Future habitat suitability for coral reef ecosystems under global warming and ocean acidification.

Elena Couce,Erica J Hendy,Andy Ridgwell

doi:10.1111/gcb.12335

Abstract

Rising atmospheric CO2 concentrations are placing spatially divergent stresses on the world's tropical coral reefs through increasing ocean surface temperatures and ocean acidification. We show how these two stressors combine to alter the global habitat suitability for shallow coral reef ecosystems, using statistical Bioclimatic Envelope Models rather than basing projections on any a priori assumptions of physiological tolerances or fixed thresholds. We apply two different modeling approaches (Maximum Entropy and Boosted Regression Trees) with two levels of complexity (one a simplified and reduced environmental variable version of the other). Our models project a marked temperature-driven decline in habitat suitability for many of the most significant and bio-diverse tropical coral regions, particularly in the central Indo-Pacific. This is accompanied by a temperature-driven poleward range expansion of favorable conditions accelerating up to 40–70 km per decade by 2070. We find that ocean acidification is less influential for determining future habitat suitability than warming, and its deleterious effects are centered evenly in both hemispheres between 5° and 20° latitude. Contrary to expectations, the combined impact of ocean surface temperature rise and acidification leads to little, if any, degradation in future habitat suitability across much of the Atlantic and areas currently considered ‘marginal’ for tropical corals, such as the eastern Equatorial Pacific. These results are consistent with fossil evidence of range expansions during past warm periods. In addition, the simplified models are particularly sensitive to short-term temperature variations and their projections correlate well with reported locations of bleaching events. Our approach offers new insights into the relative impact of two global environmental pressures associated with rising atmospheric CO2 on potential future habitats, but greater understanding of past and current controls on coral reef ecosystems is essential to their conservation and management under a changing climate.

Highlights

Anthropogenic climate change has emerged as a serious global-scale threat to the future viability of coral reef ecosystems (e.g., Hoegh-Guldberg et al, 2007; Riegl et al, 2009; Veron et al, 2009; Glynn, 2012), with studies linking widespread bleaching events to increasing sea surface temperatures (SSTs; Hoegh-Guldberg, 1999)
Areas where projected coral reef habitat suitability is most critically degraded by ocean surface warming (Fig. 2) correspond to areas with the highest mean annual SSTs today (Fig. 1), and map onto regions identified as being susceptible to future coral bleaching (e.g., Guinotte et al, 2003; Donner et al, 2005; van Hooidonk et al, 2013)
S1.7–S1.8 in Data S1 and Fig. S2.5 in Data S2). Both Boosted Regression Trees (BRT) and Maximum Entropy (MaxEnt) statistical modeling approaches assign a relatively high significance to maximum weekly SST; in 10 randomly generated versions of MaxEntOPT/BRTOPT, this parameter ranked 5th/8th of a total of 27 parameters in average relative contribution to model output

Summary

Introduction

Anthropogenic climate change has emerged as a serious global-scale threat to the future viability of coral reef ecosystems (e.g., Hoegh-Guldberg et al, 2007; Riegl et al, 2009; Veron et al, 2009; Glynn, 2012), with studies linking widespread bleaching events to increasing sea surface temperatures (SSTs; Hoegh-Guldberg, 1999). 3.25; Cao & Caldeira, 2008; Hoegh-Guldberg et al, 2007) This has raised concerns over the corals’ ability to maintain reef structures, pushing the view that declining aragonite saturation represents a pressing threat (e.g., Cao & Caldeira, 2008; Silverman et al, 2009; Ricke et al, 2013). The present-day relationship between coral reef locations and aragonite saturation is very likely modulated by other environmental factors such as temperature and nutrient availability (e.g., Cohen & Holcomb, 2009; Chauvin et al, 2011), meaning that habitat suitability implications cannot be drawn from future aragonite saturation changes in isolation

Objectives

Methods

Results

Conclusion