Temperature and predation alter metabolic scaling without changing the size-based community structure of freshwater macroinvertebrates

Body size is a key trait that influences ecological processes such as metabolism, abundance and species interactions. While the metabolic theory of ecology (MTE) proposes a universal scaling of metabolic rate with body mass, recent evidence shows that this relationship is not fixed. Environmental factors like temperature and predation can alter the metabolic scaling exponent, potentially reshaping size distributions. However, most research has examined these patterns within individual species, leaving open questions about how environmental drivers affect scaling at the community level. To address this, we performed a mesocosm experiment manipulating both temperature and fish predator presence in freshwater macroinvertebrate communities. We found that warming altered metabolic rates in a predation-dependent way: without fish, metabolic rates increased in large individuals but decreased in small individuals, whereas the opposite occurred with fish present. This suggests that larger individuals reduce their baseline metabolic rates under predation risk, especially at higher temperatures. Interestingly, the slope of the community size distribution remained stable across treatments, indicating that shifts in metabolic scaling occurred independently of changes in size structure. Together, these findings highlight the environmental sensitivity of metabolic scaling and suggest that links between metabolism scaling and size distributions may be more complex than MTE predicts. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.