Despite considerable attention to global change effects, few studies focused on the consequences of sea level rise (Menon et al. 2010; Wetzel et al. 2012; Schmidt et al. 2012). Many studies showed that global warming will be one of the biggest threat to future biodiversity, and predictions could play an important role in alerting scientists and decision makers to support the development of proactive strategies to reduce climate change impacts on biodiversity (Bellard et al. 2012). Potential effects of sea level rise are of considerable interest because of its potential impact on biodiversity and society. By 2030, 50% of the world population will live within 100 km of the coast (Small and Nicholls 2003; Bindoff et al. 2007). The two major causes of global sea level rise are thermal expansion of the oceans (water expands as it warms) and the loss of land-based ice due to increased melting. During recent years, for which the observing system is much better, thermal expansion and melting of land ice each account for about half of the observed sea level rise (Solomon et al. 2007). Several recent studies strongly suggest that sea level rise will increase by 0.5 to 2.3 m at the end of the century (Rahmstorf 2007; Pfeffer et al. 2008; Grinsted et al. 2009; Jevrejeva et al. 2010; Nicholls and Cazenave 2010; Traill et al. 2011). Most dramatic scenarios of ice sheet melting and sliding lead to potential sea level rise of 4 to 6 m (Overpeck et al. 2006). Sea level increases could lead to the total immersion of many low-lying islands, associated with potential important consequences for biodiversity. Surprisingly, few studies have focused on consequences of sea level rise on islands and their biodiversity, only specific archipelagos that represent very limited subsets of existing islands (Baker et al. 2006; Webb and Kench 2010; Wetzel et al. 2012; Bellard et al. 2013), or on continental coastlines (Menon et al. 2010; Hinkel et al. 2010; Traill et al. 2011) have been studied. Yet, islands are generally considered as important hotspots of biodiversity due to very high endemism richness, about 70, 000 vascular plant species are endemic to islands (Kier et al. 2009). In particular, among French territories, four of them are present in biodiversity hotspots. Although French islands only represent 0.08% of Earth land surface, they contain 1.4% of the plants, 3% of the molluscs, 2% of fishes, 1% of reptiles and 0.6% of birds of the world. Therefore, one could expect significant loss of insular biodiversity due to sea level rise. For example, the endangered Lower Keys Marsh rabbit (Sylvilagus palustris hefneri) already lost 64% of its habitat, the majority due to sea level rise (>48%) (Schmidt et al. 2012). While actions to mitigate climate change and its consequences in islands are being debated, few studies regard the potential impact of an increase of sea level on insular habitats.

Here, we assessed impacts of sea level rise on islands (n = 1269) under French jurisdiction. This sample includes islands of various sizes, geological types and elevations, and covers all latitudes in the Pacific, Indian and Atlantic oceans and in the Mediterranean. We investigated three scenarios of projected sea level rise on these islands, to provide estimations of loss of entire islands. First, we considered a global sea level rise by 1 m that is slightly below the average of six recent projections of sea level rise for 2100 (Overpeck et al. 2006; Rahmstorf 2007; Pfeffer et al. 2008; Grinsted et al. 2009; Jevrejeva et al. 2010; Nicholls and Cazenave 2010). We also explored another realistic upper bound of 2 m sea level rise (Nicholls and Cazenave 2010) and an extreme scenarios of 3 m (Hansen 2007). In addition, we studied sea level rise of two different types: a uniform increase over the globe or a recent projection that considers the regional variability of sea level rise based on a synthesis of 16 sea level rise models (see Figure 2B for details). The latter projection is more realistic because heterogeneous ocean warming will lead to non-uniform thermal expansion (Cazenave and Llovel 2010) and leads to average rises of 1.05, 2.10 and 3.15 m, hereafter termed ~1 m, ~2 m and ~3 m in order to compare with homogeneous rises.

For each scenario, we assessed the number of islands that would be entirely submerged, which was estimated by overlying maximal island elevation with local sea level projections. These must be considered very conservative estimates of biodiversity impacts of sea level rise on islands because we only consider losses on islands that are totally submerged. Finally, we also highlighted a case study of New Caledonia taking into account partial habitat losses of islands and potential loss of endemic plant species area distribution due to sea level rise.

Over the next 60 years, sea level rise and higher storm frequency, together with natural and human-induced subsidence, population growth and urbanization, will cost roughly 9% of global GDP (Hanson et al. 2010). However, little attention has been paid to island vulnerability directly caused by sea level rise (Menon et al. 2010; Wetzel et al. 2012). It is clear that sea level rise will be the largest driver of habitat loss for insular habitats (Wetzel et al. 2012). Sea level rise is a particularly critical consideration for preserving coastal biodiversity, yet many approaches for prioritizing conservation networks do not take this into account (Runting et al. 2013). Our results predicted that impacts of sea level rise on insular French habitats are likely to be important, especially for two regions: New Caledonia and French Polynesia. Under an increase of sea level by 1 m, at least 5% of islands will be potentially entirely submerged. Our pessimistic scenario projected a loss of up to 11% of French islands. Globally, we did not observe important differences between homogeneous and heterogeneous projections results. Other studies that attempted to study the potential impacts of sea level rise in terrestrial eco-regions (Menon et al. 2010) and on Indo-Malaysian islands (Wetzel et al. 2012) showed results of the same order of magnitude. We do regret our inability to work with a greater precision of elevation data and to take into account partial habitat losses due to marine intrusion for all the French islands. However taking into account partial losses of New Caledonia, we showed that this region will also suffer from significant partial losses of insular habitat. Therefore, neglecting partial loss of islands means that the impact of sea level rise will cause probably more dramatic land losses than projected by the study for the other regions. Actually, several factors may underestimate habitat loss estimations, especially: lateral erosion, tidal range, centennial tides or floods and increased salinity on new shorelines. For instance, beach erosion along the United States East Coast due to sea level rise was about two orders of magnitude greater than the rate of sea level rise (Zhang et al. 2004). This means that, for example, a 1 meter rise of sea level would create habitat loss of about 100 meters inland along the coasts due to lateral erosion. Therefore, results are conservative and they give evidence that sea level rise represents an important threat on insular biodiversity over the next decades. We did not either consider the indirect habitat losses following displacement of human population from inundated areas and the potential important ecological consequences if urban and intensive agricultural areas in the coastal zones of islands are relocated to the hinterland (Wetzel et al. 2012), though these effects have been shown to be sometimes greater than the losses directly due to sea level rise.

Then, regarding endemic species distributions over New Caledonia, we showed that species that are already at risk of extinction were the most vulnerable to sea level rise. This result is particularly important, because it means that endemic plants that are at risk of extinction are mainly located on the coastal areas. Although we focused on endemic plant species, rising seas will also flood other populations of different taxonomic groups including invertebrates and vertebrates. In addition, the two most important regions that are susceptible to be threatened by sea level rise are part of biodiversity hotspots. For instance, French Polynesia contains 85 of the 425 atolls of the world that shelter nearly 19, 000 vascular plant species including 3, 450 endemic species. These species may also have to face many threats such as salinity intrusion, submersion, soil erosion and climate changes.

We attempt a first approximation of the potential impact of sea level rise on French biodiversity following marine intrusion. In our study, most of islands that showed important vulnerability to sea level rise were atolls. Their remoteness from mainland, flat topography and great susceptibility to natural disturbances (e.g., hurricanes) might lead to a low biodiversity present in these islands. Thus, the biodiversity on small islands and other low-lying coastal regions appeared to be highly vulnerable to sea level rise, but it is unclear how many species could be lost under existing sea level rise projections. To conclude, some patterns emerging from this study provided useful information for conservation planning. As our climate continues to change at a faster rate than previous century, sea level rise will create further challenges for the conservation of insular ecosystem in French territories, and in low lying areas worldwide. Given that French islands covers all latitudes in the Pacific, Indian and Atlantic oceans and in the Mediterranean, with the French maritime domain being ranked as the second largest in the world, our results suggested that the implications for the 180 000 world islands around the world should be considerable. Assuming that French islands are representative of worldwide islands, roughly 10, 800 islands could be entirely lost with the 1 meter scenario. Consequently, new prioritizations programs for islands have to be established in order to mitigate the impacts of sea level rise. For example, in order to anticipate threats and prepare adequate conservation actions (e.g., local protection, identification of refuges or translocation programs), conservation managers have to target specific islands that have high risk of being permanently inundated within the next decades. Our results suggested that the effects of sea level rise will be particularly dangerous for New Caledonia, and French Polynesia. Considering their important contribution to global biodiversity and the threat of sea level rise for future biodiversity of some of these islands, there is an urgent need that islands feature prominently in global and regional conservation prioritization schemes. In addition, with accepted projections of sea level rise now exceeding one meter, the improvement of estimates of the associated loss of insular habitat and biodiversity becomes essential. In addition, it would be advantageous for decision makers to have more certainty about future sea level rise, the nonlinearity of ice-sheet melt makes accurately predicting the change in sea level at a particular date unattainable at present (Hansen 2007). We hope that this study will provide a first valuable attempt for new research efforts in this direction.