Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.4.2.6 Observed Changes to Physiology and Morphology Driven by Climate Change ====

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Impacts on species physiology in terrestrial and freshwater systems have been observed, and attributed to climate change ( ''medium confidence'' ). These include changes in tolerance to high temperatures ( [[#Healy--2012|Healy and Schulte, 2012]] ; [[#Gunderson--2015|Gunderson and Stillman, 2015]] ; [[#Deery--2021|Deery et al., 2021]] ), increased metabolic costs of living at elevated temperatures ( [[#Scheffers--2016|Scheffers et al., 2016]] ) and shifts in sex ratios in species with temperature-dependent sex determination. For example, warmer temperatures have driven the masculinisation of lizard populations ( [[#Schwanz--2008|Schwanz and Janzen, 2008]] ; [[#Schwanz--2016|Schwanz, 2016]] ; [[#Edmands--2021|Edmands, 2021]] ) and the feminisation of turtle populations ( [[#Telemeco--2009|Telemeco et al., 2009]] ). Skewed sex ratios can lead to mate shortages, reduced population growth, reduced adaptive potential and increased extinction risk, because genetic diversity decreases as fewer individuals mate and heterozygosity is lost ( [[#Mitchell--2010|Mitchell and Janzen, 2010]] ; [[#Edmands--2021|Edmands, 2021]] ).

Behavioural plasticity (flexibility) such as nest-site selection can provide a partial buffer from the effects of increasing temperature by placing the individual in a slightly cooler microclimate, but there are environmental and physical limits to this plasticity ( ''medium confidence'' ) ( [[#Refsnider--2016|Refsnider and Janzen, 2016]] ; [[#Telemeco--2017|Telemeco et al., 2017]] ). Plasticity in heat tolerance (e.g., due to reversible acclimation or acclimatisation) can also potentially compensate for rising temperatures ( [[#Angilletta%20Jr--2009|Angilletta Jr, 2009]] ), but ectotherms have relatively low acclimation in thermal tolerance and acclimation is expected to only slightly reduce the risk of overheating in even the most plastic taxa ( ''low confidence'' ) ( [[#Gunderson--2015|Gunderson and Stillman, 2015]] ).

Geographic variation in thermal tolerance plasticity is expected to influence the vulnerability and range shifts of species in response to climate change ( [[#Gunderson--2015|Gunderson and Stillman, 2015]] ; [[#Sun--2021|Sun et al., 2021]] ). In many ectotherms, plasticity in thermal tolerance increases polewards, as thermal seasonality increases ( [[#Chown--2004|Chown et al., 2004]] ), contributing to higher vulnerability to warming in tropical organisms ( ''low confidence'' ) ( [[#Huey--2009|Huey et al., 2009]] ; [[#Campos--2021|Campos et al., 2021]] ). Some species have evolved extreme upper thermal limits at the expense of plasticity, reflecting an evolutionary trade-off between these traits ( [[#Angilletta--2003|Angilletta et al., 2003]] ; [[#Stillman--2003|Stillman, 2003]] ). The most heat-tolerant species, such as those from extreme environments, may therefore be at a greater risk of warming because of an inability to physiologically adjust to thermal change ( ''low confidence'' ) ( [[#Bozinovic--2011|Bozinovic et al., 2011]] ; [[#Overgaard--2014|Overgaard et al., 2014]] ; [[#Magozzi--2015|Magozzi and Calosi, 2015]] ).

Physiological changes have observable impacts on morphology, such as changes in body size (and length of appendages), and colour changes in butterflies, dragonflies and birds ( ''medium confidence'' ) ( [[#Galeotti--2009|Galeotti et al., 2009]] ; [[#Karell--2011|Karell et al., 2011]] ). However, trends are not always linear or consistent across realms, taxonomic groups or geographic regions ( [[#Gotanda--2015|Gotanda et al., 2015]] ). Some morphological changes arise in response to environmental changes, rather than as the result of genetic adaptation or selection for an optimal body type. For example, dietary changes associated with climate change have led to changes in chipmunk skull morphology ( [[#Walsh--2016|Walsh et al., 2016]] ).

Decreased body size has been suggested as a general response of species to climate change in freshwater species, given the temperature-related constraints of metabolism with larger size. Reduced body size in response to global warming has been documented for freshwater bacteria, plankton and fish, as well as a shift towards smaller species ( [[#Daufresne--2009|Daufresne et al., 2009]] ; [[#Winder--2009|Winder et al., 2009]] ; [[#Jeppesen--2010|Jeppesen et al., 2010]] ; [[#Crozier--2014|Crozier and Hutchings, 2014]] ; [[#Jeppesen--2014|Jeppesen et al., 2014]] ; [[#Farmer--2015|Farmer et al., 2015]] ; [[#Rasconi--2015|Rasconi et al., 2015]] ; [[#Woodward--2016|Woodward et al., 2016]] ). However, the lack of systematic empirical evidence in fresh waters, and confounding effects such as interactions between temperature, nutrient availability and predation, limit generalisations in attributing observed body size changes to climate change ''(low confidence)'' ( [[#Pomati--2020|Pomati et al., 2020]] Nutrients).

Evidence is weak for a consistent reduction in body size across taxonomic groups in terrestrial animals ( ''low confidence'' ) ( [[#Siepielski--2019|Siepielski et al., 2019]] ). Decreased body size in warmer climates (as higher surface area-to-volume ratios maximise heat loss) is expected, based on biogeographic patterns such as Bergmann’s rule, but both increases and decreases have been documented in mammals, birds, lizards and invertebrates and were attributed to climate change ( [[#Teplitsky--2014|Teplitsky and Millien, 2014]] ; [[#Gotanda--2015|Gotanda et al., 2015]] ; [[#Gardner--2019|Gardner et al., 2019]] ; [[#Hill--2021|Hill et al., 2021]] ). Contrasting patterns (increased body size) may be due to short-term modifications in selection pressures (e.g., changes to predation and competition), variation in life histories or as a result of interactions with climate variables other than temperature (e.g., changes to food availability along with rainfall changes) and other disturbances ( [[#Yom-Tov--2004|Yom-Tov and Yom-Tov, 2004]] ; [[#Gardner--2019|Gardner et al., 2019]] ; [[#Wilson--2019|Wilson et al., 2019]] ) or use of different body size measurements (linear vs. volumetric dimensions) ( [[#Salewski--2014|Salewski et al., 2014]] ).

Several lines of evidence suggest the evolution of melanism in response to climate change ( ''low confidence'' ), with colour changes associated with thermoregulation being demonstrated in butterflies ( [[#Zeuss--2014|Zeuss et al., 2014]] ; [[#MacLean--2016|MacLean et al., 2016]] ; [[#MacLean--2019a|MacLean et al., 2019a]] ), beetles ( [[#de%20Jong--1998|de Jong and Brakefield, 1998]] ; [[#Brakefield--2011|Brakefield and de Jong, 2011]] ; [[#Zvereva--2019|Zvereva et al., 2019]] ), dragonflies ( [[#Zeuss--2014|Zeuss et al., 2014]] ) and phasmids ( [[#Nosil--2018|Nosil et al., 2018]] ). Such changes may represent decreased phenotypic diversity and, potentially, genetic diversity ( ''low confidence'' ), but the consequences of climate change for the genetic structure and diversity of populations have not been widely assessed ( [[#Pauls--2013|Pauls et al., 2013]] ). Simplistically, the thermal melanism hypothesis suggests that lighter (higher-reflectance) individuals should be fitter and therefore be selected for in a warmer climate ( [[#Clusella-Trullas--2007|Clusella-Trullas et al., 2007]] ). However, several biotic (e.g., thermoregulatory requirements, predator avoidance and signalling) and abiotic (e.g., UV, moisture and inter-annual variability) factors interact to influence changes in colour, making attribution to climate change across species and broad geographic regions difficult ( [[#Kingsolver--2015|Kingsolver and Buckley, 2015]] ; [[#Stuart-Fox--2017|Stuart-Fox et al., 2017]] ; [[#Clusella-Trullas--2020|Clusella-Trullas and Nielsen, 2020]] ).

Interactions between morphological changes and changes in phenology may facilitate or constrain adaptation to climate change ( ''medium confidence'' ) ( [[#Hedrick--2021|Hedrick et al., 2021]] ). For example, advancing phenology in migratory species may impose selection on morphological traits (e.g., wing length) to increase migration speed. If advancing spring phenology results in earlier breeding, this may offset the effect of rising temperatures in the breeding range and reduce the effect of increasing temperature on body size ( [[#Zimova--2021|Zimova et al., 2021]] ). A study of 52 species of North American migratory birds, based on &gt;70,000 specimens, showed that spring migration phenology has advanced over the past 40 years, concurrent with widespread shifts in morphology (reduced body size and increased wing length), perhaps to compensate for the increased metabolic cost of flight as body size decreases ( [[#Weeks--2020|Weeks et al., 2020]] ).

A lack of understanding of physiological constraints and mechanisms remains a barrier to predicting many of the ecological effects of climate change ( [[#Bozinovic--2011|Bozinovic et al., 2011]] ; [[#Vázquez--2017|Vázquez et al., 2017]] ; [[#González-Tokman--2020|González-Tokman et al., 2020]] ). Many behavioural, morphological and physiological responses are highly species- and context-specific, making generalisations difficult ( [[#Bodensteiner--2021|Bodensteiner et al., 2021]] ). Recent advances in mechanistic understanding (from experiments), in process-based modelling (including micro-climates and developmental processes) ( [[#Carter--2021|Carter and Janzen, 2021]] ) and in the sophistication of niche models ( [[#Kearney--2009|Kearney et al., 2009]] ) have improved projections, but comprehensive tests of geographic patterns and processes in thermal tolerance and plasticity are still lacking, with studies limited to a few phylogenetically restricted analyses showing mixed results ( [[#Gunderson--2015|Gunderson and Stillman, 2015]] ). Improved understanding of the mechanistic basis for observed geographic patterns in thermal tolerance and plasticity is needed to identify the physiological limits of species, the potential for adaptation and the presence of evolutionary trade-offs, which will strongly influence population declines, species range shifts, invasive interactions and the success of conservation interventions ( [[#Cooke--2021|Cooke et al., 2021]] ; [[#Ryan--2021|Ryan and Gunderson, 2021]] ).

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