DOI: 10.5061/dryad.15dv41p1m
title: A basic community dynamics experiment: Disentangling deterministic and stochastic processes in structuring ecological communities

description: 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 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.
description: Source
 community The source plant community was
 isolated from a eutrophic pond adjacent to fallowed agricultural fields on
 the Macdonald campus of McGill University, Quebec, Canada (45° 42’ N, 73°
 94’ W). The pond sustains a diverse community of floating macrophytes, the
 four most abundant being <em>Lemna</em>
 <em>minor</em> (Lm), <em>Lemna</em>
 <em>trisulca</em> (Lt), <em>Spirodela</em>
 <em>polyrhiza</em> (Sp) and <em>Wolffia</em>
 <em>columbiana</em> (Wc), all in the family Lemnaceae. Large
 samples consisting of hundreds of thousands of individuals were taken in
 June 2020 and manually separated into the constituent species.
 Experimental
 design The experiment was conducted at the
 LEAP (Large Experimental Array of Ponds) facility at  Gault Nature Reserve
 of McGill University in Quebec, Canada (45° 32’ N, 73° 08’ W), (Fugère et
 al. 2020) (Fig. 1). 48 large mesocosms (surface
 area=2.43m<sup>2</sup>) were filled each with 500L of water
 piped from Lac Hertel, a mesotrophic lake on the reserve, 1km upstream of
 the experiment. The water was sieved to remove fish, tadpoles,
 macroinvertebrates, and macrophytes, but contained intact communities of
 zooplankton and phytoplankton. The removal of these larger organisms was
 to decrease unwanted variation due to sampling. Material from the source
 community was used to assemble four
 community types defined by the initial relative abundance of each species
 (10%, 20%, 30% or 40%) (Table 1). Relative abundance was calculated as
 mass-weighted frequencies using an average value of individual mass for
 each species. Each community was seeded with a total of 1g wet mass of
 community biomass, which works out to between ~2,000–3,000 individuals,
 depending on the community type (Table 1). Abundances of the larger
 species were determined by manual counting, while Wc, only ~0.5mm wide,
 was weighed and added in bulk, using an estimate of mean frond mass.
 Initial community densities translated to roughly 5–10% surface cover.
  Communities were mixed after inoculation to remove any initial spatial
 variation.  Each of the four community types was replicated in 12
 mesocosms (total number of mesocosms = 48) which were arranged in six
 blocks of eight mesocosms, each block containing two replicates of each
 community type, with community type randomized within block.
 All mesocosms received a one-time initial addition of
 inorganic Nitrogen and Phosphorus (KNO<sub>3</sub> and
 H<sub>2</sub>KPO<sub>4</sub>), to obtain initial
 dissolved concentrations of these nutrients in the mesocosms comparable to
 those of the pond from which the source community was taken (800 µgL-1 N
 and 40 µgL-1 P).  The mesocosms were covered with 70% shade cloth to mimic
 canopy cover. Although this minimized the input of wind-carried debris
 like leaf litter, rainwater could pass through the mesh cloth, which
 roughly balanced water lost due to evaporation. Communities were then left
 to grow for 12 weeks, from the beginning of July to the end of September,
 ending shortly before the first frost. All mesocosms were randomly sampled every two
 weeks to estimate species relative abundances. This was done by first
 mixing the communities to break up species clustering
 (Hart et al. 2019, Jewell
 and Bell 2022), then
 removing a fixed percentage of the surface area (~5%) with a net. Although
 mixing eliminated communities’ spatial structure, it allowed us to
 efficiently obtain representative samples. These samples were exhaustively
 counted before being returned to the mesocosm.
 Statistical
 analysis The main goal of this experiment
 was to estimate the contributions of species sorting, ecological drift,
 and initial state to community change. Overall variation in final species
 composition among communities can be broken into these three components,
 whose contributions to variation can be partitioned using an Anova
 framework (Travisano et al. 1995, Bell 2013). If
 Y<sub>ij</sub> is the final frequency of the focal species in
 community type i and replicate j, then its deviation from that species’
 mean initial frequency, Y<sub>initial</sub>, can be
 partitioned into three additive components representing the three sources
 of variation: Y<sub>ij</sub>  -
 Y<sub>initial</sub> = (Y<sub>ij</sub> -
 Y<sub>i</sub>) + (Y<sub>i</sub> - Y) + (Y -
 Y<sub>initial</sub>)   where
 Y<sub>i</sub> is the mean final frequency of the focal species
 in community type i, and Y is the grand mean final frequency of the focal
 species across all community types and replicates. For n community types
 (communities with different initial species composition) each replicated m
 times, the total variation attributable to sorting, drift and initial
 state can be calculated as follows: nm S (Y
 - Y<sub>initial</sub>)<sup>2</sup>, the shift in
 grand mean representing an overall convergence to an equilibrium
 composition (sorting), m S (Y<sub>i</sub>
 - Y)<sup>2</sup>, the variance among community types around
 the grand mean representing the influence of a community’s initial state,
 and S S (Y<sub>ij</sub> -
 Y<sub>i</sub>)<sup>2</sup>, the variance among
 replicates of same community type representing neutral variation
 (drift). Such a partition was done for
 each species at the end of the experiment to estimate the overall
 contributions of sorting, drift and initial state, as well as for each
 intermediate census to assess how the contributions changed over
 time. We calculated a normalized value of the change in
 relative abundance of each species at each census in each mesocosm as the
 difference between the relative abundance of that species in the current
 and immediately preceding census, divided by its relative abundance in the
 preceding census. For each species, we used the regression of this
 normalized change in relative abundance on its preceding relative
 abundance to determine whether species dynamics were frequency-dependent.
 We used the X-intercept (at which change in relative abundance is zero) as
 an estimate of the equilibrium abundance of that species in a stable
 community.