Thermal history mediates the ecological role of body size in a freshwater fish

Intraspecific trait variation among populations may
strongly alter community and ecosystem structure and function.
Body size is a fundamental trait of all organisms, affecting both
organismal physiology and ecological effects, which may differ across
populations. For example, body size is predicted to decline with warming.
As such, in this study, we aim to determine how population differences in
thermal history mediate the ecological role of body size in a freshwater
fish. We conducted a mesocosm experiment in which we manipulated fish
(Gambusia affinis [Baird and Girard, 1853]) source populations (ambient
source vs warm source) and body size while holding their biomass constant.
We monitored community and ecosystem response variables including,
macroinvertebrate abundance, zooplankton biomass, phytoplankton abundance,
and greenhouse gas flux. Changes in fish body size influenced most
ecological responses, but these effects often depended on the thermal
history of the fish population. For many responses, the effects of reduced
fish body size were offset by a history of exposure to warm temperatures,
suggesting that environmental factors (including thermal acclimation) and
adaptation may offset the community and ecosystem effects of decreased
consumer body size. Our research suggests that ecological changes will
depend on changes in body size and environmental factors, as well
as on other trait changes associated with warming. Experiments
and models addressing the ecological effects of body-size decline alone
may overestimate the ecological changes expected under warming.