Sea-level rise (SLR), one of the predicted effects of climate change, is expected to significantly affect coastal biodiversity of islands in Southeast Asia and the Pacific. While its primary effects are flooding and erosion, SLR also has important secondary impacts such as human relocation into wildlife habitats that are often overlooked. To assess the primary effects of SLR, Wetzel et al. (2012) modeled levels of inundation and erosion under scenarios of 1, 3, and 6 meter sea-level rises on 1,287 islands in Australasia, Oceania, and Indo-Malaysia. They also estimated secondary effects by analyzing flooded urban and intensive agricultural areas and assuming that equal proportions of habitat were lost in the hinterland due to human migration. By comparing these areas to 106 mammal distributions, they determined the habitat areas and ranges that could potentially be lost. The authors found that depending on the scenario, islands lost 3–32% of their coastal zones and displaced 8–52 million people, leading to dramatic losses of habitat areas for terrestrial species. However, the relative importance of primary versus secondary effects ultimately differed greatly by region due to variance in influential factors such as island geographies and individual species distributions.—Katie Huang
Wetzel, F.T., Kissling, W.D., Beissmann, H., Penn, D.J., 2012. Future climate change driven sea-level rise: secondary consequences from human displacement for island biodiversity. Global Change Biology 18, 2707–2719.
Wetzel et al. studied 1,287 islands in the biogeographical realms of Australasia, Oceania, and Indo-Malaysia, regions particularly vulnerable to biodiversity loss because of coastal flooding. Although they initially considered 12,983 islands, the authors ultimately excluded those that were likely to become completely submerged as well as those that would not be exposed to secondary effects. To determine the primary effects of SLR, they modeled scenarios of 1, 3, and 6 meter sea-level rises and calculated the area of land affected by flooding and shoreline recession due to erosion. They estimated the secondary effects of SLR by determining the proportion of inundated urban and agricultural areas. Under the assumption that the populations of these areas would relocate to similarly sized regions in the hinterland, they were able to determine how much land would be converted from potential habitat to human populated areas. Wetzel et al. also assessed how primary and secondary SLR effects affected species distributions using data concerning 109 species of mammals. Assigning this data to 106 islands, they analyzed how habitat ranges were affected, making sure to account for individual habitat data for each species by assuming that species ranges were reduced proportionally to the loss of hinterland.
The authors found that primary effects led to large area losses of the coastal zone throughout the entire Southeast Asian and Pacific area, although the magnitude of effect varied greatly by region. Under a 1 m scenario, 3% of total coastal area was flooded, with losses increasing to 13% in a 3 m scenario and 32% in a 6 m scenario across all 1,287 islands. The Oceanic region was most vulnerable to inundation, suffering a 7–46% area loss, followed by Indo-Malaysia (4–35%) and Australasia (2–25%). The amount of flooded urban and intensive agricultural land, which determined the extent of the secondary effects, tended to vary by region. Islands in Indo-Malaysia and Oceania were more susceptible to urban and intensive agricultural flooding and lost 30% and 20–35% respectively. In contrast, Australasian islands only suffered 12–16% losses, making them less vulnerable to secondary SLR effects. Primary effects were most prominent on Oceanic islands, while secondary effects were more pronounced in Indo-Malaysia. Secondary effects affected only 5% of land on Oceanic islands, while losses were much greater on Australasian (14%) and Indo-Malaysian islands (18%).
Depending on the SLR scenario, islands were projected to lose 11–45% of potential terrestrial wildlife habitat due to primary effects. The loss was increased by secondary impacts, which were more pronounced on islands with high degrees of urban and intensive agricultural area, as these regions were more likely to be affected by human relocation. The authors estimated that approximately 8–52 million people, 4–27% of the population, were expected to migrate, leading to considerable effects on hinterland habitat. Although islands with large human ecological impacts were already projected to lose 13–44% of habitat area, secondary effects increased the estimate by an additional 6–24%. In contrast, islands with low proportions of urban and intensive agricultural areas only suffered minor losses of 0–1% of habitat from secondary effects. The authors suggested that range loss was hurt more by secondary than primary effects for 10–46% of species because most habitat ranges were located in the hinterland where SLR driven human relocation occurs. They also indicated that smaller islands were more greatly affected than larger islands by SLR effects. However, the extent of primary and secondary effects still varied enormously by region, depending on factors such as coastal geomorphology, species distributions, and other climate change effects.
The authors concluded their study by suggesting further research be conducted using other vertebrate classes or taxonomic groups, as vulnerability to SLR may vary. They also emphasized that their results were conservative estimates due to range loss assumptions, the exclusion of non-intensive types of land conversion, and the elimination of small islands from their analysis. They also only accounted for three types of SLR scenarios, excluded human population growth and resource extraction, and did not consider other ecological factors such as potential benefits from sea-level rise. They urged conservation planning to account for both primary and secondary affects of SLR in determining potential management actions.