On the Characterization of Density and Resource Availability

Abstract

The usual measure of population density, the ratio of total population to total resource, is actually a weighted average of local density weighted by the proportion of the total resource at that density. This resource-weighted density does not take into account the heterogeneity of resources available to each individual. An alternative, the population-weighted density, does reflect this heterogeneity. The two density measures are related as a function of the variances of local population size and resource and the covariance between them. Under some special ecological circumstances, the two measures are equivalent. Wealth can be measured as either the resource- or population-weighted reciprocal of local density, and again these measures are related by the variances and covariances of local resource and population. Organism-weighted density gives the highest estimate of effective density, whereas resource-weighted wealth gives the highest estimate of effective wealth. The appropriate measure of density and wealth depends on whether dynamic models describe pressure on resources or pressures on individuals.