Research methods in ecology

by Frederic E. Clements

HISTORICAL DEVELOPMENT

3. Geographical distribution. The systematic analysis of the great field of ecology is essential to its proper development in the future. A glance at its history shows that, while a number of essential points of attack have been discovered, only one or two of these have been organized, and that there is still an almost entire lack of correlation and coordination between these. The earliest and simplest development of the subject was concerned with the distribution of plants. This was at first merely an offshoot of taxonomy, and, in spite of the work of Humboldt and Schouw, has persisted in much of its primitive form to the present time, where it is represented by innumerable lists and catalogues. Geographical distribution was grounded upon the species, a fact which early caused it to become stereotyped as a statistical study of little value. This tendency was emphasized by the general practice of determining distribution for more or less artificial areas, and of instituting comparisons between regions or continents too little known or too widely remote. The fixed character of the subject is conclusively shown by the fact that it still persists in almost the original form more than a half century after Grisebach pointed out that the formation was the real unit of vegetation, and hence of distribution.

4. The plant formation. The corner-stone of ecology was laid by Grisebach in 1838 by his recognition of the plant formation as the fundamental feature of vegetation. Earlier writers, notably Linné (1737, 1751), Biberg (1749), and Hedenberg (1754), had perceived this relation more or less clearly, but failed to reduce it to a definite guiding principle. This was a natural result of the dominance of descriptive botany in the 18th century, by virtue of which all other lines of botanical inquiry languished. This tendency had spent itself to a certain degree by the opening of the 19th century, and both plant distribution and plant physiology began to take form. The stimulus given the former by Humboldt (1807) turned the attention of botanists more critically to the study of vegetation as a field in itself, and the growing feeling for structure in the latter led to Grisebach’s concept of the formation, which he defined as follows: “I would term a group of plants which bears a definite physiognomic character, such as a meadow, a forest, etc., a phytogeographic formation. The latter may be characterized by a single social species, by a complex of dominant species belonging to one family, or, finally, it may show an aggregate of species, which, though of various taxonomic character, have a common peculiarity; thus, the alpine meadows consist almost exclusively of perennial herbs.” The acceptance of the formation as the unit of vegetation took place slowly, but as a result of the work of Kerner (1863), Grisebach (1872), Engler (1879), Hult (1881, 1885), Goeze (1882), Beck (1884), Drude (1889), and Warming (1889), this point of view came to be more and more prevalent. It was not, however, until the appearance of three works of great importance, Warming (1895), Drude (1896), and Schimper (1898), that the concept of the formation became generally predominant. With the growing recognition of the formation during the last decade has appeared the inevitable tendency to stereotype the subject of ecology in this stage. The present need, in consequence, is to show very clearly that the idea of the formation is a fundamental, and not an ultimate one, and that the proper superstructure of ecology is yet to be reared upon this as the foundation.

5. Plant succession. The fact that formations arise and disappear was perceived by Biberg as early as 1749, but it received slight attention until Steenstrup’s study of the succession in the forests of Zealand (1844 prox.). In the development of formations, as well as in their recognition, nearly all workers have confined themselves to the investigation of particular changes. Berg (1844), Vaupell (1851), Hoffmann (1856), Middendorff (1864), Hult (1881), Senft (1888), Warming (1890), and others have added much to our detailed knowledge of formational development. Notwithstanding the lapse of more than a half century, the study of plant successions is by no means a general practice among ecologists. This is a ready explanation of the fact that the vast field has so far yielded but few generalizations. Warming (1895) was the first to compile the few general principles of development clearly indicated up to this time. The first critical attempt to systematize the investigation of succession was made by Clements (1904), though this can be considered as little more than a beginning on account of the small number of successions so far studied. Future progress in this field will be conditioned not only by the more frequent study of developmental problems by working ecologists, but also, and most especially, by the application of known principles of succession, and by the working out of new ones.

6. Ecological phytogeography. Until recent years, the almost universal tendency was to give attention to formations from the standpoint of vegetation alone. While the habitat was touched here and there by isolated workers, and plant functions were being studied intensively by physiologists, both were practically ignored by ecologists as a class. The appearance of Warming’s Lehrbuch der oecologischen Pflanzengeographie (1896) and of Schimper’s Pflanzengeographie auf physiologischer Grundlage (1898) remedied this condition in a measure by a general discussion of the habitat, and by emphasizing the importance of the ecological or physiological point of view. Despite their frank recognition of the unique value of the habitat, the major part of both books was necessarily given to what may be termed the general description of formations. For this reason, and for others arising out of an almost complete dearth of methods of investigation, ecology is still almost entirely a floristic study in practice, although there is a universal recognition of the much greater value of the viewpoint which rests upon the relation between the formation and its habitat.

7. Experimental ecology. Properly speaking, the experimental study of ecology dates from Bonnier[^[1]] (1890, 1895), though it is well understood that experimental adjustment of plants to certain physical factors had been the subject of investigation before this time. The chief merit of Bonnier’s work, however, lies in the fact that it was done out of doors, under natural conditions, and for these reasons it should be regarded as the real beginning of this subject. Bonnier’s experiments were made for the purpose of determining the effect of altitude. Culture plots of certain species were located in the Alps and the Pyrenees, and the results were compared with control cultures made in the lowlands about Paris. In 1894 he also made a comparative study of certain polydemic species common to the arctic islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these methods are fundamental to field experiment, but the results are inconclusive, inasmuch as altitude is a complex of factors. As no careful study was made of the latter, it was manifestly impossible to refer changes and differences of structure to the definite cause. In a paper that has just appeared, E. S. Clements (1905) has applied the method of polydemic comparison to nearly a hundred species of the Rocky mountains. In this work, the all-important advance has been made of determining accurately the decisive differences between the two or more habitats of the same species in terms of direct factors, water-content, humidity, and light. In his own investigations of Colorado mountain vegetation, the author has applied the method of field cultures by planting seeds of somewhat plastic species in habitats of measured value, and has thought to initiate a new line of research by applying experimental methods to the study of vegetation as an organism. In connection with this, there has also been developed a method of control experiment in the plant house under definitely measured differences of water and light.

8. Ecology of the habitat. Since the time of Humboldt, there have been desultory attempts to determine the physical factors of habitats with some degree of accuracy. The first real achievement in this line was in the measurement of light values by Wiesner in 1896. In 1898 the writer first began to study the structure of habitats by the determination of water-content, light, humidity, temperature, wind, etc., by means of instruments. These methods were used by one of his pupils, Thornber (1901), in the study of a particular formation, and by another, Hedgcock (1902), in a critical investigation of water-content. Two years later, similar methods of measuring physical factors were put into operation in connection with experimental evolution under control in the plant house. E. S. Clements (1905), as already indicated, has made the use of factor instruments the foundation of a detailed study of polydemic species, i. e., those which grow in two or more habitats, and which are, indeed, the most perfect of all experiments in the production of new forms. In a volume in preparation upon the mountain vegetation of Colorado, the writer has brought the use of physical factor instruments to a logical conclusion, and has made the study of the habitat the basis of the whole work. Out of this investigation has come a new concept of vegetation (Clements 1904), namely, that it is to be regarded as a complex organism with structures and with functions susceptible of exact methods of study.

9. The evidence from historical development. This extremely brief resume of what has been accomplished in the several lines of ecological research makes evident the almost complete absence of correlation and of system. The whole field not merely lacks system, but it also demands a much keener perception of the relative value of the different tendencies already developed. It is inevitable from the great number of tyros, and of dilettante students of ecology in comparison with the few specialists, that the surface of the field should have received all of the attention. It is, however, both unfortunate and unscientific that great lines of development should be entirely unknown to all but a few. There is no other department of botany in which the superficial study of more than half a century ago still prevails to the exclusion of better methods, many of which have been known for a decade or more. It is clear, then, that the imperative need of ecology is the proper coordination of its various points of view, and the working out of a definite system which will make possible a ready recognition of that which is fundamental and of that which is merely collateral. The historical development, as is well understood, can furnish but a slight clue to this. It is a fact of common knowledge that the first development of any subject is general, and usually superficial also. True values come out clearly only after the whole field has been surveyed. For these reasons, as will be pointed out in detail later, the newer viewpoints are regarded as either the most important or the most fundamental. Experimental ecology will throw a flood of light upon plant structure and function, while exact methods of studying the habitat are practically certain of universal application in the future.