Ecosystems are conceptual and functional units of study that entail the ecological community together with its abiotic environment. Implicit in the concept of any system, such as an ecosystem, is that of a system boundary which demarcates objects and processes occurring within the system from those occurring outside the system. This inside-outside perspective gives rise to two environments, the environment external to the system within which it is embedded, and the environment outside the object of interest but within the system boundaries (the latter has been termed environ by Patten, 1978). We typically are not concerned with events occurring wholly outside the system boundary, i.e., those originating and terminating in the environment without entering the system by crossing the system boundary. Furthermore, as open systems, energy-matter fluxes occur across the boundary; these in turn provide the ecosystem with an available source of energy input such as solar radiation and a sink for waste heat. In addition to continuous radiative energy input and output, pulse inputs are important in some ecosystems such as allochthonous organic matter in streams and deltas, and migration in Tundra.
The spatial extent of an ecosystem varies greatly and depends often on the functional processes within the ecosystem boundaries. O'Neill et al. (1986) defined an ecosystem as the smallest unit which can persist in isolation with only its abiotic environment, but this does not give an indication to the area encompassed by the ecosystem. Cousins (1990) has proposed the home range or foraging range of the local dominant top predator arbiter of ecosystem size, which he refers to as an ecosystem trophic module or ecotrophic module. Similar to the watershed approach in hydrology, Power and Rainey (2000) proposed a "resource shed" to delineate the spatial extent of an ecosystem. Taken to the extreme, one could eliminate environment altogether by expanding the boundaries outward indefinitely to subsume all boundary flows, thus making the very concept of environment a paradox (Gallopin, 1981). The idea is not to make the "resource shed" so vast as to include everything in the system boundary, but to establish a demarcation line based on gradients of interior and exterior activities. In fact, in open systems an external reference state is a necessary condition, which frames the ecosystem of interest (Patten, 1978). We give the last word to Post et al. (2005) who stated that different organisms within the ecosystem based on their resource needs and mobility will operate at different temporal and spatial scales, typically leaving the scale context-specific for the research question in hand.
Definitional difficulties aside, one must operationalize an ecosystem so following O'Neill's approach of the smallest unit that could sustain life, the minimum set for a sustainable functioning ecosystem comprises producers and consumers, specifically decomposers (see further below). One visualizes a naturally occurring biotic community to include:
(1) organisms that can draw in and fix external energy into the system, typically primary producers,
(2) additional organisms that feed on this fixed energy, consumers, and
(3) decomposers that close the cycle on material flow as well as provide additional energy pathways.
This biotic community interacts with its abiotic environment acquiring energy, nutrients, water, and physical space to form its place or habitat niche (although habitat is often comprised of other biotic entities). As a result, ecosystems are comprised of many interactions, both biotic and abiotic. This includes interactions between individuals within populations (e.g., mating), interactions between individuals from different species (e.g., feeding), and active and passive interactions of the individuals with their environment (e.g., water and nutrient uptake, excretion, and death). In ecosystem studies two approaches are employed. The first, a "black-box" approach concerns itself entirely with the inputs and outputs to the ecosystem not elucidating the processes that generated them (Likens et al., 1977). The second, generally termed ecological network analysis (ENA), is a detailed accounting of energy-nutrient flows within the ecosystem. In these studies, the focus is usually at the scale of the species or trophospecies (trophic functional groups), and how they interact rather than interactions between individuals of the same species, although these are considered in individual-based models and studies. ENA could even be called reductionistic-holism since it requires fine scale detail of the ecosystem constituents and their interconnections, but uses them to reveal global patterns that shape ecosystem structure and function.
Although interaction networks are ubiquitous, observing them is difficult and this has led to slow recognition of their importance. For example, ecological observations reveal direct transactions between individuals but do not immediately reveal the contextual network in which they play out. Sitting in a forest, one does not readily observe the network, but rather an occasional act of grazing, predation, or death. While watching a wolf take down a deer, it is not apparent what grasses the deer grazed on, now assimilated by the deer, and soon the wolf, not to mention the original source of energy, solar radiation, or nutrients in soil pore water. Since the components form a connected web, it is necessary to study and understand them in relation to the interconnection network, not in isolation or a limited subset of the system.
Each component, in fact, must be connected to others through both its input and output transactions. There are no trivial, isolated components in an ecosystem. Pulling out one species is like pulling one intersection of a spider's web, such that although that one particular facet is brought closer for inspection, the entire web is stretched in the direction of the disturbance. Those sections of the web more closely and strongly connected to the selected node are more affected, but the entire system is warped as each node is embedded within the whole network of webbed interactions. The indicator species approach works because it focuses on those organisms that are deeply embedded in the web (Patten, 2005) and therefore produce a large systemic deformation. The food web is, therefore, in fact, more than just a metaphor; it acknowledges the inherent connectivity of ecosystem interactions.
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