Research methods in ecology

by Frederic E. Clements

APPLICATIONS OF ECOLOGY

16. The subjects touched by ecology. The applications of ecological methods and results to other departments of botany, and to other fields of research are numerous. Many of these are both intimate and fundamental, and give promise of affording new and extremely fruitful points of view. It has already been indicated that ecology bears the closest of relations to morphology and histology on the one side, and to physiology on the other—that it is, indeed, nothing but a rational field physiology, which regards form and function as inseparable phenomena. The closeness with which it touches plant pathology follows directly from this, as pathology is nothing more than abnormal form and functioning. Experimental work in ecology is purely a study of evolution, and the facts of the latter are the materials with which taxonomy deals. Forestry has already been termed “applied ecology” and in its scientific aspects, which are its foundation, it is precisely the ecology of woody plants, and of the vegetation which they constitute. Apart from botany, the physical side of ecology is largely a question of soil physics, and of physiography. On the other hand, vegetation is coming more and more to be regarded as a fundamental factor in zoogeography and in sociology. Furthermore, with respect to the latter, it will be pointed out below that the principles of association which have been determined for plants, viz., invasion, succession, zonation, and alternation, apply with almost equal force to man.

17. Physiology and pathology. The effect of ecology in emphasizing the intrinsically close connection between physiology and morphology has already been mentioned. Its influence in normalizing the former by forcing it into the field as the place for experiment, and by directing the chief attention to the plant as an organism rather than a complex of organs, is also rapidly coming to be felt. Ecology will doubtless exert a corrective influence upon pathology in the near future. This is inevitable as the latter ceases to be the merely formal study of specific pathogenic organisms, and turns its attention to the cause of all abnormality, which is to be found in the habitat, whether this be physical, as when the water-content is low, or biotic, when a parasitic fungus is present. The relative ease with which specific diseases can be studied has helped to obscure the essential fact that the approach to pathology must be through physiology. Much indeed of the observational physiology of the laboratories has been pathology, and it will be impossible to draw a clear line between them until precise experiment in the habitat has come into vogue.

18. Experimental evolution. As a result of the extremely fragmentary character of the geological record, nothing is more absolute than that there can be no positive knowledge of the exact origin of a form or species, except in those rare cases of the present day, where the whole process has taken place under the eye of a trained observer. The origin of the plant forms known at present must forever lie without the domain of direct knowledge. If it were possible, by a marvel of ingenuity and patience to develop experimentally Myosurus from Selaginella, this would not be absolutely conclusive proof that Myosurus was first derived in this way. When all is said, however, this would be the very best of presumptive evidence. It must also be recognized that this is the nearest to complete proof that we shall ever attain, and with this in mind it becomes apparent at once that evidence from experiment is of paramount importance in the study of evolution (the origin of species).

The phase of experimental ecology which has to do with the plant has well been called experimental evolution. While this is a field almost wholly without development at present, there can be little question that it is to be one of the most fertile and important in the future. Attention will be directed first to those forms which are undergoing modification at the present time. The cause and direction of change will be ascertained, and its amount and rapidity measured by biometrical methods. The next step will be to actually change the habitat of representative types, and to determine for each the general trend of adaptation, as well as the exact details. By means of the methods used and the results obtained in these investigations, it will be possible to attack the much more difficult problem of retracing the development of species already definitely constituted. This will be accomplished by the study of the derived and the supposed ancestral form, but owing to the great preponderance of evolution over reversion, the study of the ancestral form will yield much more valuable results.

The general application of the methods of experimental ecology will mark a new era in the study of evolution. There has been a surplus of literary investigation, but altogether too little actual experiment. The great value of De Vries’ work lies not in the importance of the results obtained, but in calling attention to the unique importance of experimental methods in contributing to a knowledge of evolution. The development of the latter has been greatly hindered by the dearth of actual facts, and by a marked tendency to compensate for this by verbiage and dogmatism. This is well illustrated by the present position of the “mutation theory,” which, so far as the evidence available is concerned, is merely a working hypothesis. An incredible amount of bias and looseness of thought have characterized the discussion of evolution. It is earnestly to be hoped that the future will bring more work and less argument, and that the literary evolutionists will become less and less reluctant to leave the relative merits of variation and mutation to experiment.

19. Taxonomy. Taxonomy is classified evolution. It is distinct from descriptive botany, which is merely a cataloguing of all known forms, with little regard to development and relationship. The consideration of the latter is peculiarly the problem of taxonomy, but the solution must be sought through experimental evolution. The first task of the latter is to determine the course of modification in related forms, and the relationships existing between them. With this information, taxonomy can group forms according to their rank, i. e., their descent. The same method is applicable to the species of a genus, and, in a less degree, perhaps, to the genera which constitute a family. The use to which it may be put in indicating family relationships will depend largely upon the gap existing between the families concerned. While interpretation will always play a part in taxonomy, the general use of experiment will leave much less opportunity for the personal equation than is at present the case. Taxonomy, like descriptive botany, is based upon the species, but, while there may exist a passable kind of descriptive botany, there can be no real taxonomy as long as the sole criterion of a species is the difference which any observer thinks he sees between one plant and another. The so-called species of to-day range in value from mere variations to true species which are groups of great constancy and definiteness. The reasons for this are obvious when one recalls that “species” are still the product of the herbarium, not of the field, and that the more intensive the study, the greater the output in “species.” It would seem that careful field study of a form for several seasons would be the first requisite for the making of a species, but it is a precaution which is entirely ignored in the vast majority of cases. The thought of subjecting forms presumed to be species to conclusive test by experiment has apparently not even occurred to descriptive botanists as yet. Notwithstanding, there can be no serious doubt that the existing practice of re-splitting hairs must come to an end sooner or later. The remedy will come from without through the application of experimental methods in the hands of the ecologist, and the cataloguing of slight and unrelated differences will yield to an ordered taxonomy.

Experimental evolution will solve a taxonomic problem as yet untouched, namely, the effect of recent environment upon the production of species. It is well understood that some species grow in nature in various habitats without suffering material change, while others are modified to constitute a new form in each habitat. It is at once clear that these forms (or ecads) are of more recent descent than the species, i. e., of lower rank. It must also be recognized that a constant group and a highly plastic one are essentially different. If constancy is made a necessary quality of a species, one is a species, the other is not. If both are species, then two different kinds must be distinguished. Among the species of our manuals are found many ecads, alongside of constant and inconstant species. These can be distinguished only by field experiment, and their proper coordination is possible only after this has been done. Indeed, the whole question of the ability or the inability of environmental variation to produce constant species is one that must be referred to repeated and long-continued experiment in the field.

A minor service of considerable value can be rendered taxonomy by working over the diagnosis from the ecological standpoint. Many ecological facts are of real diagnostic value, while others are at least of much interest, and serve to direct attention to the plant as a living thing. The loose use of terms denoting abundance, which prevails in lists and manuals, should be replaced by the exact usage which the quadrat method has made possible for vegetation. The designation of habitats could be made much more exact, and the formation, as well as the habitat form or ecad, and the vegetation form or phyad, should be indicated in addition. The general terms drawn from pollination, seed-production, and dissemination might also be included to advantage.

20. Forestry, if the purely commercial aspects be disregarded, is the ecology of a particular kind of vegetation, the forest. Therefore, in pointing out the connection between them, it is only necessary to say that whatever contributes to the ecology of the forest is a contribution to forestry. There are, however, certain lines of inquiry which are of fundamental importance. First among these, and of primary interest from the practical point of view, are the questions pertaining to the distribution of forests and their structure. Of even greater significance are the problems of forest development, movement, and of reforestation, which are comprised in succession. The gradual invasion of the plains and prairies by the forest belt of the east and north is in full conformity with the laws of invasion, and the ecological methods to be employed here serve not merely to determine the actual conditions at present, but also to forecast them with a great deal of accuracy. The slow but certain development of forests on new soils, and their more rapid reestablishment where the woody vegetation has been destroyed by burning or lumbering, are ordinary phenomena of succession, for which the ecologist has already worked out the laws, and determined the methods of investigation. Having once ascertained the original and adjacent vegetation and the character of the habitat, the ecologist can indicate with accuracy not only the character of the new forest that will appear, but also the nature of the antecedent formations. A full knowledge of the character and laws of succession will prove of the greatest value to the forester in all studies of forestation and reforestation. Forests which now seem entirely unrelated will be seen to possess the most intimate developmental connection, and the fuller insight into the life history gained in this way will have a direct bearing upon methods of conservation, etc. It will further show that the forester must know other vegetations as well, since grassland and thicket formations have an intimate influence upon the course of the succession, as well as upon the advance of a forest frontier.

One of the greatest aids which modern ecology can furnish forestry, however, is the method of determining the physical nature of the habitat. So far, foresters have been obliged to content themselves with a more or less superficial study of the structure of forest formations, without being able to do more than guess at the physical causes which control both structure and development. This handicap is especially noticeable in the case of forest plantings in non-forested regions, where it has been impossible to estimate the chances of success, or to determine the most favorable areas except by actual plantations. Equipped with the proper instruments for measuring water-content, humidity, light and temperature, the ecologist is able to determine the precise conditions under which reproduction is occurring, and to ascertain what non-forested areas offer the most nearly similar conditions. A knowledge of habitats and the means of measuring them enables the forester to discover the causes which control the vegetation with which he is already familiar, and to forecast results otherwise hidden. Furthermore, it makes it possible for him to enter a new region and to determine its nature and capabilities at a minimum of time and energy.

21. Physiography. Physiographic features play an important part in determining the quantity of certain direct factors of the habitat. Perhaps a more important connection between physiography and ecology is to be found in succession. The beginning of all primary, and of many secondary successions is to be sought in the physiographic processes which produce new habitats, or modify old ones. On the other hand, most of the reactions which continue successions exert a direct influence upon the form of the land. The most pronounced influence of terrestrial successions is found in the stabilization which their ultimate stages exert upon land forms, even where these are highly immature. The chief effect of aquatic successions is to be found in the “silting up” and the formation of new land, which result from the action of vegetation upon silt-bearing waters. The closeness of the relation between succession and the forms of the land has led to the application of the term “physiographic ecology” to that part of the subject which deals with the development of vegetation, i. e., succession.

22. Soil physics. This subject is as much a part of ecology as is forestry. It is intrinsically that subdivision of ecology which deals with the edaphic factors of the habitat, and their relation to the plant. Since the basis is physics, there has been a general tendency to overvalue the determination of soil properties, and to ignore the fact that these are decisive only when considered with reference to the living plant. As the soil contains the water which is the factor of greatest importance to plants, soil physics is an especially important part of ecology. Its methods are discussed under the habitat.

23. Zoogeography. Since animals are free for the most part, and hence not confined so strictly to one spot as plants, their dependence upon the habitat is not so evident. The relation is further obscured by the fact that no physical factor has the direct effect upon them which water or light exerts upon the plant. Vegetation, indeed, as the source of food and protection, plays a more obvious, if not a more important part. This is especially true of anthophilous insects, but it also holds for all herbivorous animals, and, through them, for carnivorous ones. The animal ecology of a particular region can only be properly investigated after the habitats and plant formations have been carefully studied. Here, as in floristics, a great deal can be done in the way of listing the fauna, or studying the life habits of its species, without any knowledge of plant ecology; but an adequate study must be based upon a knowledge of the vegetation. Although animal formations are often poorly defined, there can be no doubt of their existence. Frequently they coincide with plant formations, and then have very definite limits. They exhibit both development and structure, and are subject to the laws of invasion, succession, zonation, and alternation, though these are not altogether similar to those known for plants, a fact readily explained by the motility of animals. Considered from the above point of view, zoogeography is a virgin field, and it promises great things to the student who approaches it with the proper training.

24. Sociology. In its fundamental aspects, sociology is the ecology of a particular species of animal, and has in consequence, a similar close connection with plant ecology. The widespread migration of man and his social nature have resulted in the production of groups or communities which have much more in common with plant formations than do formations of other animals. The laws of association apply with especial force to the family, tribe, community, etc., while the laws of succession are essentially the same for both plants and man. At first thought it might seem that man’s ability to change his dwelling-place and to modify his environment exempts him in large measure from the influence of the habitat. The exemption, however, is only apparent, as the control exerted by climate, soil, and physiography is all but absolute, particularly when man’s dependence upon vegetation, both natural and cultural, is called to mind.

The Essentials of a System

25. Cause and effect: habitat and plant. In seeking to lay the foundation for a broad and thorough system of ecological research, it is necessary to scan the whole field, and to discriminate carefully between what is fundamental and what is merely collateral. The chief task is to discover, if possible, such a guiding principle as will furnish a basis for a permanent and logical superstructure. In ecology, the one relation which is precedent to all others is the one that exists between the habitat and the plant. This relation has long been known, but its full value has yet to be appreciated. It is precisely the relation that exists between cause and effect, and its fundamental importance lies in the fact that all questions concerning the plant lead back to it ultimately. Other relations are important, but no other is paramount, or able to serve as the basis of ecology. Ecology sums up this relation of cause and effect in a single word, and it may be that this advantage will finally cause its general acceptance as the proper name for this great field.

In the further analysis of the connection between the habitat and the plant, it is evident that the causes or factors of the habitat act directly upon the plant as an individual, and at the same time upon plants as groups of individuals. The latter in no wise decreases the importance of the plant as the primary effect of the habitat, but it gives form to research by making it possible to consider two great natural groups of phenomena, each characterized by very different categories of effects. Ecology thus falls naturally into three great fundamental fields of inquiry: habitat, plant, and formation (or vegetation). To be sure, the last can be approached only through the plant, but as the latter is not an individual, but the unit of a complex from the formational standpoint, the formation itself may be regarded as a sort of multiple organism, which is in many ways at least a direct effect of the habitat. In emphasizing this fundamental relation of habitat and vegetation, it is imperative not to ignore the fact that neither plant nor formation is altogether the effect of its present habitat. A third element must always be considered, namely, the historical fact, by which is meant the ancestral structure. Upon analysis, however, this is in its turn found to be the product of antecedent habitats, and in consequence the essential connection between the habitat and the plant is seen to be absolute.

26. The place of function. In the foregoing it is understood that the immediate effect of the physical factors of the habitat is to be found in the functions of the plant, and that these determine the plant structure. Function has so long been the especial theme of plant physiology that methods of investigation are numerous and well known, and it is unnecessary here to consider it further than to indicate its general bearing. The essential sequence in ecological research, then, is the one already indicated, viz., habitat, plant, and formation, and this will constitute the order of treatment in the following pages. That portion of floristic which is not mere descriptive botany belongs to the consideration of the formation, and in consequence there will be no special treatment of floristic as a subdivision of ecology.