Eco-evolutionary contributions to community trait change in floating aquatic plants

An entire community of organisms may become modified when its environment
changes. These modifications can happen through physiological process
(plasticity), evolutionary processes (adaptation) or shifts in species
composition (sorting). The outcome of these three sources of change
constitutes the community’s phenotypic response, but how they combine to
drive community trait dynamics is not currently well understood. We have
conducted a community selection experiment in which communities of
short-lived floating aquatic plants were grown in a range of stressful
conditions, and measured changes in their body size. Determinants of
phenotypic change were assessed with a full community reciprocal
transplant which led to estimates of the contributions of plasticity,
adaptation, and sorting. Species were modified during the experiment by
both plasticity and adaptation, but in either case the magnitude and
direction of change differed among species. Sorting and adaptation were of
equal magnitude, but tended to act in opposite directions: in conditions
where species with large fronds prevailed, each species evolved smaller
fronds, and vice versa. We conclude that community trait dynamics cannot
be understood simply by extrapolating the adaptive response of any single
species to the whole community.