ECOSYSTEM STRUCTURE
4.2 ECOSYSTEM STRUCTURE
The hierarchical organization of biological systems often used as a starting point for an ecological study is the ecosystem. An ecosystem includes living organisms in a specified physical environment, the multitude of interactions between the organisms, and the nonbiological factors in the physical environment that limit their growth and reproduction, such as air, water, minerals and temperature. Viewed this way, an ecosystem practically means the house of life (Miller 1991). The definition of boundaries and the spatial scale of an ecosystem can vary. An ecosystem can be as small as a pond or as big as the entire earth. We can, therefore, refer to the ecosystem of a pond or the ecosystem of the earth in its entirety. What is important in each case is the definition of boundaries across which inputs and outputs of energy and matter can be measured (Boulding 1993).
Generally, an ecosystem is composed of four components: the atmosphere (air), the hydrosphere (water), the lithosphere (earth) and the biosphere (life). The first three comprise the abiotic or nonliving components of the ecosystem, whereas the biosphere is its biotic or living component. It is important to recognize that the living and nonliving components of an ecosystem interact with each another. The dynamic interaction of these components is critical to the survival and functioning of the ecosystem, just as breathing and eating are essential to the survival of animals. Furthermore, these components are capable of coexisting so that the ecosystem itself is alive (Schneider 1990; Miller 1991). For example, soil is a living system that develops as
a result of interactions between plant, animal and microbial communities (living components) and parent rock material (abiotic components). Abiotic factors such as temperature and moisture influence the process of soil development.
In the ecosystem, the abiotic components serve several functions. First, the abiotic components are used as a habitat (space), and an immediate source of water and oxygen for organisms. Second, they act as a reservoir of the six most important elements for life: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S) and phosphorus (P). These elements constitute 95 percent of all living organisms. Furthermore, the earth contains only a fixed amount of these elements. Thus, continual functioning of the ecosystem requires that these elements be recycled since they are critical to the overall welfare of the ecosystem.
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The biotic (living) component of the ecosystem consists of three distinct groups of organisms: the producers, consumers and decomposers. The producers are those organisms capable of photosynthesis: production of organic material solely from solar light and carbon dioxide. This organic material serves as a source of both energy and mineral nutrients, which are required by all living organisms. Examples include terrestrial plants and aquatic plants, such as phytoplankton. The consumers are organisms whose very survival depends on the organic materials manufactured by the producers. The consumers represent animals of all sizes ranging from large predators to small parasites, such as mosquitoes. The nature of the consumers’ dependence on the producers may take different forms. Some consumers (herbivores such as rabbits) are directly dependent on primary producers for energy. Others (carnivores such as lions) are indirectly dependent on primary producers. The last group of living organisms is the decomposers. These include microorganisms, such as fungi, bacteria, yeast, etc., as well as a diversity of worms, insects and many other small animals that rely on dead organisms for their survival. In their effort to survive and obtain energy they decompose materials released by plants and consumers to their original elements (C, O, H, N, S, P). This, as we shall see shortly, is what keeps material cycling within the ecosystem.