Energetic mismatch induced by warming decreases leaf litter decomposition by aquatic detritivores

1. The balance of energetic losses and gains is of paramount importance
for understanding and predicting the persistence of populations and
ecosystem processes in a rapidly changing world. Previous studies
suggested that metabolic rate often increases faster with warming than
resource ingestion rate, leading to an energetic mismatch at high
temperature. However, little is known about the ecological consequences of
this energetic mismatch for population demography and ecosystem functions.
2. Here, we combined laboratory experiments and modeling to investigate
the energetic balance of a stream detritivore (Gammarus fossarum) along a
temperature gradient and the consequences for detritivore populations and
organic matter decomposition. 3. We experimentally measured the energetic
losses (metabolic rate) and supplies (ingestion rate) of Gammarus and we
modeled the impact of rising temperatures and changes in Gammarus body
size induced by warming on population dynamics and benthic organic matter
dynamics in freshwater systems. 4. Our experimental results indicated an
energetic mismatch in a Gammarus population where losses via metabolic
rate increase faster than supplies via food ingestion with warming, which
translated in a decrease of energetic efficiency with temperature rising
from 5 to 20 °C. Moreover, our consumer-resource model predicts a decrease
in the biomass of Gammarus population with warming, associated with lower
maximum abundances and steeper abundance decreases after biomass annual
peaks. These changes resulted in a decrease of leaf litter decomposition
rate and thus longer persistence of leaf litter standing stock over years
in the simulations. In addition, Gammarus body size reductions led to
shorter persistence for both leaf litter and Gammarus biomasses at low
temperature and the opposite trend at high temperature, revealing that
body size reduction was weakening the effect of temperature on resource
and consumer persistence. 5. Our model contributes to identifying the
mechanisms that explain how thermal effects at the level of individuals
may cascade through trophic interactions and influence important ecosystem
processes. Considering the balance of physiological processes is crucial
to improve our ability to predict the impact of climate change on carbon
stocks and ecosystem functions.