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

==== 3.4.1.2 Impacts on biodiversity: Plant and wildlife ====

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''Plant biodiversity''

Over 20% of global plant biodiversity centres are located within drylands (White and Nackoney 2003 <sup>[[#fn:r618|618]]</sup> ). Plant species located within these areas are characterised by high genetic diversity within populations (Martínez-Palacios et al. 1999 <sup>[[#fn:r619|619]]</sup> ). The plant species within these ecosystems are often highly threatened by climate change and desertification (Millennium Ecosystem Assessment 2005b <sup>[[#fn:r620|620]]</sup> ; Maestre et al. 2012 <sup>[[#fn:r621|621]]</sup> ). Increasing aridity exacerbates the risk of extinction of some plant species, especially those that are already threatened due to small populations or restricted habitats (Gitay et al. 2002 <sup>[[#fn:r622|622]]</sup> ). Desertification, including through land-use change, already contributed to the loss of biodiversity across drylands ( ''medium confidence'' ) (Newbold et al. 2015 <sup>[[#fn:r623|623]]</sup> ; Wilting et al. 2017 <sup>[[#fn:r624|624]]</sup> ). For example, species richness decreased from 234 species in 1978 to 95 in 2011 following long periods of drought and human driven degradation on the steppe land of south-western Algeria (Observatoire du Sahara et du Sahel 2013 <sup>[[#fn:r625|625]]</sup> ). Similarly, drought and overgrazing led to loss of biodiversity in Pakistan to the point that only drought-adapted species can now survive on the arid rangelands (Akhter and Arshad 2006 <sup>[[#fn:r626|626]]</sup> ). Similar trends were observed in desert steppes of Mongolia (Khishigbayar et al. 2015 <sup>[[#fn:r627|627]]</sup> ). In contrast, the increase in annual moistening of southern European Russia from the late 1980s to the beginning of the 21st century caused the restoration of steppe vegetation, even under conditions of strong anthropogenic pressure (Ivanov et al. 2018 <sup>[[#fn:r628|628]]</sup> ). The seed banks of annual species can often survive over the long term, germinating in wet years, suggesting that these species could be resilient to some aspects of climate change (Vetter et al. 2005 <sup>[[#fn:r629|629]]</sup> ). Yet, Hiernaux and Houérou (2006) <sup>[[#fn:r630|630]]</sup> showed that overgrazing in the Sahel tended to decrease the seed bank of annuals, which could make them vulnerable to climate change over time. Perennial species, considered as the structuring element of the ecosystem, are usually less affected as they have deeper roots, xeromorphic properties and physiological mechanisms that increase drought tolerance (Le Houérou 1996 <sup>[[#fn:r631|631]]</sup> ). However, in North Africa, long-term monitoring (1978–2014) has shown that important plant perennial species have also disappeared due to drought ( ''Stipa tenacissima and Artemisia herba alba'' ) (Hirche et al. 2018 <sup>[[#fn:r633|633]]</sup> ; Observatoire du Sahara et du Sahel 2013 <sup>[[#fn:r634|634]]</sup> ). The aridisation of the climate in the south of Eastern Siberia led to the advance of the steppes to the north and to the corresponding migration of steppe mammal species between 1976 and 2016 (Ivanov et al. 2018 <sup>[[#fn:r635|635]]</sup> ). The future projection of impacts on plant biodiversity is presented in Section 3.5.2.

''Wildlife biodiversity''

Dryland ecosystems have high levels of faunal diversity and endemism (MEA 2005 <sup>[[#fn:r636|636]]</sup> ; Whitford 2002 <sup>[[#fn:r637|637]]</sup> ). Over 30% of the endemic bird areas are located within these regions, which is also home to 25% of vertebrate species (Maestre et al. 2012 <sup>[[#fn:r638|638]]</sup> ; MEA 2005 <sup>[[#fn:r639|639]]</sup> ). Yet, many species within drylands are threatened with extinction (Durant et al. 2014 <sup>[[#fn:r640|640]]</sup> ; Walther 2016 <sup>[[#fn:r641|641]]</sup> ). Habitat degradation and desertification are generally associated with biodiversity loss (Ceballos et al. 2010 <sup>[[#fn:r642|642]]</sup> ; Tang et al. 2018 <sup>[[#fn:r643|643]]</sup> ; Newbold et al. 2015 <sup>[[#fn:r644|644]]</sup> ). The ‘grazing value’ of land declines with both a reduction in vegetation cover and shrub encroachment, with the former being more detrimental to native vertebrates (Parsons et al. 2017 <sup>[[#fn:r645|645]]</sup> ). Conversely, shrub encroachment may buffer desertification by increasing resource and microclimate availability, resulting in an increase in vertebrate species abundance and richness observed in the shrub-encroached arid grasslands of North America (Whitford 1997 <sup>[[#fn:r646|646]]</sup> ) and Australia (Parsons et al. 2017 <sup>[[#fn:r647|647]]</sup> ). However, compared to historically resilient drylands, these encroached habitats and their new species assemblages may be more sensitive to droughts, which may become more prevalent with climate change (Schooley et al. 2018 <sup>[[#fn:r648|648]]</sup> ). Mammals and birds may be particularly sensitive to droughts because they rely on evaporative cooling to maintain their body temperatures within an optimal range (Hetem et al. 2016 <sup>[[#fn:r649|649]]</sup> ) and risk lethal dehydration in water limited environments (Albright et al. 2017 <sup>[[#fn:r650|650]]</sup> ). The direct effects of reduced rainfall and water availability are ''likely'' to be exacerbated by the indirect effects of desertification through a reduction in primary productivity. A reduction in the quality and quantity of resources available to herbivores due to desertification under changing climate can have knock-on consequences for predators and may ultimately disrupt trophic cascades ( ''limited evidence, low agreement'' ) (Rey et al. 2017 <sup>[[#fn:r651|651]]</sup> ; Walther 2010 <sup>[[#fn:r652|652]]</sup> ). Reduced resource availability may also compromise immune response to novel pathogens, with increased pathogen dispersal associated with dust storms (Zinabu et al. 2018 <sup>[[#fn:r653|653]]</sup> ). Responses to desertification are species-specific and mechanistic models are not yet able to accurately predict individual species’ responses to the many factors associated with desertification (Fuller et al. 2016 <sup>[[#fn:r654|654]]</sup> ).

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