31. The need of exact measurement. Any serious endeavor to find in the habitat those causes which are producing modification in the plant and in vegetation can not stop with the factors merely. The next step is to determine the quantity of each. It is not sufficient to hazard a guess at this, or to make a rough estimate of it. Habitats differ in all degrees, and it is impossible to institute comparisons between them without an exact measure of each factor. Similarly, one can not trace the adaptations of species to their proper causes unless the quantity of each factor is known. It is of little value to know the general effect of a factor, unless it is known to what degree this effect is exerted. For this purpose it becomes necessary to appeal to instruments, in order to determine the exact amount of each factor that is present in a particular habitat, and hence to determine the ratio between the stimulus and the amount of structural adjustment which results. The employment of instruments of precision is clearly indispensable for the task which we have set for ecology, and every student that intends to strike at the root of the subject, and to make lasting contributions to it, must familiarize himself with instrumental methods. One great benefit will accrue to ecology as soon as this fact is generally recognized. The use of instruments and the application of results obtained from them demand much patience and seriousness of purpose upon the part of the student. As a consequence, there will be a general exodus from ecology of those that have been attracted to it as the latest botanical fad, and have done so much to bring it into disrepute.
32. The value of meteorological methods. At the outset there must be a very clear understanding that weather records and readings have only a very general value. This is in spite of the fact that the instruments employed are of standard precision. An important reason for this lack of value is that readings are not made in a particular habitat; as a rule, indeed, they are made in towns and cities, and hence are far removed from masses of vegetation. They are usually taken at considerable heights, and give but a general indication of the conditions at the level of vegetation. The chief difficulty, however, is that the factors observed at weather stations—temperature, pressure, wind, and precipitation—are those which have the least value for the ecologist. It is true that a knowledge of the temperature and rainfall of a great region will afford some idea of the general character of its vegetation. A proper understanding of such a vegetation is, however, to be gained only through the exact study of its component formations. Ecology has already incurred sufficient censure as a subject composed of very general ideas, and the use of meteorological data, which can never be connected definitely with anything in the plant or the formation, should be discontinued. This must not be understood to mean that meteorological instruments can not be used in the proper place and manner, i. e., in the habitat.
33. Habitat determination. It is self-evident that determinations of factors by instruments can only be of value in the habitat where they are made. In other words, a habitat is a unit for purposes of measuring its factors, and measures of one habitat have no exact value in another. This fact can not be overstated. Thus, while it is perfectly legitimate, and indeed highly desirable, to locate thermographs in different mountain zones for ascertaining the rate at which temperature decreases with altitude, the data obtained in this way are not directly applicable in explanation of plant or formation changes, except when the same species occurs at different altitudes. Special methods are valuable and often absolutely necessary, but in view of the fact that the plant as well as the formation is the definite product of a definite habitat, the fundamental rule in instrumentation is that complete readings must be made within a habitat for that habitat alone. This necessarily presupposes a certain preliminary acquaintance with the habitat to be investigated, as it is imperative that the station for making readings be located well within the formation, in order to avoid transition conditions. In vegetation, there are as many habitats as formations, and in addition a large number of new and denuded habitats upon which successions have not yet started; a knowledge of each formation or succession must rest ultimately upon the factors of its particular habitat.
34. Determinable and efficient differences. The instruments employed in studying habitats can not be too exact, as there is no adequate knowledge as yet concerning the real differences which exist between related or contiguous formations. This is particularly true of differences which are efficient in producing a recognizable structural change in plant or formation. Investigations made by the writer have shewn that standard instruments will measure differences of quantity quite too small to produce a visible reaction. Efficient differences are not the same for different factors, and perhaps also for the same factor when found in various combinations. They vary widely for different species, being in direct relation to the plasticity of the latter. The point necessary to bear in mind in formulating methods for habitat investigation and in making use of instruments is that standard instruments should be used for the very reason that we do not yet know the relation between determinable and efficient differences. On the other hand, it is unnecessary to insist upon absolute exactness as soon as it is found that the determinable difference lies well within the efficient one. This by no means indicates that instruments are not to be carefully standardized and frequently checked, or that accurate readings should not be made. It means that a slight margin of error may be permitted in a machine which registers well within the efficient difference for that factor, and that instruments that read to the last degree of nicety are not absolutely necessary. In the fundamental work of determining efficient differences, however, instruments can not have too great precision. Moreover, these differences must be based upon the most plastic species of a formation, and the readings must be made under normal conditions.
Instrumentation
35. Methods. In the field use of instruments two methods have been developed. The first in point of time was the method of simple instruments, devised especially for class work, and capable of being used only where a number of trained students are available. The method of automatic instruments was an immediate outgrowth of this, due to the necessity which confronts the solitary investigator of being in different habitats at the same time. In the gradual evolution of this subject, it has become possible to combine the two methods in such a way as to retain all the advantages of the automatic method, and most of those of the method of simple instruments.
36. Method of simple instruments. By simple instruments are denoted those that do not record, but must be read by the observer at the time. They are standard instruments of precision, but possess the disadvantage of requiring an observer for each one. They are well illustrated by the thermometers and psychrometers used by the Weather Bureau. In the hands of trained observers the results obtained are unimpeachable; in fact, standard simple instruments must be constantly employed to check automatic ones. As physical factors vary greatly through the day and through the year, it is all-important that the readings in habitats which are being compared should be made at the same instant. This requires a number of observers; as many as twelve stations have been read at one time, and there is of course no limit to the number. It is very important, also, that observers be carefully trained in the handling of instruments, and in reading them accurately and intelligently at the proper moment. In practice it has been found impossible to do such work in elementary classes, and, even in using small advanced classes, prolonged drill has been necessary before trustworthy results could be obtained. When a class has once been thoroughly trained in making accurate simultaneous readings, there is practically no limit, other than that set by time, to the valuable work that can be done, both in instruction and investigation.
37. Method of automatic instruments. The solitary investigator must replace trained helpers by automatic instruments or ecographs. These have the very great advantages of giving continuous simultaneous records for long periods, and of having no personal equation. They must be regulated and checked, to be sure, but as this is all done by the same person, the error is negligible. There is nothing more satisfactory in resident investigation than a series of accurate recording instruments in various habitats. Ecographs have two disadvantages. The chief perhaps is cost. The expense of a single “battery” which will record light, water-content, humidity, and temperature is about $250. Another difficulty is that they can be used only within a few miles of the base, since they require attention every week for regulation, change of record, etc. While this means that ecographs in their present form are not adapted to reconnaissance, this is not a real disadvantage, as the scattered observations possible on such a journey can best be made by simple instruments.
38. Combined methods. The best results by far are to be obtained by the combined use of simple and automatic instruments. This is particularly true in research, but it applies also to class instruction. The ecographs afford a continuous, accurate basal record, to which a single reading made at any time or place can be readily referred for comparison. On the other hand, it is an easy matter to carry a full complement of simple instruments on the daily field trips, and to make accurate readings in a score or more of formations in a single day. An isolated reading, especially of a climatic factor, has little or no value in itself, but when it can be compared with a reading made at the same time in the base station by an ecograph, it is the equivalent of an automatic reading. This method renders a set of simple instruments more desirable for a long trip or reconnaissance than a battery of automatic ones. It is practically impossible to carry the latter into the field, and in any event a continuous record is out of the question. As there are other tasks at such times also, it becomes evident that the taking of single readings which can be compared with a continuous record offers the most satisfactory solution.