A test of metabolic and consumptive responses to local and global perturbations: enhanced resources stimulate herbivores to counter expansion of weedy species

Abstract

The capacity of natural systems to resist environmental change underpins ecosystem stability, e.g. the persistence of kelp-dominated states which are sometimes displaced by subordinates or weedy species (i.e. algal turfs). Perturbation by resource enhancement at global (e.g. CO2 emissions) through local scales (e.g. nutrient pollution) increases the probability of turf domination, yet these same resources stimulate an increase in per capita consumption of turfs by herbivores. We test whether such resource perturbation can stimulate herbivores to absorb the additional productivity of turfs that cause kelp displacement. We tested the hypotheses that (1) elevated nitrogen (N) and carbon dioxide (CO2) not only stimulate an increase in consumptive rates, but also stimulate an increase in underlying metabolic rates of gastropod herbivores, so that (2) enhanced primary productivity is countered by herbivory. We reveal that elevated nitrogen and CO2 stimulated an elevation in rates of consumption in proportion to an increase in metabolic rate of grazers. Subsequently, grazers consumed proportionately greater cover of turfs to counter turf expansion. Resource enrichment, therefore, can stimulate metabolic and consumptive activity of herbivores to absorb the additional productivity of opportunistic species. Hence, the competitive potential of subordinates to displace community dominants may be checked by the very resources that otherwise drive instability.