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

==== 3.7.3.1 Introduction ====

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The spread of invasive plants can be exacerbated by climate change (Bradley et al. 2010 <sup>[[#fn:r1603|1603]]</sup> ; Davis et al. 2000 <sup>[[#fn:r1604|1604]]</sup> ). In general, it is expected that the distribution of invasive plant species with high tolerance to drought or high temperatures may increase under most climate change scenarios ( ''medium to high confidence'' ) (Bradley et al. 2010 <sup>[[#fn:r1605|1605]]</sup> ; Settele et al. 2014 <sup>[[#fn:r1606|1606]]</sup> ; Scasta et al. 2015 <sup>[[#fn:r1607|1607]]</sup> ). Invasive plants are considered a major risk to native biodiversity and can disturb the nutrient dynamics and water balance in affected ecosystems (Ehrenfeld 2003 <sup>[[#fn:r1608|1608]]</sup> ). Compared to more humid regions, the number of species that succeed in invading dryland areas is low (Bradley et al. 2012 <sup>[[#fn:r1609|1609]]</sup> ), yet they have a considerable impact on biodiversity and ecosystem services (Le Maitre et al. 2015, 2011; Newton et al. 2011 <sup>[[#fn:r1610|1610]]</sup> ). Moreover, human activities in dryland areas are responsible for creating new invasion opportunities (Safriel et al. 2005 <sup>[[#fn:r1611|1611]]</sup> ).

Current drivers of species introductions include expanding global trade and travel, land degradation and changes in climate (Chytrý et al. 2012 <sup>[[#fn:r1612|1612]]</sup> ; Richardson et al. 2011 <sup>[[#fn:r1613|1613]]</sup> ; Seebens et al. 2018 <sup>[[#fn:r1614|1614]]</sup> ). For example, Davis et al. (2000) suggests that high rainfall variability promotes the success of alien plant species – as reported for semi-arid grasslands and Mediterranean-type ecosystems (Cassidy et al. 2004 <sup>[[#fn:r1615|1615]]</sup> ; Reynolds et al. 2004 <sup>[[#fn:r1616|1616]]</sup> ; Sala et al. 2006 <sup>[[#fn:r1617|1617]]</sup> ). Furthermore, Panda et al. (2018) demonstrated that many invasive species could withstand elevated temperature and moisture scarcity caused by climate change. Dukes et al. (2011) observed that the invasive plant yellow-star thistle ( ''Centaurea solstitialis'' ) grew six time larger under the elevated atmospheric CO <sub>2</sub> expected in future climate change scenarios.

Climate change is ''likely'' to aggravate the problem as existing species continue to spread unabated and other species develop invasive characteristics (Hellmann et al. 2008 <sup>[[#fn:r1619|1619]]</sup> ). Although the effects of climate change on invasive species distributions have been relatively well explored, the greater impact on ecosystems is less well understood (Bradley et al. 2010 <sup>[[#fn:r1620|1620]]</sup> ; Eldridge et al. 2011 <sup>[[#fn:r1621|1621]]</sup> ).

Due to the time lag between the initial release of invasive species and their impact, the consequence of invasions is not immediately detected and may only be noticed centuries after introduction (Rouget et al. 2016 <sup>[[#fn:r1622|1622]]</sup> ). Climate change and invading species may act in concert (Bellard et al. 2013 <sup>[[#fn:r1623|1623]]</sup> ; Hellmann et al. 2008 <sup>[[#fn:r1625|1625]]</sup> ; Seebens et al. 2015 <sup>[[#fn:r1626|1626]]</sup> ). For example, invasion often changes the size and structure of fuel loads, which can lead to an increase in the frequency and intensity of fire (Evans et al. 2015). In areas where the climate is becoming warmer, an increase in the likelihood of suitable weather conditions for fire may promote invasive species, which in turn may lead to further desertification. Conversely, fire may promote plant invasions via several mechanisms (by reducing cover of competing vegetation, destroying native vegetation and clearing a path for invasive plants or creating favourable soil conditions) (Brooks et al. 2004 <sup>[[#fn:r1627|1627]]</sup> ; Grace et al. 2001 <sup>[[#fn:r1628|1628]]</sup> ; Keeley and Brennan 2012 <sup>[[#fn:r1629|1629]]</sup> ).

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'''Figure 3.14'''

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'''Difference between the number of invasive alien species (n=99, from Bellard et al. (2013)) predicted to occur by 2050 (under A1B scenario) and current period ‘2000’ within the dryland areas'''
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[[File:f7e78c29625c2d159d611f7fff53955a Figure-3.14.png]]

Difference between the number of invasive alien species (n=99, from Bellard et al. (2013) <sup>[[#fn:r1808|1808]]</sup> ) predicted to occur by 2050 (under A1B scenario) and current period ‘2000’ within the dryland areas

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At a regional scale, Bellard et al. (2013) <sup>[[#fn:r1809|1809]]</sup> predicted increasing risk in Africa and Asia, with declining risk in Australia (Figure 3.14). This projection does not represent an exhaustive list of invasive alien species occurring in drylands.

A set of four case studies in Ethiopia, Mexico, the USA and Pakistan is presented below to describe the nuanced nature of invading plant species, their impact on drylands and their relationship with climate change.

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