Research methods in ecology

by Frederic E. Clements

METHOD OF HABITAT CULTURES

191. Scope and advantages. By means of experiments actually made in the field, practically every species that is capable of modification can be made to produce new forms, the origin of which can be traced in the manner already indicated. Field experiments of this sort are especially favorable to the production of ecads from adaptable species. No attempt has yet been made to apply it to mutable or variable species, but its ultimate application to these does not seem at all impossible. The chief advantage of the method of habitat cultures is seen in the great range of choice in selecting the plant for experiment, and the habitat or area in which the experiment is carried out. A polydemic species which already has one or more ecads can be extended to a number of different habitats of known value, and a complete series of ecads obtained, based either upon water-content, or light, or upon both. On the other hand, an endemic species, or one brought from a remote flora, can be placed in as many habitats as desired, and the appearance of ecads followed in each. Frequently, results of much value are obtained in a diversified habitat by growing its most plastic species in those areas which show the greatest differences in water-content or light intensity. Habitat cultures give results which are practically as perfect as those obtained from natural experiments, since the course of adaptation in no wise depends upon whether the agent by which the seed or propagule is carried into the new habitat is natural or artificial. Cultures of this kind further possess the distinct advantage of permitting more or less modification of the physical factors themselves. However, when it is desirable to have the factors under as complete control as possible, it is necessary to use the method of control cultures in the planthouse.

192. Methods. All field experiments in evolution are based upon a change of habitat. The latter is accomplished by the modification of the habitat itself, or by the transfer of the species to one or more different habitats, or to different areas of the same habitat. In both cases the choice of habitats is made upon the basis of efficient differences of water-content or light. Saline situations do not constitute an exception, since the chresard is really the effective stimulus. Cultures at different altitudes, which afford striking results, appear to concern several factors, but in the final analysis, water-content and humidity are alone found to be really formative. Cultures may furthermore be distinguished as simple or reciprocal. Simple cultures are those in which a species is transferred to one or more habitats, or in which a habitat is modified in one or more ways. Reciprocal cultures are possible only with polydemic species, or with endemics after ecads have been produced by experiment. Modification or transfer is made in the usual way, but reciprocally, i. e., the original form is transferred to the habitat of the ecad, and the latter to the habitat of the former; or the shade in which some individuals of the ecad are growing may be destroyed, and at the same time individuals of the type may be shaded. Both transfer and modification may be applied to the same species, but since the same measured change of factor can be obtained in either way, the use of both is undesirable, with the exception of the rare cases where they serve as checks upon each other. The transfer of a seed or plant is so much simpler and more convenient that this method is the one regularly used. It sometimes happens, however, that a change of water-content or light intensity is readily and conveniently made, and is desirable for other reasons.

It is evident that both transfer and modification require that the factor records of the various habitats or areas be as full as possible, at least so far as water-content, humidity, and light are concerned. In the case of the areas that are to be modified, these factors are determined before the change is made. Afterward they are read from time to time during the growing season, and are also checked by readings made near at hand in the unmodified formation. The readings made in the beginning should correspond closely to the check readings, but in case of disagreement the latter are to be taken as conclusive.

193. Transfer. After the species to be used for experiment has been chosen, the various habitats or areas selected, and the direct factors measured by instruments, the actual transfer of the individuals is made by means of seeds, preferably in autumn, though the results are practically the same if seeds are kept over the winter and planted at the opening of spring. The natural method is to scatter the seeds in the place selected, as though they had been carried by the usual agents of migration. The mortality is usually great in such case, however, and the chances of success are increased by actually planting the seeds. This is the method which has been used in making cultures of species of the European Alps on the summit of Mount Garfield in the Rocky mountains. The number of seeds used is recorded in order to obtain some estimate of germination and competition. While the use of the seed or disseminule possesses the great advantage of making the experiment essentially a natural one, the transfer of rosettes, seedlings, or young plants makes the results more certain, and consequently saves time, even though the actual transfer is somewhat more difficult. It is hardly necessary to point out that the removal of the plant should be made with the greatest care. The best success is obtained by making the transfer on cloudy or rainy days, and when shade plants are to be placed in sunny situations, they should be transplanted late in the afternoon. When the task of carrying them is not too great, it is a distinct advantage to move a number of individuals in the same block of earth. The transfer of mature plants is inadvisable, except for those perennials which can not readily be secured in an early stage. This naturally does not apply to woody plants, evergreen herbs, mosses and lichens; the last two may be transferred at any time with satisfactory results. Each culture is carefully marked with stakes, and definitely located by means of landmarks.

Fig. 46. Series for producing hydrophytic forms under control: 1, amphibious; 2, floating; 3, competition; 4, submerged.

Reciprocal transfers may be made by means of seed or plant. Since the experiment is a complex one, all the care possible should be taken to make sure that the plants become established in the reciprocal situations, and consequently, it is often advisable to transfer both seeds and plants. Reciprocal transfer is of paramount value in solving the problem which bog plants present. A slight modification of the method makes it possible to obtain experimental evidence of the polyphyletic origin of species in consequence of adaptation. In an experiment mentioned elsewhere, the transfer of Kuhnistera purpurea to the area occupied by K. candida, and vice versa, is designed to show whether one has been derived from the other. If the two species are moved into an area which contains more water than that usually occupied by K. purpurea, and less water than is found where K. candida habitually grows, the resulting modifications will throw much light upon the origin of polyphyletic species. In this connection, it hardly needs to be pointed out that this simple transfer of a species to several separated areas of a new habitat may often furnish complete proof that a new form may arise at different times, and at different places.

Fig. 47. Control ecad of Ranunculus sceleratus, holard 10% (50 cc.).

194. Modification of the habitat. Efficient changes in the habitat are brought about by increasing or decreasing the water-content, or by varying the light intensity between sunshine and the diffuse light of deep forests. Humidity can not well be regulated except in so far as it is connected with water-content. Since its effects merge with those of the latter, its modification is unnecessary. An increase in water-content is readily brought about by irrigation. A stream may be dammed and its water allowed to spread over the area to be studied, or the water may be carried to the proper place by deflecting the stream or by digging a canal. The construction of earth reservoirs makes it possible to obtain almost any per cent of soil water by varying the size of the reservoir or the height of the wall or bank. Near a base station, such as Minnehaha, where there is a simple system of water-works, the experimental area may be watered whenever desirable by means of a hose. Water-content may be readily decreased by drainage, or by the deflection of a stream. When such means are not available, as in the case of extensive marshes, hummocks may be used or constructed, and the soil blocks containing plants placed upon them. By the use of sand or gravel, the water-content of mesophytic areas can be reduced in a similar manner, or by surrounding the plant in situ with either of these soils which hold little water. In meadows, especially, the addition of a large quantity of alkaline salts decreases the amount of available water, while the holard may be reduced by denuding the soil about the plants concerned.

Fig. 48. Control ecad of Ranunculus sceleratus, holard 40% (200 cc.).

In sunny habitats, the light intensity is most easily reduced by means of cloth awnings, which can be put in place conveniently. It is not a difficult matter to produce effective shade by using shrubs or small trees for this purpose. This plan is especially advantageous in habitats too remote to make frequent visits feasible. When a shrub or tree is used, the experiment necessarily requires a longer time, though this disadvantage is partly compensated by the fact that the shelter requires practically no attention after the shrub is once established. Forest plantations furnish excellent examples of this kind of experiment. On the other hand, clearings afford the only examples of habitats modified in such manner as to increase the light. In nature, the diffuse light in which shade plants grow is due to the presence of tall plants, chiefly shrubs and trees, and an increase in the light intensity is possible only through the thinning-out or removal of the plant screen. This is a task of considerable magnitude in forests, but it can be readily accomplished in thickets and at the edges of woodlands. It is quite practicable to establish a series of awnings or clearings of various light values, but the labor required is hardly worth while when it is recalled that the method of transfer makes it possible to take advantage of the various intensities already found in nature.