Spatial insurance against a heatwave differs between trophic levels in experimental aquatic communities

Climate change-related heatwaves are major threats to biodiversity and
ecosystem functioning. However, our current understanding of the
mechanisms governing community resistance to and recovery from extreme
temperature events is still rudimentary. The spatial insurance hypothesis
postulates that diverse regional species pools can buffer ecosystem
functioning against local disturbances through the immigration of
better-adapted taxa. Yet, experimental evidence for such predictions from
multi-trophic communities and pulse-type disturbances, like heatwaves, is
largely missing. We performed an experimental mesocosm study to test
whether species dispersal from natural lakes prior to a simulated heatwave
could increase the resistance and recovery of plankton communities. As the
buffering effect of dispersal may differ among trophic groups, we
independently manipulated the dispersal of organisms from lower
(phytoplankton) and higher (zooplankton) trophic levels. The experimental
heatwave suppressed total community biomass by having a strong negative
effect on zooplankton biomass, probably due to a heat-induced increase in
metabolic costs, resulting in weaker top-down control on phytoplankton.
While zooplankton dispersal did not alleviate the negative heatwave
effects on zooplankton biomass, phytoplankton dispersal enhanced biomass
recovery at the level of primary producers, providing partial evidence for
spatial insurance. The differential responses to dispersal may be linked
to the much larger regional species pool of phytoplankton than of
zooplankton. Our results suggest high recovery capacity of community
biomass independent of dispersal. However, community composition and
trophic structure remained altered due to the heatwave, implying
longer-lasting changes in ecosystem functioning.