The term adaptation is sometimes used to describe short-term changes which are actually a form of responsiveness. We often say that man is very adaptable to climatic conditions, for instance, since he can adjust from a warm climate to a very cold one.
A more clear-cut example of responsive “adaptation” is seen in the case of less complex organisms, such as protozoa, where certain chemical substances may be added to their aqueous environment by degrees until concentrations are reached that would have killed them is such a quantity of chemical were added all at one time.
Evidently, such adjustment is possible through chemical and physical chances in the protoplasm, whose capacity for such adjustment is, of course, limited. However, the very occurrence of such adjustments illustrated the tremendous versatility, of protoplasm in responding to environmental changes, and as a type of responsiveness; it is of considerable biological significance.
All organisms exhibit the capacity for at least some versatility in this regard. However, this is not what is meant by adaptation. This term describes the characteristic of living forms to develop, over a period of time, certain structural and functional features which enable them to survive and reproduce within the limits of a particular environment.
Hence, when a biologist says that adaptation is a characteristic of living forms, he has in mind a process, and one that has gone on throughout past ages, the result of which is the variety of present day organisms.
However, any structural or functional feature of an organism that has been developed by this process may be called an adaptation, and so the same term is at once used to describe an overall process in nature and any given result of that process.
The leaves of cactus are greatly modified as spines, and these have very little surface area from which water might evaporate. Actually, they are of little use to the plant except that they prevent desert animals from eating it for the water that it contains.
Reduction of exposed surface and loss of photosynthetic ability by these modified leaves have been accompanied in the overall modification of the plant by an emphasis on the stem as a photosynthetic organ. These structural adaptations relate the plant to its environment, but they developed over time by the overall process.
In general, any characteristics exhibited by an organism which re of benefit to it in relation to a giver, environment are primarily either structural or functional in nature. Structural adaptations are the more obvious, and it would be difficult for even an amateur biologist to miss the direct connection between the morphological features of almost any organism and its environment.
We have already called attention to the spines and stems of cacti; these plants exhibit many other structures which make it possible for
For purposes of discussion, therefore, we shall distinguish between environmental factors that are physical in nature, those that are biogeochemical, and those that are biotic.
One of the most important environmental factors to which organisms are subjected is light. This should be obvious in the case of green plants since they are dependent upon sunlight as an energy source in the photosynthetic process. The leaves of higher plants are adapted in various ways to sunlight; in many cases, they are arranged on the plant in such a way that maximum exposure to sunlight is realized.
However, the influence of light is not limited to photosynthetic effects; the flowering process in many plants is rigidly controlled by the time pattern and quality of light; received.
One example such control is photoperiodicity, to the response of an organism to the length of time it is exposed to light. The principle of photoperiodicity is exploited by commercial nurserymen in producing blooms out of season.
By supplying artificial light or by subjecting plants to periods of darkness, as the case may be, it is possible to regulate the time of flowering. Some species are termed long-day plants, because they normally bloom only when days are long, whereas others are called short-day plants, since they respond in their flowering processes to shorter periods of light.
As might be expected, there are species which are intermediate in their light requirements for the development of flowers, and there are some whose flowering process if not influenced at all by length of light exposure. In addition to the regulation of lowering, a great many other light-controlled processes are known in plants, some of which are seed germination, leaf fall, the development of colour in leaves, and growth rates of plant organs.
A number of adaptations to light are also seen in the animal kingdom, among the most obvious of which are eyes of higher animals. Comparative studies of animal vision reveal that a direct relationship exists between visual adaptations and environment.
This, in turn, is reflected by the adaptations of flowers and berries. Most insect-pollinated flowers are not red, whereas many which depend upon birds for pollination are. Furthermore, the berries and fruits of many plants are red; as a consequence of which birds are attracted to them.
Such species may become widely distributed, since birds ingest and later eliminate many seeds. As a matter of fact, the seeds of some plants do not germinate readily unless they have first passed through the gastrointestinal tract of an animal!
This illustrated how far-reaching one factor of adaptation may be. Again, the eyes of nocturnal animals such as cats and alligators have extremely sensitive light receptors. These receptors are protected from daylight intensities by slit pupils, which admit far less light than round pupils. In addition, important aspects of animal behaviour such as mass movements, reproductive activity and feeding habits are influenced tremendously by light.
‘Another important factor of the physical environment is temperature. If a survey is made of plans and animals that exist at various temperature zones of the earth, it is found that there is a direct correlation between this factor and those features which adapt organisms to their respective habitats.
Many animals are adapted to seasonal changes, and in some cases even change their camouflage patterns. In addition to its influence upon animal and plant distribution, temperature is of considerable ecological significance in other ways.
Seeds of most plants do not germinate until the temperature if fairly warm; some will not germinate even at the proper temperature unless they have previously been exposed to a period of cold. One rather unusual adaptation to temperature is seen in the case of seed cones of the jack-pine, which do not open readily to release the seeds until they have been scorched by fire.
Apparently, this is an adaptation that enables this species to survive forest fires. Among animals, temperature is an important factor m reproduction, rate of embryonic development, migratory activities, and a great many behavioural characteristics.
As an example of temperature adaptations in animals, the birds and mammals are able to maintain constant body temperatures by virtue of a feature of the nervous system which is absent in “cold-blooded” animals.
This is the temperature-regulating centre of the brain, a kind of built-in “thermostat”. This feature, plus accompanying heat- regulating adaptations such as the feathers and hair exhibited by these two groups of animals, respectively, makes it possible for them to exist under a wide range of temperature conditions. Water is another factor that is ecologically important. Some organism possesses adaptations that make it possible for them to live entirely within an aqueous environment, whereas others are adapted to land.
Structural adaptations in these respective environments are frequently quite obvious; gills of fishes and lungs of land vertebrate are among those that are better known. We have mentioned the contractile vacuoles featured by many unicellular freshwater organisms which are adaptations to osmotic pressure.
In oceans, it has been observed that certain species are characteristic of various depth zones, indicating adaptations for withstanding pressured that exist at different levels of the sea. Organisms that live on land and are adapted variously to water some plants and animals are restricted to moist environments, while others inhabit only dry areas.
Cacti are typical plants that possess features making possible their existence in desert areas, and among animals, the camel and the kangaroo rat have become almost proverbial for their capacity to withstand dry conditions.
In addition to these physical factors, a number of others assume great importance. Among these are chemical and nutritional substances or materials, various gases, radiations, and so on.
In general, however, light, temperature, and water may be considered the most important physical factors of the environment. It should be pointed out that these environmental factors are merely separate parts of a total situation, and they frequently exert effects together.
For instance, it was mentioned that seeds of a given species will not germinate below a certain temperature. However, even if an ideal temperature is provided, there will be germination unless enough moisture is present to activate enzymes and to enter into reactions within cells of the embryonic plant.
In this case, neither temperature nor water are independent of each other; a seed if adapted to germinate only when certain conditions of both factors are met.
Similarly, the temperature of an environment may be ideal for some animal to exist successfully, but if its water needs are not met, it cannot survive. This important principle is called the law of the minimum, and stated more precisely, it holds that regardless of how satisfactory one or more requirements of an organism may be, it cannot survive unless all requirements are met.
As further example of this “law” and its application, let us suppose that a certain plant is supplied with all physical and chemical requirements in adequate amounts except one essential element. Regardless of how adequate other environmental conditions maybe, the minimum requirements of the plant have not been met, and it cannot survive. Put in terms of an analogy, a chain is no stronger than its weakest link.
If the list of requirements for a given organism be considered a chain, and if only one item is missing, then the rest of the chain is worthless in terms of keeping the organism alive. Since plant and animal bodies are composed of matter, they depend upon their surroundings for those elements which make them up and keep them alive. Significantly, matter is not ordinarily lost to the world of life with usage; a given atom of carbon.
Since the atoms of such elements may be used over and over, we say that they travel through cycles. Because certain phases of a given elemental cycle may involve other factors than purely biological ones, we refer to the entire Pathway as a biogeochemical cycle, a term which tells us that biological, geological and chemical factors are all involved. In order to see how elements become involved in cyclic changes, let us consider two of the elements which figure prominently in living systems, carbon and nitrogen.
Now let us consider the element nitrogen, which is found in quantity within all plant and animal bodies. It is not present as molecular nitrogen, of course, which is a gas; rather, it is an essential part of proteins and certain other types of organic molecules.
Of course, just as carbon may be trapped in organic molecules for long periods of time, nitrogen may become temporarily unavailable at some point of the cycle. At any rate, the nitrogen of waste products and dead bodies eventually appears in the form of ammonia. Further bacterial action makes possible the formation of nitrites and then nitrates.
In the latter form, nitrogen becomes available to green plants, which combine it with photosynthetic products in the synthesis of plant proteins. It should be noted that a lesser cycle occurs between nitrates and atmospheric nitrogen; nitrates may be decomposed by certain bacteria in soil and water to release gaseous nitrogen to the atmosphere, which is something of a “loss” to the main cycle.
However, this loss is partially compensated for by the phenomenon that lightning converts gaseous nitrogen to nitric acid which becomes deposited in the soil, and by the ability of certain bacteria and algae to “fix” gaseous nitrogen in the form of organic compounds.
Carbon and nitrogen are only two of the many elements that undergo cycles of this sort, and their changes in form may be considered typical of others.
Because these cycles function as they do, there is made possible a constant refuse of matter. In the sense, therefore, matter is conserved; energy may be lost, organisms die, and species undergo changes, but the fundamental units of matter travel their cycles in unending fashion.
A given organism may be influenced by two other groups of organisms: those that are members of its own species and those that are not. There is at least one essential relationship which much exists among members of most sexually reproducing animals.
A great many adaptations are seen in both the animal and plant kingdoms which allow for biparental reproduction. Some of these are highly specialized. With regard to essential requirements such as food and sunlight, members of the same species are frequently found to be in competition with one another. One of the most fundamental of all ecological principles is that the reproductive potential of a species tends to run far ahead of its food supply or available space, and it is inevitable that some members survive at the expense of others.
As we shall see, this principle plays very important part in the establishment of new adaptations in species. Some species are so organized that cooperation, rather than individual competition, is exhibited. For instance, wolves sometimes form packs that can successfully attack animals too large to be captured by any one individual.
This type of cooperative group is called an aggregation. A much more highly organized cooperative unit is the society, which achieves its higher degree of specialization in certain insect species. In the consideration of relationships that exist between members of different species, competition is of great ecological important. Plants of different species compete in nature for sunlight and root space; animals whose food or shelter requirements are similar may be in keen competition, especially when food or space becomes scarce.
Perhaps the most important ecological consideration of inter-specific relationships, however, is the principle of the food chain. Essentially, a food chain is a series that begins with a chemical medium, either soil or water, within or upon which photosynthetic producers serve as food for varying numbers of animal consumers.
These support certain microscopic animals which, along with the algae, constitute a mass of small, living forms collectively known as plankton. Certain fishes, especially small forms, are adapted to live upon plankton by virtue of mechanisms which strain these organisms out of the water as it passes into the mouth over the gills. Eventually these fishes are eaten by larger forms which may in turn be eaten by others, and so on.
Hence, there is a chain beginning with the ultimate consumers. Eventually, of course, the consumers themselves die, and they may figure prominently in the same food chain or in a different one. Somewhere along the line, animal materials are decomposed by bacteria or fungi, and the products of decomposition are made available to the producers.
In fact, materials of dying and dead organisms and their wastes are decomposed by bacteria and fungi throughout the chain. Food chains necessarily involve predation, or the feeding of certain forms on others.
Except for a few species of plants such as the Venus fly-trap, which we have mentioned, predators are all animals. Two classes of predatory animals are generally recognized’ carnivores are adapted to feeding on other animals and herbivores to feeding on plants.
Some species exist on a mixed diet, however, and thus a third group, the omnivores, maybe recognized. Predation is frequently thought of as a somewhat unpleasant side of nature, and many persons build up some rather anthropomorphic views with regard to carnivorous animals.
Hence, a lion is often pictured as a “bad” animal, and a sheep is evaluated as a “good” one. Some people actually think that a carnivore is capable of making value judgments, and “ought” not to kill and devour other animals since this is cruel and wicked.
What these people fail to see is that an animal which is adapted through heredity for a carnivorous existence cannot be expected to shake off these structural and functional limitations and start eating grass. Their behaviour,, at least with respect to the acquisition of food, is quite determined. Furthermore, predation is not as bizarre a phenomenon as some people might think.
In the first place, many plants and small animal forms, which apparently lack any sort of consciousness, are involved. They are no worse off for having been eaten than a rock is for having been broken. As for higher animals, there is no reason to believe that they are capable of the mental anguish with which human beings regard death. Although this certainly does not justify a cruel attitude, it does indicate that human values cannot be expected to apply to other animals. Aside from such considerations as these, predation is very important in the maintenance of natural populations.
We mentioned that reproductive potential in organisms runs far ahead of available food and space; were it not for predators, animals and plants would overproduce, and certain forms would become extremely numerous.
Such an imbalance is often seen when normal population rations of nature are upset in some way. When it is excessively hunted by man so that numbers of individuals are greatly reduced in a given area, the result is usually one that is ecologically worse for man than the former situation.
With the decline of the coyote population, smaller animals upon which they would normally prey increase in number until they become more detrimental than coyotes. The natural prey of coyotes is the rabbit, whose reproductive potential is proverbial. Within a short time after the disappearance of coyotes in a given area, rabbits generally increase to such an extent that they consume pasture grass and other plants that support livestock.
The rancher may thus be choosing the worse of two evils when he destroys coyotes. To cite another example, hunters sometimes urge the eradication of wolves and mountain lions because they prey upon deer. Studies have shown quite clearly that, in the long run, predators exert a favourable influence upon the long-range survival of deer populations.
For the most dispensable members of the herd. Almost without exception, the elimination of predators has led to difficulties in maintaining healthy herds of deer. Organisms of different species often become closely associated in their environmental; adaptations and these relationships are of special interest to the ecologist.
One such relationship is that of mutualism, wherein two associated organisms belonging to different species both derive benefit from living together. We have observed that lichen is composed of an alga and a fungus, both of which are adapted to live under conditions where neither could exist separately.
The alga is able to furnish photosynthetic materials to the fungus, while the fungus provides a suitable environment for the alga by supporting its cells and by holding water which it can use. We also mentioned the association of cellulose-digesting protozoa with termites, an arrangement that enables these insects to subsist on wood
At the same time, the protozoa are provided with a place to live, and they also receive essential materials such as water and inorganic salts from their mutuality partners. Another type of relationship is that of commensalism, in which two members of different species are so associated that one derives benefit while the other is neither harmed nor helped.
By virtue of this arrangement, a given barnacle may take in bit of food that drift away from the other organism when it eats, and in terms of species distribution, it may be carried to points it could not possibly reach otherwise.
At the same time, it does not harm to its partner organism. A third important special relationship between members of different species is that of parasitism in which one member. Parasites such as these that live within the body of the host are known as end parasites. Others, called ectoparasites, exist upon the bodies of animals or plants. Lice, ticks, and leeches are representative ectoparasites.