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.

Variation in Allee effects: evidence, unknowns, and directions forward

“…1b Table 1 Five phenomenological theoretical population models with Allee efects (reviewed in Courchamp et al 2008) capable of producing patterns of variation in low-density population dynamics similar to those in Fig. 1 We indicate what pattern of variation is produced by independently varying diferent model parameters Figure 1c Aviles (1999) and Arnott 2011; Kramer et al 2011;Dooley et al 2013;Kaul et al 2016), it appears that such an outcome could result only when there are tradeofs between component Allee efects and other processes, and there is no a priori reason to believe that this pivot point would be located at the Allee threshold.…”

“…Additionally, spatiotemporally non-uniform invasion patterns were attributed to variation in the Allee threshold (Tobin et al 2007). The authors did not investigate the causes of this variability, but other investigations corroborate that Allee efects vary in natural populations and shed light on likely causes and their implications Hofman et al 2010;Gray and Arnott 2011;Dooley et al 2013;Budroni et al 2014;Bürgi et al 2014;Kaul et al 2016).…”

“…Existing models can be studied to explicitly address how variability in component Allee efects and other demographic factors inluence the density-growth rate relationship, or form the basis for development of new models. Controlled experiments, for example using artiicial laboratory populations (Kramer and Drake 2010;Kaul et al 2016), can develop further insights and help bridge the gap between theory and the dynamics of wild populations.…”

Allee dynamics: growth, extinction and range expansion

“…Examples include mate shortages in sexually reproducing species, less efficient feeding and reduced effectiveness of vigilance and antipredator defences [5,6]. Allee effects have been demonstrated in all major taxonomic groups of animals, in plants [6] as well as in microbial populations [8,9].…”

Allee dynamics: Growth, extinction and range expansion

“…Examples include mate shortages in sexually reproducing species, less efficient feeding and reduced effectiveness of vigilance and antipredator defences [5,6]. Allee effects have been demonstrated in all major taxonomic groups of animals, in plants [6] as well as microbial populations [8,13].…”