Units of the BiosphereEdit
You have been presented with a definition of the term biosphere above. This chapter will consider the question: How is the biosphere organized? We will initially look at this organization over a range of scales so to familiarize you to important terms used in ecology. In later chapters, we will cover the details and significance of the concepts only introduced here.
Ecologists regard ecosystems as basic structural units, implying that the biosphere is a collection or mosaic of definable ecosystems. However, an ecosystem is really more conceptual than actual. By this we mean that the definable boundaries of an ecosystem are more in the mind of the ecologist than geographical. We can certainly observe that there are many boundaries in nature—some fairly sharp—and what we are observing are physical manifestations between volumes wherein different conditions prevail. These are biocycles, subdivisions of the biosphere. Consider a lake shore, or even the water surface for that matter; the transitions between the biocycles—land, the air or atmosphere, and the body of water—are sharp and recognizable upon casual inspection. But are they really?
There is no doubt that under gravity, water in the liquid phase sorts itself out from gaseous air—a lake surface exists. But along the shore, the water moves into the earth wherever the soil or voids in the rock allow. Constantly water-saturated soil becomes a wetland. Further, atmospheric gases dissolve in the water of the lake, and molecules of water move off the lake as gaseous moisture, evident perhaps as fog on a cool morning. The sharpness of the boundaries is both actual and illusionary. Actual, because conditions encountered within each of the separate spaces or volumes (air, land, water) are very different. Illusionary, because no biocycle is completely isolated from adjacent biocycles, and typically each has influence over the other.
We have, so far considered just the physical environment. Ecosystems include organisms. And we can expect that most, though certainly not all, organisms will sort themselves out along the same physical boundaries described. That is, fish will find life conditions suitable only in the lake; conifers only in the uplands. Yet organisms are no more isolated from adjacent biocycles than are the substances of the physical world. The food of many fish will be insects living only on the land and moving through the atmosphere, which upon death or by accident, fall into the water. The conifers draw up moisture from the earth, some directly from the lake or the groundwater body connected to the lake. Ecologists use the term biotope to mean a topographic unit characterized by a particular set of physical conditions and a uniform assemblage of plant and animal species. Considering our example above, the lake is part of the major freshwater biocycle and would be a biotope of the region in which it is located, being perhaps just one of many ponds and lakes comprising that biotope. A biochore is a subdivision of the biosphere consisting of the biotopes that resemble one another; thus, our lake is but one example of the temperate zone, freshwater, lake biochore.
The terms "biotope", "biochore", and "biocycle" fill a need in descriptive ecology. We can speak in a general way about the life forms and climatic conditions that constitute a desert biochore, or a northern forest biochore. And we can describe, from our example illustrated by the photograph, lake, marsh, and upland forest biotopes, each with a distinct listing of species perhaps unique to just that geographical region near Minsk (in our photo example), or more broadly represented across the continent.
Consider the importance of the functional relationships between species comprising an ecosystem, and how those relationships contribute to the very survival of the ecosystem. Consider that ecosystems are not, in most cases, isolated units, but parts of a larger functioning whole, at its broadest being the biosphere. Is it possible that biosphere—all life on earth and at least those physical things utilized and modified by life on earth—is one, large integrated system?
- Read Gaia Theory (Follow links that interest you.)
Species and HabitatEdit
In biology, the concept of species is basic. A species comprises a group of related organisms that share a more or less distinctive form and are capable of interbreeding. According to Ernst Mayr, species are:
- groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.
- Read Species (Although you will be familiar with this material from biology studies, you should explore the topic to satisfy a more complete understanding)
In biology, it is generally the case that focus on the species concept is largely the concern of taxonomy and taxonomists. In ecology, the more important aspect is that of the species population (or population as we shall more often use the term). Note in Mayr's simple definition, that "interbreeding populations" is somewhat central. In ecology we are less concerned with all considerations of what "potentially interbreeding" might mean and more interested in the group of individuals that are, in fact, interbreeding. By which is meant, not that sexual reproduction must take place between all extant (living at the same time) members, but that there exists a gene pool that is open and potentially contributed to by each member. The concept of species would admit all those individuals separated by great distances and physical barriers as included if breeding could still take place. The species population takes into account the isolation that might actually exist, frequently dividing a single species into more than one population. Within the population, alleles presumably flow freely through time, held (at any given point in time) within the nuclei of the cells of the individual organisms constituting the population.
Using the term habitat in describing the ecological conditions surrounding an assemblage of species or a community, we are speaking of a biotope. Many ecologists extend the term habitat in the manner originally defined to cover physical parameters relative to a community of species, but others would substitute the term "biotope" for "habitat" in such cases. Note that this is not exactly the same as "biotope" defined above, which included the biota as well as the physical or abiotic environment. Odum (1959, p. 28) stated that "habitat... includes other organisms as well as the abiotic environment... [but a] description of the habitat of the community would include only the latter"—a point of view opposite to that expressed by most ecologists today.
Any term in ecology that attempts to separate the biotic and abiotic components of an ecological unit of organisation will encounter this difficulty. Consider that if the habitat of a bird is the place it lives, and the bird lives in a pine forest (actually up among the branches of the pine trees), then the physical environment (the "habitat") of that bird species must include a life form (pine trees); and now extend this reasoning to any manner of invertebrates, such as flea or tapeworm, that really do live most of their life on or in other organisms. A reasonably solid definition of habitat would be simply the environment of a species (Sweet, 2005).
- Read Habitat (You need follow links only as necessary to clarify terms; include the following:)
- Read Biotope
A niche or ecological niche is essentially what a species does. We do not mean so much what activities it participates in (although these could be pertinent), but the role the species plays in the functioning of the ecosystem: the "functional status of an organism in its community" (Charles Elton, in Odum, 1959).
- Read Niche (You need follow links only as necessary to clarify terms)
The choice of habitat affects reproductive success. How do species choose which place to inhabit? Wecker (1963) and Klopfer (1963) determined this choice is partly genetic. It is also partly psychological. Habitat selection is a common behavior among vertebrates. Garter snakes (Thamnophis elegans) choose rocks of intermediate thickness over thicker or thinner rocks, maintaining their preferred body temperature for extended periods (Huey et al. 1989, Huey 1991). Many species show some flexibility in habitat selection. Organisms are often forced to make these choices, sometimes choosing less suitable habitats. Optimal habitats are occupied more quickly, then marginal habitats, then poor habitats, where reproductive success is limited (Fretwell and Lucas 1969).
We have just touched upon the many aspects of the biology of species populations that concern or interest us as ecologists and this topic will be much expanded upon in Chapter 5 of the Guide.
There is but one known biosphere, that of the planet earth. The biosphere can be subdivided into a number of biocycles, each with fairly specific gross physical characteristics and limited by geography and topography as determined by planetary dynamics, especially gravity. Biocycles are divided into biochores, units with both similar basic physical attributes and life forms. Biochores are made up of a multitude of biotopes, divisions of the landscape that feature both similar physical attributes and a more or less consistent species composition. Some ecologists would use "biotope" for just the physical properties of this level of organisation, and biocoenosis for the biotic component. However, such definitional division of highly integrated and complex ecological systems is difficult to uphold. At the level of the species population (essentially an organism and its relatives), the habitat describes where it is found, where that species occurs in the landscape. The very different term, niche, describes what that organism does as a functional role in the ecosystem. The ecosystem concept differs from traditional (or the "old") biology by its clear emphasis on functional relationships among organisms.
- Clements, Frederic E., and Victor E. Shelford. 1939. Bio-ecology. John Wiley & Sons, New York. 425 pp.
- Forbes, S.A. 1887. The lake as a microcosm. Bull. Sc A. Peoria. Reprinted in Ill. Nat. His. Surv. Bull., 15: 537-550 (1925).
- Fretwell, S. D., and H. L. Lucas. 1969. On territorial behavior and other factors influencing habitat distribution in birds. Acta Biotheoretica 19:16-36.
- Friedrichs, K. 1930. Die Grundfragen und Gesetzmässigkeiten der landund forstwirtschaftlichen Zooligie. Paul Parey, Berlin. 2 vols.
- Hess, R. W., W. C. Allee, and K. P. Schmidt. 1951. Ecological animal geography. John Wiley & Sons, Inc., New York. 715 pp.
- Huey, R. B. 1991. Physiological consequences of habitat selection. American Naturalist 131:S91-S115.
- Huey, R. B., C. R. Peterson, S. J. Arnold, and W. P. Porter. 1989. Hot rocks and not-so-hot rocks: Retreat site selection by garter snakes and its thermal consequences. Ecology, 70:931-944.
- Klopfer, P. 1963. Behavioral aspects of habitat selection: The role of early experience. Wilson Bull. 75:15-22.
- Odum, E. P. 1959. Fundamentals of ecology. W. B. Saunders Co., Philadelphia and London. 546 p.
- Sweet, M. H. 2005. Biology 357, Ecology Lecture Syllabus, Spring 2005. Texas A & M University. URL: 
- Thienemann, August. 1939. Grundzüge einer allgemeinen Oekologie. Arch. Hydrobiol., 35: 267-285.
- Vernadsky, W. I. 1944. Problems in biogeocemistry. II. Trans. Conn. Acad. Arts, Sci., 35: 43-494.