THE EXPRESSION OF PHYSICAL FACTOR RESULTS
137. The form of results. It is almost inevitable that the general adoption of precise methods of measuring the habitat will result in a common form for expressing the physical character of the latter. An actual diagnosis of each habitat is not a difficult matter, after the factors are carefully measured, and will unquestionably lead to very desirable definiteness and precision. The accurate investigation of the physical factors of a number of habitats for one growing season furnishes the necessary material for a diagnosis based upon the mean for the growing season. Similar results for two or three seasons will yield a diagnosis as accurate and as final as that of a formation, or, indeed, as that of many species. The author’s investigations have not yet gone far enough to warrant proposing a final form for this, but the following diagnosis is offered as a suggestion:
Elymus-Muhlenbergia-chalicium. Habitat: holard 9 per cent, chresard 8 per cent, relative humidity 40 per cent, light 0.6, soil colluvial gravel (gravel 70 per cent, sand 27 per cent, silt 3 per cent), air temperature 65°, surface 82°, soil 59°, wind 10 miles, rainfall 8 inches, altitude 2,800 m., slope 23°, exposure south, surface even, cover open, no active biotic agencies.
The detailed comparison of habitats is made most readily by the graphic method of curves, which constitute the most desirable form of expression in connection with the original record upon which they are based. Factor means are particularly desirable for diagnostic purposes, and they furnish valuable curves also. Factor sums are impracticable at present, and it seems doubtful that they will ever be of much value. It is by no means impossible, however, that a more detailed and exact knowledge of the physiology of adaptation, coupled with methods of precision in the habitat, will render them necessary.
Factor Records
138. Experience has shown that the practice of making hasty and often formless records in the field is unwise and is apt to be inaccurate as well. The time saved in the field is more than counterbalanced by that consumed in copying the results into the permanent form. The danger of error in field notes rapidly taken is very grave, and the chance of confusion and the waste of time in deciphering them are great. Moreover, the task of checking a copy with the original, which is absolutely necessary for accuracy, involves a further expenditure of time and energy. For these reasons the field record should be made in permanent form. Definite record sheets are used, and the invariable rule is made that all readings are to be noted in ink at the time and spot where they are taken. On a long journey, or in the face of many observations, the tendency to take notes or to record observations rapidly is very great, but this will correct itself after a few attempts to use such notes. The record forms for various factors have been indicated in the proper place, as well as the one for simultaneous readings. Ecograph sheets are carefully filed, and constitute permanent records. With a little practice they may be read almost as easily as tables, and any attempt to put them into tabular form is a mere waste of time. For purposes of study and of publication, it often becomes necessary to bring together all the results obtained for a particular habitat, both by simple instruments and by ecographs. The form of record used for this is essentially that already indicated for simultaneous readings on page [92], since general features and constant factors can not well be included in the table. Record sheets of this type have been printed at a cost of $5 per thousand, and the various factor records can be obtained at about the same rate. The size of sheet used is 9½ × 7¾ inches. The record book is the usual notebook cover, which has been found neither too large nor too small. It is protected from dirt and rain by a covering of oilcloth which overlaps the edges. Record books should be carefully labeled, and each one should contain a single year’s records.
Factor Curves
139. Plotting. The paper employed is divided into centimeter squares which are subdivided into 2–millimeter units. For ordinary curves the size of sheet is 9½ × 7¾ inches, which makes it possible for curve sheets to be filed in the record book. Tablets containing 60 of these sheets can be obtained for 20 cents each from the Central School Supply House, Chicago. For curves longer than 9 inches special sizes of sheets must be used. On account of their inconvenience large sheets are avoided whenever possible. This can usually be accomplished by increasing the numerical value of the intervals. The inks employed in plotting are the waterproof inks of Chas. Higgins & Co., Brooklyn, New York. These are made in ten or more colors, black, violet, indigo, blue, green, yellow, orange, brown, brick red, carmine, and scarlet, and cost 25 cents per bottle. In addition to being waterproof, they make it possible to combine curves in all conceivable ways without destroying their identity. Furthermore, it is a great advantage to use the same color invariably for the same kind of curve: thus, it has been the practice to indicate the 3–foot, surface, 5, 10, and 15–inch temperature curves by violet, green, yellow, blue, and carmine respectively. A fine-pointed pen, such as the Spencerian No. 1, is most satisfactory for inking; drawing pens, such as Gillott’s Crowquill, are too finely pointed for ordinary use.
In plotting a curve, it is first necessary to determine the value of the interval, and the extreme range of the curve or combination. For example, in the case of temperature, it is most convenient to assign a value of 1° Centigrade to each centimeter, since the thermometers used read to one-fifth of a degree, which corresponds exactly to the 2–millimeter units of each square. The length of the sheet permits a range of 22 degrees Centigrade, and the actual limits must be determined for the particular results to be employed. For the same region, it is very desirable that the unit interval and the range be the same, in order that all curve sheets may admit of direct comparison. Indeed, it is greatly to be hoped that in the future ecologists will agree to a uniform system of curve-plotting, cartography, etc., as the geographers are beginning to do in the construction of maps. The major intervals are written, or, better, typewritten, at both sides of the sheet, and the time or space intervals are indicated at the top. Each curve sheet is properly labeled, and essential data indicated. The readings are taken from the field record, and their proper positions indicated by a dot. These are connected first by a pencil line, the curves being made abrupt rather than flowing; and the line, after having been carefully checked, is traced in ink.
140. Kinds of curves. Curves are named both with reference to the factor concerned and the position or sequence of the readings. The factors which lend themselves most readily to this method of representation are the variable ones, water-content, humidity, light, temperature, and wind, and corresponding curves are distinguished. Altitude and slope may likewise be shown by means of curves, but the use of cross section or contour lines serves the same purpose and is more natural. With regard to time and position, curves are distinguished as level, station, and point curves. A level curve is one based upon readings made at the same level through a series of stations or of habitats, e. g., the level curve of surface temperature. The station curve represents the various levels or points at which readings are made in a single station. The point curve has for a basis the hourly or daily variation of a factor at a particular point or level in a station. All of these may be simple curves, when established upon a single reading for a series, or mean curves when they are based upon the mean of a number of readings. Curves which show the extremes of a factor, i. e., the maximum and minimum, are also extremely valuable, though a combination of the two for comparison is preferable.