Ectothermic omnivores increase herbivory in response to rising temperature

Higher temperatures as a consequence of global climate change may
considerably alter trophic interactions. Ectothermic herbivores and
carnivores generally ingest more food with rising temperature as their
metabolic rates increase with rising temperature. However, omnivorous
ectotherms may respond in two ways: quantitatively by consuming more food
and qualitatively by altering their degree of herbivory or carnivory
through a diet shift. We hypothesize that rising temperature will increase
herbivory of ectothermic omnivores as herbivory increases towards the
equator. We tested the hypothesis in a freshwater model system in which
ectothermic omnivores are prevalent, by applying two approaches, a
temperature manipulation experiment and a literature study. We performed
feeding trials with a juvenile aquatic ectothermic omnivore (pond
snail Lymnaea stagnalis) at different temperatures ranging from
12 to 27°C, supplying them with both animal food and plant material, and
directly quantified their consumption rates over time. The results showed
that snails cultured at high temperatures (> 21°C) increased the
proportion of plant material in their diets after 17 days, which supports
our hypothesis. In the literature survey, we found that rising temperature
increased herbivory in multiple aquatic animal taxa, including
zooplankton, amphibians, crayfish, fish and snails. This suggests that
aquatic ectothermic omnivores might commonly increase herbivory with
rising temperature. The mechanisms underlying this temperature‐induced
diet shift are not sufficiently explained by current theories related to
the physiology, metabolism and stoichiometry of omnivores. We propose to
incorporate the animals’ ontogenetic development in the temperature
metabolic stoichiometry hypothesis as a complementary explanation for the
diet shift, namely that the diet shift could be due to faster development
of the ectotherms and an earlier ontogenetic diet shift at higher
temperatures. We conclude that future global warming will most likely
alter food webs by increasing the top–down control of aquatic herbivores
and omnivores on primary producers.