Editing IPCC:AR6/WGII/Chapter-15 (section)

==== 15.3.3.3 Impacts on Terrestrial Biodiversity Systems ====

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Despite encompassing approximately 2% of the Earth’s terrestrial surface, oceanic and other high-endemicity islands are estimated to harbour substantial proportions of existing species including ~25% extant global flora, ~12% birds and ~10% mammals ( [[#Alcover--1998|Alcover et al., 1998]] ; [[#Wetzel--2013|Wetzel et al., 2013]] ; [[#Kumar--2017|Kumar and Tehrany, 2017]] ). Islands also have higher densities of critically endangered species, hosting just under half of all species currently considered to be at risk of extinction ( [[#Spatz--2017a|Spatz et al., 2017a]] ; 2017b), hence making the loss of terrestrial biodiversity and related ecosystem services a KR (KR3) for small islands (Figure 15.5). Impacts from developing synergies between changing climate, natural and anthropogenic stressors on islands (Cross-Chapter Box DEEP in Chapter 17) could lead to disproportionate changes in global biodiversity. The most prominent drivers include: SLR, increasing intensities of extreme events (human activities—especially continuing/accelerating habitat destruction/degradation) and the introduction of invasive alien species (IAS) ( [[#Tershy--2015|Tershy et al., 2015]] ). When coupled with characteristic small island traits such as spatial and other resource limitations, these synergies play a critical role towards increasing the vulnerability of these insular ecosystems (Box [https://www.ipcc.ch/chapter/15#CCP1.1 CCP1.1] ). This is likely to hinder the adaptation response of terrestrial biota–increasing the risk of biodiversity loss and, in turn, impairing the resilience capacity of ecosystem functioning and services ( ''high confidence'' ) ( [[#Heller--2009|Heller and Zavaleta, 2009]] ; [[#Ferreira--2016|Ferreira et al., 2016]] ; [[#Vogiatzakis--2016|Vogiatzakis et al., 2016]] ).

Current observations of insular species response to climate change generally report geographic range shifts/reductions for species and vegetation associations in addition to resulting impacts on local ecology ( [[#Virah-Sawmy--2016|Virah-Sawmy et al., 2016]] ; [[#Koide--2017|Koide et al., 2017]] ; [[#Maharaj--2019|Maharaj et al., 2019]] ). These include changes in plant/animal phenology and resulting community alterations such as for the common Mediterranean island species ''Quercus ilex'' (holly oak) and ''Ficus carica'' (common fig). Species have been shifting greater distances to access not only suitable climate conditions but also, by association, suitable breeding conditions and seasonal food. Examples include: migratory birds such as ''Coturnix coturnix'' now having earlier spring arrival dates in the Mediterranean compared to six decades ago and the increased mortality of the iconic ''Argyroxiphium sandwicense'' (Hinahina) as result of warmer drier trends at Hawaiian high altitudes ( [[#Krushelnycky--2012|Krushelnycky et al., 2012]] ; [[#Taylor--2016a|Taylor and Kumar, 2016a]] ; [[#Vogiatzakis--2016|Vogiatzakis et al., 2016]] ). There have also been die-offs of some species from temperature extremes (e.g., flying fox species: ''Pteropus'' species) within the Pacific islands ( [[#Taylor--2016a|Taylor and Kumar, 2016a]] ).

Recorded alterations of ecological interactions include increased competition, changes to migratory routes ( [[#Harter--2015|Harter et al., 2015]] ) and mismatches between species, such as increased pathogen attacks on Mediterranean forest species ( [[#Vogiatzakis--2016|Vogiatzakis et al., 2016]] ). Also, in some areas of Madagascar there has been increased vulnerability to fire, due to the replacement of succulents by less fire-resilient species ( [[#Virah-Sawmy--2016|Virah-Sawmy et al., 2016]] ). Further, the low functional redundancy of island ecosystems implies a comparatively higher proportion of keystone species than continents, many of them being endemic ( [[#Harter--2015|Harter et al., 2015]] ), with potentially unpredictable system consequences due to climate-induced ecological changes. For example, Caribbean land crabs have been observed to alter their food intake as a response to drying conditions ( [[#McGaw--2019|McGaw et al., 2019]] ) and Aldabra giant land tortoises have reduced their activity in response to increasing temperature and decreasing precipitation ( [[#Falcon--2018|Falcon and Hansen, 2018]] ); such changes in both these ecosystem engineers are of potential consequence for seed dispersal, among other ecological functions.

The majority of studies modelling geographical range changes of small island species, to even the most optimistic 21st century climate change scenarios, imply a reduction in climate refugia (Table 15.3, Box [https://www.ipcc.ch/chapter/15#CCP1.1 CCP1.1] ). This is due to projected strong shifts, reductions or even complete losses of climatic niches resulting from inadequate geographic space for species to track suitable climate envelopes ( ''high confidence'' ) (e.g., [[#Maharaj--2013|Maharaj and New, 2013]] ; [[#Fortini--2015|Fortini et al., 2015]] ; [[#Struebig--2015b|Struebig et al., 2015b]] ). Because of the high proportion of global endemics hosted within small and especially isolated islands, the resulting increased extinction risk of such species (up to 100%) could lead to disproportionate losses in global biodiversity ( ''medium'' to ''high confidence'' ) ( [[#Harter--2015|Harter et al., 2015]] ; [[#Manes--2021|Manes et al., 2021]] ).

SLR has been projected to impact the terrestrial biodiversity of low-lying islands and coastal regions via large habitat losses both directly (e.g., submergence) and indirectly (e.g., salinity intrusion, salinisation of coastal wetlands and soil erosion) at even the 1-m scenario ( ''medium'' to ''high confidence'' ). However, these impacts vary depending on the islands’ topographical differences. In a study of SLR impacts on insular biodiversity hotspots, Bellard et al. (2013a) reported that the Caribbean islands, Sundaland and the Philippines were projected to suffer the most habitat loss while the East Melanesian islands were projected to be less (but not minimally) affected. The most threatened of these, the Caribbean, was projected to have between 8.7% and 49.2% of its islands entirely submerged, respectively, from 1-m to 6-m SLR ( [[#Bellard--2013a|Bellard et al., 2013a]] ). However, many current projection studies consider marine flooding directly and seldom incorporate other indirect impacts such as increased habitat losses from horizontal erosion loss, increased salinity levels, tidal ranges and extreme events. These projections are considered to be conservative, underestimating the extent of habitat loss to terrestrial biodiversity ( [[#Bellard--2013b|Bellard et al., 2013b]] ).

Marine flooding is expected to destroy habitats of coastal species, particularly range-restricted coastal and/or single-island endemics (many already listed as ''at least'' ‘threatened’ by the International Union for Conservation of Nature) within the limited terrain on atoll islands. These species have limited opportunities to accommodate such direct impacts of climate change apart from shifting further inland or to other neighbouring atolls which might have favourable habitat. However, fragmentation of habitat due to anthropogenic activity may hinder migration further inland, while shifting to neighbouring islands is not viable due to the water barrier between islands ( ''high confidence'' ) ( [[#Bellard--2013b|Bellard et al., 2013b]] ; [[#Wetzel--2013|Wetzel et al., 2013]] ; [[#Kumar--2017|Kumar and Tehrany, 2017]] ). Additionally, migratory birds, which use small islands (e.g., atolls) for stopovers or breeding/nesting sites, are projected to become impacted. Within the Mediterranean and Caribbean, significant losses to coastal wetlands—critical habitat for migratory birds—has already been observed, with further significant habitat losses, redistribution and changes in quality being projected across island systems such as the Bahamas (Caribbean) and Sardinia (Mediterranean) ( [[#Vogiatzakis--2016|Vogiatzakis et al., 2016]] ; [[#Wolcott--2018|Wolcott et al., 2018]] ).

Indirect impacts of SLR may potentially result in equal or more biodiversity loss than direct impacts ( ''medium confidence'' ). Relocation of displaced coastal human populations and associated intensive agriculture and urban areas inland to natural habitat may result in greater biodiversity loss than direct impacts—especially on islands with large coastal populations and urban centres ( [[#Wetzel--2012|Wetzel et al., 2012]] ; [[#Bellard--2013b|Bellard et al., 2013b]] ). Given the dense population of insular hotspots (~31.8% of existing humans within ~15.9% of inhabited global land area) and the fact that on many islands, large proportions of human populations live within coastal regions, it has been suggested that immense impacts from such relocations should be factored into projection and adaptation studies ( [[#Wetzel--2012|Wetzel et al., 2012]] ).

Tropical island natural habitats/systems are highly vulnerable to extreme weather events such as TCs, due to their small size, unique ecological systems and often low socioeconomic capacity ( ''high confidence'' ) (Box 15.2; [[#Goulding--2016|Goulding et al., 2016]] ; [[#Schütte--2018|Schütte et al., 2018]] ). Growing evidence suggests high resilience of forest habitats ( [[#Keppel--2014|Keppel et al., 2014]] ; [[#Luke--2017|Luke et al., 2017]] ), especially within intact forest ecosystems to hurricanes and cyclones ( [[#Goulding--2016|Goulding et al., 2016]] ). While initial damage can be high, relatively fast recovery rates have been reported for both floral and faunal components of these ecosystems ( [[#Cantrell--2014|Cantrell et al., 2014]] ; [[#Shiels--2014|Shiels et al., 2014]] ; [[#Monoy--2016|Monoy et al., 2016]] ; [[#Richardson--2018|Richardson et al., 2018]] ). Within the Caribbean in particular, high resilience of forest types has been associated with the ''current'' intensity and return rate of hurricanes over the past 150 years.

It should, however, be underscored that these relatively fast recovery rates are associated with the ''present'' intensity and return rate of TCs. They do not reflect the impacts of increasingly intense events such as Hurricane Dorian (2019), which resulted in almost complete inundation of several low-lying islands of the Bahamas from storm surges. Severe weather events also have indirect effects on the biodiversity of islands—interacting synergistically with other stressors, such as increased invasion by non-native species and land use change. For example, TCs within Papua New Guinea resulted in the destruction of subsistence gardens, which led inhabitants to clear forest areas for new farming areas and for harvesting of timber resources to rebuild ( [[#Goulding--2016|Goulding et al., 2016]] ).

The most recent projections suggest that TC intensity is predicted to increase as climate continues to change ( [[#Walsh--2016|Walsh et al., 2016]] ; [[#Kossin--2017|Kossin et al., 2017]] ). There are too few studies available to suggest potential future response trends of these ecosystems to this increased intensity; however, it seems plausible that present resilience capacities may be adversely impacted ( ''medium confidence'' ) ( [[#Marler--2014|Marler, 2014]] ). Further, the potential for stressors such as forest fragmentation/degradation or IAS combining with these increasingly intense events to cause precipitating ecosystem cascades is a real concern ( [[#Goulding--2016|Goulding et al., 2016]] ).

Continued high rates of habitat loss and degradation have been reported for many small islands as natural habitats continue to be cleared to meet increasing demands upon natural resources from rising human populations, agriculture, urbanisation, unsustainable tourism, overgrazing and fires. This increases the vulnerability of ecosystems within especially oceanic islands—where isolation has given rise to high levels of endemism but simple biotic communities, with low functional redundancy (Box [https://www.ipcc.ch/chapter/15#CCP1.1 CCP1.1] ). There is ''high confidence'' that climate change may exacerbate the effects of this habitat loss upon the biodiversity of these islands as the climate refugia (Table 15.3) and the upslope shifts of range-restricted, dispersal-limited and poorly competitive species, confined within narrow latitudinal (and decreasing altitudinal) gradients, are increasingly challenged by fragmented and degraded landscapes (e.g., [[#Struebig--2015a|Struebig et al., 2015a]] ; [[#IPBES--2019|IPBES, 2019]] ). Additionally, high-altitude ecosystems such as cloud forests which harbour high levels of endemism are projected to shrink due to increasing atmospheric temperature and competition from upward-shifting lowland species ( [[#Taylor--2016a|Taylor and Kumar, 2016a]] ). These may ultimately increase the risk of multiple extinctions, negatively impacting upon global biodiversity levels ( ''high confidence'' ) ( [[#Taylor--2016a|Taylor and Kumar, 2016a]] ; [[#Portner--2021|Portner et al., 2021]] ).

Analyses of historical and current threats indicate that IAS and disease have been the primary drivers of insular extinctions in modern history ( [[#Bellard--2016|Bellard et al., 2016]] ). Impacts of IAS on islands are projected to increase with time due to synergies between climate change and other traditional drivers such as increasing global trade, tourism, agricultural intensification, overexploitation and urbanisation ( [[#Bellard--2014|Bellard et al., 2014]] ; [[#Russell--2017|Russell et al., 2017]] ). Changing climate conditions may not necessarily increase the rate of IAS introductions but is expected to improve chances of IAS establishment via (a) altering IAS transport and introduction mechanisms, (b) increasing the impacts and distributions of existing IAS and (ci) altering the effectiveness of existing control strategies ( [[#Hellmann--2008|Hellmann et al., 2008]] ; [[#Russell--2017|Russell et al., 2017]] ). These are likely to enhance IAS impacts on islands including: restructuring of ecological communities leading to declines and extinctions/extirpations in flora and fauna, habitat degradation, declining ecosystem functioning, services and resilience and, in extreme cases, potential community homogenisation ( ''high confidence'' ) ( [[#Russell--2017|Russell and Blackburn, 2017]] ; [[#IPBES--2019|IPBES, 2019]] ). Given the high degree of endemicity within oceanic islands and their associated vulnerabilities, such exacerbation by changing climate poses a serious threat to decreasing global biodiversity ( ''medium'' to ''high confidence'' ) ( [[#van%20Kleunen--2015|van Kleunen et al., 2015]] ).

Compared to continents, terrestrial IAS are disproportionately prevalent on islands (almost three quarters of global species currently threatened by IAS and disease are found on islands) and also generate stronger impacts (e.g., within alpine ecosystems of high islands) than on continents ( ''high confidence)'' ( [[#Bellard--2014|Bellard et al., 2014]] ; [[#Bellard--2016|Bellard et al., 2016]] ; [[#Frazier--2019|Frazier and Brewington, 2019]] ). [[#Russell--2017|Russell and Blackburn (2017)]] suggested a correlation between small island size and increased numbers of IAS. SIDS within the Indian Ocean and in particular the Pacific SIDS region were reported to have significantly more IAS ( ''medium confidence'' ), while the Caribbean and Atlantic SIDS have fewer numbers but faster accumulation of IAS. Finally, while there have been developments in the eradication of IAS on islands ( [[#Jones--2016|Jones et al., 2016]] ), there is sparse evidence and hence assessment of the degree to which measures designed to prevent introduction and to manage invasion pathways and establishment have been successful.

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