Taylor's Law holds in experimental bacterial populations but competition does not influence the slope

Abstract

Populations vary in time and in space, and temporal variation may differ from spatial variation. Yet, in the past half century, field data have confirmed both the temporal and spatial forms of Taylor's power Law, a linear relationship between log(variance) and log(mean) of population size. Recent theory predicted that competitive species interactions should reduce the slope of the temporal version of Taylor's Law. We tested whether this prediction applied to the spatial version of Taylor's Law using simple, well-controlled laboratory populations of two species of bacteria that were cultured either separately or together for 24 h in media of widely varying nutrient richness. Experimentally, the spatial form of Taylor's Law with a slope of 2 held for these simple bacterial communities, but competitive interactions between the two species did not reduce the spatial Taylor's Law slope. These results contribute to the widespread usefulness of Taylor's Law in population ecology, epidemiology and pest control.

Figure 1.

Competition lowers abundance. The effect of competitor presence on bacterial densities (expressed in log₁₀ of CFU ml⁻¹) for (a) P. fluorescens and (b) S. marcescens, by level of KB dilution, between treatments alone and in competition. (a,b) Filled circles with solid line, in competition; open circles with dashed line, alone. Dilution levels 1 through 8 correspond to actual dilutions of 1, 3, 9, 27, 81, 243, 729, and 2187 times, respectively.

Figure 2.

Taylor's Law holds. Linear regression of log(variance) to log(mean) of the bacterial densities of (a) P. fluorescens and (b) S. marcescens either grown alone in monoculture or in competition with one another. (a,b) Filled circles with solid line, in competition; open circles with dashed line, alone.