The distribution of stream bottom faunas
Very few animals are continuously distributed through their environment, and this is, in fact, only possible for sessile forms which can both feed and reproduce without moving from one spot. The great majority need room to move about for these purposes and therefore there must be, on the average, considerable spaces between individuals. It is the existence of these spaces which make it possible far each individual to occupy any of a number of positions, and thus for the population as a whole to be distributed in a great variety of ways. Considerable attention has already been given to the distributional patterns found in animal populations, particularly by the application of statistical techniques. These methods make it possible to test whether or not a population is randomly distributed; that is, whether the distribution is one which could reasonably occur if each individual was equally likely to be found at every point withinthe environment. Departures from randomness can be in one of two directions; either the animals are too evenly spaced out, that is, they are under-dispersed; or they tend to be aggregated, or gathered together in clusters—they are over-dispersed. Many actual distributions which have been studied have been found to depart markedly from randomness generally in the direction of aggregation.