A Statistical Study of Plant Distribution During the Colonization and Early Development of Vegetation on China Clay Residues

Abstract

Following upon an earlier review (1936), Ashby has recently (1948) summarized the present position in statistical ecology. He considers that, while attempts to classify communities have up to now been disappointing, the analysis of the distribution of the individuals of a species is becoming clearer and is more amenable to further attack by statistical techniques. There is an increasing amount of evidence indicating that overdispersion, that is, contagious distribution, is common. Ashby points out that this habitual *aggregation is due to a number of different causes, some of which are still obscure: indeed Blackman (1935, 1942) as well as Pidgeon & Ashby (1940) found the same species randomly distributed in one habitat and over-dispersed in another. Recently, Archibald (1948) has shown that the contagious distribution of Neyman often gives a good fit to plant distributions, and in a second paper (1949) she has applied a series recently developed by Thomas (1949). Anscombe (1949) has indicated a number of theoretical distributions which might fit population counts of plant individuals (see also Williams, 1944, 1947). The observed distribution of the individuals of a species can be considered both as a function of the biological characters of the species, particularly in relation to its mode of propagation and means of dispersal, and as a function of the habitat, whilst modification may result from the interaction of vegetation and habitat. In a developing community there is continuous modification of the habitat, and data collected under relatively simple conditions where some of the complicating factors are minimal might help to clarify the position. Ashby (1948) has pointed out that the small amount of available evidence suggests that species with no evident biological predisposition to over-dispersion are randomly distributed when they first occupy an area, as found, for example, by Singh & Chalam (1937) for the establishment of weeds on arable land at low population densities. We have made a study (not yet published) of the colonization and subsequent development of a series of artificial habitats which are produced in great numbers around china clay works. As a result of working the china clay there is an immense and continuous output of several types of waste material, only one of which (mica) will be considered. The fine suspension of mica (remaining after sedimentation of the china clay) is carried by water in narrow channels and deposited, often to a depth of several feet, in the so-called mica dams, which are roughly enclosed areas of the adjacent ground. As the water drains off, these dams present restricted virgin areas for plant colonization, the underlying vegetation with the exception of willow trees (see later) being completely obliterated. These dams, usually of the order of 5000 sq.yd. in area are found both on the moorland immediately surrounding the clay pits, and occasionally at lower levels in old meadow land. The surface is (except for a very slight slope for drainage) a uniform level expense of bare