Variability in the abundance of animal and plant species

Abstract

A striking and consistent observation in ecology is that variability in population abundance of a species and average population density are related in both space and time1,2. Taylor and his colleagues have shown that this relationship between variance (V) and mean (M) conforms well to a simple power law for a very wide range of animal and plant species, with the logarithms of sample variance and average sample density following a linear relationship1-3 (V = âM6 where â and b̂ are constants). They suggest that the slope of this line, b̂, which generally falls within the range 1 to 2, is a species-specific characteristic reflecting the balance between the opposing behavioural tendencies to aggregate within, and migrate from, centres of population density4. Taylor and co-workers1-6 have stressed that migration between patches is rarely a random process, on the evidence of the observed 'density-dependent' relationships between the degree of dispersion (the variance to mean ratio, V/M and mean abundance per patch). We show here that the relationship between variability and average abundance can be a simple, and inevitable, consequence of chance demographic events in the dynamics of population growth and decline. The precise form of these relationships is determined by the relative magnitude of the various rate processes which govern the dynamics of population change (the birth, death, immigration and emigration rates), and by the degree of spatial and temporal heterogeneity. We do not need to invoke any complex behavioural mechanisms to explain observed patterns. Show more