Statement
The habitat of an organism is the place where it lives, or the place wherc Olle would go to find it. The ecological niche, on the other hand, is the position or status of an organism within its com- munity and ecosystem resulting from the organism's structural adaptations, physiological responses and specific behavior (in- herited and/or learned). The ecological niche of an organism de- pends not only on where it lives but also on what it does. By analogy, it may be said that the habitat is the organism's "ad- dress," and the niche is its "profession," biologically speaking.
Explanation and examples
The term habitat is Widely used, not only in ecology but else.
where. It is generally understood to mean simply the place where an organism lives. Thus, the hahitat of the water 'backswimmer,"
Notonecta, is the shallow, vegetation-choked areas (littoral zone) of ponds and lakes; that is where (,De would go to collect this particular organism. The hahitat of a Trillium. plant is a moist, shaded situation in a mature deciduous forest; that is where one would go to find Trillium plants. Different species in the genus Notonect-a. or Trillium may occur in the same general habitat but exhibit small differences in location, in which event we would say that the rnicrohabitat is different. Other species in these genera ex- hibit large habitat, or mac1'01wbitat, differences.
Habitat may also refer to the place occupied by all entire com- munity. For example, the habitat of the "sand sage grassland com- munity" is the series of ridges of sandv soil occurring along the
28 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CJ-I. 2 north sides of rivers in the southern Great Plains region of the United States. Habitat in this case consists mostly of physical or abiotic complexes, whereas habitat as used with reference to Notonecta and Trillium, mentioned above, includes living as well as nonliving objects. TIlllS, the habitat of an organism or group of organisms (population) includes other organisms as well as the abiotic environment. A description of thc habitat of the community would include only the latter. It is important to recognize these two possible uses of the term habitat in order to avoid confusion.
Ecological niche is a more recent concept and is not sO gencrally understood outside the field of ecoJogy. It is, however, a very im- portant concept. Charles Elton in England was one of the first to hegin using thc term "niche" in thc sense of the "functional status of an organism in its community"; it has gradually become gen- erally accepted as it became clear that niche is by no means a synonym for habitat. Let us rcturn, for the momcnt, to the simple analogy of the "address" and the "profession" mentioned above.
If we wished to become acquainted wilh some person in our human community we would need to know, first of all, his address, that is, where he could be found. To really get to know him, however, we would have to lea111 morc than the neighborhood where he lives or works. vVc would want to know something about his occupation, his interests, his associates, and the part he plays in general community life. So it is with the study of organ- isms; learning the habitat is just the beginning. To determine the organism's status in the natural community we would need to know something of its activities, especially its nutrition and encrgy source; also its rate of metabolism and growth, its effect on other organisms with which it comes into contact, and the extent to which it modifies or is capable of modifying important operations in the ecosystem.
Returning to the Notonecta and the Trillium, we can see that discovery of the habitat, or where to go to collect it, does not necessarily result in detennination of the ecological niche. By studying the structure of organisms in the laboratory we can infer certain things about their activities, but we would most certainly also have to do some observing or experimenting in nature. We would soon find that Notonecta is an active predator, swimming about and grasping and eating other animals of its general size range. It is thus a secondary consumer in its marginal pond com-
TIlE ECOSYSTEM AND BIOGEOCHEMICAL CYCLES: §2 29 munity, and is fed upon in turn by other, larger animals or tertiary consumers. In contrast, other water bugs of the genus Corixa ..
found in the same habitat and looking much like N otonecta (see Figure 6), feed on decaying vegetation. Corixa thus plays a very different role in community life and, therefore, occupies an en- tirely different niche. The Trillium, because it has chloroplasts and is capable of food manufacture, can be immediately classed as a producer. As far as the forest community as a whole is con- cerned, it is a very minor producer, in no way influencing and con- trolling community life, as do the great forest trees. However, be·
cause of its ability to live in dense shade and to convert the sparse light energy into food energy, it docs occupy a definite and im- portant place wil'hin its particular stratum, the forest Hoar, of the forest community. It is important to other forest floor plants com- peting with it for nutrients and sunlight, and to the animals which eat its foliage, are attracted to its flowers or use jts body as a resting or hiding place.
The description of ecological niches of organisms is necessary as a foundation for all understanding of the functioning of the community and ecosystems operating in major habitats, just as the knowledge of the basic morpbology of the parts (cells, tissues, organs) of the individual organism is prerequisite to an under- standing of its physiology. In attempting to lliscover and describe the niches of organisms, the number of individuals present (i.e., the population density), their collective metabolism, the effects of
Figure 6. Notonecta (left) and Corixa (right), two aquatic bugs (Hemip- tera) which occupy the same general habitat but very different ecological niches because of differences in food habits. (Redrawn from Mellanby, 1938.)
30 BASIC ECOLOGICAL PRINClPLES AND CONCEPTS: CH. 2 the abiotic enviromnent on the organisnls, the effect of the organ- isms on tIle abi(ltic environment, and the interaction of organisms on each other all enter the pietme. Sometimes the niche is rela- tively easy to discover and descrihe, but llsllally it is not. There are various complications. The same species often occupies somewhat different niches in different regions, dcpendillg on the local com- munity organization. Within the same taxonomic group, such as tIw gel111S or family, the species often sIww a Similarity in nic11(') relations (as might he expected from their similar morphological adaptations), but they almost never actuany occupy the same niche in the same llabitat. This is an important principle which will be discussed in Chapter 7 (Section 2). Many organisms, especially animals with distinct life 11islory stages, occupy successively sev- eral widely different niches. Thus, the larval stage of a mosquito is a primar)' consumer in shallow water habitats of ponds, etc., whereas the adult occupies an entirely difTerent habitat and lliche- one that often hrings ,_ it-into violent interaction with mankind!