Functionally redundant herbivores: urchin and gastropod grazers respond differently to ocean warming and rising CO2

Future ocean CO2 and temperatures are predicted to increase primary productivity across tropical marine habitats, potentially driving a shift towards algal-dominated systems. However, increased consumption of algae by benthic grazers could potentially counter this shift. Yet, the response of different grazer species to future conditions will be moderated by their physiologies, meaning that they may not be functional equivalents. Here, we experimentally assessed the physiological response of key grazers—the sea urchin Heliocidaris crassispina and 2 gastropod species, Astralium haematragum and Trochus maculatus—to predicted CO2 concentrations (400, 700 and 1000 ppm) and temperature conditions (ambient, +3 and +5°C). In line with metabolic theory, we found that urchin metabolic rate increased at future temperatures regardless of CO2 conditions, with evidence of metabolic acclimation to higher temperatures. The metabolic rate of A. haematragum was depressed only by CO2, whereas T. maculatus initially had elevated metabolic rates at moderate CO2, which were depressed by the combination of the highest CO2 concentration and temperatures. Taxa showed differential survival, with no urchin mortality under any future conditions but substantial mortality of both gastropods under elevated temperatures regardless of CO2 concentration. Importantly, all species had substantially reduced algal consumption in response to elevated CO2, though the urchins only demonstrated an energetic mismatch under combined future CO2 and temperature. Therefore, despite sharing an ecological niche, these key grazers are likely to be differentially affected by future environmental conditions, potentially reducing the strength of ecological compensatory responses depending on the functional redundancy in this grazing community.