Editing IPCC:AR6/WGII/Cross-Chapter-Paper-1 (section)

=== Box CCP1.1 | Climate change and terrestrial biodiversity hotspots on small islands ===

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Despite covering approximately 2% of the Earth’s land area, islands harbour more than 20% of extant terrestrial species ( [[#Wetzel--2013|Wetzel et al., 2013]] ). Islands have disproportionately higher rates of endemism and threat when compared to continents, with 80% of historical extinctions (since 1500 CE) having occurred on islands ( ''high confidence'' ) ( [[#Taylor--2016|Taylor and Kumar, 2016]] ; [[#Spatz--2017|Spatz et al., 2017]] ; [[#Dueñas--2021|Dueñas et al., 2021]] ). Current climate change projections suggest that insular species are particularly sensitive and, even at mild warming levels, substantial losses are expected ( ''high confidence'' ) ( [[#Pouteau--2016|Pouteau and Birnbaum, 2016]] ; [[#Taylor--2016|Taylor and Kumar, 2016]] ; [[#Dawson--2017|Dawson et al., 2017]] ; [[#Manes--2021|Manes et al., 2021]] ). Given islands’ characteristic high endemicity, current high threat levels and the fact that islands host almost half of all species currently considered to be at risk of extinction, especially at higher warming levels ( ''high confidence'' ) ( [[#Taylor--2016|Taylor and Kumar, 2016]] ; [[#Spatz--2017|Spatz et al., 2017]] ), further losses could contribute disproportionately to global biodiversity decline ( ''medium evidence, high agreement'' ) ( [[#Harter--2015|Harter et al., 2015]] ; [[#Pouteau--2016|Pouteau and Birnbaum, 2016]] ; [[#Manes--2021|Manes et al., 2021]] ).

The high vulnerability of terrestrial biodiversity on islands to global change can be explained by a number of limitations, characteristic of both islands and insular species. Older, isolated islands tend to have fewer species and lower functional redundancy but a higher proportion of endemism ( [[#Pouteau--2016|Pouteau and Birnbaum, 2016]] ; [[#Médail--2017|Médail, 2017]] ). Many of these islands contain species with inherently high sensitivity to environmental change (narrow habitat ranges, small population sizes, low genetic diversity and poor adaptive, dispersal and defensive capabilities) ( [[#Harter--2015|Harter et al., 2015]] ). Unlike continental environments, insular species often have limited opportunities for autonomous adaptation from not having enough geographic space to shift their ranges to track suitable climatic conditions ( ''high confidence'' ) ( [[#Fortini--2015|Fortini et al., 2015]] ; [[#Manes--2021|Manes et al., 2021]] ). Local extinction risks are amplified by even small losses of habitat due to global change including human-induced disturbances, extreme events, sea level rise (Chapter 15; Cross-Chapter Box SLR in Chapter 3) and invasive species.

However, some insular species have shown resilience to climate change. Intact island forests, for example, have shown rapid recovery rates after tropical cyclones, despite high levels of initial damage, especially in the Caribbean ( ''medium confidence'' ) ( [[#Luke--2016|Luke et al., 2016]] ; [[#Richardson--2018|Richardson et al., 2018]] ). Additionally, many Mediterranean islands are ‘disturbance adapted’, with continued persistence of some single-island endemic plants, despite exposure to multiple threats ( [[#Vogiatzakis--2016|Vogiatzakis et al., 2016]] ). This continued persistence has been attributed, at least partially, to climate refugia, oceanic buffering and high habitat heterogeneity within topographically complex mountainous regions ( [[#Pouteau--2016|Pouteau and Birnbaum, 2016]] ; [[#Médail--2017|Médail, 2017]] , Chapter 15, Table 15.1). However, this climate resilience will not be sustained under climate change, especially when coupled with habitat degradation ( ''high confidence'' ) ( [[#Wiens--2016|Wiens, 2016]] ).

Adaptation strategies depend on the ability to project future impacts from climate change, but this is hampered by lack of fine-scale climate data, especially for developing small island nations. There is a paucity of robust impacts-based modelling output for terrestrial biodiversity from these islands due to the wide, chronic unavailability of Regional Climate Model (RCM) data premised on the most recent suite of scenarios (RCPs and especially SSPs) ''(medium evidence'' , ''high agreement'' ) (Gutiérrez et al., 2021, Ch.15.8; [[#Pörtner--2021|Pörtner et al., 2021]] ; [[#WMO--2021|WMO, 2021]] ). Additionally, realistic assessments of changing climate on such small ecosystems require further RCM downscaling and verification to sub-island resolutions of &lt;5 km. Furthermore, widely used statistically (bias-corrected) downscaled data at sub-5 km resolutions, such as WorldClim are often unsuitable due to limited spatial and temporal resolutions of observation station data from small islands ( [[#Maharaj--2013|Maharaj and New, 2013]] ; Gutiérrez et al., 2021) and higher errors associated with statistical downscaling and locations with complex topography and coastlines ( [[#Fick--2017|Fick and Hijmans, 2017]] ; [[#Lanzante--2018|Lanzante et al., 2018]] ). Widespread unavailability of such data constrains accurate simulations of climatic variation within the small-scale mountainous and coastal regions of islands, associated with climate refugia and high habitat heterogeneity ( ''high confidence'' ) ( [[#Balzan--2018|Balzan et al., 2018]] ). This is a key element contributing to the continued delay in development of robust adaptation strategies towards not only biodiversity conservation but other important cross-sectoral issues ( ''medium confidence'' ) ( [[#Robinson--2020b|Robinson, 2020b]] ).

Due to islands’ limited size and isolation, conventional conservation measures focused on expanding protected areas, dispersal corridors and buffer zones are of limited effectiveness on islands ( ''high confidence'' ) ( [[#Vogiatzakis--2016|Vogiatzakis et al., 2016]] ). Instead, multifaceted, locally driven holistic climate-smart strategies across mosaics of human-impacted, often heavily degraded and fragmented, landscapes are required. These should ideally be long-term, flexible and sustainable solutions that incorporate social and biocultural knowledge as well as economic co-benefits to island communities in order to ‘buy time’ ( [[#Betzold--2015|Betzold, 2015]] ; [[#Robinson--2020a|Robinson, 2020a]] ). Examples include ecosystem-based approaches such as ridge-to-reef management ( [[#Struebig--2015|Struebig et al., 2015]] ; [[#Ferreira--2019|Ferreira et al., 2019]] , Figure CCP1.1 5.4), which incorporates conservation partnerships among lands inside and outside protected areas to increase connectivity and reduce land use impacts, while building on the interconnections among terrestrial, freshwater, coastal and marine ecosystems. Such strategies require raising awareness of biodiversity values among local communities, and cross-sectoral planning and policy at both island, regional and trans-boundary scales. These lend to private–public partnerships, increasing the potential of solutions reaching beyond protected areas boundaries and affecting socio-political change ( ''high confidence'' ) ( [[#Scobie--2016|Scobie, 2016]] ).

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Box CCP1.1

Limited terrain, natural, economic and data resources across small developing nation islands mean that unconstrained habitat destruction and degradation cannot be sustained, as this harms both people and the biodiversity upon which they depend. This limitation of resources compromises climate adaptation, which is often further complicated by varying governance and states of economic development ( [[#Petzold--2019|Petzold and Magnan, 2019]] ). With changing climate conditions, there is an increased urgency to re-think how progress can be measured, and to create opportunities building on synergies between disaster risk reduction, food security and social justice, so that islands can most benefit from their natural resources and biodiversity in a sustained manner (Box 15.2; [[IPCC:Wg2:Chapter:Chapter-15#15.3.4.4|Section 15.3.4.4]] ).

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