Results:
There was an initial sharp drop in the relative abundances ofLemna trisulca (Lt) and Wolffia columbiana (Wc) because of
transfer shock, with a fraction of plants immediately sinking to the
bottom of the mesocosms. Because of this, we have used week 2 as the
initial time point. After this initial settling time, community
composition continued to change during the 12-week experiment withLemna minor (Lm) coming to dominate most communities, regardless
of its initial frequency, at the expense of Lt and Wc which became rare
(but not extinct) in most communities (Fig. 2). Spirodela
polyrhiza (Sp), maintained moderate abundances in most communities. The
overall trajectory of community composition seemed to be largely
independent of initial composition and was consistent among replicates.
This was the case regardless of whether we relative abundance was
weighted by mass (as for the main analysis), or left as un-weighted
individual counts (Fig. S1 & S2 in Supporting information).
By the end of the experiment, each community type had diverged
substantially from its initial composition. The extent of this
divergence has three components: sorting as the source of directional
change, drift as idiosyncratic divergence among replicates, and initial
state as retention of differences among community types. To quantify the
relative contributions of sorting, drift and initial state to final
community composition we partitioned the overall sum of squares into
components representing these three processes (Fig. 3). This analysis
was done for each time point (see Appendix S3 in Supporting
information), to evaluate how these three sources of community change
varied over the course of the experiment (Fig. 4).
We regressed the normalized change in relative abundance between time
points against previous relative abundance for each species to obtain
estimates of equilibrium species frequencies (Fig. 5). These regressions
are autocorrelated, but they can be used to estimate the equilibrium
frequency of a species as the X-intercept, yielding 0.76 for Lm, 0.05
for Lt, 0.27 for Sp and 0.01 for Wc. When constrained to sum to 1, these
frequencies are 0.70 for Lm, 0.04 for Lt, 0.25 for Sp and 0.01 for Wc.