In the short period of five years since the publication of the first edition, interest in ecology has increased significantly. In preparing the second edition, I am very grateful to the following people for their specific contributions: Frank A.
The subdivisions of ecology
Synecology can be further subdivided according to the level of organization, as outlined in the previous section. Therefore, subdivisions according to the natural environment are most convenient for field research, as outlined in Part II of this book.
Concept of the ecosystem
For additional discussions of the ecosystem concept see Forbes' (1887) classic essay, Tansley (1935), and especially the important paper by Lindeman (1942). Consequently, it may be worthwhile to consider the role of the decomposers in the ecosystem in more detail.
Concepts of habitot and ecological niche
It is important to recognize these two possible uses of the term habitat to avoid confusion. 2 the abiotic environment on the organisms, the effect of the organisms on the abiotic environment, and the interaction of organisms on each other all come into play.
Biogeochemical cycles
Other cycles may be less perfect; that is, some of the supply may be 'lost' for extended periods of time in places or in chemical forms that are inaccessible to organisms. As shown in Figure 7 (A), the nitrogen in the protoplasm is broken down from organic to inorganic form by a series of decomposing bacteria, each specialized for a particular part of its job.
UREA, E~C
By comparison, the amount of phosphorus in the circulation is only about 1 percent of calcium. This strontium is new material added to the biosphere as it is the result of the fission of uranium.
The food chain
And only a very small percentage of the available solar energy is fixed by the plant in the first place. Thus, the efficiency of fixation of the total insolation reaching plants is about 1 percent.
Metabolism and size of individuals
An example of the "pyramid of num commitment." The number of organisms (excluding decomposers) in a bluegrass field is ranked accordingly·. Actually, the biomass of decomposers is very small in relation to their importance in the functioning of the community. Energy pyramids depict in a different way some of the same relationships shown in an energy flow diagram.
The output rate is in equilibrium with the supply or input rate of the minimum limiting factor (in other words, the law of the minimum applies, see Chapter 3). Therefore, the sum of the oxygen produced in the light bottle and the oxygen used in the dark bottle is the total oxygen production.
Liebig's " law" of the minimum
PRINCIPLES OF LIMITING FACTORS: §2 89 from the minimum to chemical materials (oxygen, phosphorus, etc.) necessary for physiological growth and reproduction, as originally intended, and to include other factors and the limiting effect. of the maxim in the "law" of tolerance. TIlUs, the "law" of the minimum is but one aspect of the concept of limiting factors which in turn is but one aspect of the environmental control of organisms. Thus, high concentration or availability of some substance, or the action of some factor other than the minimum one, can change the utilization rate of the latter.
Sometimes organisms are able to substitute, at least partially, a chemically closely related substance for a deficient substance in the environment. Some plants have been shown to require less zinc when grown in shade than when grown in full sunlight; thus a given amount of zinc in soil would be less limiting to plants in shade than under the same conditions in sunlight.
Shelford's "law" of tolerance
4 knowledge helped us understand the distribution of organisms in nature; but we must hurry that this is only part of the story. For example, McMillan (19:56) found that prairie grasses of the same (and apparently identical) species transplanted into experimental gardens from different parts of the area responded quite differently to light. Northern individuals of the sea jellyfish, Aurelia, can swim at optimal speeds in temperatures that would completely boil southern individuals.
Relation of temperature to swimming movement in northern (Halifax) and southern (Tortugas) individuals of the same jellyfish species Aurelia aurita. An interesting case that has not yet been fully elucidated is that of the ring-necked pheasant, a game bird introduced to North America.
Combined concept of limiting factors
In other words, it is not necessary to consider all the numerous phenomena in the sounding in a given situation. Regarding the occurrence and distribution of the tiger beetle in the region, soil conditions would be the main limiting factor. The diagram shows (1) the convergence effect on the western edge of the Sierra, (2) the area of maximum precipitation on the central western slope of the Sierra, and (3) the landward rain shadows of the two mountain ranges.
In a sense, this is another aspect of the principle of factor interaction, which was discussed earlier in the chapter. In the ocean, however, hydrostatic pressure is important because of the tremendoll gradient from surface to depth.
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Ecological indicators
In the western United States, for example, plants have been used extensively as indicators of water and soil conditions (especially as they affect grazing and agricultural potentials) since the early work of Shantz (1911) and Clements (1916). The use of vertebrate animals, as well as plants, as indicators of temperature zones (developed by Merriam) has also been much studied. Before relying on particular species or groups of species as indicators, there must be ample field evidence, and , if possible, experimental evidence that the factor in question is limiting.
This has proven particularly good in the search for biological indicators of various types of pollution (see page 440). Since selenium is often associated with uranium ore, plants that indicate selenium, such as the Astragalus species of the Rocky Mountain region, can also be useful in locating deposits.
Population density and indices of relative abundance
The range of population densities (as biomass per hectare) of different species of mammals, as reported from the species' preferred habitat in places where humans are not overly restrictive. World population of the gannet (Sula's perch (Jna), a large seabird that nests in a few densely populated colonies on the northern Atlantic coast of North America and Europe (data from Fisher and Vevers, 1944). Note that the growth rate increases and decreases as the slope of the growth curve increases and decreases.
In terms of the growth curve, the slope (tangent of the straight line) at any point is the growth rate. In the example above, the percent growth rate is in terms of the original population present at the beginning of the measurement (ie, 50 protozoa).
Natality
An approximation could of course be obtained by taking a census of the population at very short intervals. It is not a constant for a population, but can vary with the size and composition of the population and the physical environmental conditions. In the case of the bluebird, the exact number of females in the population was not known, so rate calculations shown in Part III of the table are approximations.
A rather different situation can be seen in the case of the blind gull, for which I also plotted the survival curve. The shape of the survival curve can vary with population density, as shown in Figure 44.
Population age distribution
Those in the middle of the figure show age distribution of a population of weivols (left) under conditions of maximum population increase rate with a stable age distribution, and (right) the same population with birth equal to mortality and rate of increase equal to zero. The lower left diagrams in Figw-e 45 show the age distribution of ring-necked pheasants in the Dakotas for two specific years. When the environment is unlimited (space, food, other organisms that do not exert a limiting effect), the percentage growth rate (i.e. the population growth rate per individual) becomes constant and.
For others it would simply and more specifically mean the maximum number of eggs, seeds, spores, etc., that the most fertile individual was known to produce, despite the fact that this would make little sense in a population sense in most cases. , since most populations do not contain individuals all of whom are constantly capable of maximum production. Under such conditions the population as a whole is expanding at an extraordinary rate even though each organism reproduces at the same rate as before, i.e., the specific rate of growth is constant.
Population growth form and concept of carrying capacity
In the case of the rice beetle, it will be noted that the variation of the temperature above and below the optimum has a noticeable effect on the ability of the population to multiply. In the J-shape there may be no equilibrium level, but the limit on N represents the upper limit imposed by the environment. It is interesting to note that these two basic growth forms are similar to the two metabolic or growth types described in the case of individual organisms (Bertalanffy, 1957).
As will be noted, this is the same equation as the exponent1 written in the previous section with the addition of the expression (K-N)/K or (rIK)N2• The latter expressions are two ways of indicating the environment. the resistance created by the growing population itself, which brings about an increasing reduction in the potential rate of reproduction as the population size approaches carrying capacity. There must be evidence that the factors in the equation actually act to control the population before an attempt to compare can be made.
