Trophic levels describe the various stages within ecological food chains or webs. Examples of trophic levels, all of which will be described below, are primary producers, primary consumers or herbivores, and secondary and higher-level consumers, or predators.
Food webs are based on the productivity of photosynthetic organisms, such as blue-green bacteria , algae , and plants. These are autotrophic organisms, which are capable of fixing some of the diffuse energy of solar radiation into simple organic compounds. This fixed energy can then be utilized by the primary producers to metabolically synthesize a diverse array of biochemicals and to support the growth of these organisms.
The solar energy fixed by photosynthesis is the energetic base that all heterotrophic organisms utilize to achieve their own productivity. Heterotrophic organisms include any animals and microorganisms that feed on the living or dead biomass of plants, or that of other heterotrophs. Heterotrophs include herbivores that feed directly on autotrophs, carnivores that feed on other animals, detritivores that feed on dead biomass, and omnivores that feed on any or all of the above.
Therefore, the food web is a diverse assembly of organisms that are ecologically linked through their feeding relationships, and is ultimately based on the fixation of solar radiation through photosynthesis.
Primary producers are autotrophic organisms that are capable of fixing solar radiation into biochemical energy, through the process of photosynthesis. Photosynthesis is comprised of a series of enzyme-mediated chemical reactions which result in the combination of carbon dioxide and water into glucose, a simple sugar. This chemical reaction requires an input of energy to proceed, and this energy is provided by red and blue wavelengths of solar radiation, which are captured by the photosynthetic pigment chlorophyll . The fixed-energy content of the glucose can then be utilized to drive a great diversity of other metabolic reactions, which are used to synthesize the myriad other biochemicals that are found in the tissues of primary producers.
Primary producers include green plants, algae, and blue-green bacteria. If the rate of photosynthesis by these organisms exceeds their metabolic requirements, then they are able to grow, and their biomass increases.
The accumulating biomass of primary producers is a source of fixed energy that can be utilized by heterotrophic organisms by directly feeding on the autotrophic biomass. The primary consumers of autotrophic biomass are also known as herbivores and include the tiny crustacean zooplankton that filter microscopic algal cells out of the surface waters of lakes, ponds, and oceans, as well as much larger, mammalian herbivores, such as mice , deer , cows, and elephants. Herbivores utilize the fixed energy and nutrients in their food of autotrophic biomass to drive their own metabolic processes and to achieve their own growth.
Secondary and higher-order consumers
Herbivores may be fed upon by other heterotrophs, which are known as secondary consumers. If the herbivore must be killed before it is eaten, the secondary consumer is known as a predator . However, if the herbivore does not have to be killed to be eaten, the secondary consumer is known as a parasite. Predators of the tiny zoo-plankton described in the previous section include somewhat larger, carnivorous zooplankton, as well as small fish . In terrestrial ecosystems, herbivorous mice may be fed upon by predatory weasels and hawks , while deer are killed and eaten by coyotes and cougars.
If the resource base of the ecosystem is large enough, the secondary consumers may be killed and eaten by higher-order consumers, which will generally be the top predators in the system. For example, mature lake trout may be at the top of the food web of a temperate-lake ecosystem, in which the trophic structure is organized as: algae...herbivorous zooplankton...predatory zooplankton and small fish...and the largest predatory fish, such as lake trout. If bald eagles or humans subsequently predate on the lake trout, they would be considered the top predators in the system as well as a trophic linkage to the terrestrial part of the larger ecosystem.
All organisms eventually die, and detritivores are a class of organisms that feed on their dead bodies. Actually, detritivores can themselves be divided into a food web, based on the feeding relationships among the species . In this sense, primary detritivores feed directly on the dead biomass, while secondary detritivores feed on these direct consumers of detritus.
Omnivores are animals that feed at various places within the food web and are therefore difficult to classify in terms of trophic level. For example, grizzly bears are highly opportunistic animals that feed quite widely, on sedges and berries, small mammals , fish, and dead animals (or carrion). Of course, humans are the most omnivorous of all species (we eat just about anything that is not acutely poisonous), and in turn are not eaten by many other creatures, except, eventually, by detritivores.
Ricklefs, R.E. Ecology. New York: W.H. Freeman and Co., 1990.
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—This refers to organisms that can synthesize their biochemical constituents using inorganic precursors and an external source of energy.
—Organism that requires food from the environment since it is unable to synthesize nutrients from inorganic raw materials.
—Pertaining to the means of nutrition.