Editing IPCC:AR6/WGII/Cross-Chapter-Paper-5 (section)

==== CCP5.3.2.3 KR3: Risks of Ecosystem Change and Species Extinction ====

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Risks to mountain ecosystems and the services they provide to people are varied in magnitude, timing, likelihood and potential to adapt and place specific. However, many mountain ecosystems are already showing impacts of climate change (CCP5.3.1), reflecting the strong influence climate exerts in many situations and indicating that risks are significant and immediate and will ''likely'' increase in the near as well as long term. There is ''robust evidence'' ( ''high agreement'' ) of vegetation zones and individual species shifting to higher elevations (CCP5.2.1 and [[IPCC:Wg2:Chapter:Chapter-2#2.4|Section 2.4]] ), and projections indicate that current trends will continue and accelerate at higher rates of warming ( ''medium evidence, high agreement'' ) ( [[IPCC:Wg2:Chapter:Chapter-2#2.5|Section 2.5]] ).

Many mountain species are at risk of range contraction and ultimately extinction if dispersal at the upper range limit is slower than losses due to mortality at the lower range limit (observed for trees in the neotropics ( [[#Feeley--2013|Feeley et al., 2013]] ; [[#Duque--2015|Duque et al., 2015]] ) or if mountains are not high enough to allow species to move to higher elevations. [[#Ramirez-Villegas--2014|Ramirez-Villegas et al. (2014)]] modelled 11,012 species of birds and vascular plants in the Andes and found large decreases by 2050 (SRES-A2 scenario); in the absence of dispersal, 10% of species could become extinct. Even assuming unlimited dispersal, most of the Andean endemics would become severely threatened. Other modelling studies have also projected declines in a range of communities and species, including rare endemics ( [[#Zomer--2014a|Zomer et al., 2014a]] ; [[#Rashid--2015|Rashid et al., 2015]] ; [[#Bitencourt--2016|Bitencourt et al., 2016]] ; [[#Li--2017|Li et al., 2017]] ; [[#Rehnus--2018|Rehnus et al., 2018]] ; [[#Ashrafzadeh--2019|Ashrafzadeh et al., 2019]] ; [[#Zhang--2019b|Zhang et al., 2019b]] ; [[#Cuesta--2020|Cuesta et al., 2020]] ; [[#Hoffmann--2020|Hoffmann et al., 2020]] ).

Many treelines will continue to shift to higher elevations with increasing temperatures ( [[#Chhetri--2018|Chhetri and Cairns, 2018]] ), although very few are changing as fast as the climate ( [[#Liang--2016|Liang et al., 2016]] ; [[#Hansson--2021|Hansson et al., 2021]] ) and some are not moving or even shifting to lower elevations (CCP5.2.1). If treelines fail to shift uphill, this will pose a risk for species of the upper-montane forest that experience range contraction at their lower range limit but lack a suitable habitat to expand into beyond their upper range limit ( [[#Rehm--2015|Rehm and Feeley, 2015]] ). Changes in phenology can also pose risks to species and ecosystems (Chapter 2), including a potential desynchronisation of mutualistic relationships like pollination and increased freezing damage due to premature emergence from winter dormancy. In European broadleaf trees, for example, the upper elevational limits of different species involve a trade-off between maximising growing season length and limiting the risk of spring freeze damage ( [[#Vitasse--2012|Vitasse et al., 2012]] ; [[#Körner--2016|Körner and Spehn, 2016]] ).

A wide range of mechanisms can cause changes within ecological communities, some of which are hard to predict, but an increasing number of studies illustrate some of the risks which are expected to be most common. If treelines shift upwards, this will pose a risk for alpine species, which cannot compete with trees. This may lead to the extinction of alpine species on mountains where there is insufficient room for the alpine zone to shift uphill. Shifts in species distributions, and in particular shifts in ecosystem types, can cause changes in ecosystem function, which may in turn have cascading impacts on people, for example leading to increased exposure to diseases such as malaria at high elevation ( [[IPCC:Wg2:Chapter:Chapter-2#2.4.2.7.2|Section 2.4.2.7.2]] ) as vector distribution changes and wider impacts on ecosystem services ( [[IPCC:Wg2:Chapter:Chapter-2#2.5.3|Section 2.5.3]] ) such as water supply, flood alleviation and food.

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