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. 1). 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.
By the end of the experiment, each community type had diverged 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. 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. 2). This analysis was done for
each time point (see Appendix S1 in Supporting information), to evaluate
how these three sources of community change varied over the course of
the experiment (Fig. 3).
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. 4). 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.