Der Artikel geht auf die unmittelbare Bedrohung ein, die von einem Anstieg des Meeresspiegels für die Artenvielfalt der Wirbeltiere auf den Inseln Floridas ausgeht. Anhand von Prognosen zum Anstieg des Meeresspiegels wird die Überschwemmung von Küsteninseln und der potenzielle Verlust von Lebensraum für zahlreiche Arten vorhergesagt. Die Studie konzentriert sich auf die Anfälligkeit niedrig gelegener Inseln und die einzigartigen Herausforderungen, vor denen nichtflüchtige Arten stehen, die sich nicht so leicht an sich ändernde Bedingungen anpassen können. Die Autoren heben das potenzielle Aussterben mehrerer endemischer Arten hervor und diskutieren die Notwendigkeit von Schutzstrategien, um die Auswirkungen des Klimawandels auf diese empfindlichen Ökosysteme abzumildern. Die Forschung unterstreicht die Dringlichkeit, den Anstieg des Meeresspiegels anzugehen, um Floridas reiche Artenvielfalt und die einzigartigen Lebensräume zu erhalten, die er unterstützt.
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Abstract
Islands are some of the most biodiverse places on earth, but they are also hotspots of biodiversity loss. The coastline of Florida, U.S.A., is surrounded by thousands of islands, many of which are home to species that occur nowhere else. A rapidly emerging threat to these low-lying islands is inundation as sea levels rise. The capacity of island-dwelling species to adapt to climate change and sea level rise may be limited because many species do not have the ability to shift their distribution off the island to track favorable conditions. We assessed the vulnerability of Florida’s islands to inundation from sea level rise and estimated the terrestrial biodiversity on Florida’s islands that could be lost. Our models predicted that by 2100, over 80% and up to 90% of Florida’s islands could be completely inundated from sea level rise, depending on the sea level rise projection (1.2 m or 2.2 m). Of the 85 mammalian, reptilian, and amphibian species on our subset list of Florida’s Species of Greatest Conservation Need, over half occur on Florida’s islands for at least part of their range, highlighting the importance of these islands for housing Florida’s rich biodiversity. Notably, at least 12 mammal species and 7 reptile species have their entire distribution on Florida’s islands, and this count is likely an underestimate. Projections of future sea level rise mean that these island-endemic species face the threat of extinction in the wild if their island habitat is submerged.
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Introduction
The most biodiverse places on earth are also some of the most threatened (Myers et al. 2000; Roberts et al. 2002). Islands, in particular, are rich with endemic species (Kier et al. 2009; Fernández-Palacios et al. 2021); over 20% of the world’s biodiversity can be found on islands even though islands make up only ~ 5% of the earth’s land area (Mittermeier et al. 2011; Tershy et al. 2015). Islands worldwide face many threats, including habitat conversion, species introductions, disease, overexploitation, and extreme weather events (Simberloff 2000; Leclerc et al. 2018). Further, insular biota are particularly vulnerable to these threats because they tend to have small ranges, relatively low abundance, and low genetic diversity (Fernández-Palacios et al. 2021). As such, islands are centers of biodiversity loss: islands are home to 41% of the world’s highly threatened terrestrial vertebrate species and 80% of historically recorded species’ extinctions (Ricketts et al. 2005; Spatz et al. 2017).
Climate change is a growing threat to island ecosystems (Fordham and Brook 2010; Russell and Kueffer 2019; Veron et al. 2019). A rapidly emerging consequence of climate change for low-lying islands is inundation as sea levels rise (Noss 2011; Wetzel et al. 2013). Over the last 100 years, sea levels along the contiguous U.S. coast have risen by ~ 28 cm (11 inches; Sweet et al. 2022), and the rate is accelerating (Church and White 2011; Hay et al. 2015; Chen et al. 2017). Continued thermal expansion of ocean waters and the addition of water from melting glaciers and ice sheets mean that a further global mean sea level rise of 0.3–2.0 m by 2100 is projected (Sweet et al. 2022).
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Already, some islands are experiencing the effects of anthropogenic climate change. For example, in the last 20 years at least 3 islands that were once crucial pupping grounds for endangered Hawaiian Monk Seals (Neomonachus schauinslandi) and nesting grounds for threatened Green Sea Turtles (Chelonia mydas) in Hawaii, U.S.A. have disappeared (Antonelis et al. 2006; Baker et al. 2006). In 2016, the Bramble Cay Melomys (Melomys rubicola) became the first mammal pronounced extinct due to anthropogenic climate change and rising seas (Woinarski et al. 2017; Fulton 2017). In 2024, the Key Largo Tree Cactus (Pilosocereus millspaughii) became the first known species in the United States to be extirpated due to sea level rise (Possley et al. 2024). However, inundation of habitat is not the only consequence of rising seas for island ecosystems. Long before islands are under water, rising seas will cause saltwater intrusion, erosion, redistribution of sediment, and in some areas, could cause a transition from mesophytic upland forests and freshwater wetlands to halophilic vegetation (Ross et al. 1994, 2009), all contributing to an entire restructuring of coastal ecosystems (Burkett et al. 2008; Cazenave and Le Cozannet 2014). For island biota that do not have the ability to swim, fly, or distribute seeds across relatively expansive ocean, the expectation that species will adapt to climate change by shifting their distribution to follow favorable climatic and habitat conditions is unlikely (Courchamp et al. 2014; Taylor and Kumar 2016).
The Southeast U.S.A. is a global biodiversity hotspot (the North American Coastal Plain; Noss et al. 2015), and the state of Florida is home to some of the highest numbers of endemic bird, mammal, and reptile species in the United States (Jenkins et al. 2015). Florida’s coastline is surrounded by thousands of islands, many of which represent important habitat for nesting seabirds (Gore et al. 2007) and sea turtles (Bjorndal et al. 1983; Hays et al. 1995), and wintering habitat for migrant shorebirds (Nicholls and Baldassarre 1990; Sprandel et al. 2000). For several subspecies, these coastal islands are the only place on earth where they occur (e.g., Key Ring-Necked Snake (Diadophis punctatus acricus), Key Deer (Odocoileus virginianus clavium), Cedar Key Mole Skink (Plestiodon egregius insularis), and Sanibel Island Rice Rat (Oryzomys palustris sanibeli)). At less than 1.5 m above sea level on average (Kosovich 2008), most of the low-lying islands surrounding Florida’s coast are threatened by the myriad effects of climate change. Here, we assessed the vulnerability of Florida’s coastal islands to inundation from sea level rise and estimated the terrestrial vertebrate biodiversity on Florida’s islands that could be lost. Using rasters of predicted sea level rise, we predicted how many islands will disappear underwater completely, and how much of the total land area on islands will be inundated from sea level rise. Separately, we compiled a list of 85 reptile, amphibian, and mammal species on Florida’s list of Species of Greatest Conservation Need with some or all of their distribution on islands to estimate the insular vertebrate biodiversity that could be lost to sea level rise.
Methods
How vulnerable are Florida’s coastal islands to inundation?
Sea level rise projections
For all analyses, we used projections of sea level rise for United States coastal waters, published by the Sea Level Rise and Coastal Flood Hazard and Tools Interagency Task Force and the National Oceanic and Atmospheric Administration (NOAA), which represents the most up-to-date sea level rise projections out to the year 2150 (Sweet et al. 2022). These projections include five scenarios (low, intermediate-low, intermediate, intermediate-high, and high) based on historical tide gauge data and satellite altimetry and draw upon new green-house gas emission science from the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (Sweet et al. 2022). Projections incorporate local estimates of vertical land motion (i.e., subsistence or uplift of coastal land), projections of thermal water expansion from ocean–atmosphere warming, projections of increased ocean water mass from melting glaciers and ice sheets, and shifts in ocean current patterns (Sweet et al. 2022).
NOAA produced spatial predictions of the depth of inundation along the United States coast, as 5 m × 5 m rasters, given sea level rise above current mean higher high water (MHHW) in ~ 0.30 m increments (ranging from 0 to 3 m; Sweet et al. 2022, available for download at https://coast.noaa.gov/slrdata/). In other words, these raster datasets predict areas along the coast that would be inundated for every ~ 0.30 m rise in sea level. NOAA’s data are derived from 5 m resolution Lidar-based digital elevation models and NOAA’s tidal surface models, using a modified bathtub model that accounts for local tidal variability and hydrological connectivity. We used these raster datasets to calculate which island land pixels (within island polygons; see "Mapping Florida’s coastal islands") would be above or below water for each ~ 0.30 m rise in sea level. Because each raster represents a ~ 0.3 m incremental rise in sea level, they do not directly correspond to predictions per decade for each sea level rise scenario (i.e., low, intermediate-low, intermediate, intermediate-high, and high). For context, Sweet et al. (2022) predicted that sea levels for the contiguous United States will reach 1.2 m by 2100 under the intermediate scenario and 2.2 m by 2100 under the high scenario (Table S1).
Mapping Florida’s coastal islands
We mapped 2504 islands within 210 km of the coast of mainland Florida, U.S.A. using the United States Geological Survey’s 30 m resolution Global Shoreline Vector and Global Islands database (Sayre et al. 2018, Fig. 1). The islands in our analyses ranged in area from 774 m2 to 442 km2 (mean = 0.74 km2, SD = 9.64 km2); 94.4% of the islands had an area less than 1 km2. Sayre et al. (2018) derived the island polygons in the Global Islands database using a semi-automated classification of 2014 Landsat 7 satellite imagery. The island polygons were not tidally corrected (Sayre et al. 2018), meaning that island polygons derived from imagery obtained at low tide might appear larger in area than they would if the imagery was obtained at high tide.
Fig. 1
A subset of islands along the coast of Florida, U.S.A. Icons depict the approximate location of island-endemic species, subspecies, or populations (grouped by Order) on Florida’s list of Species of Greatest Conservation Need (SGCN, Table S2) that occur in the vicinity of each depicted island-group (numbers signify the number of species, subspecies, or populations within each Order that are endemic to that island group; lack of a number means one). The inset map of the panhandle region (top left) shows only a portion of the entire panhandle coast. Island-endemic species on the SGCN list in the panhandle region include the Perdido Key Beach Mouse and the Santa Rosa Beach Mouse; Cedar Keys region: Cedar Key Mole Skink; Sanibel Island region: Sanibel Island Rice Rat and Insular Hispid Cotton Rat; Anastasia Island region: Anastasia Island Beach Mouse; Merritt Island region: Southeastern Beach Mouse; Florida Keys: Key Largo Woodrat, Key Largo Cotton Mouse, Silver Rice Rat, Lower Keys Cotton Rat, Key Deer, Lower Keys Marsh Rabbit, Florida Keys Mole Skink, Key Ring-Necked Snake, and lower Keys populations of Florida Brown Snake, Cornsnake, Peninsula Ribbon Snake, and Striped Mud Turtle (Table S2). Note that most of Florida’s 2,504 coastal islands mapped in the United States Geological Survey’s 30 m resolution Global Islands database (Sayre et al. 2018) are too small to be visible on a map of the entire state
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We explored alternative spatial databases to identify island polygons, but each had its own limitations, such as polylines that could not consistently be converted into polygons or relatively large portions of Florida’s coast with missing data. Because the Global Islands database is global, it, too, had shortcomings when used for projects at smaller spatial extents, such as Florida. Some of Florida’s coastal islands were not included in the Global Islands database, including some islands connected to the mainland by bridges and some islands separated from the mainland by intercoastal waterways (e.g., Siesta Key and Clearwater Beach Island, FL), and thus our analyses did not include these islands. We do not have a count of how many of Florida’s coastal islands were not mapped in the dataset, but our qualitative scan of Florida’s coastline suggests that the majority of unmapped islands were connected to the mainland by a bridge. By not including these islands, our estimates of the proportion of inundated islands could be overestimated because unmapped islands likely have a higher elevation due to built-up human infrastructure. We omitted from the Global Islands database 286 islands in Franklin and Gulf counties, on the Gulf coast of Florida, and 5 islands from the lower Florida Keys region because corresponding NOAA sea level rise raster data were not available for these areas. Note that we excluded the aforementioned islands from our island inundation estimate (see "Predicting coastal island inundation" ), but we did not omit them from our count of the number of island-dwelling species that occur on islands (see "How many insular terrestrial species could be lost due to sea level rise?").
Predicting coastal island inundation
We used two metrics to predict the impact of sea level rise on Florida’s islands—how many islands we predict will disappear underwater completely, and how much of the total land area on islands we predict will be submerged. First, we used NOAA’s raster of sea level rise along the coast of Florida to calculate the proportion of each island (derived from the Global Islands database) predicted to be under water at each ~ 0.30 m increment of rise in sea level and recorded the water height at which the entire island would be inundated. Second, we estimated the total area of land on islands that would be under water at each ~ 0.30 m increment of sea level rise, relative to the current area of all mapped islands off the coast of Florida.
Next, we estimated how much of the land that we predict will remain above water is currently developed. Specifically, we estimated the proportion of islands above the predicted sea level rise of 1.2 m, which corresponds to the intermediate sea level rise scenario in 2100 (Sweet et al. 2022), that is already human modified by combining NOAA’s raster representing 1.2 m sea level rise (above current MHHW) with the Cooperative Land Cover (CLC; v 3.5) for Florida (10 m raster grids; Florida Fish and Wildlife Conservation Commission and Florida Natural Areas Inventory 2021). We considered all land-cover classes in the cultural-terrestrial category (class 1800) of the CLC to be human modified, which includes all residential, urban, industrial, mowed grass, orchards, plantations, crops, feeding operations, and transportation. For every island map pixel predicted to be above water in 2100, we identified whether it was human modified or not, according to the CLC. We note that this estimate is likely underestimated because the islands omitted from the Global Islands database were often connected to the mainland by a bridge and thus were more likely to have human-modified land (see "Mapping Florida’s coastal islands" ). We did not produce a corresponding estimate for the high scenario because NOAA’s set of predictive rasters were at 0.30 m increments and did not directly correspond to sea level rise projections for the high scenario as published in Sweet et al. (2022; Table S1).
We conducted all analyses in ArcMap 10.8.1 (Environmental Systems Research Institute, Redlands, CA, USA) and R (R Core Team 2023); we used the raster (Hijmans 2022), sf (Pebesma 2018), rgeos (Bivand et al. 2017), and rgdal (Bivand et al. 2022) packages for spatial analyses and dplyr (Wickham et al. 2022) and ggplot2 (Wickham 2016) for data manipulation and visualization, respectively.
How many insular terrestrial species could be lost due to sea level rise?
We quantified the number of at-risk, nonvolant terrestrial vertebrate species occurring on Florida’s coastal islands that could be lost due to island inundation from sea level rise. To do this, we assembled a list of mammalian (n = 30), reptilian (n = 38), and amphibian (n = 17) species classified by the Florida Fish and Wildlife Conservation Commission as Florida’s Species of Greatest Conservation Need (SGCN; Florida Fish and Wildlife Conservation Commission 2019). This list included species, subspecies, or isolated populations of species or subspecies that are native to Florida and designated as special concern, threatened, or endangered under the federal U.S. Endangered Species Act (ESA; United States Fish and Wildlife Service 2022) or at the state level under Florida’s Endangered and Threatened Species List (Florida Fish and Wildlife Conservation Commission 2021). The list also included biologically vulnerable taxa in Florida with rankings from NatureServe (i.e., S1, G1, or S2G2), Florida Fish and Wildlife Conservation Commission (FWC Species Ranking System biological score ≥ 27), or the International Union for Conservation of Nature (IUCN list as vulnerable or above; Florida Fish and Wildlife Conservation Commission 2019). We also included Lower Keys populations of three reptile species (Striped Mud Turtle Kinosternon baurii, Cornsnake Pantherophis guttatus, and Peninsula Ribbon Snake Thamnophis sauritus sackenii) that are no longer included on the current SGCN list because the distinctiveness of the subspecies are currently in question (Table S2). We included only nonvolant species that may be less likely to move from an island to find suitable habitat elsewhere when their preferred habitat on the island is inundated. We note that many nonvolant, island-dwelling species are able to swim among islands and may even thrive in regularly-flooded habitats (e.g., Silver Rice Rat Oryzomys palustris natator; Taillie et al. 2021). Our decision to exclude from our analysis birds, fishes, invertebrates, some mammal species (bats and marine mammals), and sea turtles does not, however, suggest that these taxa are not vulnerable to habitat loss from sea level rise. We did not include species such as the Florida Reef Gecko (Sphaerodactylus notatus notatus) that occurs on islands (Mays and Enge 2016) or the Mimic Glass Lizard (Ophisaurus mimicus) that has experienced substantial habitat loss, because they are not yet on the SGCN list, although both species meet the criteria for listing as threatened and endangered species in Florida (Clements et al. 2021; Stevenson et al. 2023).
After compiling our list of 85 species, we used published literature, government reports, occurrence records from the Global Biodiversity Information Facility (GBIF), or distribution information contained in biological status reviews to determine the number of mammalian, reptilian, and amphibian species from our list with some or all of their distribution on at least one of Florida’s coastal islands (Table S2, S3). We did not count species as insular if we could find only historical observations on islands with no evidence that they currently occur on islands (e.g., Long-Tailed Weasel (Mustela frenata); Table S2, S3), nor did we count species as insular if individuals of a species occasionally move on and off islands (e.g., Florida Panthers (Puma concolor coryi) have been observed on Keewaydin Island on Florida’s Gulf Coast (Bodine 2022). In our analysis, we classified species that occur on Merritt Island, FL as occurring on an island, with the exception of the Striped Newt (Notophthalmus perstriatus) because the Striped Newt breeding pond on Merritt Island is at the north end of Merritt Island National Wildlife Refuge, a portion that is connected to the mainland (Enge et al. 2015; Figure S1). Much of Merritt Island, however, is separated from the mainland by canals (Figure S1).
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We note that several species on Florida’s list of SGCN do not occur on Florida’s coastal islands per se but given the location and extent of their current distributions, the impact of sea level rise on their population(s) may be similar to that of Florida’s island-dwelling species. Specifically, the St. Andrew Beach Mouse (Peromyscus polionotus peninsularis) and the Choctawhatchee Beach Mouse (P. p. allophrys) occur entirely within small, isolated protected areas on coastal peninsulas that are connected to the mainland (Cronin et al. 2021). We did not include these species in our count of island-dwelling taxa, however, because they do not technically occur on islands.
Our analysis described above produces a count of the number of species on Florida’s list of SGCN that have some or all of their distribution on Florida’s islands and is not a direct prediction of the impact of sea level rise on their habitat or distributions. Indeed, most of these species are relatively rare, thus there is often a lack of fine-resolution distribution data. Our classification of species as occurring on Florida’s islands (or not) may change as more research and monitoring occurs; our analysis here reflects our current knowledge.
Results
By 2050, at least 35% of Florida’s coastal islands could be under water at high tide given both the intermediate and high sea level rise scenarios (0.4 m and 0.5 m predicted rise, respectively; Fig. 2). Our models predicted that by 2100, over 80% and up to 90% of Florida’s islands could be completely inundated, depending on the sea level rise projection (1.2 m intermediate or 2.2 m high scenarios; Fig. 2).
Fig. 2
The number of Florida’s islands, as a percent of the total number of islands in our analysis (n = 2504), that will be completely inundated as sea level rises, shown here in 0.3 m increments above the current mean higher high water (MHHW) with a Loess curve. Vertical lines indicate projections of sea level rise for the intermediate and high scenarios by 2050 and 2100 for the contiguous United States (Sweet et al. 2022). We estimated sea level rise with NOAA’s raster datasets (available for download at https://coast.noaa.gov/slrdata/) that use a modified bathtub model. We delineated island polygons using the Global Shoreline Vector and Global Islands database (Sayre et al. 2018)
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By 2050, over 42% of the current land area of Florida’s islands could be under water (both intermediate and high scenarios; Fig. 3). By 2100, with 1.2 m of sea level rise (as predicted by the intermediate scenario), over 66% of the land that is currently above water on islands will be under water; this estimate increases to over 76% by 2100 given the high scenario (Fig. 3). Of the 510 islands that are predicted to have land still above water in 2100 (intermediate scenario), an average of 13.1% (SD 29.8%, min = 0%, max = 100%) of that land is currently human-modified.
Fig. 3
The total area of land on Florida’s islands (as a percent of all island area) that will be under water as sea levels rise, shown here in 0.3 m increments above the current mean higher high water (MHHW) with a Loess curve. We estimated sea level rise with NOAA’s raster datasets (available for download at https://coast.noaa.gov/slrdata/) that use a modified bathtub model. We delineated island polygons using the Global Shoreline Vector and Global Islands database (Sayre et al. 2018). Vertical lines indicate projections of sea level rise for the intermediate and high scenarios by 2050 and 2100 for the contiguous United States (Sweet et al. 2022)
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Of the 85 mammalian, reptilian, and amphibian species on our subset list of Florida’s Species of Greatest Conservation Need, 48.2% (n = 41) had some or all of their distribution located on Florida’s coastal islands (Fig. 4, Table S2). Sixty percent (n = 18) of the 30 mammal species on our subset SGCN list had some or all of their distribution on islands (Fig. 4). Notably, at least 12 of these mammal species have their entire distribution on islands, including the Santa Rosa Beach Mouse (Peromyscus polionotus leucocephalus) and the Lower Keys Marsh Rabbit (Sylvilagus palustris hefneri). More than 57% (n = 22) of the 38 reptile species had some or all of their distribution on islands (Fig. 4). Seven of the reptile species on our list are endemic to islands, and all but one of these (Cedar Key Mole Skink) occur in the Florida Keys (Fig. 1, Table S2). Sixty percent (n = 9) of the 15 Testudines on our list have some of their distributions on Florida’s coastal islands, including all 5 subspecies of diamondback terrapin that occur in Florida (3 subspecies of which are endemic to FL). Only one amphibian species on the list of SGCN is known to occur on islands—the Gopher Frog (Lithobates capito) occurs in the Merritt Island National Wildlife Refuge off Florida’s Atlantic coast (Table S2, Koen et al. 2024a).
Fig. 4
Percent of taxa (by class, panel a) on our subset list of Florida’s Species of Greatest Conservation Need (SGCN, n = 85 species total) with some or all of their distribution on Florida’s islands. Panel b shows the number of species (by order) on Florida’s list of SGCN with some or all of their distribution on Florida’s islands. White numbers on bars indicate the total number of species considered, by taxon. For example, 60% of the 30 mammals on the list of SGCN have some or all of their distribution on Florida’s islands, and 40% are island endemics (panel a); 22 of the 30 mammals are rodents, and 10 of those are island-endemics (panel b)
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Discussion
Up to 90% of Florida’s coastal islands could be completely under water from sea level rise within this century. This could represent the extirpation of genetically unique island populations or the extinction of endemic island species that currently occur only on Florida’s islands. Given future sea level rise projections, we predict that over 66% of the land that is currently above water on Florida’s islands will soon be under water, representing significant, or in some cases complete habitat loss for many island species. This is especially true for species that depend on the lowest-lying habitats that are likely to be inundated first (Romañach et al. 2020; Koen et al. 2023).
Sea level rise could result in the degradation or loss of Florida’s unique insular biodiversity. Almost half of the mammalian, reptilian, and amphibian species on our subset list of Florida’s Species of Greatest Conservation Need have some or all of their distribution on islands. Of these, 12 mammal species and 7 reptile species have their entire distributions on islands. Thus, sea level rise represents a significant and immediate threat to the persistence of these 19 species. For example, the Rim Rock Crowned Snake (Tantilla oolitica) and the Key Ring-Necked Snake are small, primarily fossorial snakes that are restricted to the imperiled rockland habitat of south Florida and the Florida Keys. Models predict that most of the rockland habitat within their distributions will be inundated from sea level rise by 2080 (Subedi et al. 2022), threatening both species with extinction. Similarly, the Florida Keys Mole Skink (Plestiodon egregius egregius) and the Cedar Key Mole Skink (P. e. insularis) are endemic to low-lying islands along Florida’s Gulf coast. Both subspecies burrow under sand and tidal beach wrack along the beach, dune, coastal berm, and dry coastal hammock, thus their primary habitats are at risk from flooding, storm surge, saltwater intrusion, and inundation from sea level rise (U.S. Fish and Wildlife Service 2022, 2023; Koen et al. 2024b). Given current sea level rise projections, models predict that > 90% of the primary habitat for both subspecies will be inundated by 2140, resulting in possible extinction of both subspecies (Koen et al. 2024b). In the Florida Keys, Key Deer and Lower Keys Marsh Rabbits are predicted to lose almost all of their habitat with just 1 m of sea level rise (Climate Adaptation Explorer 2024), a scenario that could occur within this century.
The magnitude and timing of impacts of sea level rise on coastal species will vary based on many factors, including a species’ geographic range, its habitat associations, the area and elevation of the islands that it inhabits, and the proportion of its distribution that occurs on islands. For those species whose range occurs on both islands and mainland (e.g. Eastern Indigo Snake Drymarchon couperi, Gopher Tortoise Gopherus polyphemus), the impact of sea level rise is much less certain and will greatly depend on the extent of the species' range that occurs on islands. Even if mainland populations are not vulnerable to the effects of sea level rise, possible consequences for these species if their island populations are extirpated include loss of representation and redundancy, as well as potential loss of unique genetic variation (e.g., U.S. Fish and Wildlife Service 2018).
Our prediction of the insular biodiversity loss that could occur in Florida due to sea level rise is underestimated. We focused here on nonvolant, at-risk, terrestrial vertebrate species, but there are many island-dwelling or island-endemic species that we did not include in our tally. The Miami Blue Butterfly (Cyclargus thomasi bethunebakeri), the Key Tree Cactus (Pilosocereus robinii), and the Florida Duskywing (Ephyriades brunnea floridensis), for example, occur in the Florida Keys and are among the species most likely to be extinct from sea level rise and other synergistic threats (Reece et al. 2013). Further, we did not count island-dwelling species in Florida that are also at risk of significant habitat loss from sea level rise if they were not currently on the list of Florida’s SGCN, such as the Florida Reef Gecko. Indeed, many of Florida’s coastal islands are rich with flora and fauna: Blaney (1971) reported 30 reptile and amphibian species on three barrier islands in the Apalachicola region of Florida, and Franz et al. (1992) reported at least 14 reptile and amphibian species on Egmont Key, FL; some of these island populations may have been isolated from mainland since sea levels rose after the last glacial maximum and thus may have a unique genetic makeup that is yet undiscovered.
There are many species that do not reside on Florida’s islands and thus were not included in our tally, but that depend on island habitat for portions of their life cycle. For example, the beaches and coastal upland habitat on Florida’s islands provides essential foraging habitat for migrating shorebirds (Nicholls and Baldassarre 1990; Sprandel et al. 2000), and nesting habitat for seabirds (Gore et al. 2007) and sea turtles (Bjorndal et al. 1983; Hays et al. 1995). Although these species could presumably fly or swim to find suitable habitat elsewhere when Florida’s islands are submerged, suitable habitat at higher elevations or along the mainland coastline may not exist as sea levels rise because much of the coastline is already developed (Defeo et al. 2009; Vitousek et al. 2017, our study). These species may not face extinction from sea level rise in the same way that island-endemic species do (i.e., inundation of their entire current distribution), but the loss of these important island habitats for foraging and reproducing will likely have a negative impact on population viability (Galbraith et al. 2002; Mazaris et al. 2009). Likewise, there are coastal species in Florida that do not occur on islands at all but are at risk of losing a substantial proportion of their habitat to inundation from sea level rise, such as the Choctawhatchee Beach Mouse and the St. Andrew Beach Mouse (Climate Adaptation Explorer 2024), highlighting that the threat of sea level rise to biodiversity extends beyond island biota and will also impact coastal mainland species.
There are relatively few amphibian species on the list of Florida’s SGCN that occur on Florida’s coastal islands, likely because many of these islands lack suitable conditions, such as freshwater ponds to support breeding (Gibbons and Coker 1978). However, this does not mean that sea level rise will not influence mainland Florida’s amphibian biodiversity. For example, nearly half of the current distribution of the Frosted Flatwoods Salamander (Ambystoma cingulatum) and the Reticulated Flatwoods Salamander (A. bishopi) is predicted to be inundated from sea level rise (Climate Adaptation Explorer 2024). Many coastal amphibian species, including the Frosted Flatwoods Salamander, are already being impacted by storm surge that can flood freshwater breeding ponds, increasing pond salinity (Walls et al. 2019). The magnitude of storm surge volume is expected to increase, and is exacerbated by sea level rise and increasing frequency of the most intense storms as the climate warms (Christensen et al. 2013; Camelo et al. 2020).
Sea level rise and the more immediate impacts of storm surge are not the only threats faced by island-dwelling fauna. Many at-risk species on islands along the US coast are threatened by invasive flora and fauna (Cove et al. 2018; Dueñas et al. 2018). Development along Florida’s coast has also been increasing (Volk et al. 2017), contributing to the degradation and loss of coastal habitat for many species. Indeed, we showed that much of the land on Florida’s islands that is expected to remain above water given sea level rise projections, is already human-modified. Florida has already lost some of its insular biodiversity within the last century, due in part to habitat loss from coastal development. For example, both the Pallid Oldfield Beach Mouse (Peromyscus polionotus decoloratus) and the Anastasia Island Cotton Mouse (P. gossypinus anastasae), endemic to barrier islands on the Atlantic coast, are now thought to be extinct (Humphrey and Barbour 1981; Humphrey et al. 1988). The Chadwick Beach Cotton Mouse (P. g. restrictus) from Manasota Key, an isolated section of a peninsula on Florida’s Gulf coast, is also presumed extinct (Repenning and Humphrey 1986). The interacting effects of coastal development and sea level rise on the amount and integrity of coastal habitat contribute to the vulnerability of coastal species (Reece et al. 2013).
The capacity of island-dwelling species to adapt to climate change and sea level rise may be limited because many species do not have the ability to shift their distribution off the island to track favorable conditions and colonize new areas. The shorter-term prognoses for these species as sea levels rise, however, are largely unknown for most island biota (Braun de Torrez et al. 2021), but recent species-specific studies provide some insight. The Insular Hispid Cotton Rat (Sigmodon hispidus insulicola) and the Sanibel Island Rice Rat, for example, are native, island-endemic species that co-occur on some of Florida’s Gulf coast islands. The Insular Hispid Cotton Rat is expected to decline as sea level rise floods interior freshwater wetlands, whereas the Sanibel Island Rice Rat may be less impacted, at least in the short term, as sea level rise may facilitate an increase in the distribution of their preferred mangrove forest habitat (Boone and McCleery 2023). Similarly, Silver Rice Rats in the lower Florida Keys region have been undergoing elevational range shifts on par with recent sea level rise (Taillie et al. 2023) and may even thrive as sea levels rise if mangrove forests increase in extent (Taillie et al. 2021). Indeed, historical data in the lower Florida Keys region suggest a transition from mesophytic upland forests and freshwater wetlands to more halophilic vegetation with sea level rise and associated saltwater intrusion (Ross et al. 1994, 2009). This transition may benefit species that can capitalize on these landscapes (e.g., Silver Rice Rats), but concomitantly may result in the loss of other species as their habitats disappear (e.g., Key Ring-Necked Snake). There is uncertainty about the resiliency of coastal wetlands to sea level rise. While some models predict that coastal wetlands may keep pace with sea level rise via gains in elevation from sediment accretion, others predict that accelerated sea level rise and surrounding anthropogenic infrastructure may limit the ability of these ecosystems to adjust (Morris et al. 2002; Crosby et al. 2016; Kirwan et al. 2016; Schuerch et al. 2018, Osland et al. 2022). Given uncertainty in how ecosystems and species will respond and adapt to rising sea levels, continued research can help to develop adaptation and mitigation plans, and this leaves room for both optimism and action (Maschinski et al. 2011; Braun de Torrez et al. 2021).
Potential conservation actions for rare and endemic insular taxa may include translocation of individuals among islands or captive breeding to generate stock for both future reintroductions or augmentation of wild populations (Maschinski et al. 2011). The long-term success of these actions is limited, however, if there are few nearby islands with suitable habitat that are safe from both sea level rise and coastal development. Managed relocation of island endemics to sites outside of their current distributions may be possible for some species (Hoegh-Guldberg et al. 2008). This action is fraught with ethical, social, ecological, political, and policy concerns, however, such as the potential for pathogen introduction, competition, and hybridization among translocated individuals and native congeners at recipient sites (Camacho et al. 2010; Maschinski et al. 2011; Schwartz et al. 2012; Butt et al. 2021). Further, because habitat requirements for many rare island endemics are narrow, identifying suitable recipient sites at high enough elevations to withstand projected sea level rise can be challenging, and ultimately, success of conservation actions in general is uncertain (Maschinski et al. 2011). Even so, in some situations, agencies have been using managed relocation to help avert extinction (Willis et al. 2009; Groenewegen et al. 2017; Bouma et al. 2020). When conservation dollars are limited, managers may be faced with the necessity of triage to prioritize conservation actions (Bottrill et al. 2008; Reece and Noss 2014; Johnson et al. 2015). Alternatively, taking no action to conserve our biodiverse island ecosystems may lead to more species experiencing the fate of Australia’s Bramble Cay Melomys, declared the first mammal to become extinct from human-induced climate change due to inundation of its island habitat (Fulton 2017).
Our results mirror those from around the globe—that sea level rise projections translate to a significant loss of island habitat for species at least partially dependent on islands and the threat of extinction for island-endemic taxa. Wetzel et al. (2013) found that 15–62% of islands in the Pacific and Southeast Asia are at risk of complete inundation from sea level rise, threatening 37–118 island-endemic species with extinction. Similarly, Bellard et al. (2014) showed that 6–19% of islands in 10 biodiversity hotspots worldwide could be entirely submerged from sea level rise, threatening up to 300 island-endemic species with extinction. The islands off Florida’s coast are rarely included in global analysis of insular biodiversity because they are relatively small, requiring a finer-scale dataset than can typically be obtained at a global scale. We have shown that even relatively small islands that are close to the mainland can harbor a considerable number of endemic taxa that are vulnerable to extinction from sea level rise, and that many other coastal species occurring on islands for part of their range may experience less certain population impacts.
Acknowledgements
We thank Terry Doonan, Kevin Enge, Jeff Gore, Kendyl Hassler, Mark Lotz, Nick Moore, Dave Onorato, and Lisa Smith from Florida Fish and Wildlife Conservation Commission for sharing their expertise of Florida’s coastal wildlife, and Hance Ellington for analytical advice. We thank Mary Brown and Jonathan Freedman (U.S. Geological Survey ), and two reviewers whose comments greatly improved the manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This manuscript is contribution #925 of the USGS Amphibian Research and Monitoring Initiative.
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The authors have no relevant financial or non-financial interests to disclose. The authors have no conflicts of interest to declare that are relevant to the content of this article. All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript. The authors have no financial or proprietary interests in any material discussed in this article.
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