Introduction:
Patterns in the composition and diversity of species in a community are the result of many interacting processes. Borrowing concepts from population genetics, Vellend (2010, 2016) distilled these down to four fundamental processes in a conceptual synthesis of community ecology: selection, ecological drift, speciation, and dispersal. Since speciation and dispersal are responsible for the introduction of new variation into the community, the dynamics of a closed community is essentially governed by selection (also referred to as species sorting) and ecological drift alone. Species sorting is natural selection at the level of species, which will produce distinct assemblages of species in different habitats, each local community consisting of those species best adapted to their local conditions of growth. This classical “niche-based” view asserts that species coexistence is due to functional differences between species and predicts deterministic dynamics. Ecological drift is genetic drift at the level of species, which will produce a distinctive assemblage of species in any given place whose composition is unrelated to local conditions. This “neutral-based” view assumes the functional equivalency of species and predicts stochastic community dynamics. The relative importance of these two processes in structuring communities has been vigorously debated in the last two decades and many attempts have been made to show that one of these processes is much more important than the other (Wright 2002, Hubbell 2006, Rosindell et al. 2011, Wennekes et al. 2012). It would be difficult to support either extreme view, however, because both processes will be active at all times everywhere, and the main goal of community ecology should be to understand how the balance between them depends on the underlying physical and biotic characteristics of sites. Many recent developments have been proposed to resolve the niche-neutral controversy (Leibold and McPeek 2006, Adler et al. 2007, Haegeman and Loreau 2011, Chase 2014, Fisher and Mehta 2014, Matthews and Whittaker 2014, Shoemaker et al. 2020, Siqueira et al. 2020), but experimental work directly quantifying the contributions of these processes has not been attempted.
Species sorting and ecological drift will have directly opposed effects on the species composition of communities under a given set of environmental conditions. Under species sorting, communities that initially differ in composition will converge on the same composition, which represents the stable equilibrium community for that set of conditions. Under ecological drift, communities that are initially identical in composition will diverge over time. The relative contributions of these two processes to community dynamics (changes over time in species composition) can therefore be estimated by setting up replicated communities with different initial composition. Ecological drift will cause divergence of replicate communities of any given initial composition. Species sorting will cause convergence of communities that initially differ in composition.
A third factor which might influence how a community changes over time is its initial state (Chase 2003). For example, the likely direction of ecological drift will depend on the initial frequencies of species, and species sorting might depend on a mechanism that favoured abundant species, such as priority effects or growth inhibition by exudates. The contributions of sorting, drift and initial state will sum to the overall change in composition observed over a given period of time.
Such experiments have been done for single-species populations to estimate the contributions of natural selection, genetic drift and ancestry to the evolution of fitness and of phenotypes such as cell size in bacteria and the evolution of heterotrophy in Chlamydomonas(Travisano et al. 1995, Bell 2013). Despite the clear analogy of these processes in population genetics to community ecology (Vellend 2010), no similar work has yet been done in multi-species communities. Here we extend experimental evolution into ecology to estimate the relative contributions of species sorting (the ecological equivalent of natural selection), ecological drift (genetic drift), and initial state (ancestry) to community species dynamics.
We assembled experimental communities of floating aquatic macrophytes from the family Lemnaceae that frequently coexist in the field. These are highly reduced angiosperms that consist of a single leaf-like frond which may or may not bear a submerged unbranched root, depending on the species. Reproduction is nearly always asexual and vegetative, which results in extremely short generation times of less than a week in eutrophic conditions. Many species are widespread and abundant in lentic ecosystems and often coexist in multi-species communities consisting of hundreds of thousands to millions of individuals. Because of their small size and short generation time, they are being increasingly used as a model system in ecology and evolution (Laird and Barks 2018, Hart et al. 2019, Vu et al. 2019) and enable us to run highly-replicated experiments lasting more than a dozen generations in a single season. Here we report the results of a basic community dynamics experiment using semi-natural communities consisting of four such species of Lemnaceae . By manipulating initial relative abundances of species and following changes in species composition over time, we can estimate the relative contributions of these opposing processes to community species dynamics.