Community dynamics are governed by two opposed processes: species sorting, which produces deterministic dynamics leading to an equilibrium state, and ecological drift, which produces stochastic dynamics. Despite a great deal of theoretical and observational work aiming to demonstrate the predominance of one or the other of these processes, experimental work remains rare. Here we present the results of a basic community dynamics experiment using floating aquatic plants, designed to measure the relative contributions of species sorting and ecological drift to community change over about a dozen generations. We found that species sorting became overwhelmingly dominant as the experiment progressed, and directed communities towards a stable equilibrium state maintained by negative frequency-dependent selection. The dynamics of any particular species depended on how far its initial frequency was from its equilibrium frequency, however, and consequently the balance of sorting and drift varied among species.

Much recent work has focused on characterising the mutualistic elements of the plant microbiome, often aiming to identify bacterial strains that can increase plant fitness. Although most work has focused on terrestrial plants, Lemna minor, a floating aquatic angiosperm, is increasingly used as a model in host-microbe interactions. Here we assess the fitness and phenotypic consequences of the full microbiome for L. minor by assaying plants from eight natural sites, with and without their microbiomes, over a range of environmental conditions. We find that the microbiome supresses plant fitness, for all genotypes and across all environmental conditions. This decrease in fitness was accompanied by phenotypic changes, with plants forming smaller colonies and producing smaller fronds and shorter roots with the microbiome present. Although the L. minor microbiome clearly includes important symbionts, our findings suggest that we cannot discount the important pathogenic, parasitic, and competitive interactions, whose influence can override that of mutualists.

Community dynamics are governed by two opposed processes: species sorting, which produces deterministic dynamics leading to an equilibrium state, and ecological drift, which produces stochastic dynamics. Despite a great deal of theoretical and empirical work aiming to demonstrate the predominance of one or the other of these processes, the importance of drift in structuring communities and maintaining species diversity remains contested. Here we present the results of a basic community dynamics experiment using floating aquatic plants, designed to measure the relative contributions of species sorting, ecological drift to community change over about a dozen generations. We found that species sorting became overwhelmingly dominant as the experiment progressed, and directed communities towards a stable equilibrium state maintained by negative frequency-dependent selection. The dynamics of any particular species depended on how far its initial frequency was from its equilibrium frequency, however, and consequently the balance of sorting and drift varied among species.