Ecosystems

Ecosystems consist of co-occurring organisms, their connections to each other and to surrounding environments, and the parts of their environments that are controlled or incorporated into their lives to such an extent that sep

Figure 1

The Flow of Carbon among Components of the Chesapeake Bay Ecosystem

Figure 1

Source: Baird, Daniel, and Robert E. Ulanowicz. 1989. "The Seasonal Dynamics of the Chesapeake Bay Ecosystem." Ecological Monographs 59(4) 329-364, fig 2.(Reprinted with permission)

Note: The complex connections and the numerous components are typical of such systems, which may be much more complicated than this example. The "bullets" represent autotrophic system elements (plants); hexagons, heterotrophic taxa (fauna); and "birdhouses," nonliving storages. DOC = dissolved organic carbon; POC = particulate organic carbon. Numbers inside each box are the standing stocks in mg/m2.

Source: Baird, Daniel, and Robert E. Ulanowicz. 1989. "The Seasonal Dynamics of the Chesapeake Bay Ecosystem." Ecological Monographs 59(4) 329-364, fig 2.(Reprinted with permission)

Note: The complex connections and the numerous components are typical of such systems, which may be much more complicated than this example. The "bullets" represent autotrophic system elements (plants); hexagons, heterotrophic taxa (fauna); and "birdhouses," nonliving storages. DOC = dissolved organic carbon; POC = particulate organic carbon. Numbers inside each box are the standing stocks in mg/m2.

aration of the life-forms from physical-chemical factors becomes hard to justify. One way to look at ecosystems is to think of them as communities with all of the energy and materials pathways mapped out—communities, by comparison, would be the representations of the composition and structure of local ecosystems. An ecosystem, in a sense, is a dynamic picture of a community with the functional and developmental processes painted in, so that assemblages are seen as a kind of collective living entity. This viewpoint considers ecosystems to be natural, localized divisions of the biosphere. Another way to look at them is from a systems point of view, in which inter acting populations are connected in a network of compartments representing the different organisms, with the connections being all of the quantifiable energy transfers or material cycles. This is a more pragmatic point of view that does not necessarily admit the natural reality of ecosystems but simply explores such systems using the perspective of a circuitry diagram or a flow chart. The functional identity of the ecosystem depends mostly on the flow of energy and chemicals. In ecosystem ecology, the emphasis is on processes. In community ecology, the emphasis is mostly on composition and pattern.

Although ecosystems are usually pictured as local functional networks of organisms together with their connections to environments, larger systems are often studied using the same systems approach. These more inclusive, regional systems appear to consist of smaller local ecosystems, the functional components of which are population systems (sometimes called avatars). This nested pattern of dynamic systems has led some ecologists to consider the economic aspects of life on earth as an eco-logic hierarchy consisting of differently scaled entities that process energy and materials, undergo developmental changes over time, and simultaneously interact with other systems at the same level of organization, their own component parts, and their encompassing system. This is a complicated picture of the economy of life, but it is probably a more realistic one than thinking of ecosystems as simply the parts of nature that are interesting or significant enough to study using systems methodology and flow charts. This means that in order to understand how a local ecosys tem functions and develops over time, one must investigate the patterns and processes at the focal level of the local system, properties of the component parts (providing what are called initiating mechanisms) that characterize each local system, and the processes going on in the surrounding larger system (providing what we call the boundary conditions). In other words, ecosystems do not function and develop in isolation: they are the products of their own internal dynamics and the larger-scale interactions taking place at local to regional levels of organization.

Some ecologists think that ecosystems can be studied adequately by dissecting the systems and measuring all the characteristics of organization and movements of energy and materials (this is referred to as the reductionist approach). Others argue that such systems display what are referred to as emergent properties: not only do the larger, more inclusive systems have process rates obviously different from the faster rates in the component sys-

Table 1

Major Divisions of the Ecologic Hierarchy

Biosphere1

Provincial systems

Regional ecosystems

Biotope systems2

Local ecosystems

Interaction cells3

Population systems (avatars)

Individual organisms and colonies

Functional divisions of organisms4

Cells involved in economic functions

Molecular systems5

Each system in the hierarchy interacts simultaneously with similar systems at the same level, its component systems, and with a larger encompassing system. Systems at relatively higher positions in the hierarchy tend to be larger, longer lived, and exhibit slower process rates compared to systems at lower levels.

1All living organisms and their interactions; the most inclusive level of ecologic organization on Earth

2Constellations of closely connected local ecosystems

3Population systems ("entourage") within local ecosystems organized around a "hub" species that provides a significant resource (food, space, habitat structure)

4Includes organs and tissues involved in moment-to-moment survival of an organism

5When decomposed, this level reveals parts of cellular metabolic cycles and pathways of molecular synthesis tems, but, in addition, differently scaled systems appear to be able to do different things. A single population system cannot form and maintain a food web or undergo succession, but a local ecosystem can; individual organisms may damage or destroy one another, but a predator-prey interaction results in a transfer of energy between processor levels, giving the overall system a part of its functional identity. In ecosystem ecology, ecologists probably have done more to own up to the complex organization of life than in any other branch of biology, because here they investigate various levels in the ecologic hierarchy simultaneously.

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