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

===== 2.4.2.3.1 Overall patterns of community change =====

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The most common type of community change takes the form of ''in situ'' decreases in cold-adapted species and increases in warm-adapted species ( [[#Bowler--2017|Bowler et al., 2017]] ; [[#Hughes--2018|Hughes et al., 2018]] ; [[#Kuhn--2019|Kuhn and Gégout, 2019]] ; [[#Feeley--2020|Feeley et al., 2020]] ). This process has lead to increases of species richness on mountaintops and decreased richness at adjacent lower elevations ( ''medium evidence'' , ''high agreement'' ) ( [[#Forister--2010|Forister et al., 2010]] ; [[#Steinbauer--2018|Steinbauer et al., 2018]] ). While it is also expected from observed range shifts of individual species that species richness should increase along tropical/temperate ecotones and along temperate/boreal ecotones, to date this has not been well documented. Lewthwaite et al ( [[#Lewthwaite--2022|Lewthwaite and Mooers, 2022]] ) documented a small increase in local richness across Canada for 265 species of butterflies, but the stronger effect was an homogenization across the region, with generalist species generally expanding into new sites and leading to lower Beta-diversity (lower diversity among sites). In a study of 66 bumble bee species across North America and Europe, Soroye et al ( [[#Soroye--2020|Soroye et al., 2020]] ) did not find the expected pattern, with most sites, regardless of latitude, declining in species richness, even when individual species benefited from warming or increased precipitation at some sites. Observed shifts in community composition have consequences for species’ interactions. Such indirect effects of climate change have been shown to often have greater impacts on species than the direct effects of climate itself, particularly for higher-level consumers ( [[#Cahill--2013|Cahill et al., 2013]] ; [[#Ockendon--2014|Ockendon et al., 2014]] ).

Analyses indicated that responses in range shifts and timing were lagging behind the changes expected from regional warming. This type of lag, where biological response is less than expected from known underlying physiology or general climatic limits, is called ‘climate debt’. Examples of climate debt, measured from community composition changes, come from birds and butterflies in Europe ( [[#Devictor--2012|Devictor et al., 2012]] ) and lowland forest herbaceous plants in France ( [[#Bertrand--2011|Bertrand et al., 2011]] ). The French study found that larger debts occurred in communities with warmer baseline conditions and that some of the apparent debt stemmed from the ability of species to tolerate warming ''in situ'' , so no range shift was observed.

Prominent changes in freshwater community composition, such as increases in cyanobacteria and warm-tolerant zooplankton species, the loss of cold-water fish, gains in thermo-tolerant fish, macro-invertebrates, and floating macrophytes, are occurring ( ''medium evidence'' , ''high agreement'' , ''medium confidence'' ) ( [[#Adrian--2016|Adrian et al., 2016]] ; [[#Hossain--2016|Hossain et al., 2016]] ; [[#Short--2016|Short et al., 2016]] ; [[#Huisman--2018|Huisman et al., 2018]] ; [[#Gozlan--2019|Gozlan et al., 2019]] ). Geothermal streams have provided evidence about community structure and ecosystem function at high temperatures. A study of 14 such habitats reported simplified food web structures and shortened pathways of energy flux between consumers and resources ( ''high confidence'' ) ( [[#O’Gorman--2019|O’Gorman et al., 2019]] ). Changes in the relative abundance of species, species composition and biodiversity due to warming trends, and non-climate-driven changes are to be expected in lakes and rivers globally. However, thus far, empirical evidence and mechanistic understanding to inform modelling is too limited to draw general conclusions about the nature of current and future climate change-driven changes within entire food webs on a global scale ( [[#Urban--2016|Urban et al., 2016]] ).

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