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 Yij is the final frequency of the focal species in community type i and replicate j, then its deviation from that species’ mean initial frequency, Yinitial, can be partitioned into three additive components representing the three sources of variation:
Yij - Yinitial = (Yij - Yi) + (Yi - Y) + (Y - Yinitial)
where Yi 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:
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.