Statement
The presence and success of an organism depend upon the com- pleteness of a complex of conditions. Absence or failure of an organism can be controlled by the qualitative or quantitative de- ficiency or excess with respect to anyone of several factors which may approach the limits of tolerance for that organism.
Explanation
Not only may too little of something be a limiting factor, as pro- posed by Liebig, but also too much, as in the case of such factors as heat, light, and water. Thus, organisms have an ecological mini- mum and maximum, with a range in between which represents the limits of tolerance. The concept of the limiting effect of maxi- mum as well as minimum was incorporated into the "law" of tolerance by V. E. Shelford in 1913. From about 1910, much work has been done in "toleration ecology," so that the limits within which various plants and animals can exist are known, and this
90 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: Cli. 4 knowledge has helped us to understand the distribution of or- ganisms in nature; however, we should hasten to say, it is only part of the story. All phySical requirements may be well within the limits of tolerance for an organism and tIle organism may still fail as a result of biological interrelations. There are other princi- ples of ecology, as we shall see.
Some subsidiary principles to the "law" of tolerance may be stated as follows:
1. Organisms may have a wide range of tolerance for one factor and a narrow range for another.
2. Organisms with wide ranges of tolerance for all factors are likely to be most widely distributed.
3. When conditions are not optimum for a species with respect to one ecological factor, the limits of tolerance may be reduced with respect to other ecological factors. For example, Penman (1956) reports that when soil nitrogen is limiting, the resistance of grass to drought is reduced. In other words, he found that more water was required to prevent wilting at low nitrogen levels than at high levels.
4. The limits of tolerance and the optimum range for a physical factor often vary geographically (and also seasonally) within the same species; that is to say, organisms often adjnst their rate functions to local conditions. McMillan (19.56), for example, found that prairie grasses of the same species (and to all appear- ances identical) transplanted into experimental gardens from dif- ferent parts of the range responded quite difFcrently to light. In each case the timing of growth and reprodllction was adapted to the area from which the grasses were transplanted. A .good exam- ple of temperature compensation is shown in Figure 19. Northern individuals of the marine jellyfish, Aurelia, are able to swim at an optimum rate at temperatures which would completely inhibit southern individuals. This sort of temperature compensation is apparently widesprcad, although some species show a much greater ability to acclimate than others (Bullock, 1955). Com- pensatory mechanisms help to explain how northern seas can be equally productive (as was discussed in the previous chapter) as southern seas even when it would appear that low temperatures should be limiting to the whole ecosystem. Sometimes a given individual is able to acclimate if conditions are changed slowly, but frequently limits of tolerance have become genetically fixed in local races or strains (with or wHhout morphological manifes-
PRINCIPLES PERTAINING TO LIMITING FACTORS: §2 91 tations). This important possibility has often been overlooked in applied ecology with the result that restocking attempts have often failed simply because individuals from remote regions were used, and these proved unable to adapt to conditions in the area to which they were transplanted. It would seem logical to expect that racial differences in transplants would likely be more critical under the competitive conditions of nature as compared with the semiprotection of agriculture, horticulture or animal husbandry, but even ill the latter it is often necessary to choose or breed strains adapted to the particular climatic region in question. In ecology, locally adapted variants are often called ecotypes.
5. Sometimes it is discovered that organisms in nature are not actually living at the optimum range (as determined experimen- tally) with regard to a particular physical factor. In such cases some other factor Or factors are found to have greater importance.
Certain tropical orchids, for example, actually grow better in full sunlight than in shade provided they are kept cool (see Went, 1957); in nature they grow only in the shade because they cannot tolerate the heating effect of direct sunlight. In many cases of this sort biological factors (competition, predators, parasites, etc.) apparently prevent organisms from taking advantage of optimum
20 1S 10
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_ _ _ - - 0 - 0 ~ Tortugas. ' .
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0 Halifax.
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summer animals \ummer animals ""-
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O~~0---+5----~10~--~1~S----~----~--~~-~3S~
Temperature (OC.)
Figure 19. The relation of temperature to swimming movement in northem (Halifax) and southem (Tortugas) individuals of the same species of jellyfish Aurelia aurita. The habitat temperatures were 140 and 290 C., respectively. Note that each population is acclimated to swim at a maximum rate at the temperature of its local environment. The cold-adapted form shows an especially high degree of temperature independence. (From Bullock, 1955, after Mayer.)
92 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CR. 4 physical conditions (examples of this are described in the next section of this chapter).
6. The period of reproduction is usually a critical period when environmental factors are most likely to be limiting. The limits of tolerance for reproductive individuals, seeds, eggs, embryos, seed- lings, larvae, etc., are usually narrower than for non-reproducing adult plants or animals. Thus, an adult cypress tree will grow on dry upland or continually submerged in water, but it cannot re- produce unless there is moist unflooded ground for seedling devel- opment. Adult blue crabs and many other marine animals can tolerate brackish water, or fresh water which has a high chloride content; thus, individuals are often found for some distance up rivers. The larvae, however, cannot live in such waters; therefore, the species cannot reproduce in the river environment and never become established permanently. An interesting case which l,as not yet been fully worked out is that of the ring-necked pheasant, a game bird introduced into North America. Successfully estab- lished in many parts of the United States and Canada, the bird has failed to take hold in southern United States despite large- scale introductions. The adults appear to survive well enough, but reproduction fails. Some experimental evidence (Yeatter, 1950) indicated that the eggs are unable to tolerate high temperatures during the egg-laying period, when they lie unattended in the nest before the beginning of incubation. The problem is not settled and is a good one for further ecological work. The search for rea]
limiting factors is rarely a simple or easy job.
To express the relative degree of tolerance, a series of terms have come into general use in ecology utilizin~ the prefixes
"steno-" meaning naITOW, and "eury-" meaning wide. Thus, stenothermal - eurythermal
stenohydrie - euryhydrie stenohaline - euryhaline stenophagie - euryphagic stenoecious - euryecious
refers to temperature refers to water refers to salinity refers to food
refers to habitat selection As an example, let us compare the conditions under which brook b'out (Salvelinus) eggs and leopard frog (Rana pipiens) ggs will develop and hatch. Trout eggs develop between 0° .and 12° C with optimum at about 4°C. Frog eggs will develop be- tween 0° and 30° C with optimum at about 22°. Thus, trout eggs
PRINCIPLES PERTAINING TO LIMITING FACTORS: §3 93 are stenothermal, low temperature tolerant, compared with frog eggs, which are euryth erm aI, both low and high temperature tolerant. Trout in general, both eggs and adults, are relatively stenothermal, but some species are more eurythermal than is the brook trout. Likewise, of course, species of frogs differ. These con- cepts, and the use of terms in regard to temperature, are illus- trated in Figure 20. In a way, the evolution of narrow limits of tolerance might bc considered a form of specialization, as dis- cussed in the ecosystem chapter, which results in greater effi- ciency at the expense of adaptability. "Steno" organisms often become very abundant when their conditions are favorable and stable.