are organized into cells consisting of a central species with an entourage

Figure 3

Simplified Classification of Ecosystems

Figure 3

Source: Based on Miller, William III. 2002. "Regional Ecosystems and the Origin of Species." Neues Jahrbuch Für Geologie Und Paläontologie, Abhandlungen 225:137-156, fig. 1. (Reprinted with permission)

Note: Ecosystems can be classified based on whether function and development are caused largely by external or by internal processes. Environmental pacing (such as climate) could control local ecosystems that fluctuate irregularly in terms of vital functions and composition or remain stable or undergo regular cyclic changes. When internal processes become important, systems may be dominated by key predators or grazers, or by hub species that organize retinues of mutualists. Most of the local ecosystems that have been studied so far fit into one of the four major categories shown here.

of intimately associated organisms, and experiencing only small-scale disturbances from occasional tree falls, would be an example of a biologically controlled system.

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