The Influence of Environmental Factors upon The Invertebrates of the Soil.
Since animals are endowed with powers of independent locomotion: they are not necessarily tied to their environment to the same extent that plants are. The investigation of the influence of environmental factors sooner or later involves a study of the tropisms of the animals concerned. Until these are adequately understood it is scarcely possible to arrive at any exact conclusions relative to their behaviour in the soil. Insects, for example, respond to the stimuli of various, and often apparently insignificant forces, acting upon their sensory organs. Such responses are known as chemotropism, phototropism, hydrotropism, thermotropism, and so forth according to the nature of the stimuli. Tropisms are automatic and, so far as they distinguish sensations, are independent of any choice, and consequently of psychic phenomena. Animal automatism, however, does not present the rigidity of mechanical automatism. Differential sensibility, vital rhythms, or periodicity, etc., are other important aspects of animal behaviour.
The environmental factors, affecting more especially the insect population of the soil, have been discussed by Cameron (1917) and Hamilton (1917), and certain broader aspects of animal ecology by Adams (1915) and Shelford (1912). These factors are so numerous and so inter-connected, that it is only possible to refer to them briefly in the space available. As might be expected, soils that are of a light and open texture are the ones most frequented by soil insects, nutritional and other factors being equal. Furthermore, it has already been shown that in arable land insects and other animals penetrate to a greater depth than in pastures. This fact is primarily due to the greater looseness of the soil occasioned by agricultural operations, which ensure at the same time better drainage aeration, and greater facilities for penetration. Hamilton found that soil insect larvæ are very sensitive to evaporation, and especially so if the temperature is 20° C. or over. In their natural habitat the relative humidity of the air, in moist or wet soil, is not far below saturation, and the temperature of the soil rarely goes above 20°-23°C., and then only in exposed, dry, hard soil in which these larvæ do not occur.
The significance of the rate of evaporation as an environmental factor was first emphasised by Shelford. According to him the best and more accurate index of the varying physical conditions affecting land animals, wholly or in part exposed to the atmosphere, is the evaporating power of air. By means of a porous cup-atmometer, as devised by Livingston, Shelford has carried out an important series of experiments on the reactions of various animals to atmospheres of different evaporation capacities, and reference should be made to his text-book.
The importance of the organic matter present in the soil is well illustrated in the [table] on [p. 152]. The great increase in the number of insects and other animals is partly due to their direct introduction along with the manure, and partly to their entry into the soil in response to chemotropic stimuli exerted by fermentation. Organic matter influences the fauna in other ways also; it increases the moisture content of the soil, and it provides many species with an abundance of food material. Also, the amount of carbon dioxide present in the soil is partly dependent upon decaying organic matter. Hamilton conducted experiments on the behaviour of certain soil insects in relation to varying amounts of carbon dioxide. Although his work is of too limited a nature to be accepted without reserve, it lends support to the conclusions of Adams who says: “The animals which thrive in the soil are likely to be those which tolerate a large amount of carbon dioxide, and are able to use a relatively small amount of oxygen, at least for considerable intervals, as when the soil is wet during prolonged rains. The optimum soil habitat is therefore determined, to a very important degree, by the proper ratio or balance between the amount of available oxygen and the amount of carbon dioxide which can be endured without injury.”
Little is known concerning the occurrence of ammonia in the soil atmosphere, but its presence in minute quantities is probably an important chemotropic factor in relation to saprophagous organisms which are the largest constituent of the fauna. A great increase in Dipterous larvæ occurs on the addition of farmyard manure, and this is noteworthy in the light of Richardson’s experiments (1916), which indicate that ammonia exercises a marked attraction for Diptera, which spend some part of their existence in animal excrement in some form or another.
The nature of the vegetation supported by the soil is of paramount importance in relation to phytophagous organisms, and examples need scarcely be instanced of certain species of soil insects being dependent upon the presence of their specific food plants.