Seedling‐Scale Environmental Heterogeneity Influences Individual Fitness and Population Structure

Abstract

This study examines the potential of small—scale environmental heterogeneity to influence population structure and fitness of individuals within a population. Two populations of Abutilon theophrasti were established at each of three densities in the greenhouse. At each density, one population was grown on substrate made heterogeneous before planting, with the addition of stone, localized nutrients, or sand, and soil compactions on a scale corresponding to individual seedlings. These were termed "patch types" and wee chosen to represent factors that would be unpredictable in both time and space to seedlings emerging in nature. Another population was established on homogeneous substrate at each density. Seedling height, final height, biomass, and seed number were measured. The variation in biomass of individuals within a population was significantly greater on heterogeneous than on homogeneous substrate at the lowest density. When the data where analyzed with ANOVA, there was a significant interaction of patch type with presence or absence of heterogeneity. Patch type explained 47—62% of the variance in 2—d seedling height and 27—33% or 30—37% of the variance in seed output or final biomass, respectively. A multiple regression model including patch type, seedling size, and random block together explained 58—76% of the total variance in final biomass, depending on density. Size of immediate neighbors showed no correlation with individual size. The data support our hypothesis that small—scale stochastic events can influence the position of an individual in a population hierarchy. This conclusion implies that genetic determinants of fitness may be overridden by commonly occurring stochastic events. The resulting lack of predictable selection against certain genotypes should contribute to the maintenance of genetic diversity in populations.