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Abstract
Climate driven range shifts are driving the redistribution of marine species and threatening the functioning and stability of marine ecosystems. For species that are the structural basis of marine ecosystems, such effects can be magnified into drastic loss of ecosystem functioning and resilience. Rhodoliths are unattached calcareous red algae that provide key complex three-dimensional habitats for highly diverse biological communities. These globally distributed biodiversity hotspots are increasingly threatened by ongoing environmental changes, mainly ocean acidification and warming, with wide negative impacts anticipated in the years to come. These are superimposed upon major local stressors caused by direct destructive impacts, such as bottom trawling, which act synergistically in the deterioration of the rhodolith ecosystem health and function. Anticipating the potential impacts of future environmental changes on the rhodolith biome may inform timely mitigation strategies integrating local effects of bottom trawling over vulnerable areas at global scales. This study aimed to identify future climate refugia, as regions where persistence is predicted under contrasting climate scenarios, and to analyze their trawling threat levels. This was approached by developing species distribution models with ecologically relevant environmental predictors, combined with the development of a global bottom trawling intensity index to identify heavily fished regions overlaying rhodoliths. Our results revealed the importance of light, thermal stress and pH driving the global distribution of rhodoliths. Future projections showed poleward expansions and contractions of suitable habitats at lower latitudes, structuring cryptic depth refugia, particularly evident under the more severe warming scenario RCP 8.5. Our results suggest that if management and conservation measures are not taken, bottom trawling may directly threaten the persistence of key rhodolith refugia. Since rhodoliths have slow growth rates, high sensitivity and ecological importance, understanding how their current and future distribution might be susceptible to bottom trawling pressure, may contribute to determine the fate of both the species and their associated communities.
Highlights
This study aims to investigate the global future distributional shifts of the rhodolith biome under different climate change scenarios, using species distribution models, to identify crucial refugial regions providing persistent suitable habitat in the longterm
Benthic predictors for the annual average values of temperature, pH, light, nitrate, phosphate, salinity and currents, were downloaded from Bio-ORACLE (Tyberghein et al, 2012; Assis et al, 2017b) for the present and for two contrasting best and worst future scenarios: the Representative Concentration Pathway (RCP) 2.6 characterized by a large reduction of greenhouse emissions over time and RCP 8.5, where emissions are predicted to continue to increase over time (Moss et al, 2010)
Future projections varied depending on the climatic scenarios used, with RCP 8.5 causing the most profound distributional changes (Figures 3, 4)
Summary
In a world where climate is changing rapidly affecting the distribution of marine species (Cheung et al, 2009; Leadley et al, 2010; Poloczanska et al, 2013; Pecl et al, 2017), climate refugia, i.e., regions with stable habitat conditions through variable time intervals, may play a fundamental role in enabling population persistence, preserving local ecosystem functioning and serving as sources for the replenishment of impacted regions (Hewitt, 2004; Provan and Bennett, 2008; Keppel et al, 2012). A collapse of such species, already observed for coral reefs (Carpenter et al, 2008; Hudges et al, 2018), kelp forests (Wernberg et al, 2015; Assis et al, 2017a) and seagrass meadows (Marbà and Duarte, 2010; Arias-Ortiz et al, 2018), has produced cascading effects reducing local biodiversity levels and disrupting ecosystem services (García Molinos et al, 2015; Pecl et al, 2017) In this scope, climate refugia function as regions of climate stability providing effective conservation of global biodiversity (inter and intra-specific) under climate change (Keppel et al, 2012; Morelli et al, 2016, 2020). Analyses show that there are almost no pristine or unaffected ecosystems left globally, with overfishing and pollution being the primary threats to biodiversity (Costello et al, 2010; Halpern et al, 2015)
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