Research methods in ecology

by Frederic E. Clements

METHOD OF NATURAL EXPERIMENT

188. Selection of species. Species that are producing variants or ecads are found everywhere in nature; those which give rise to mutants seem, however, to be extremely rare. Consequently, mutants can not be counted upon for experimental work, and their study scarcely needs to be considered. When a mutant is discovered by some fortunate chance, the mutable species from which it has sprung, and related species as well, should be subjected to the most critical surveillance, in the hope that new mutants will occur or the original one reappear. On account of the suddenness with which they appear, mutants do not lend themselves readily to natural experiment, and after they have once been discovered, inquiry into the causes and course of mutation is practicable only by means of habitat and control cultures. Among variable species, those are most promising that show a wide range of variation and are found in abundance over extensive areas. A species which occurs in widely separated, or more or less isolated areas, furnishes especially favorable material for investigation, since distance or physical barriers partly eliminate the leveling due to constant cross-fertilization. The individuals or groups which show appreciable departure from the type are marked and observed critically from year to year. The direction of the variation and the rapidity with which small changes are accumulated can best be determined by biometrical methods. Representative individuals of the species and each of its variants should likewise be selected from year to year. After being photographed, these are preserved as exsiccati, and with the photographs constitute a complete graphic record of the course of variation. When the latter is made evident in structural feature also, histological slides are an invaluable part of the record.

Polydemic species are by far the best and most frequent of all natural experiments. In addition to plants that are strictly polydemic, i. e., grow in two or more distinct habitats, there are a large number which occur in physically different parts of the same habitat. The recognition of polydemics is the simplest of tasks. As a rule, it requires merely a careful examination of contiguous formations in order to ascertain the species common to two or more of them. The latter are naturally most abundant along the ecotones between the habitats, and, as a result, transition areas and mixed formations are almost inexhaustible sources of ecads. Many adaptable species are found throughout several formations, however, and such are experiments of the greatest possible value. Not infrequently species of the manuals are seen to be ecads, in spite of their systematic treatment, and to constitute natural experiments that can be readily followed. Finally, it must be kept in mind that some polydemics are stable, and do not give rise to ecads by structural adaptation. They not only constitute extremely interesting experiments in themselves, but they should also be very carefully followed year by year, since it seems probable that the responses are merely latent, and that they will appear suddenly in the form of mutants. In natural experiments it is sometimes difficult to distinguish which form is the ecad and which the original form of the species. As a rule, however, this point can be determined by the relative abundance and the distribution, but in cases of serious doubt, it is necessary to appeal to experimental cultures.

Although habitats differ more or less with respect to all their factors, the study of polydemics needs to take into account only the direct factors, water-content, humidity, and light. Humidity as a highly variable factor plays a secondary part, and in consequence the search for ecads may be entirely confined to those habitats that show efficient differences in the amount of water-content or of light. Temperature, wind, etc., do not produce ecads, and may be ignored, except in so far as they affect the direct factors. Complexes of factors, such as altitude, slope, and exposure, are likewise effective only through the action of the component simple factors upon water and light. The influence of biotic factors is so remote as to be negligible, especially in view of the fact that ecads are necessarily favorable adaptations, and are in consequence little subject to selective agencies. The essential test of a habitat is the production of a distinguishable ecad, but a knowledge of the water-content and light values of the habitats under examination is a material aid, since a minute search of each formation is necessary to reveal all the ecads. It is evident that habitats or areas that do not show efficient differences of water or light will contain no ecads of their common species, and also that extreme differences in the amount of either of these two factors will preclude origin by adaptation to a large degree, on account of the need for profound readjustment. The general rule followed by most polydemics is that sun species will give rise to shade forms, and vice versa, and that xerophytes will produce forms of hydrophytic tendency, or the converse, when the areas concerned are not too remote, and the water or light differences are efficient, but not inhibitive. Some species are capable of developing naturally two series of ecads, one in response to light, the other to water-content, but they, unfortunately, have been found to be rare. Greatly diversified regions, such as the Rocky mountains, in which alternation is a peculiarly striking feature of the vegetation, are especially favorable to the production of ecads, and hence for the study of natural experiments in origin by adaptation.

189. Determination of factors. For the critical investigation of the origin of new forms, an exact knowledge of the factors of the habitat, both physical and biotic, is imperative. In the case of variable species, these factors determine what variations are of advantage, and thereby the direction in which the species can develop. They are the agents of selection. With mutants, the factors of the habitat are apparently neither causative nor selective, though it seems probable that further study of mutants will show an essential connection between mutant and factor. In any event, the persistence of a mutant in nature, and its corresponding ability to initiate new lines of development, is as much dependent upon the selection exerted by physical and biotic factors as is the origin of variants. Physical factors are causative agents in the production of ecads, as has been shown at length elsewhere. The form and structure of the ecad are the ultimate responses to the stimuli of light or water-content, and the quantitative determination of the latter is accordingly of the most fundamental importance. The measurement of factors has been treated so fully in the preceding chapters that it is only necessary to point out that the thorough investigation of habitats by instruments is as indispensable for the study of experimental evolution as for that of the development and structure of the formation. Furthermore, it is evident that a knowledge of physical factors is as imperative for habitat and control cultures as for the method of natural experiment. In the latter, however, the biotic factors demand unusual attention, since pollination, isolation, etc., are often decisive factors in origin by variation and in the persistence of mutants.

Measurements of adjustment, i. e., functional response to the direct factor concerned, are extremely valuable, but not altogether indispensable to research in experimental evolution. This is due to the fact that a knowledge of adjustment is important in tracing the origin of new forms only when adjustment is followed by adaptation, and in all such cases the ratio between the two processes seems to be more or less constant. In the present rudimentary development of the subject, however, it is very desirable to make use of all methods of measuring functional responses to water and light that are practicable in the field. Certain methods that are difficult of application in nature may be used to advantage in control cultures, and the results thus secured can be used to interpret those obtained from natural experiments and field cultures.

190. Method of record. As suggested elsewhere, there are four important kinds of records, which should be made for natural experiments, and likewise for habitat and control cultures. These are exsiccati, photographs, biometrical formulae and curves, and histological sections. These serve not merely as records of what has taken place, but they also make it possible to trace the course of evolution through a long period with an accuracy otherwise impossible, and even to foreshadow the changes which will occur in the future. The possibility of doing this depends primarily upon the completeness of the record, and for this reason the four methods indicated should be used conjointly. In the case of ecads and mutants, exsiccati, photographs, and sections are the most valuable, and in the majority of cases are sufficient, since both ecads and mutants bear a more distinctive impress than variants do. On the other hand, since variations are more minute, the determination of the mean and extreme of variation by biometrical methods is almost a prerequisite to the use of the other three methods, which must necessarily be applied to representative individuals.

Exsiccati and photographs are made in the usual way for plants, but it is an advantage to photograph each ancestral form alongside of its proper ecads, mutants, or variants, in addition to making detail pictures of each form and of the organs which show modification. In the collection of material for histological sections, which deal primarily with the leaf or with stems in the case of plants with reduced leaves, a few simple precautions have been found necessary. Whenever possible, material should be killed where it is collected, since in this way the chloroplasts are fixed in their normal position. In case leaves that can not be replaced easily have become wilted, an immersion of 5–6 hours in water will make it possible to kill them without shrinkage. In selecting leaves, great pains must be taken to collect only mature leaves. When the plants have a basal rosette, or distinct radical leaves, mature leaves are taken from both stem and base. In all cases where the two surfaces of the leaf can not be readily distinguished, the upper one is clearly marked.