BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.3. Ecosystems and Energy Flow

10.3.3. Food Webs

A food chain implies that each species feeds on only one other species. In practice, however, trophic relationships within ecosystems are far more complex. Most organisms rely on multiple food sources, and some belong to both grazing and detritus food chains. This is particularly true for carnivores at higher trophic levels, many of which have highly diverse diets, making them secondary, tertiary, and higher-order consumers all at once. Some animals, including humans, consume organisms from every trophic level—plants, animals, and Fungi. These are known as omnivores.

Grazing and detritus food chains are intricately intertwined. For example, earthworms consume dead organic matter as well as small living roots, functioning simultaneously as detritivores and herbivores. Waste products and dead remains from all trophic levels provide the starting point for the detritus food chain. This complex web that mirrors the real world is called a food web. Fig. 10.5 illustrates simplified food webs for a forest and a freshwater habitat. These diagrams show only a fraction of the numerous possible interconnections and conventionally feature only one or two so-called apex predators at the highest trophic level. Yet, even in simplified form, such schemes are valuable because they serve as a foundation for the quantitative study of Energy Flow and nutrient cycling among the biotic components of an ecosystem.

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Fig. 10.5. Food webs: A — forest; B — freshwater ecosystem (adapted from: Popham (1955) Some aspects of life in fresh Water, Heinemann).