Allee effects: Scaling down to the microbial level

Background/Question/Methods

The microbial Baas-Becking hypothesis also known as “everything is everywhere ”, and the zoological observation of positive density dependence in small populations or Allee effects were described in the early 20th century. The theories which stress abiotic and biotic factors, respectively, as drivers of population establishment have persisted with little influence from the other. However, as our understanding of microbial populations grows so does the need to reconcile these opposing theories. We looked for the presence of both intrinsic and predator-induced Allee effects in a microbial system: Vibrio fisheri, strain ES114 containing green fluorescent protein. V. fisheri populations of varying densities (10^-2 to 10 cells/ul) were crossed with Cafeteria roenbergensispresence, a natural predator. Accurate inoculation of the necessarily small numbers of predator and prey was achieved using flow cytometry gated by size and fluorescence. The presence of an Allee effect was quantitatively assessed by testing for a sigmoidal relationship (as defined by the Weibull function) between probability of survival and initial population size. Conveniently, the Weibull function shape is parameter dependent, ranging from exponential (lambda>0, k<1; no Allee effect), to sigmoidal (lambda>0 , k>1; Allee effect). Lambda and k were simultaneously estimated by maximum likelihood.

Results/Conclusions

The Weibull functions parameterized by V. fischeri populations with and without predators were sigmoidal, with the 95% confidence region of the bivariate contour plots greater than k=1. At face value, this indicates a  demographic Allee effect is inherent to V. fisheri populations. However, the inflection point of the sigmoidal curve, which is analogous to the deterministic unstable equilibrium, for non-­predated populations is between zero and one individual. This implies a weak Allee effect. The critical threshold for populations of predated V. fisheri ranges from 2 to 6 individuals. The resolution of these relationships can be improved by manipulating predation strength. Of the isolated populations that survived, initial population size did not influence equilibrium size (ANOVA p­value =0.11). Preliminary data suggest this trend is not maintained in predated populations. The development of this system allows us to test theoretical predictions about Allee effects in a highly tractable, replicated manner. Additionally, understanding the relative importance of abiotic and biotic factors with respect to microbial population establishment and persistence will add to the existing body of knowledge used to assess and predict microbial community assemblages, assemblages that can impact ecosystem level processes.