Trophic Structure
The trophic structure (from Greek trophos = feeder) of an ecosystem is the arrangement of organisms based on their feeding relationships. All organisms interact with each other through the food web, based on what they eat and what eats them. Thus the trophic structure controls the passage of energy and nutrients from one organism to another in an ecosystem.
The trophic structure of an ecosystem can be broken up into various trophic, or feeding levels. At the base of the trophic structure are the producers, which are sometimes called primary producers. These autotrophic organisms produce their own food using the processes of photosynthesis or chemosynthesis. The most common types of producers are green plants on land and algae in water. All other organisms in the trophic structure ultimately depend upon these producers for their energy and organic material. In a field ecosystem, the grasses would be an example of the producers.
Consumers are heterotrophic organisms that cannot make their own food; they rely on eating other organisms to obtain their energy and other nutritional requirements. Herbivores, animals that eat the producers, are the first level of consumers. They are often called primary or first-order consumers. A field mouse that eats grasses would be an example of one herbivore in a field ecosystem. Carnivores are organisms that eat meat. There are two types of carnivores, predators, which actively hunt and kill the animals they eat, and scavengers, which eat organisms that they did not kill. Both types of carnivores might eat herbivores or they might eat other carnivores. Those that eat herbivores, such as a snake eating our field mouse, are the secondary or second-order consumers. The tertiary or third-order consumers eat the secondary consumers. For example, a hawk that consumes the snake would be a tertiary consumer. In some ecosystems there are fourth and fifth level consumers, however trophic structure rarely gets to higher levels than this. In many cases, humans are at the top of the trophic structure.
Some organisms, such as most scavengers, can be on more than one trophic level, depending on what they eat. For example, a vulture in the example of the field used above, might eat the snake after it dies, making it a tertiary consumer. However, the same vulture could also eat the mouse, in which case it would be a secondary consumer. Omnivores, organisms that eat both plants and animals, are another example of organisms that are found on more than one trophic level.
Decomposers are also part of the trophic structure. They are responsible for recycling nutrients from dead organisms and organisms' wastes back into a form that is usable by plants and other autotrophs. Decomposers are usually bacteria and fungi, and they play a dual role in an ecosystem's trophic structure: they both recycle nutrients and are a source of food for other organisms.
In any ecosystem, the amount of stored energy differs within the trophic structure. Stored energy of lower trophic levels, particularly the producers, is much greater than higher levels. This is because energy is "lost" between successive trophic levels. This energy is used up in a variety of ways, including by an organism's daily activities and metabolic processes (e.g., moving around, caring for young, respiration), and as excess body heat. Only a small portion of this energy (approximately 10%) is passed on to the next trophic level. As a result, there are usually far fewer top level consumers in an ecosystem than lower level consumers, and fewer lower level consumers than producers. The trophic structure of an ecosystem can be very complex, depending on the number of organisms in the ecosystem and their feeding relationships.
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