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

==== 2.4.2.3 Observed Changes in Community Composition Driven by Climate Change ====

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===== 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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===== 2.4.2.3.2 Freshwater systems: mechanistic drivers and responses =====

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Physical changes in lakes (see [[#2.3.3|Section 2.3.3]] ) have affected primary production (see [[#2.4.4.5|Section 2.4.4.5.2]] ), algal-bloom formation and composition, zooplankton and fish size distribution, and species composition ( [[#Urrutia-Cordero--2017|Urrutia-Cordero et al., 2017]] ; [[#Gozlan--2019|Gozlan et al., 2019]] ; [[#Seltmann--2019|Seltmann et al., 2019]] ). Declines in the abundance of cold-stenothermal species, particularly the Arctic charr ( ''Salvelinus alpinus'' ), coregonids and smelt, and increases in eurythermal fish (e.g., the thermo-tolerant carp ''Cyprinus carpio'' , common bream, pike perch, roach and shad) have been observed in northern temperate lakes associated with warming trends ( ''medium evidence, high agreement'' ) ( [[#Jeppesen--2012|Jeppesen et al., 2012]] ; [[#Jeppesen--2014|Jeppesen et al., 2014]] ). These changes increase predation pressure on zooplankton and reduce grazing pressure on phytoplankton, which may result in higher phytoplankton biomass ( [[#De%20Senerpont%20Domis--2013|De Senerpont Domis et al., 2013]] ; [[#Jeppesen--2014|Jeppesen et al., 2014]] ; [[#Adrian--2016|Adrian et al., 2016]] ). Reduction in lake mixing lowers the concentration of nutrients in the epilimnion and may lead to higher silicon-to-phosphorous ratios that negatively affect diatom growth ( [[#Yankova--2017|Yankova et al., 2017]] ) or overall primary productivity (see [[#2.4.4.5|Section 2.4.4.5.2]] ).

In a study of 1567 lakes across Europe and North America, [[#Kakouei--2021|Kakouei (2021)]] identified climate change as the major driver of increases in phytoplankton biomass in remote areas with minimal LULCC. Greater temperature variability can be more important than long-term temperature trends as a driver of zooplankton biodiversity ( [[#Shurin--2010|Shurin et al., 2010]] ). Reductions of winter severity attributed to anthropogenic climate change are increasing winter algal biomass, and motile and phototropic species, at the expense of mixotrophic species ( [[#Özkundakci--2016|Özkundakci et al., 2016]] ; [[#Hampton--2017|Hampton et al., 2017]] ).

Tropical lakes are prone to loss of deep-water oxygen due to lake warming, with negative consequences for their fisheries and their biodiversity ( [[#Lewis%20Jr--2000|Lewis Jr, 2000]] ; [[#Van%20Bocxlaer--2012|Van Bocxlaer et al., 2012]] ). Many ancient tropical lakes (Malawi, Tanganyika, Victoria, Titicaca, Towuti and Matano) hold thousands of endemic animal species ( [[#Vadeboncoeur--2011|Vadeboncoeur et al., 2011]] ).

Observed effects of climate change on freshwater invertebrates are variable ( [[#Knouft--2017|Knouft and Ficklin, 2017]] ). In glacier-fed streams globally, climate change has caused community turnover and changes in abundances in terms of increased generalist and decreased specialist species ( [[#Lencioni--2018|Lencioni, 2018]] ; [[#Cauvy-Fraunié--2019|Cauvy-Fraunié and Dangles, 2019]] ). In turn, dragonflies in flowing waters, monitored during the warming period from 1988 through 2006 in Europe, did not show consistent changes in their distribution ( [[#Grewe--2013|Grewe et al., 2013]] ), reviewed in Knouft and Ficklin (2017). Long-term trends in the species composition and community structure of stream macro-invertebrates, specifically a general trend for decreases in species characteristic of cold, fast-flowing waters and increases of thermophilic species typical of stagnant or slow-moving waters, have been attributed to climate change ( ''robust evidence, high agreement'' ) ( [[#Daufresne--2007|Daufresne et al., 2007]] ; [[#Chessman--2015|Chessman, 2015]] ). A study of 14 geothermal streams reported simplified food web structures and shortened pathways of energy flux between consumers and resources ( [[#O’Gorman--2019|O’Gorman et al., 2019]] ). Macrophytes benefit from rising water temperatures, but increased shading from increased phytoplankton biomass could offset this ( [[#Hossain--2016|Hossain et al., 2016]] ; [[#Short--2016|Short et al., 2016]] ; [[#Zhang--2017a|Zhang et al., 2017a]] ).

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===== 2.4.2.3.3 Emergence of novel communities and invasive species =====

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As climate change is increasing the movements of species into new areas, there is concern about how exotic species are being impacted, either by becoming invasive or by already invasive species gaining even more advantage over native species. Modelling predicts that the effects of climate warming on food web structure and stability favour the success of invading species ( [[#Sentis--2021|Sentis et al., 2021]] ). Both simulated warming experiments ( [[#Zettlemoyer--2019|Zettlemoyer et al., 2019]] ) and long-term observations ( [[#Willis--2010|Willis et al., 2010]] ) have found phenologies of exotic species to respond more adaptively to warming than those of natives; in the long-term observations, the success of exotics was attributed to their greater phenological responsiveness. In an expert assessment of the future relative importance of different drivers of the impacts of biological invasions, climate change was named as the most important driver in polar regions, second-most important in temperate regions (after trade/transport), and third-most important in the tropics (after trade/transport and human demography/migration) ( [[#Essl--2020|Essl et al., 2020]] ).

However, not all exotic species become invasive. As novel climate conditions develop, novel communities made up of new combinations of species are emerging as populations and species adapt and shift their ranges differentially, not always with negative consequences ( ''high confidence'' ) ( [[#Dornelas--2014|Dornelas et al., 2014]] ; [[#Evers--2018|Evers et al., 2018]] ; [[#Teixeira--2020|Teixeira and Fernandes, 2020]] ). Novel communities differ in composition, structure, function and evolutionary trajectories, as the proportions of specialists and generalists, native, introduced and range-shifting species change and species interactions are altered, ultimately affecting ecosystem dynamics and functioning ( [[#Lurgi--2012|Lurgi et al., 2012]] ; [[#Hobbs--2014|Hobbs et al., 2014]] ; [[#Heger--2019|Heger and van Andel, 2019]] ). The exact nature of novel communities is difficult to predict because species-level uncertainties propagate at the community level due to ecological interactions ( [[#Williams--2007|Williams and Jackson, 2007]] ). However, observations, experimental mesocosms ( [[#Bastazini--2021|Bastazini et al., 2021]] ), and theoretical models ( [[#Lurgi--2012|Lurgi et al., 2012]] ; [[#Sentis--2021|Sentis et al., 2021]] ) provide support that novel communities will continue to emerge with climate change ''(medium confidence)'' .

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