Large conservation opportunities exist in >90% of tropic-subtropic coastal habitats adjacent to cities

Abstract

•Urban areas have <1% of coastal habitat within their extent•Large conservation opportunities for coastal habitats within 50 km from urban areas•Conservation potential for coastal habitats is high in Asia and Africa•Nature-based solutions (NBS) have co-benefits for society and biodiversity Coastal areas are urbanizing rapidly, currently supporting >40% of the world's population. Yet, coastal cities are facing greater vulnerability to climate-change-related hazards (e.g., flooding), causing significant social and economic disruptions. Investing in the protection of existing coastal habitats is considered a low-cost nature-based solution (NBS) with co-benefits for society and biodiversity. In this study, we examine the global applicability of this solution across 5,096 coastal urban areas in tropical and subtropical regions. We discover large conservation opportunities for urban areas within an adjacent 50 km buffer zone, where a large proportion of coastal habitat remains (93%). This work prompts actions of coastal habitat protection across coastal cities of Asia and Africa where nature still exists in close proximity to cities, threats from sea-level rise and storm surge are high, and populations are large and growing. This work aims to initiate a pathway whereby decision makers can evaluate interdisciplinary characteristics of an area to prescribe suitable NBS to efficiently address these global challenges. Coastal habitats have faced decades of loss caused by urbanization. Global recognition of the ecosystem services that coastal habitats provide has led to an emphasis on cities to adopt nature-based solutions (NBS). However, a broad assessment of urban areas and their potential to conserve remaining coastal habitat has not been undertaken. Here we apply spatial analytics to investigate 5,096 coastal urban areas in tropical and subtropical regions within the distribution of mangroves, tidal flats, seagrass meadows, and coral reefs, and find <50% of urban areas have natural coastal habitats within their extent. Large conservation opportunities for urban areas exist within an adjacent 50 km buffer zone where a significant proportion (93%) of urban-influenced coastal habitat lies and where 26% is currently protected. Potential high-conservation areas across the globe provide a unique opportunity to increase the resilience of urbanizing coasts and NBS for long-term socioeconomic and conservation goals. Coastal habitats have faced decades of loss caused by urbanization. Global recognition of the ecosystem services that coastal habitats provide has led to an emphasis on cities to adopt nature-based solutions (NBS). However, a broad assessment of urban areas and their potential to conserve remaining coastal habitat has not been undertaken. Here we apply spatial analytics to investigate 5,096 coastal urban areas in tropical and subtropical regions within the distribution of mangroves, tidal flats, seagrass meadows, and coral reefs, and find <50% of urban areas have natural coastal habitats within their extent. Large conservation opportunities for urban areas exist within an adjacent 50 km buffer zone where a significant proportion (93%) of urban-influenced coastal habitat lies and where 26% is currently protected. Potential high-conservation areas across the globe provide a unique opportunity to increase the resilience of urbanizing coasts and NBS for long-term socioeconomic and conservation goals. Coastal habitats provide unique ecological functions, benefits, and services.1Kallesøe M.F. Bambaradeniya C.N.B. Iftikhar U.A. Ranasinghe T. Miththapala S. 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Urbanization is a major cause of coastal habitat decline, and urban or otherwise modified coastlines cover as much as 3.4 × 106 km2.26Bugnot A.B. Mayer-Pinto M. Airoldi L. Heery E.C. Johnston E.L. Critchley L.P. Strain E.M.A. Morris R.L. Loke L.H.L. Bishop M.J. et al.Current and projected global extent of marine structures.Nat. Sustain. 2020; 4: 33-41Crossref Scopus (41) Google Scholar It is estimated that >40% of the global population lives within 100 km of the coast,27IOC/UNESCO, IMO, FAO, UNDPA Blueprint for Ocean and Coastal Sustainability. IOC/UNESCO, 2011Google Scholar,28Neumann B. Vafeidis A.T. Zimmermann J. Nicholls R.J. Future coastal population growth and exposure to sea-level rise and coastal flooding-a global assessment.PloS one. 2015; 11: e0118571Google Scholar and 15 of the world's 23 megacities (population >10 million) are located in low-lying coastal areas (<20 m above mean sea level29Blackburn S. Pelling M. Marques C. 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Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 2013: 1137-1216Google Scholar The predicted growth of urbanized coasts is expected to result in higher rates of habitat loss, fragmentation, and degradation, and highlights urban centers as a focal point for immediate conservation initiatives. Hence, knowledge deficits on the association between human settlements and coastal habitat loss and function need to be addressed before effective actions can be implemented.38Todd P.A. Heery E.C. Loke L.H. Thurstan R.H. Kotze D.J. Swan C. Towards an urban marine ecology: characterizing the drivers, patterns and processes of marine ecosystems in coastal cities.Oikos. 2019; 128: 1215-1242Crossref Scopus (88) Google Scholar Protecting natural habitats on urban coastlines remains a common goal, whether centered on biological conservation or socioeconomic demands, and has the potential to provide positive “win-win solutions”39Conti J. Cummin N. Gentry B. George L. Martin A. Molnar J. Rogers M. South J. Stoneburner L. Strategies for Operationalizing Nature-Based Solutions in the Private Sector. The Nature Conservancy Business Council, 2019https://www.nature.org/content/dam/tnc/nature/en/documents/NBSWhitePaper.pdfGoogle Scholar or “no-regret” options.18Seddon N. Chausson A. Berry P. Girardin C.A. Smith A. Turner B. Understanding the value and limits of nature-based solutions to climate change and other global challenges.Philos. Trans. R. Soc. B. 2020; 375: 20190120Crossref PubMed Scopus (214) Google Scholar,24Cohen-Shacham E. Walters G. Janzen C. Maginnis S. Nature-Based Solutions to Address Global Societal Challenges. IUCN, 2016Crossref Google Scholar Despite growing acknowledgment of the value of coastal habitats, a fundamental question has not been addressed: how much nature remains along urban coasts? Here we undertake three key investigations: (1) we assess the distribution of coastal habitats (mangroves, tidal flats, seagrasses, and coral reefs) associated with urban areas; (2) we examine the current protection status of those habitats; and (3) we evaluate conservation opportunities. We synthesize global datasets on urban areas, coastal habitats, and protected areas and use spatial analysis tools to quantify their association (see experimental procedures for details). Furthermore, we apply a conservation potential (CP) metric to determine conservation opportunity for urban coastal areas. Key findings include the limited proportion of coastal habitat within urban extents (<1%; with <50% of urban extents containing coastal habitat) and the identification of large conservation opportunities for urban areas within an adjacent 50 km buffer zone, where a significant portion (93%) of largely unprotected urban-influenced coastal habitat lies (only 26% is currently protected). Our results aim to provide insight into the extent of remaining coastal habitat across the urbanized landscape and guide conservation efforts toward areas of high CP, where large opportunities exist for coastal conservation and the application of NBS. We assessed the distribution of 5,096 urban areas in the LECZ (<10 m above sea level as defined by McGranahan et al.40McGranahan G. Balk D. Anderson B. The rising tide: assessing the risks of climate change and human settlements in low elevation coastal zones.Environ. Urban. 2007; 19: 17-37Crossref Scopus (1489) Google Scholar) across 129 marine ecoregions in tropical and subtropical regions where our target coastal habitats exist (Figure 1A). Urbanized areas were defined by the Global Rural-Urban Mapping Project (GRUMPv141CIESIN—Center for International Earth Science Information Network Columbia University - CUNY Institute for Demographic Research—CIDR International Food Policy Research Institute—IFPRI The World Bank Centro Internacional de Agricultura tropical—CIATGlobal Rural-Urban Mapping Project, Version 1 (GRUMPv1): Urban Extent Polygons, Revision 01. Socioeconomic Data and Applications Center (SEDAC), 2017https://doi.org/10.7927/H4Z31WKFCrossref Google Scholar,42Balk D.L. Deichmann U. Yetman G. Pozzi F. Hay S.I. Nelson A. Determining global population distribution: methods, applications and data.Adv. Parasitol. 2006; 62: 119-156Crossref PubMed Scopus (346) Google Scholar), which distinguishes urban areas by combining the presence of contiguous night-time lights (data extracted from meteorological satellites) with population count data (>5,000 people) and settlement points. To determine the amount of coastal habitat (mangrove, tidal flats, seagrass, and coral reef) associated with an urban area, we used three criteria (Figure 1B): (1) urban extent—area of habitat that intersects the urban extent (polygon area defined by GRUMPv141CIESIN—Center for International Earth Science Information Network Columbia University - CUNY Institute for Demographic Research—CIDR International Food Policy Research Institute—IFPRI The World Bank Centro Internacional de Agricultura tropical—CIATGlobal Rural-Urban Mapping Project, Version 1 (GRUMPv1): Urban Extent Polygons, Revision 01. Socioeconomic Data and Applications Center (SEDAC), 2017https://doi.org/10.7927/H4Z31WKFCrossref Google Scholar,42Balk D.L. Deichmann U. Yetman G. Pozzi F. Hay S.I. Nelson A. Determining global population distribution: methods, applications and data.Adv. Parasitol. 2006; 62: 119-156Crossref PubMed Scopus (346) Google Scholar) with no buffer applied; (2) 10 km buffer zone—urban extent and a 10 km buffer extending the boundary of the urban area polygon; and (3) 50 km buffer zone—urban extent and a 50 km buffer extending the boundary of the urban area polygon. A 50 km buffer zone was analyzed given the reported limit (50 km Euclidean distance from urban boundaries) of impacts and ecological interactions between urban and protected areas.43McDonald R.I. Forman R.T. Kareiva P. Neugarten R. Salzer D. Fisher J. Urban effects, distance, and protected areas in an urbanizing world.Landsc. Urban Plan. 2009; 93: 63-75Crossref Scopus (128) Google Scholar, 44Güneralp B. Seto K.C. Futures of global urban expansion: uncertainties and implications for biodiversity conservation.Environ. Res. Lett. 2013; 8: 014025Crossref Scopus (180) Google Scholar, 45The Nature ConservancyNature in the urban century assessment.https://www.nature.org/en-us/what-we-do/our-insights/perspectives/nature-in- the-urban-century/Date: 2018Google Scholar A 10 km buffer was considered as the semi-rural area most vulnerable to population expansion. Buffer distances were also chosen to be consistent with common usage within other literature.45The Nature ConservancyNature in the urban century assessment.https://www.nature.org/en-us/what-we-do/our-insights/perspectives/nature-in- the-urban-century/Date: 2018Google Scholar According to our three criteria, most urban areas (85.4%) have some associated natural coastal habitat (Figure 2A). However, less than half (48.5%) of the urban areas in our study have natural coastal habitat within the urban extent, and of these the majority (91%) contain only a small area of up to 10 km2 each (Figure 2A). Importantly, the low proportion of habitat found within the urban extent is also attributed to the inclusion of coral reefs and seagrass meadows, which are subtidal features occurring further offshore. Thus, our results may under-represent coastal habitats given the urban extent boundaries used in the analysis. A greater portion of urban areas have coastal habitat associated within the 10 km and 50 km buffer zones (68.6% and 85.4%, respectively; Figure 2A). The majority of urban areas (76.8%) with associated habitat in the 10 km buffer zone have up to 100 km2 of habitat each, whereas for urban areas that contain habitat within the 50 km buffer zone, more than half (55.1%) support habitat areas of >100 km2. Our results indicate extensive natural coastal habitat near urban areas (Figure 2), but only small amounts remain within the urban extent itself (25,229 km2, 1% of habitat in the study region; Table 1), with larger areas of coastal habitat only found within the wider 10 km (1,028,992 km2, 39%; Table 1) and 50 km (2,447,466 km2, 93%; Table 1) buffer zones. Tidal flats, followed by mangroves, are the most widespread habitats found in association with urban areas, although their habitat area is small (Figures 2B and 3). We acknowledge that this is also a result of their habitat distribution being more aligned to the coastal zone in comparison with seagrass meadows and coral reefs, which are located further offshore. The small area of mangroves within the urban extent supports observations of their fragmentation reported by Bryan-Brown et al.47Bryan-Brown D.N. Connolly R.M. Richards D.R. Adame F. Friess D.A. Brown C.J. Global trends in mangrove forest fragmentation.Sci. Rep. 2020; 10: 7117Crossref PubMed Scopus (63) Google Scholar Seagrass habitat was confined to a small portion of urban areas (27.3%; Figure 2B), occurring almost entirely (99%) within the 50 km urban buffer zone (Figure 3), with 42% in the 10 km buffer zone and 0.63% habitat in the urban extent.Table 1Amount of coastal habitat associated with urban areas across the tropics and subtropicsCoastal habitatUrban extentBuffer 10 kmBuffer 50 kmMean (km2)SD (km2)Total habitat (km2) (%)Mean (km2)SD (km2)Total habitat (km2) (%)Mean (km2)SD (km2)Total habitat (km2) (%)Mangrove1.096.335,786 (4%)9.1728.6538,602 (27%)75.53190.43109,645 (76%)Tidal flats0.763.894,158 (3%)9.9826.3740,433 (30%)83.33153.3595,239 (71%)Seagrass2.7048.0213,762 (0.6%)217.571,263.25931,135 (42%)1747.957,604.482,181,251 (99%)Coral reef0.305.261,523 (1%)4.2216.0818,822 (12%)26.0473.4461,331 (38%)Total25,229 (1%)1,028,992 (39%)2,447,466 (93%)The mean, standard deviation (SD), and total area of coastal habitat (km2) per urban area (n = 5,096) within the urban extent (urban areas defined by GRUMPv141CIESIN—Center for International Earth Science Information Network Columbia University - CUNY Institute for Demographic Research—CIDR International Food Policy Research Institute—IFPRI The World Bank Centro Internacional de Agricultura tropical—CIATGlobal Rural-Urban Mapping Project, Version 1 (GRUMPv1): Urban Extent Polygons, Revision 01. Socioeconomic Data and Applications Center (SEDAC), 2017https://doi.org/10.7927/H4Z31WKFCrossref Google Scholar,42Balk D.L. Deichmann U. Yetman G. Pozzi F. Hay S.I. Nelson A. Determining global population distribution: methods, applications and data.Adv. Parasitol. 2006; 62: 119-156Crossref PubMed Scopus (346) Google Scholar), 10 km buffer zone, and 50 km buffer zone (Figure 1B). The total amount of habitat per coastal habitat type as found within the three urban area proximity measures, and the proportion (%) of this habitat area relative to the total amount of habitat within the study region. Note that habitat was not summed across urban areas to avoid potential overlap of habitat areas and inflation of calculated area; rather, habitat area was calculated using ArcGIS clip tools.48ESRIArcGIS Desktop: Release 10.8. Environmental Systems Research Institute, 2019Google Scholar Open table in a new tab Figure 3Current protection of urban coastal habitatsShow full captionPercentage of coastal habitat within our study region that overlaps with urban extents (area defined by GRUMPv141CIESIN—Center for International Earth Science Information Network Columbia University - CUNY Institute for Demographic Research—CIDR International Food Policy Research Institute—IFPRI The World Bank Centro Internacional de Agricultura tropical—CIATGlobal Rural-Urban Mapping Project, Version 1 (GRUMPv1): Urban Extent Polygons, Revision 01. Socioeconomic Data and Applications Center (SEDAC), 2017https://doi.org/10.7927/H4Z31WKFCros