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

===== 3.4.3.3.2 Observed and projected changes in contemporary community structure and biodiversity =====

<div id="h4-10-siblings" class="h4-siblings"></div>

Ocean temperature is a major driver of species richness in the global ocean at evolutionary time scales ( [[#Tittensor--2010|Tittensor et al., 2010]] ; [[#Chaudhary--2021|Chaudhary et al., 2021]] ). This, together with temperature-driven range and phenology shifts evident across taxa and ocean ecosystems (Sections 3.4.3.1, 3.4.3.2), suggests that recent ocean warming ( [[#3.2.2.1|Section 3.2.2.1]] ) should alter biodiversity at regional to global scales. Since previous assessments (Table 3.20), the most common evidence supporting these expected changes is replacement of cold-adapted species by warm-adapted species within an ecosystem as waters warm ( [[#Worm--2021|Worm and Lotze, 2021]] ). Known as tropicalisation ( [[#3.4.2.3|Section 3.4.2.3]] ), this phenomenon has been attributed to ocean warming ( ''medium to high confidence'' ) in communities as diverse as kelp, invertebrates, plankton and fish ( [[#Burrows--2019|Burrows et al., 2019]] ; [[#Flanagan--2019|Flanagan et al., 2019]] ; [[#Ajani--2020|Ajani et al., 2020]] ; [[#Villarino--2020|Villarino et al., 2020]] ; [[#Punzón--2021|Punzón et al., 2021]] ; [[#Smith--2021|Smith et al., 2021]] ).

'''Table 3.20 |''' Summary of previous IPCC assessments of community composition and biodiversity

{| class="wikitable"
|-
! Observations

! Projections

|-
| ''AR5 ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ; [[#Pörtner--2014|Pörtner et al., 2014]] )''

|
|-
| The paleoecological record shows that global climate changes comparable in magnitudes to those projected for the 21st century under all scenarios resulted in large-scale biome shifts and changes in community composition, and that for rates projected under RCP6 and 8.5 those changes were associated with species extinctions in some groups ( ''high confidence'' ).

Loss of corals due to bleaching has a potentially critical influence on the maintenance of marine biodiversity in the tropics ( ''high confidence'' ).

| Spatial shifts of marine species due to projected warming will cause high-latitude invasions and high local-extinction rates in the tropics and semi-enclosed seas ( ''medium confidence'' ).

Species richness and fisheries catch potential are projected to increase, on average, at mid and high latitudes ( ''high confidence'' ) and decrease at tropical latitudes ( ''medium confidence'' ).

‘Shifts in the geographical distributions of marine species [...] cause changes in community composition and interactions [...]. Thereby, climate change will reassemble communities and affect biodiversity, with differences over time and between biomes and latitudes ( ''high confidence'' ).’

‘Models are currently useful for developing scenarios of directional changes in net primary productivity, species distributions, community structure, and trophic dynamics of marine ecosystems, as well as their implications for ecosystem goods and services under climate change. However, specific quantitative projections by these models remain imprecise ( ''low confidence'' ).’

|-
|
|-
| ''SROCC ( [[#Bindoff--2019a|Bindoff et al., 2019a]] )''

|
|-
| ‘Ocean warming has contributed to observed changes in biogeography of organisms ranging from phytoplankton to marine mammals ( ''high confidence'' ), consequently changing community composition ( ''high confidence'' ), and in some cases altering interactions between organisms and ecosystem function ( ''medium confidence'' ).’

| Poleward range shifts are projected to decrease species richness in tropical oceans, counterbalanced by increases in mid- to high-latitude regions, leading to global-scale species turnover ( ''medium confidence'' on trends, ''low confidence'' on magnitude because of model uncertainties and the limited number of published model simulations). ‘The projected intensity of species turnover is lower under low-emission scenarios ( ''high confidence'' ).’

‘Projections from multiple fish species distribution models show hotspots of decrease in species richness in the Indo-Pacific region, and semi-enclosed seas such as the Red Sea and Persian Gulf ( ''medium evidence, high agreement'' ). In addition, geographic barriers, such as land, [bounding the] poleward species range edge in semi-enclosed seas or low-oxygen water in deeper waters are projected to limit range shifts, resulting in a larger relative decrease in species richness ( ''medium confidence'' ).’

‘The large variation in sensitivity between different zooplankton taxa to future conditions of warming and ocean acidification suggests elevated risk on community structure and inter-specific interactions of zooplankton in the 21st century ( ''medium confidence'' ).’

|}

At local to regional scales, tropicalisation often increases species richness where warm-water species extend their ranges to overlap with existing communities and decreases species richness where warming waters extirpate species ( ''medium to high confidence'' ) ( [[#Friedland--2020a|Friedland et al., 2020a]] ; [[#Chaudhary--2021|Chaudhary et al., 2021]] ; [[#Worm--2021|Worm and Lotze, 2021]] ). Latitudinal estimates from catalogued observations show declining species richness in equatorial waters over the past 50 years, with concomitant increases in species richness at mid-latitudes; the pattern is especially prominent in free-swimming pelagic species (Figure 3.18; [[#Chaudhary--2021|Chaudhary et al., 2021]] ). Similar patterns among marine animals have been described previously for historical warming events ( [[#Song--2020b|Song et al., 2020b]] ). Tropicalisation is associated with increased representation of herbivorous species ( [[#Vergés--2016|Vergés et al., 2016]] ; [[#Zarco-Perello--2020|Zarco-Perello et al., 2020]] ; [[#Smith--2021|Smith et al., 2021]] ), although observations and theory suggest that dietary generalism can also favour range-shifting species ( [[#Monaco--2020|Monaco et al., 2020]] ; [[#Wallingford--2020|Wallingford et al., 2020]] ).

<div id="_idContainer068" class="Figure"></div>

[[File:775d59d4d5b1b9873ed3788209234bbd IPCC_AR6_WGII_Figure_3_018.png]]

'''Figure 3.18 |''' '''Changes in the latitudinal distribution of marine species richness.'''

'''(a)''' Observed species richness for three historical periods. The observed latitudinal patterns in species richness are for a suite of taxonomic groups based on 48,661 marine species ( [[#Chaudhary--2021|Chaudhary et al., 2021]] ).

'''(b)''' Projected changes in species richness under RCP4.5 and RCP8.5 are calculated as differences by grid cell by 2100 relative to 2006. Latitudinal global median (5° moving average). (Based on Figure 1b,c in [[#García%20Molinos--2016|García Molinos et al., 2016]] .) The projected latitudinal patterns in changes in species richness under climate change are based on a numerical model that includes species-specific information across a suite of taxonomic groups, based on 12,796 marine species ( [[#García%20Molinos--2016|García Molinos et al., 2016]] ).

At the community level, the magnitude and shape of projected future biodiversity changes differ depending on which groups are considered ( ''medium confidence'' ) ( [[#Chaudhary--2021|Chaudhary et al., 2021]] ). Molecular-based richness measures indicate that the most dramatic increases in diversity relative to current conditions are expected for photosynthetic eukaryotes and copepods in the Arctic Ocean ( [[#Ibarbalz--2019|Ibarbalz et al., 2019]] ). However, component eukaryotic taxa, for example diatoms ( [[#Busseni--2020|Busseni et al., 2020]] ), are projected to lose diversity by 2100 under RCP8.5. Ecosystem models project a decline in nutrient supply that drives the disappearance of less-competitive and larger phytoplankton types, leading to extinction of up to 30% of diatom types, particularly in the Northern Hemisphere, by 2100 under RCP8.5 ( [[#Henson--2021|Henson et al., 2021]] ). Models further suggest that high latitudes are ''likely'' to encounter entirely novel phytoplankton communities by 2100 under RCP8.5 (100% change in community composition; [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ; [[#Reygondeau--2020|Reygondeau et al., 2020]] ). At the polar edges, the increased richness is projected to coincide with high species turnover and increasing dominance of smaller phytoplankton types ( [[#Henson--2021|Henson et al., 2021]] ). These imply pronounced changes to both the oceans’ ecological and biogeochemical function, as regions dominated by small phytoplankton typically support less-productive food webs ( [[#3.4.3.4|Section 3.4.3.4]] ; [[#Stock--2017|Stock et al., 2017]] ; [[#Armengol--2019|Armengol et al., 2019]] ) and sequester less particulate organic carbon (POC) in the deep ocean ( [[#3.4.3.5|Section 3.4.3.5]] ; [[#Mouw--2016|Mouw et al., 2016]] ; [[#Cram--2018|Cram et al., 2018]] ) than areas dominated by larger size classes ( ''high confidence'' ).

The profound climatic and environmental changes projected for the Arctic region by 2100 (Cross-Chapter Paper 6) are also anticipated to alter the composition of apex assemblages like marine mammals (see Box 3.2; [[#Albouy--2020|Albouy et al., 2020]] ). Under both RCP2.6 and 8.5 scenarios the most vulnerable marine mammal species will be the North Pacific right whale ( ''Eubalaena japonica'' , listed as an endangered species; [[#IUCN--2020|IUCN, 2020]] ) and the grey whale ( ''Eschrichtius robustus'' , which has critically endangered subpopulations; [[#IUCN--2020|IUCN, 2020]] ). The extinction of the most-vulnerable species will disproportionately eliminate unique and important evolutionary lineages as well as functional diversity, with consequent impacts throughout the entire marine ecosystem ( [[#3.3.4|Section 3.3.4]] ). More generally, future warming and acidification simulated in mesocosm experiments support projections of a substantial increase in biomass and productivity of primary producers and secondary consumers, but a decrease by &gt;40% of primary consumers ( [[#Nagelkerken--2020|Nagelkerken et al., 2020]] ). On longer time scales, alteration of energy flow through marine food webs may lead to ecological tipping points ( [[#Wernberg--2016|Wernberg et al., 2016]] ; [[#Harley--2017|Harley et al., 2017]] ) after which the food web collapses into shorter, bottom-heavy trophic pyramids ( ''medium confidence'' ).

Global projections anticipate a ''likely'' future reorganisation of marine life of variable magnitude, contingent on emission scenario ( [[#Beaugrand--2015|Beaugrand et al., 2015]] ; [[#Jones--2015|Jones and Cheung, 2015]] ; [[#Barton--2016|Barton et al., 2016]] ; [[#García%20Molinos--2016|García Molinos et al., 2016]] ; [[#Nagelkerken--2020|Nagelkerken et al., 2020]] ; [[#Henson--2021|Henson et al., 2021]] ). Marine organism redistributions projected under RCP4.5 and RCP8.5 include extirpations and range contractions in the tropics, strongly decreasing tropical biodiversity, and range expansions at higher latitudes, associated with increased diversity and homogenisation of marine communities (Figure 3.18b). Under continuing climate change, the projected loss of biodiversity may ultimately threaten marine ecosystem stability ( ''medium confidence'' ) ( [[#Albouy--2020|Albouy et al., 2020]] ; [[#Nagelkerken--2020|Nagelkerken et al., 2020]] ; [[#Henson--2021|Henson et al., 2021]] ), altering both the functioning and structure of marine ecosystems and thus affecting service provisioning ( ''medium confidence'' ) ( [[#3.5|Section 3.5]] ; [[#Ibarbalz--2019|Ibarbalz et al., 2019]] ; [[#Righetti--2019|Righetti et al., 2019]] ).

However, biodiversity observations remain sparse, and statistical and modelling tools can provide conflicting diversity information (e.g., [[#Righetti--2019|Righetti et al., 2019]] ; [[#Dutkiewicz--2020|Dutkiewicz et al., 2020]] ) because correlative approaches assume that the modern-day relationship between marine species distribution and environmental conditions remains the same into the future, whereas mechanistic models permit marine species to respond dynamically to changing environmental forcing. Moreover, existing global projections of future biodiversity disproportionately focus on the effects sea surface temperature ( [[#Thomas--2012|Thomas et al., 2012]] ), typically overlooking other factors such as ocean acidification, deoxygenation and nutrient availability ( [[#3.2.3|Section 3.2.3]] ), and often failing to account for natural adaptation (e.g., [[#3.3.4|Section 3.3.4]] ; see Box 3.1; [[#Barton--2016|Barton et al., 2016]] ; [[#Henson--2021|Henson et al., 2021]] ).

<div id="3.4.3.3.3" class="h4-container"></div>

<span id="abrupt-ecosystem-shifts-and-extreme-events"></span>