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

===== Case Study 2: Chytrid fungus and climate change =====

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Infection by the chytrid fungus, Bd ''(Batrachochytrium dendrobatidis),'' can cause chytridiomycosis in amphibians. Bd is widely distributed globally and has caused catastrophic disease in amphibians, associated with the decline of 501 species and extinction of a further 90 species, primarily in tropical regions of the Americas and Australia ( [[#Scheele--2019|Scheele et al., 2019]] ; [[#Fisher--2020|Fisher and Garner, 2020]] ). Bd successfully travelled with high-elevation Andean frog species as they expanded their elevational ranges upward, driven by regional warming, to &gt; 5200 m ( [[#Seimon--2017|Seimon et al., 2017]] ).

New findings since AR5 from controlled laboratory experiments (manipulating temperature, humidity and water availability), intensive analyses of observed patterns of infection and disease in nature, and modelling studies have led to an emerging consensus that interactions between chytrids and amphibians are climate-sensitive, and that the interaction of climate change and Bd has driven many of the globally observed declines and extinctions of ~90 amphibian species ( ''robust evidence'' , ''high agreement'' ) ( [[#Rohr--2010|Rohr and Raffel, 2010]] ; [[#Puschendorf--2011|Puschendorf et al., 2011]] ; [[#Rowley--2013|Rowley and Alford, 2013]] ; [[#Raffel--2015|Raffel et al., 2015]] ; [[#Sauer--2018|Sauer et al., 2018]] ; [[#Cohen--2019a|Cohen et al., 2019a]] ; [[#Sauer--2020|Sauer et al., 2020]] ; [[#Turner--2021|Turner et al., 2021]] ).

The ‘thermal mismatch hypothesis’ posits that vulnerability to disease should be higher at warm temperatures in cool-adapted species and higher at cool temperatures in warmth-adapted species and is generally supported ( [[#Pounds--2006|Pounds et al., 2006]] ). However, the most recent studies reveal more complex mechanisms underlying amphibian disease–climate change dynamics, including variation in thermal preferences among individuals in a single amphibian population ( ''robust evidence'' , ''high agreement'' ) ( [[#Zumbado-Ulate--2014|Zumbado-Ulate et al., 2014]] ; [[#Sauer--2018|Sauer et al., 2018]] ; [[#Cohen--2019b|Cohen et al., 2019b]] ; [[#Neely--2020|Neely et al., 2020]] ; [[#Sauer--2020|Sauer et al., 2020]] ).

Bd is not universally harmful; it has been recorded as endemic in frog populations that do not suffer disease, where it may be commensal rather than parasitic ( [[#Puschendorf--2006|Puschendorf et al., 2006]] ; [[#Puschendorf--2011|Puschendorf et al., 2011]] ; [[#Rowley--2013|Rowley and Alford, 2013]] ). Projections of future impacts are difficult, as the virulence is variable across Bd populations and dependent upon the evolutionary and ecological history and evolutionary potential of both a local amphibian population and the endemic or invading Bd ( ''robust evidence'' , ''high agreement'' ) ( [[#Retallick--2004|Retallick et al., 2004]] ; [[#Daskin--2011|Daskin et al., 2011]] ; [[#Puschendorf--2011|Puschendorf et al., 2011]] ; [[#Phillips--2013|Phillips and Puschendorf, 2013]] ; [[#Rowley--2013|Rowley and Alford, 2013]] ; [[#Zumbado-Ulate--2014|Zumbado-Ulate et al., 2014]] ; [[#Sapsford--2015|Sapsford et al., 2015]] ; [[#Voyles--2018|Voyles et al., 2018]] ; [[#Bradley--2019|Bradley et al., 2019]] ; [[#Fisher--2020|Fisher and Garner, 2020]] ; [[#McMillan--2020|McMillan et al., 2020]] ). Further, specific local habitats might serve as regional climate refugia from chytrid infection (e.g., hot and dry) ( ''medium evidence'' , ''high agreement'' ) ( [[#Zumbado-Ulate--2014|Zumbado-Ulate et al., 2014]] ; [[#Cohen--2019b|Cohen et al., 2019b]] ; [[#Neely--2020|Neely et al., 2020]] ; [[#Turner--2021|Turner et al., 2021]] ).

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