Understanding the ecological impacts of climate change is a crucial
challenge of the twenty-first century. There is a clear lack of
general rules regarding the impacts of global warming on biota.
Here, we present a metaanalysis of the effect of climate change on
body size of ectothermic aquatic organisms (bacteria, phyto- and
zooplankton, and fish) from the community to the individual level.
Using long-term surveys, experimental data and published results,
we show a significant increase in the proportion of small-sized
species and young age classes and a decrease in size-at-age. These
results are in accordance with the ecological rules dealing with the
temperature–size relationships (i.e., Bergmann’s rule, James’ rule
and Temperature–Size Rule). Our study provides evidence that
reduced body size is the third universal ecological response to
global warming in aquatic systems besides the shift of species
ranges toward higher altitudes and latitudes and the seasonal
shifts in life cycle events.