Editing IPCC:AR6/WGII/Chapter-3 (section)

==== 3.3.3.2 Effects of Multiple Drivers on Animals ====

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When changing CO 2 concentrations affect marine ectotherms, they typically combine additively or synergistically with warming ( ''medium confidence'' ) (e.g., [[#Lefevre--2016|Lefevre, 2016]] ; [[#Reddin--2020|Reddin et al., 2020]] ; [[#Sampaio--2021|Sampaio et al., 2021]] ), and their cumulative effects can lead to detrimental, neutral or beneficial effects ( ''high confidence'' ) (Figure 3.9a; [[#Bennett--2017|Bennett et al., 2017]] ; [[#Büscher--2017|Büscher et al., 2017]] ; [[#Dahlke--2017|Dahlke et al., 2017]] ; [[#Foo--2017|Foo and Byrne, 2017]] ; [[#Johnson--2017b|Johnson et al., 2017b]] ; [[#Cominassi--2019|Cominassi et al., 2019]] ). Higher ocean CO 2 influences the thermal tolerance of species adapted to extreme but stable habitats in tropical and polar regions, more than that of thermally tolerant generalists ( ''high confidence'' ) ( [[#Byrne--2013|Byrne et al., 2013]] ; [[#Schiffer--2014|Schiffer et al., 2014]] ; [[#Flynn--2015|Flynn et al., 2015]] ; [[#Kunz--2016|Kunz et al., 2016]] ; [[#Pörtner--2017|Pörtner et al., 2017]] ; [[#Kunz--2018|Kunz et al., 2018]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; but see [[#Ern--2017|Ern et al., 2017]] ), especially in early life stages ( [[#Dahlke--2020a|Dahlke et al., 2020a]] ). In thermal generalists from temperate and subtropical species, warming and ocean acidification generally have detrimental effects on growth and survival (e.g., [[#Gao--2020|Gao et al., 2020]] ), but warming can also alleviate the detrimental effects of ocean acidification by increasing metabolic rate and/or growth ( [[#Garzke--2020|Garzke et al., 2020]] ), provided that other conditions (e.g., thermal niche, food availability) are beneficial. For example, larval growth and survival of Australasian snapper ( ''Pagrus auratus'' ) appear to benefit from combined acidification and warming (but see [[#Watson--2018|Watson et al., 2018]] ; [[#McMahon--2020|McMahon et al., 2020]] ), introducing major uncertainties to population modelling ( [[#3.3.4|Section 3.3.4]] ; [[#Parsons--2020|Parsons et al., 2020]] ).

As with ocean acidification, reduced oxygen availability further alters the influence of warming on metabolic rates ( ''high confidence'' ). Acidification and hypoxia can contribute to a decrease or shift in thermal tolerance, while the magnitude of this effect depends on the duration of exposure ( [[#Tripp-Valdez--2017|Tripp-Valdez et al., 2017]] ; [[#Cattano--2018|Cattano et al., 2018]] ; [[#Calderón-Liévanos--2019|Calderón-Liévanos et al., 2019]] ; [[#Schwieterman--2019|Schwieterman et al., 2019]] ). Warming and hypoxia are mostly positively correlated and tolerances to both phenomena are often linked after long-term acclimation (e.g., [[#Bouyoucos--2020|Bouyoucos et al., 2020]] ). Acute short-term heat shocks can impair hypoxia tolerance, for instance, in intertidal fish ( [[#McArley--2020|McArley et al., 2020]] ). This is relevant for shallow waters, specifically for MHWs ( [[#3.2.2.1|Section 3.2.2.1]] ; [[#Hobday--2016a|Hobday et al., 2016a]] ; [[#IPCC--2018|IPCC, 2018]] ; [[#Collins--2019a|Collins et al., 2019a]] ). Ocean acidification can increase hypoxia tolerance in some cases, possibly by downregulating activity ( [[#Faleiro--2015|Faleiro et al., 2015]] ) and/or changing blood oxygenation ( [[#Montgomery--2019|Montgomery et al., 2019]] ). Other studies, however, reported additive negative effects of acidification and warming on hypoxia tolerance ( [[#Schwieterman--2019|Schwieterman et al., 2019]] ; [[#Götze--2020|Götze et al., 2020]] ), in line with the oxygen- and capacity-limited thermal tolerance (OCLTT) hypothesis presented in AR5 ( [[#Pörtner--2014|Pörtner et al., 2014]] ): Warming causes increased metabolic rates and oxygen demand in ectotherms, which at some point exceed supply capacities (which also depend on environmental oxygen availability) and reduce aerobic scope. In consequence, expansion of OMZs and other regions where warming, hypoxia and acidification combine will further reduce habitat for many fish and invertebrates ( ''high confidence'' ) (Sections 3.4.3.2, 3.4.3.3).

Food availability modulates, and may be more influential than, other driver responses by affecting the energetic and nutritional status of animals ( [[#Cole--2016|Cole et al., 2016]] ; [[#Stiasny--2019|Stiasny et al., 2019]] ; [[#Cominassi--2020|Cominassi et al., 2020]] ). Laboratory studies conducted under an excess of food risk underestimating the ecological effects of climate-induced drivers, because increased feeding rates may help mitigate adverse effects ( [[#Nowicki--2012|Nowicki et al., 2012]] ; [[#Towle--2015|Towle et al., 2015]] ; [[#Cominassi--2020|Cominassi et al., 2020]] ). Lowered food availability from reduced open-ocean primary production (Sections 3.2.3.3, 3.4.4.2.1) will act as an additional driver, amplifying the detrimental effects of other drivers. However, warming and higher CO 2 availability may increase primary productivity in some coastal areas ( [[#3.4.4|Section 3.4.4.1]] ), ameliorating the adverse direct effects on animals (e.g., [[#Sswat--2018|Sswat et al., 2018]] ). Due to the few studies addressing food availability under multiple-driver scenarios ( [[#Thomsen--2013|Thomsen et al., 2013]] ; [[#Pistevos--2015|Pistevos et al., 2015]] ; [[#Towle--2015|Towle et al., 2015]] ; [[#Ramajo--2016|Ramajo et al., 2016]] ; [[#Brown--2018a|Brown et al., 2018a]] ; [[#Cominassi--2020|Cominassi et al., 2020]] ), there is ''medium confidence'' in its modulating effect on climate-induced driver responses.

Animal behaviour can be affected by ocean acidification, warming and hypoxia. While warming and hypoxia mostly induce avoidance behaviour, potentially leading to migration and habitat compression ( [[#3.4|Section 3.4]] ; [[#McCormick--2017|McCormick and Levin, 2017]] ; [[#Limburg--2020|Limburg et al., 2020]] ), the effects of acidification appear more complex. Some studies reported that acidification dominates behavioural effects ( [[#Schmidt--2017|Schmidt et al., 2017]] ), although outcomes vary with experimental design and duration of exposure ( ''low confidence, low agreement'' ) ( [[#Maximino--2010|Maximino and de Brito, 2010]] ; [[#Munday--2016|Munday et al., 2016]] ; [[#Laubenstein--2018|Laubenstein et al., 2018]] ; [[#Munday--2019|Munday et al., 2019]] ; [[#Sundin--2019|Sundin et al., 2019]] ; [[#Clark--2020|Clark et al., 2020]] ; [[#Munday--2020|Munday et al., 2020]] ; [[#Williamson--2021|Williamson et al., 2021]] ). Behaviour represents an integrated phenomenon that can be influenced both directly and indirectly by multiple drivers. For instance, increased ''p'' CO 2 can directly act on neuronal signalling pathways (e.g., Gamma-aminobutyric acid hypothesis; [[#Nilsson--2012|Nilsson et al., 2012]] ; [[#Thomas--2020|Thomas et al., 2020]] ) and influence learning ( [[#Chivers--2014|Chivers et al., 2014]] ), vision ( [[#Chung--2014|Chung et al., 2014]] ), and choice and escape behaviour ( [[#Watson--2014|Watson et al., 2014]] ; [[#Wang--2017b|Wang et al., 2017b]] ). There is further evidence that observed alterations in fish olfactory behaviour under ocean acidification may result from physiological and molecular changes of the olfactory epithelium, influencing olfactory receptors ( [[#Roggatz--2016|Roggatz et al., 2016]] ; [[#Porteus--2018|Porteus et al., 2018]] ; [[#Velez--2019|Velez et al., 2019]] ; [[#Mazurais--2020|Mazurais et al., 2020]] ). Temperature mainly drives metabolic processes and thus energetic requirements, which can indirectly influence behaviour, including increased risk-taking during feeding ( [[#Marangon--2020|Marangon et al., 2020]] ). Ocean warming also accelerates the biochemical reactions and metabolic processes that are primarily influenced by acidification. It is therefore difficult to generalise to what extent co-occurring ocean warming ameliorates or exacerbates effects of acidification on behaviour ( [[#Laubenstein--2019|Laubenstein et al., 2019]] ); outcomes depend upon species and life stage ( [[#Faleiro--2015|Faleiro et al., 2015]] ; [[#Chan--2016|Chan et al., 2016]] ; [[#Tills--2016|Tills et al., 2016]] ; [[#Wang--2018b|Wang et al., 2018b]] ; [[#Jarrold--2020|Jarrold et al., 2020]] ), interactions between species (e.g., [[#Paula--2019|Paula et al., 2019]] ) along with confounding factors including food availability and salinity ( ''medium confidence'' ) ( [[#Ferrari--2015|Ferrari et al., 2015]] ; [[#Pistevos--2015|Pistevos et al., 2015]] ; [[#Pimentel--2016|Pimentel et al., 2016]] ; [[#Pistevos--2017|Pistevos et al., 2017]] ; [[#Horwitz--2020|Horwitz et al., 2020]] ).

While hypoxia can dominate multiple-driver responses locally ( [[#Sampaio--2021|Sampaio et al., 2021]] ), warming is the fundamental physiological driver for most marine ectotherms, globally, as it directly affects their entire biochemistry and energy metabolism. Other influential drivers include ocean acidification, salinity ( ''high confidence'' ) ( [[#Lefevre--2016|Lefevre, 2016]] ; [[#Whiteley--2018|Whiteley et al., 2018]] ; [[#Reddin--2020|Reddin et al., 2020]] ) or food availability/quality ( ''medium confidence'' ) ( [[#Nagelkerken--2016|Nagelkerken and Munday, 2016]] ; [[#Gao--2020|Gao et al., 2020]] ). Fluctuating and decreasing salinity may aggravate the detrimental effects of warming and elevated CO 2 , because dilution with freshwater lowers acid–base buffering capacity, resulting in lower pH and calcium carbonate saturation state ( [[#Dickinson--2012|Dickinson et al., 2012]] ; [[#Shrivastava--2019|Shrivastava et al., 2019]] ; [[#Melzner--2020|Melzner et al., 2020]] ).

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