Comparison of the Physical Factors of Habitats

Theoretically, the differentiation between the floras or faunas of two habitats should be an expression of the differences which obtain in their physical factors, including among such physical factors those which are due to the reactional effects of organisms on the environment. When these factors are measured, however, the variability which is found (either from place to place or from time to time) in any habitat is often so great as to be a source of despair to the ecologist who is seeking to interpret biological consequences in terms of physical causes. Most ecologists now probably realize that, because of the great variability which may be found in one and the same habitat with respect to any physical factor which may be under consideration for two or more habitats which are being compared, a few measurements in each are practically valueless as a basis for generalization concerning the nature or magnitude of the differences between them. Except in the more extreme cases, in which differences are so great as to be obvious without the making of actual measurements, trustworthy conclusions concerning differentiation in the physical conditions of two environments must depend upon the comparison of large series of simultaneous measurements. Such comparisons are essentially statistical in nature. They may in many cases be most advantageously made by methods long employed in biometric work but heretofore little used in ecological work. Let us take two or more stations in which simultaneous measurements of some environmental variable have been made for N days. The method of comparison ordinarily employed by ecologists is to plot the magnitudes of the variables in the two stations as ordinates on the axis of time throughout the period of time during which the measurements were made. Thus diagram i shows the daily evaporation rate at four stations near Bemidji, Minnesota, for the month of August, I926. Notwithstanding the enormous variability in each of the stations, the diagram shows clearly that the evaporation rate in the open burn is conspicuously higher than in any other habitat, and that there are clearly marked differences between certain other habitats. It also shows that there is a tendency for the evaporation rates in these habitats to vary together, higher