In this article we will discuss about the trophic structure of an ecosystem.
From the standpoint of trophic structure (from trophe = “nourishment”), an ecosystem is two-layered. It has (1) an upper, autotrophic (“self-nourishing”) stratum or “green belt” of chlorophyll-containing plants in which the fixation of light energy, the use of simple inorganic substances, and the buildup of complex organic substances predominate; and (2) a lower, heterotrophic (“other-nourished”) stratum or “brown belt” of soils and sediments, decaying matter, roots, and so on, in which the use, rearrangement, and decomposition of complex materials predominate.
It is convenient to recognize the following components as constituting the ecosystem:
(1) Inorganic substances (C, N, CO2, H2O, and others) involved in material cycles;
(2) Organic compounds (proteins, carbohydrates, lipids, humic substances, and so on) that link biotic and abiotic components;
(3) Air, water, and substrate environment, including the climate regime and other physical factors;
(4) Producers (autotrophic organisms), mostly green plants that can manufacture food from simple inorganic substances;
(5) Phagotrophs (fromphago = “to eat”), heterotrophic organisms, chiefly animals, that ingest other organisms or particulate organic matter; and
(6) Saprotrophs (from sapro = “to decompose”) or decomposers, also heterotrophic organisms, chiefly bacteria and fungi, that obtain their energy either by breaking down dead tissues or by absorbing dissolved organic matter exuded by or extracted from plants or other organisms. Saprophages are organisms that feed on dead organic matter. The decomposing activities of saprotrophs release inorganic nutrients that are usable by the producers; they also provide food for the macro-consumers and often excrete substances that inhibit or stimulate other biotic components of the ecosystem.
One of the universal features of all ecosystems—whether terrestrial, freshwater, marine, or human-engineered (for example, agricultural)—is the interaction of the autotrophic and heterotrophic components. The organisms responsible for the processes are partially separated in space; the greatest autotrophic metabolism occurs in the upper “green belt” stratum, where light energy is available. The most intensive heterotrophic metabolism occurs in the lower “brown belt,” where organic matter accumulates in soils and sediments.
Also, the basic functions are partially separated in time, as there may be a considerable delay in the heterotrophic use of the products of autotrophic organisms. For example, photosynthesis predominates in the canopy of a forest ecosystem. Only a part, often only a small part, of the photosynthate is immediately and directly used by the plant and by herbivores and parasites that feed on foliage and other actively growing plant tissue.
Much of the synthesized material (leaves, wood, and stored food in seeds and roots) escapes immediate consumption and eventually reaches the litter and soil (or the equivalent sediments in aquatic ecosystems), which together constitute a well-defined heterotrophic system. Weeks, months, or years (or many millennia, in the case of the fossil fuels now being rapidly consumed by human societies) may pass before all the accumulated organic matter is used.
The term organic detritus (product of disintegration, from the Latin deterere, “to wear away”) is borrowed from geology, in which it is traditionally used to designate the products of rock disintegration. Detritus refers to all the organic matter involved in the decomposition of dead organisms. Detritus seems the most suitable of many terms that have been suggested to designate this important link between the living and the inorganic world.
Environmental chemists use a shorthand designation for two physically different products of decomposition as follows- POM is particulate organic matter; DOM is dissolved organic matter. We can also add volatile organic matter (VOM), which mostly functions as “signals”—for example, the fragrance of flowers that attracts pollinators.
As a general principle, from the operational standpoint, the living and nonliving parts of ecosystems are so interwoven into the fabric of nature that it is difficult to separate them; hence, operational or functional classifications do not sharply distinguish between biotic and abiotic.
Most of the vital elements (such as carbon, nitrogen, and phosphorus) and organic compounds (such as carbohydrates, proteins, and lipids) are not only found inside and outside of living organisms but are also in a constant state of flux or turnover between living and nonliving states. Some substances, however, appear to be unique to one or the other state.
The high-energy storage compound ATP (adenosine triphosphate), for example, is found only inside living cells (or at least its existence outside is very transitory), whereas humic substances, which are resistant end products of decomposition, are never found inside cells, yet they are a major and characteristic component of all ecosystems. Other key biotic complexes, such as DNA (deoxyribonucleic acid) and the chlorophylls, occur both inside and outside organisms but become nonfunctional when outside the cell.
The ecological classification (producers, phagotrophs, and decomposers) is one of function rather than of species as such. Some species occupy intermediate positions, and others can shift their mode of nutrition according to environmental circumstances. The separation of heterotrophs into large and small consumers is arbitrary but justified in practice because of the very different methods of study required.
The heterotrophic micro-consumers (bacteria, fungi, and others) are relatively immobile (usually embedded in the medium being decomposed), are very small, and have high rates of metabolism and turnover. Their functional specialization is more evident biochemically than morphologically; consequently, one cannot usually determine their role in the ecosystem by such direct methods as visual observation or counting their numbers.
Organisms designated as macro-consumers obtain their energy by heterotrophic ingestion of particulate organic matter. These are largely “animals” in the broad sense. These higher forms tend to be morphologically adapted for active food seeking or herbivore, with the development of complex sensory-neuromata, digestive, respiratory, and circulatory systems in the higher forms. The micro-consumers, or saprotrophs, have been typically designated as decomposers. However, it seems preferable not to designate any particular organisms as decomposers but to consider decomposition as a process involving all of the biota and abiotic processes as well.
We recommend that students of ecology read Aldo Leopold’s “The Land Ethic” (first published in 1933, and in 1949 included in his best-seller, A Sand County Almanac: And Sketches Here and There), an eloquent, often quoted and reprinted essay on environmental ethics that has special relevance to the ecosystem concept.
We also recommend reading Man and Nature by the Vermont prophet George Perkins Marsh, who analyzed the causes of the decline of ancient civilizations and forecast a similar doom for modern ones unless an “eco-systematic” view of the world is taken. B. L. Turner (1990) edited a book that reiterates this theme in a review of Earth as transformed by human action over the past 300 years.
From another viewpoint, Goldsmith (1996) argued the need for a major paradigm shift from reductionist science and consumption economics to an ecosystem worldview that would provide a more holistic and long-term approach to dealing with increasingly endangered Earth. Especially recommended are Flader and Callicott’s (1991) and Callicott and Freyfogle’s (1999) reviews of the Leopold philosophy.