Ecosystems

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Systems are assemblages of interacting objects that are linked by transfers of energy and matter, behave in specific ways under certain conditions, and are often governed by cybernetic controls that involve the flow of information through positive and negative feedback. In 1935 British ecologist Arthur Tansley (1871-1955) described functioning organisms and their physical environment as the "basic units of nature on the face of the Earth" and referred to them by the term "ecosystem." The components are both living (within the biotic realm) and nonliving (abiotic). The biotic components comprise the communities of organisms formed by interacting populations. While ecosystems are real, functioning places, they are also the abstractions, or models, that are developed to characterize the function and potentially predict the behavior of these real places.

One important aspect of ecosystems is the definition of their boundaries. In some cases this is superficially obvious. A pond can be thought of as an ecosystem with the boundaries between the water and the terrestrialenvironment forming a shoreline, the interface between the water surface and the atmosphere defining the top, and the lower extent of wet sediments in the ooze at its bottom as recognizable surfaces. Even these, however, are not quite as clear-cut as they may seem when viewed in closer detail. The shoreline is much longer when measured with centimeter segments than with a meter stick. The water surface boundary has a layer of air saturated by water vapor that may or may not be considered to be part of the ecosystem; and the bottom could be complicated by the presence of the inlet from an underground spring. Boundaries are even harder to define within an expanse of seemingly continuous grassland or forest, and are therefore at times assigned in an arbitrary manner by researchers.

Interior of Biosphere II, an enclosed ecosystem, at Oracle, Arizona.

Size alone does not necessarily help resolve the question. In some cases the interactions within an ecosystem occur over many kilometers, and the boundaries are formed by decreasing probabilities of transfers of matter and energy with other parts of the system. On the other hand, sometimes very small units can be thought of as ecosystems. The moss-covered back of a sloth, a pile of bear dung, or the surface of your skin can be treated theoretically as a microcosm or miniature ecosystem. The frequent indistinctness of boundaries, and the fact that energy and matter enters and leaves the ecosystem, makes them open systems. Even if energy gains and losses are in balance, it is more appropriate to describe an ecosystem as a steady state rather than equilibrium, because equilibrium (which is only possible in a completely isolated, thermodynamically closed system), does not adequatelymodel ecosystems. They are always dynamically interacting with adjacent ecosystems to form a complex landscape.

One of the most powerful tools emerging from the ecosystem concept is the development of models that abstract the structure and function of the real world. Pictures and graphs describe physical arrangement of objects. Flow charts characterize highly probable pathways for energy or nutrients to pass through the system. In the case of energy, this flow is a one-way street with its ultimate dissipation outside of the boundaries as heat and entropy. Nutrients, however, can be retained and recycled within the ecosystem. The extent to which this happens is one measure of stability.

The beauty of ecosystem models is that they can be quantified. This allows them to be analyzed mathematically on computers and ultimately, if the models are based on real, natural behaviors, they can be used to predict the future of ecosystems. The rapidly developing field of general systems theory can be applied to ecosystems resulting in insights about how they function. These tools also allow ecologists to make predictions about the behavior of ecosystems when disturbed, stressed, or altered by evolutionary time, questions that society is finding pressing with increasing pollution, global warming, and other environmental threats.

Agricultural Ecosystems; Aquatic Ecosystems; Biome; Coastal Ecosystems; Ecology, Energy Flow; Ecology, Fire; Ecology, History Of; Plant Community Processes.

Tansley, Arthur G. "The Use and Abuse of Vegetational Concepts and Terms." Ecology 16, no. 3 (1935): 284-307.