Research methods in ecology

by Frederic E. Clements

The quadrat constitutes the initial concept from which all the methods have grown. In itself, it has given rise to a variety of quadrats applicable to the most fundamental problems of vegetation. From it have come, on the one hand, the migration circle, and on the other, the transect. The latter in turn has yielded the ecotone chart, and the layer chart. All of these are based upon direct and detailed contact with vegetation itself, and permit accurate recording of all the results obtained.

QUADRATS

199. Uses. In its simplest form, the quadrat, as the name implies, is merely a square area of varying size marked off in a formation for the purpose of obtaining accurate information as to the number and grouping of the plants present. As indicated above, it was first used for determining the abundance of the various species of a formation. This made it possible to ascertain the relative rank of the species of layers and formations, and enabled one for the first time to gain some idea of the minute structure of a bit of vegetation. The results were at once applied to the task of establishing a numerical basis for abundance, and of working out a new system of abundance to correspond. The quadrat method was also used to determine the character of seasonal aspects, and to yield a knowledge of the exact differences in diverse areas of the same formation. Incidentally, the determinations of abundance were made the basis of an actual census of certain alpine formations. This, while it was extremely interesting to find that a square mile of alpine meadow contained approximately 1,500,000,000 plants, was confessedly destitute of ecological value. The most important applications of the quadrat idea were made by Clements[^[21]] in the chart, the permanent and the denuded quadrats. The development of these was due to the fact that zones or formations permit of comparison upon floristic as well as physical grounds, and that a detailed record of their structure is necessary for this purpose. Similar comparisons are necessary for the consocies, zones, and patches of the same formation, and the quadrat becomes an indispensable means for studying alternation and zonation. For the investigation of invasion year by year, and especially for succession, the method of permanent quadrats is imperative, and the denuded quadrat an invaluable aid. Changes, which would otherwise be incompletely observed and imperfectly recorded, are followed in the minutest detail and recorded with perfect accuracy.

200. Possible objections. The use of the quadrat has led to the criticism that it is needlessly detailed and thorough, and that, after all, the space covered is but a minute part of the entire formation. The first objection is one that has also been urged against the use of instruments of precision in the habitat. It is always brought forward by those who have not used instruments, and as witnesses they are of necessity incompetent. No one who is familiar with the instrument or the quadrat by actual practice has felt that the methods based upon them were too thorough. In no case has the writer ever listed or mapped a quadrat without discovering some new fact or relation, or clearing up an old question. It can not be denied that quadrat methods require both time and patience, but this is true of any kind of research work that is at all worth while. Every ecologist, moreover, that has the interests of his field at heart and deprecates the present slipshod work, will appreciate the necessity of methods which seem like drudgery to the mere dabbler.

The second objection, that the quadrat is at best but a small bit of the area under investigation, seems at first to be a valid one. It can not be gainsaid that the actual space studied is insignificant as compared with the whole formation; still, it must be obvious that even a single quadrat can add at least some facts of value, which can never be obtained by the best of general methods. Furthermore, if the formation be an actual and not an imaginary one, a single quadrat will be in some measure representative. In the more homogeneous ones, it will have much the same value that a type specimen bears to the species established upon it. In formations which are less uniform, its value is correspondingly reduced, so that in formations which show marked zones, consocies, or patches, it becomes necessary to locate a quadrat in each. In the matter of representation alone, the graphic method of the quadrat map with its close-focus detail photograph, is far superior to anything that can be obtained by the ordinary description and photograph. Finally, the scientific study and recording of succession, and particularly of competition, is an impossibility without the aid of the permanent and denuded quadrat. The stoutest champion of the practice of walking through a formation, and jotting down impressions, can not avoid their use if he would attack these problems, and, once familiar with the quadrat, his objections to the drudgery of thoroughness will soon vanish.

Kinds of Quadrats and Their Use

201. Size and kinds. The unit size of quadrat is the meter, and when the term is used without qualification, it refers to the meter quadrat. To make them strictly comparable, and exactly divisible, unit quadrats are always grouped in squares; thus a major quadrat is a square of four units, and a perquadrat one of sixteen units, or four meters square. Quadrats of greater size are necessary in woodland and forest, where the rule, however, is that the woody plants alone are recorded for the whole quadrat, the herbaceous growth being listed or mapped for but one or two representative units. For special purposes, quadrats of 3, 5, 6, etc., meters may be used, but they are much less convenient. Quadrats are further distinguished with respect to their use. A list quadrat is one in which the plants are merely listed and the number of individuals of each species indicated. Chart quadrats are those in which the area concerned is accurately mapped on plotting paper. Both list and chart quadrats are rendered permanent by careful labeling, so that their changes can be followed from year to year. The greater value of the chart causes practically all permanent quadrats to be of this type, and for the same reason only permanent chart quadrats are converted into denuded ones.

202. Tapes and stakes. The lines for marking out quadrats are made of strong white tape, ⅝ inches wide. This is doubled and sewed firmly at both edges. Under moderate stretching, the tape is carefully marked off into decimeters, and eyelets 5 mm. in diameter are set in at each end and at the marks. This can readily be done by any shoemaker at slight expense. The usual lengths are one and two meters, as these are most frequently used, and they can also be easily combined to make larger quadrats. The tapes are slightly longer than one meter in order that the distance between the end eyelets may be exact. The tapes of the larger forest quadrats should be divided into lengths of one meter, as these permit ready plotting and also make it possible to interpolate a meter quadrat for the study of the undergrowth at any point. The intervals of the tape are numbered from left to right, as conspicuously and clearly as possible. For this a waterproof ink or paint is very desirable. For holding the tapes in position, hatpins, nails, and meat-skewers have been used with more or less satisfaction. The ideal stake, however, is one which holds the tape close to the ground, and can be readily moved. It is merely a stout wire, 3 mm. in diameter and 8 inches long, looped at the top, sharpened at the tip, and with a small ring of solder 3 inches from the tip.

203. Locating quadrats. In staking a quadrat, the end tapes are invariably placed so that the numbers read from left to right, and the side tapes so that they read down. In mapping, a fifth tape is stretched parallel to the top, and as each decimeter strip is marked, the outer tape is shifted to delimit the new strip. Indeed, the side tapes can be placed alone, and the plotting tapes moved down one at a time as the mapping proceeds, but it is usually more satisfactory to locate the quadrat exactly and to square it first, a task most easily done by enclosing the whole quadrat, and then using a fifth tape. In the case of list quadrats in open vegetation, the measuring strip is unnecessary, but as a rule it facilitates counting, as well as mapping.