Plant-herbivore interactions under ocean acidification: Changes in herbivore body size and abundance outweigh minimal shifts in biomass-specific consumption rates
Herbivores have the potential to mediate competitive interactions between primary producers, and thereby maintain biodiversity and regulate primary productivity. Increased carbon dioxide concentrations in the ocean causes changes in its carbonate cycle, a process termed ocean acidification (OA), which may shift the balance of plant-herbivore interactions. This problem is twofold: (1) OA may directly affect herbivory rates by affecting herbivore metabolism, body size, or population size, or (2) indirectly via changes algal palatability. Here, we explore whether changes in body and population size have a greater impact on herbivory rates than changes in per biomass interaction strength driven by changes in metabolic demand and palatability.
Herbivory trials were run under acidified and control conditions for four herbivore taxa that varied in the degree to which their growth was likely to be limited by calcification. We tested the effects of acidification on both biomass-specific feeding rates and herbivore growth rates. In a complimentary set of experiments, we focused on a single herbivore species, the amphipod Cymadusa pemptos. Using a long-term (multi-generational) mesocosm experiment coupled with short-term feeding trials, we examined the effects of CO2 on C. pemptos per capita and population level rates of herbivory, as well as algal palatability.
Despite high statistical power, CO2 had no impact on the biomass specific feeding rates of any species we examined. However, both of our highly calcified herbivores, the gastropod Chlorostoma funebralis and the urchin Strongylocentrotus franciscanus, experienced reduced growth with elevated CO2, as well as a strong positive correlation between body size and per capita consumption rates. When manipulated separately CO2 had no effect on feeding rates of C. pemptos either, but did have a negative effect on algal palatability. However, when both algae and herbivores were simultaneously exposed to treatment levels, CO2 had no effect on consumption. While we did not observe body size changes in our amphipods, our multigenerational manipulation showed a 27x increase in population size with increased CO2.
In summary, changes in body size may drive an overall reduction in the feeding rates of highly calcified herbivores (i.e. urchins and gastropods) in response to OA, while increases in the population sizes of less calcified species (e.g., amphipods) may lead to an increase in herbivory by these taxa. Therefore, changes in the growth of individuals and the dynamics of populations, but not biomass specific feeding rates, will determine the ways in which plant-herbivore interactions respond to ocean acidification.