Ecosystems and Ecology - How it works



The Biosphere and Food Webs

In the sciences, a system is any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement. Modern earth scientists regard the planet as a massive ecosystem, the stage on which four extraordinarily complex earth systems interact. These systems are the atmosphere, the hydrosphere (all the planet's waters, except for moisture in the atmosphere), the geosphere (the soil and the extreme upper portion of the continental crust), and the biosphere.

The biosphere consists of all living things—microorganisms, plants, insects, birds, marine life, and all other forms of animals—as well as formerly living things that have not yet decomposed. (Decomposed remains of organic materials become part of the geosphere, specifically the soil.) Organisms in the biosphere, whether living or formerly living, are united by the interrelation of energy transfer that takes place through the food web.

A food web is similar to the more well known expression food chain. In scientific terms, however, a food chain is defined as a series of singular organisms in which each plant or animal depends on the organism that precedes or follows it. This rarely exists in naturas instead, the feeding relationships between organisms in the real world are much more complex and are best described as a web rather than a chain. (For more on the biosphere and earth systems, see The Biosphere.)

Energy Flow and Nutrients

Energy is the ability of objects or systems to accomplish work. (The latter is defined as the exertion of force over a given distance: for example, a plant growing from the ground, an insect or bird flying, or a human or pack animal moving an object.) Food webs are built around energy transfer, or the flow of energy between organisms, which begins with plant life. Hence the importance of plants to ecosystems, as we illustrate later in discussing various types of forest, which are defined by their dominant varieties of tree.

Plants absorb energy in two ways. From the Sun they receive electromagnetic energy in the form of visible light and invisible infrared waves, which they convert to chemical energy by means of photosynthesis. In addition, plants take in nutrients from the soil, which contains energy in the forms of various chemical compounds. These compounds may be organic, which typically means that they came from living things, though, strictly speaking, organic refers to characteristic carbon-based chemical structures. Plants also receive inorganic compounds from minerals in the soil. (See Paleontology for an explanation of the scientific distinction between organic and inorganic.)

Contained in these minerals are six chemical elements essential to the sustenance of life on planet Earth: hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. These are the elements involved in biogeochemical cycles, through which they are circulated continually between the living and nonliving worlds—that is, between organisms on the one hand and the inorganic realms of rocks, minerals, water, and air on the other. (See The Biosphere for more about biogeochemical cycles.)

FROM PLANTS TO MEAT EATERS.

As plants take up nutrients from the soil, they convert them into other forms. Eventually, the plants themselves become food either for herbivores (plant-eating organisms) or for omnivores (organisms that eat both plants and other animals), thus passing along these usable, energy-containing chemical compounds to other participants in the food web. It is likely that an herbivore will be eaten in turn either by an omnivore (for example, humans as well as a number of bird species and many others) or by a carnivore, an organism that eats only meat.

Carnivores and omnivores are not usually prey for other carnivores or omnivores, but this does happen in the case of what are known as tertiary consumers (see Food Webs). There is also the matter of cannibalism, discussed in Biological Communities. For the most part, however, the only creatures that eat carnivores and omnivores do so after these organisms have died or been killed.

DETRITIVORES AND DECOMPOSERS.

An animal that obtains its energy in this way, from consuming the carcasses of carnivores and omnivores (as well as herbivores and perhaps even plants), is known as a detritivore. Large and notable examples include vultures and hyenas, though most detritivores—earthworms or maggots, for instance—are much smaller.

Nonetheless, detritivores are relatively large, complex organisms compared with another variety of species that occupies a level or position in the food web that comes "after" carnivores and omnivores: decomposers, including bacteria and fungi.

This illustrates the reason why ecological sciences treat the expression food chain with disfavor. There is no such thing as "the top of the food chain," rather, there are simply stages, like a circular assembly line, with detritivores occupying a position between meat-eating animals and plants. Earthworms, in particular, help convert animal bodies into soil nutrients useful to the growth of plant life, yet even these and other detritivores must themselves eventually be converted to soil as well.

This "final" stage of conversion—that is, the last stop after the animal portion of the food web and before the cycle comes back around to ordinary plants—is occupied by decomposers, such as bacteria and fungi. These decomposers obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. Like detritivores, they aid in decomposition, a chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements.

Often an element such as nitrogen appears in forms that are not readily usable to organisms, and therefore such elements (which may appear individually or in compounds) need to be processed chemically through the body of a decomposer or detritivore. This process brings about a chemical reaction in which the substance (whether an element or compound) is transformed into a more usable version. By processing these chemical compounds, decomposers and detritivores provide nutrients necessary to plant growth.

User Contributions:

Comment about this article, ask questions, or add new information about this topic: