Abstract
Environmental anomalies that trigger adverse physiological responses and mortality are occurring with increasing frequency due to climate change. At species' range peripheries, environmental anomalies are particularly concerning because species often exist at their environmental tolerance limits and may not be able to migrate to escape unfavourable conditions. Here, we investigated the bleaching response and mortality of 14 coral genera across high-latitude eastern Australia during a global heat stress event in 2016. We evaluated whether the severity of assemblage-scale and genus-level bleaching responses was associated with cumulative heat stress and/or local environmental history, including long-term mean temperatures during the hottest month of each year (SSTLTMAX ), and annual fluctuations in water temperature (SSTVAR ) and solar irradiance (PARZVAR ). The most severely-bleached genera included species that were either endemic to the region (Pocillopora aliciae) or rare in the tropics (e.g. Porites heronensis). Pocillopora spp., in particular, showed high rates of immediate mortality. Bleaching severity of Pocillopora was high where SSTLTMAX was low or PARZVAR was high, whereas bleaching severity of Porites was directly associated with cumulative heat stress. While many tropical Acropora species are extremely vulnerable to bleaching, the Acropora species common at high latitudes, such as A. glauca and A. solitaryensis, showed little incidence of bleaching and immediate mortality. Two other regionally-abundant genera, Goniastrea and Turbinaria, were also largely unaffected by the thermal anomaly. The severity of assemblage-scale bleaching responses was poorly explained by the environmental parameters we examined. Instead, the severity of assemblage-scale bleaching was associated with local differences in species abundance and taxon-specific bleaching responses. The marked taxonomic disparity in bleaching severity, coupled with high mortality of high-latitude endemics, point to climate-driven simplification of assemblage structures and progressive homogenisation of reef functions at these high-latitude locations.
Highlights
The distribution of global biodiversity is undergoing substantial modifications as climate change accelerates and environmental anomalies become more frequent and severe (Butchart et al, 2010; Cheung et al, 2009; Parmesan & Yohe, 2003; Sala et al, 2000)
Despite the similarity in the bleaching index (BI) values of the two severely impacted genera, they differed in patterns of mortality; mortality rose as BI increased for Pocillopora, whereas mortality and BI were not correlated for Porites (Fig. 3d; Fig. 3e; Table S10)
While high-latitude reefs have been considered as potential climate refugia for tropical corals under climate change (Beger et al, 2014; Riegl, 2003; but see Lybolt et al, 2011), our findings suggest that resident high-latitude corals are vulnerable to thermal anomalies, potentially without suitable ex situ climate refugia equivalent to those of tropical corals (Harriott & Banks, 2002; Schleyer et al, 2018)
Summary
The distribution of global biodiversity is undergoing substantial modifications as climate change accelerates and environmental anomalies become more frequent and severe (Butchart et al, 2010; Cheung et al, 2009; Parmesan & Yohe, 2003; Sala et al, 2000). High-latitude regions are predicted to experience greater heat stress than the tropics over the coming decades (Hobday & Pecl, 2013; Wu et al, 2012), which is likely to result in increasingly frequent and intense regional bleaching events (Heron, Maynard, van Hooidonk, & Eakin, 2016; van Hooidonk, Maynard, & Planes, 2013; van Hooidonk et al, 2016) Unlike their tropical counterparts, poleward range shifts and/or expansions are unlikely for many high-latitude coral species because suitable habitats are progressively unavailable toward the poles, such as in the high-latitude east coast of Australia and South Africa The findings from this study improve our knowledge of the vulnerability of high-latitude corals under climate change
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