Temperature and hypoxia-driven shifts in Daphnia interspecific competition

Previous research has shown that elevated temperature and environmental hypoxia are associated with a reduced representation of larger-bodied species in communities of ectotherms, suggesting their competitive disadvantage resulting from increased oxygen demands and limited oxygen availability. To investigate this, we conducted a comparative analysis of two pairs of competing zooplankton species: Daphnia galeata (smaller-bodied) and Daphnia pulex (larger-bodied), as well as D. pulex (smaller-bodied) and Daphnia magna (larger-bodied). We monitored their population dynamics under different oxygen concentrations and temperatures, and additionally measured hemoglobin concentration in the latter pair. Contrary to expectations, the results demonstrated that the larger-bodied species outcompeted the smaller-bodied ones in all experimental treatments involving the first pair of species. In the second pair, elevated temperature favored the smaller-bodied species, while environmental hypoxia had the opposite effect. The competitive advantage of D. pulex under elevated temperature likely stemmed from their ability to allocate energy from somatic growth to reproduction, as indicated by the increased number of reproducing females. Conversely, the competitive advantage of D. magna under environmental hypoxia may be attributed to their ability to reallocate energy from reproduction to somatic growth, as indicated by lower reproductive effort and greater plasticity in hemoglobin production. These findings indicate that elevated temperature and hypoxia can independently and jointly influence interspecific competitive abilities by impacting the pace of life. However, the numerical dominance of smaller-sized species in warm and hypoxic zooplankton communities cannot be explained by their enhanced competitive abilities resulting from a more favorable oxygen supply demand balance.