Experimental demonstration of an Allee effect in microbial populations

Abstract

Microbial populations can be dispersal limited. However, microorganisms that successfully disperse into physiologically ideal environments are not guaranteed to establish. This observation contradicts the Baas-Becking tenet: 'Everything is everywhere, but the environment selects'. Allee effects, which manifest in the relationship between initial population density and probability of establishment, could explain this observation. Here, we experimentally demonstrate that small populations of Vibrio fischeri are subject to an intrinsic demographic Allee effect. Populations subjected to predation by the bacterivore Cafeteria roenbergensis display both intrinsic and extrinsic demographic Allee effects. The estimated critical threshold required to escape positive density-dependence is around 5, 20 or 90 cells ml(-1)under conditions of high carbon resources, low carbon resources or low carbon resources with predation, respectively. This work builds on the foundations of modern microbial ecology, demonstrating that mechanisms controlling macroorganisms apply to microorganisms, and provides a statistical method to detect Allee effects in data.

Keywords: Allee effect; microbial; micrososm; positive density-dependence.

Figure 1.

Probability of establishment from fitted Weibull curve (symbol and colour by treatment). The outcome of each population per inoculum size (small points) were used to calculate the average probability of establishment with a binomial confidence interval (open symbols) and fitted to the Weibull function (solid line). The estimated critical thresholds (filled symbols; 95% CI in shaded region) are 4.85 (2.76–6.67), 23.8 (19–29.6) and 89.4 (60.2–126) cells ml⁻¹ for the HC (40 mM glycerol), LC (20 mM glycerol) and LCP (20 mM glycerol plus 133 C. roenbergensis ml⁻¹) conditions, respectively. (Online version in colour.)

Figure 2.

Parameter estimates suggest Allee effect present in all treatments. The shape parameter, k, tests for the presence of an Allee effect; values greater than 1 indicate a sigmoidal relationship between density and probability of establishment. The critical threshold, as determined by the inflection point, has a theoretical upper bound of Values of λ less than zero imply a critical threshold of less than 1 cell ml⁻¹, and were considered biologically irrelevant. Point estimates are presented with 95% confidence region. (Online version in colour.)