Changes in the Structure of Plant Populations

Abstract

The patterns of numbers and sizes of individuals are frequently used in studies of animal populations. This demographic approach has, however, been relatively ignored in ecological studies of plant populations. There have also been few detailed studies of variation between individuals within populations. The tendency has been to describe the 'average' individual and the general behaviour of a population, scant attention being paid to variation between individuals. A normal distribution of individual quantities is usually assumed in most statistical analyses of biological data, though normality is rarely tested and cannot be checked for the limited data presented. In many cases the way in which the data is collected precludes a check of normality. Koyama & Kira (1956) showed that increasing departure from normality occurred with time in measures of total fresh weight per plant of populations of Impatiens balsamina L., radish, turnip, Hibiscus moschentos L., soybean, Ambrosia elatior L. and Phaseolus chrysanthos Savi. In each of these cases, and in the case of individual plant dry weights of Linum usitatissimum L. (Obeid, Machin & Harper 1967), the distribution was positively skewed with many small individuals and a few very large individuals. The same workers showed, however, that the distribution of shoot length of Impatiens balsamina L., and the number of seeds per capsule of Linum usitatissimum were negatively skewed. This study was designed to examine the effect of density upon the frequency distribution of various attributes of both tillers and plants in populations of several gramineous species.