Trophic levels and ecological niches

Categories: Classification and systematics; ecology; ecosystems

For many years, ecologists referred to niche in terms of an organism’s place in the food pyramid. The food pyramid is a simplified scheme showing organisms’ interactions with one another while obtaining nourishment. The food pyramid is represented visually as a triangle, often with four horizontal divisions, each division being a different trophic level.

The base of the food pyramid is the first trophic level and contains the primary producers, photosynthetic plants. At the second trophic level are the primary consumers; these are the herbivores, such as deer and rabbits, which feed directly on the primary producers. At the third trophic level are the secondary consumers are the omnivores, which eat both plants and animals. The fourth trophic level contains the tertiary consumers, animals that eat only meat—carnivores such as the mountain lion. The members of the uppermost trophic level are the scavengers, such as hyenas and buzzards, and the decomposers, including fungi and bacteria. The organisms in this trophic level break down all the nutrients in the bodies of plants and animals and return them to the soil to be absorbed and used by plants.

All living things are dependent on the first trophic level, because plants alone have the capability to convert solar energy to energy found in, for example, glucose and starch. The food pyramid takes the geometric form of a triangle to show the flow of energy through a system. Because organisms lose a percentage of the energy they absorb from the sun or consume by eating, less energy is found at each higher level of the pyramid. Because of this reduced energy, fewer organisms can be supported by each higher trophic level. Consequently, the sections of the pyramid get smaller at each higher trophic level.

Through the years, two concepts of niche have evolved in ecology. The first is the place niche, the physical space in which an organism lives. The second is the ecological niche, which encompasses the particular location occupied by an organism and its functional role in the community. The functional role of a species is not limited to its position within a food pyramid; it also includes its interactions with other organisms while obtaining food. Specific methods of tolerating climate, water or nutrient conditions, soil conditions, parasites, and other factors of the environment are part of its functional role. In other words, the ecological niche of an organism is its natural history: all the interactions and interrelationships of the species with other organisms and the environment.

Niche Overlap

The study of relationships among organisms has been the focus of ecological science since the 1960’s. Before that time, researchers had focused on the food pyramid and the effects of population changes of a single species upon predator-prey relationships. The goal of understanding how species interact with one another can be better accomplished by defining the degree of niche overlap, the sharing of resources among species. When two species use one or more of the same elements of an ecological niche, they exhibit interspecific competition. It was once believed that interspecific competition would always lead to survival of only the better competitor of the two species—that no two species can utilize the same ecological niche. It was conjectured that the weaker competitor would migrate, begin using another resource not used by the stronger competitor, or become extinct. It is now believed that the end result of two species sharing elements of ecological niches is not always exclusion.

Ecologists theorize that similar species do, in fact, coexist, despite the sharing of elements of their ecological niches. Character displacement leads to a decrease in niche overlap and involves a change in the morphological, behavioral, or physiological state of a species without geographical isolation. The more specialized a species, the more rigid it will be in terms of its ecological niche. A species that is general in terms of its ecological niche needs will be better able to find and use an alternative for the common element of the niche. Because a highly specialized species cannot substitute whatever is being used, it cannot compete as well as the other species. Therefore, a specialized species is more likely to become extinct.

For example, some species of tropical orchids are so specialized that they rely on a single species of bee for pollination. The flowers so closely resemble female bees that male bees attempt to copulate with them, and in the process transfer pollen from flower to flower. If one of these species of bees were to become extinct, the associated orchid species, unable to reproduce, would soon follow. On the other hand, many species of daisies are freely pollinated by bees, flies, beetles, and a number of other insects. Even if a few of these pollinator species were to become extinct, daisies would be able to continue to reproduce using the remaining pollinator species.

Hence, species with specialized ecological niche demands (specialists) are more in danger of extinction than those with generalized needs (generalists). Although this fundamental difference in survival can be seen between specialists and generalists, it must be noted again that exclusion is not an inevitable result of competition. There are many cases of ecologically similar species that coexist.

When individuals of the same species compete for the same elements of the ecological niche, it is referred to as intraspecific competition. Intraspecific competition results in niche generalization, the opposite result from that of interspecific competition. In increasing populations, the first inhabitants will have access to optimal resources. The opportunity for optimal resources decreases as the population grows; hence, intraspecific competition increases. Deviant individuals may begin using marginal resources that are in less demand; those individuals will slowly come to use fewer optimal resources. That can lead to an increase in the diversity of ecological niches used by the species as a whole. In other words, the species may become more generalized and exploit wider varieties of niche elements.

Why Study Niches?

The shift in meaning and study from mere space and trophic level placement in the food pyramid to ecological niche has been beneficial for the field of ecology and for human activities. This focus on community ecology is much more productive for the goal of ecology, the understanding of how living organisms interact with one another and with the nonliving elements in the environment.

Perhaps more important is the attempt to describe niches in terms of community ecology, which can be essential for some of humankind’s confrontations with nature. One relevant function of community-oriented studies of ecological niches involves endangered species. In addition to having aesthetic and potential medicinal values, an endangered organism may be a keystone species, a species on which the entire community depends. A keystone species is so integral to keeping a community healthy and functioning that if it is obliterated the community no longer operates properly and is not productive.

Habitat destruction has become the most common cause of drastic population declines of species. To enhance the habitat of the endangered species, it is undeniably beneficial to know what conditions cause a species to favor its particular preferred habitat. This knowledge involves details of many of the dimensions of an ecological niche integral to specific population distribution. Danger to the survival of a species also occurs when an introduced organism competes for the same resources and displaces the native species. Solving such competition between native and introduced species would first involve determining niche overlap.

Researching and understanding all the dimensions of ecological niches can prevent environmental manipulations by humankind that might lead to species extinction. Many science authorities have agreed that future research in ecology and related fields should focus on solving three main problems: species endangerment, soil erosion, and solid waste management.

This focus on research in ecology often means that studies of pristine, undisturbed communities is the most helpful for future restoration projects. Although quantitative and qualitative descriptions of pristine areas seem to be unscientific at the time they are made, because there is no control or experimental group, they are often the most helpful for later investigations. For example, after a species has shown a drastic decline in its population, the information from the observations of the once-pristine area may help to uncover what niche dimension was altered, causing the significant population decrease.

Bibliography

Ehrlich, Paul R. The Machinery of Nature. New York: Simon & Schuster, 1987. The chapter “Who Lives Together, and How” cites many examples of interactions between species in terms of community ecology. Includes references for further reading.

Giller, Paul S. Community Structure and the Niche. New York: Chapman and Hall, 1984. This excellent book is intended to give an introduction to the theories and ideals of community structure and to provide an opening into the vast and detailed information available. It contains many charts and diagrams detailing topics under discussion. Includes extensive list of references.

Knight, Clifford B. Basic Concepts of Ecology. New York: Macmillan, 1969. This general ecology book covers a broad range of topics. Ecological niche is discussed in chapter 6, with several photographs and tables supporting the text. An ample list of references is provided at the end of the chapter.

National Science Foundation Staff. Ecology: Impacts and Implications. Wayne, N.J.: Avery, 1983. A collection of articles that describe the ecology in nontechnical terms. Habitats discussed range from Antarctic waters to tropical forests, and emphasis is on the relationships between organisms and their environment. Includes photographs of habitats and diagrams of ecological processes.

Odling-Smee, F. John, Kevin N. Laland, and Marcus W. Feldman. “Niche Construction.” American Naturalist 147, no. 4 (April, 1996): 641-649. Shows how organisms not only adapt to their environment but also modify their habitat to suit their needs. Niche construction is described as a feedback to the evolutionary process.

Rayner, Alan D. M. Degrees of Freedom: Living in Dynamic Boundaries. River Edge, N.J.: World Scientific, 1997. Describes the fundamental indeterminacy that enables life-forms to thrive in inconstant circumstances. For biology students as well as general readers.

Ricklefs, Robert E., and Gary L. Miller Ecology. 4th ed. New York: W. H. Freeman, 2000. A textbook written with exceptional clarity. The chapter “The Niche Concept in Community Ecology” gives an in-depth discussion of the ecological niche and the principles involved. Many charts, figures, tables, and mathematical equations are used, further enhancing the topic. Describes the food pyramid and its relevance to the ecological niche. Includes glossary and bibliography.

Smith, Robert Leo. Ecology and Field Biology. 6th ed. San Francisco: Benjamin/Cummings, 2001. A text easily readable by college and senior-level high school students. It has many informative illustrations, and the extensive appendices make the book a useful source. Includes an excellent index and glossary.

Stone, Richard. “Taking a New Look at Life Through a Functional Lens.” Science 269, no. 5222 (July, 1995): 316-318. Describes how ecologists are revising the theory that a single dominant predator is the key to the integration of an entire ecosystem. They are discovering the importance of small organisms organized in functional groups.

Whittaker, Robert H., and Simon A. Levin, eds. Niche: Theory and Application. Stroudsburg, Pa.: Dowden, Hutchinson & Ross, 1975. A compilation of papers written on various elements of the ecological niche. Essays 5 through 8 deal with the competitive exclusion principle. The authors’ comments preceding the papers are excellent summarizations. The papers do tend to be technical and might be more suitable for the college-level reader.