Impact of sea level rise on the 10 insular biodiversity hotspots. by Celin Bellard, Camille Leclerc and Frank Courchamp. 2013. Global Ecology and Biogeography.
A recent paper investigates the effect of rising sea levels on islands and their biodiversity. Here is the scenario explored in this paper: Global sea levels are predicted to rise by 0.5 – 3 meters over the next few centuries due to polar ice melting. This increase in sea level will decrease the total land area, but may have disproportionately large effects on islands. The reason is that, 1) many islands are low lying and can therefore be completely inundated by even modest (1 meter) increases in sea level, and 2) islands are hotspots of biodiversity, that is, they often contain more unique species than comparable patches of continental habitats (islands contain more than 20% of the worlds biodiversity). This paper explores the potential effects of rising sea level on 10 island biotas using projections of extinction due to sea level rise. The projections are 1, 2, 3, and 6 meters! The hotspots were the Caribbean islands, Japanese islands, Phillipines, East Melanesia, Polynesia, Micronesia, Sundaland, Wallacea, New Caledonia, New Zealand, and Madagascar (see above figure).Ok so what did they find? The results of this study were a bit surprising. First their estimates of land area lost from sea level rise were striking. Globally, about 1% of land area would be eliminated by a 1 meter increase in sea level. That figure rises to 4% at 6 meter rise. The global dataset reflects island chains of varying topography. The islands ringing the pacific are characterized by high relief and rising sea levels wouldn’t eliminate a significant portion of those islands. However, areas with primarily low relief such as The Bahamas would suffer more. This is born out in their regional projection for the Caribbean (11% of land area under a 6 meter increase).
The number of islands totally submerged is another interesting measure of impact. Globally, under a 1m rise 6 % of islands will be totally submerged. That number jumps to almost 20% under the 6m scenario! As with land area loss, the Caribbean is at a relatively high level of impact with about 9% of its islands lost at the 1m increase and almost half at the 6 meter increase!!
So these losses of island area and rates of inundation are quite high, especially for the Caribbean. While The Bahmaas were not considered separately, the low-lying islands of the Bahamas Archipelago are obviously contributing a lot to these estimates. And what about the effect on biodiversity you ask? Well that was less extreme according to the projections.
Surprisingly few species were projected to be lost due to rising sea level. Under the 1 meter scenario only about 30 endemic species were expected to go extinct! That number jumps to about 325 species with a 6m rise. The vast majority of the species are plants.
In summary, large portions and large numbers of the worlds islands are going to be affected by rising sea levels. But the effects on biodiversity are not as extreme. Why? The authors offer several interpretations. First, they cite the limitations of their methodology. Their estimates of biodiviersity on each island are just estimates. They do not actually reflect island-specific measures, but are just averages based on land area and region. Second, the authors only examine endemic species, that is, species found only in the region. Therefore, the estimates of loss are restricted to a subset of species, not total island biodiversity. Third, endemic species are often distributed among several islands of varying size and relief, and therefore probability and severity of inundation (consider the distribution of Bahamas Rock Iguanas which includes low-lying cays and larger islands like Andros). Fourth, the extinction projections are simple, area-based estimates. While habitat loss is a critical agent of species loss, extinction is a multifarious issue. This paper does not reflect the interaction of area loss, invasive species, pollution, and growing human populations on shrinking landmasses. These agents all interact to cause extinctions, and in many cases are probably more important than the direct effects of moderate increases (1m) in sea level. Sixth, there is probably a natural correlation between island elevation and endemism. This relationship means that the islands most likely to be affected by increasing sea level have less species to lose. For example, per area the low relief Bahamas have far fewer endemic species than Cuba. Therefore a loss of 100,000 hectares in Cuba would cause extinctions while the same of area in the Bahamas would not (from an endemic species perspective). That is just a natural correlation between the risk of island submergence and the number of extinctions. Same thing can be said for island size. Finally, the loss of area and islands increases extinction risk by shrinking the range sizes of endemic species. By reducing the number (and connectivity) of populations, rising sea level increases the extinction probability of almost all species even if their specific ranges are not completely inundated.
Despite the limited extinctions projected in this study and sundry caveats of methodology and scope, there are some important results here. The estimates of land loss due to realistic scenarios of climate change are worrisome for the Caribbean, the Bahamas in particular. The loss of from 2% (1m) to 11% (6m) of land area in the Caribbean is quite large, and will surely be much greater for the Bahamas. Understanding the fuller range of these impacts and how they interact with other stressors such as invasive species and habitat alteration is an important aspect of maintaining healthy populations and ecosystems in perpetuity.
Bellard, C., Leclerc, C., and Courchamp, F. 2013. Impact of sea level rise on the 10 insular biodiversity hotspots. Global Ecology and Biogeography. DOI: 10.1111/geb.12093
Publisher site: http://onlinelibrary.wiley.com/doi/10.1111/geb.12093/abstract