Future herbivory: the indirect effects of enriched CO2 may rival its direct effects

Abstract

Variation in rates of herbivory may be driven by direct effects of the abiotic environment on grazers, as well as indirect effects mediated by their food. Disentangling these direct and indirect effects is of fundamental importance for ecological forecasts of changing climate on species interactions and their influence on biogenic habitat. Whilst elevated atmospheric CO 2 may have direct effects on grazers with calcareous structures via 'ocean acidification', it may also have indirect effects via changes caused to their food. In our study we initially tested, and confirmed, that enriched CO 2 altered per capita rates of grazing before assessing the relative importance of indirect and direct effects in driving this response. Our results eliminated the model of a direct effect of CO 2 enrichment on the grazers themselves and supported the model of an indirect effect driven by a change in the food (i.e. turf algae). We suggest that this indirect effect manifested as grazers responded to the increased nitrogen content (i.e. %N) of algal tissue that resulted under CO 2 enrichment. Understanding such indirect effects of modified environmental conditions provide important mechanistic links between climate conditions and the ecological processes they influence.KEY WORDS: Ocean acidification · Carbon dioxide · Climate change · Turf-forming algae · Habitat loss · Phase-shifts Resale or republication not permitted without written consent of the publisher

Effects of climate change on the physiology of giant kelp, Macrocystis pyrifera, and grazing by purple urchin, Strongylocentrotus purpuratus

“…Finally, the majority of studies have focused on either the physiological or demographic impacts on single species and have largely ignored how climate change will impact species interactions. Changes in such interactions may aggravate or ameliorate the impacts of environmental stressors on an organism (O'Connor 2009, Falkenberg et al 2013, ultimately resulting in outcomes different from what studies on single species would predict. As a result, care must be taken when extrapolating the physiological effects on single species to community-level impacts on the ecosystem as a whole.…”

“…In addition to altering organism physiology, environmental stressors can alter the manner in which species interact (O'Connor 2009, Falkenberg et al 2013. S. purpuratus held under both elevated temperature and pCO 2 (i.e., future conditions) exhibited significantly lower growth rates, reduced feeding rates, and smaller gonad indices than urchins held under present-day conditions.…”

“…One reason for this is that environmental stressors, such as increases in temperature and / or pCO 2 , may not support the same relationships in all circumstances, but instead may interact in complex ways depending on their relative strength and on the organisms in which they are examined (Crain et al 2008). In addition, the direct effects of these stressors on organism physiology may be altered by changes in the strength of species interactions (O'Connor 2009, Falkenberg et al 2013, resulting in outcomes different than those observed when species are studied in isolation. In this study, we demonstrated that the physiological responses of the giant kelp, M. pyrifera, to predicted increases in temperature and pCO 2 (Intergovernmental Panel on Climate Change 2007) vary greatly between situations where these factors are examined individually versus together.…”

Urchins in a high CO2 world: partitioned effects of body-size, ocean warming and acidification on metabolic rate

“…Increases of 44% in metabolism in the combined treatments suggest that nearfuture climate change will result in a substantial increase in energetic costs in H. erythrogramma, and that this will affect all age classes within the species. These increased energetic costs could be minimised through acclimation or adaptation of the species over the coming decades, but it is likely to be subject to some permanent increased energetic demand, and as a primary grazer this could cause substantial ongoing ecological effects (Falkenberg et al, 2013). …”

Ocean acidification through the lens of ecological theory

“…As alluded to above, the biggest unknown in OA research is how species will respond within the context of their communities. It is almost certain that many of the most striking consequences of OA will arise through altered species interactions (e.g., Fabricius et al 2011, Falkenberg et al 2013, Kroeker et al 2013b, McCormick et al 2013). The ''what and the how'' of these interspecific linkages is where ecological theory has much to offer.…”