Changing consumer strength in a changing climate

Abstract

The intensity at which organisms interact is affected by abiotic conditions. Ocean warming and acidification alter the metabolic demands of organisms and the strength at which they interact with each other. The metabolic costs of changing abiotic conditions vary between interacting pairs of species, and as such, their strength of influence on one another may change with changing climate. Ocean warming and acidification are anticipated to alter competitive dominance among primary producers such as perennial kelp and ephemeral turf algae, increasing the potential for ecosystems to undergo phase shifts, e.g. from kelp-dominated to persistent turf-dominated states. However, in order to meet greater metabolic demands imposed by elevated temperature, herbivorous invertebrates need to increase feeding rates and may counter turf productivity as a result. Whilst strong top-down control of primary productivity is supported by metabolic theory of ecology (MTE), it assumes that consumption rates of herbivores keep pace with metabolism and mirror increased growth of producers. At moderate warming, both metabolic rates and feeding of herbivorous gastropods were elevated, yet as temperature increased further consumption rates peaked earlier than turf growth rates. Imposed costs to resource allocation where consumption does not meet metabolic demands may result in reduced fitness and survivorship. These results suggest that future strength of top-down control is dependent on whether consumer-producer responses are synchronous, with mismatches between interacting pairs producing outcomes not predicted by metabolic theory. Further, moderate increases of temperature and CO₂ lead to reduced herbivore ingestion efficiency, ultimately resulting in reduced growth. Elevated metabolism generally requires increased foraging to meet energetic demands; however, foraging may also need to be mediated by predator avoidance. This thesis identified that the need for greater foraging activity imposed by future warming and ocean acidification was opposed by elevated predation risk. Avoidance may be heightened in calcifying herbivores such as gastropods as a way to mitigate increased costs of inducible defences like shell building. Nevertheless, reduced foraging rates may compound energetic deficiencies and lead to reduced fitness. Compensatory responses of gastropod and amphipod herbivores that buffered the accelerated effects of ocean warming and acidification on turf productivity may indicate the potential for this kelp-turf system to resist abiotic change. Moreover, this role was filled by more than one species, such that the one species could compensate for the effects of climate in the absence of the other, but not over compensate when together. Such functional redundancy of trophic compensation was underpinned by individual and population level responses to altered conditions, and offers an account for why some systems may be able to withstand both short- and long-term disturbances. Species interactions are mediated by the abiotic environment, and the strength of interactions may be altered through the influence of abiotic change on physiological demands. This thesis contributes new knowledge to recognising idiosyncratic and predictable responses of interacting species to future conditions and their ensuing consequences for ecological communities. Finally, it expands on the theory of compensatory dynamics by exploring adjustability in strength of buffering responses of consumers to the effects of altered environments on productivity.