Previous studies of the impacts of sea level rise (SLR) on human inhabitance patterns and terrestrial biodiversity have focused mainly on primary effects, which include land area loss due to inundation and erosion in coastal areas. However, secondary effects, or the ecological impacts of human displacement and relocation from low-lying areas because of primary effects, are very important when considering biodiversity loss. Wetzel et al. (2012) examined the impacts of secondary effects on biodiversity in the Southeast Asian and Pacific (SEAP) regions and found that in many cases secondary effects may dominate range loss. In the predicted levels of SLR, 4–27% of the human coastal population (2-52 million people) will be forced to relocate. Human migration will be especially pronounced on the Indo-Malaysian islands, where 4–28% of the population will be forced to migrate, and 30% of the inundated land will be urban and intensive agricultural land. This relocation, as well as the increase in natural land conversion to agricultural and urban land in the hinterlands will likely cause significant population declines in many species.–Olivia Jacobs
Wetzel, F.T., Kissling, D., Biessmann, H., Penn, D.J. Future climate change driven sea-level rise: secondary consequences from human displacement for island biodiversity. Global Change Biology 18, 2707–2719. [GSSS Wetzel kissling secondary]
Wetzel et al.(2012) studied the potential effects of sea level rise (SLR) on more than 1,200 islands in the Southeast Asian and Pacific region (SEAP). They further divided this area into three broad categories, Australia, Oceania, and Indo-Malaysia, and used Digital Elevation Model (DEM) data to map potential SLR scenarios. The three scenarios addressed were 1 m, 3 m, and 6 m of total SLR, which are values that represent the most common estimates of SLR for this century and higher ones for the coming centuries. Using these maps of SLR, the scientists analyzed primary effects on coastal areas by estimating the inundated and eroded area in the coastal zones. Secondary effects, or the land converted from natural habitat to agricultural or urban areas due to population migration, were estimated by using mammal distribution data for 54 species on 109 Indo-Malaysian islands. Each mammal was analyzed individually using information from previous studies. Notably, the scientists also assumed that there was an equal-area land-conversion of inundated urban and intensive agricultural areas in the hinterlands.
The data indicate that primary effects will result in a large loss of coastal zone in the SEAP region. On average, 3% of land area in the SEAP will be inundated from the 1 m scenario, and 32% will be lost in the 6 m scenario. This will cause an estimated 8–52 million people (or 4–27% of the total population) to migrate to the hinterlands. Further, the Oceanic area will be most vulnerable to inundation, with an estimated 7–46% loss for the 1–6 m scenarios. Australia will be less affected, experiencing a land area loss of 2–25% in these scenarios, and Indo-Malaysia will experience a land loss of 4–35%. In these inundated areas, about 30% of the lost area is urban and intensive agricultural land in Indo-Malaysia and 4–28% of the population will be forced to migrate, whereas only 2–6% of the population in Oceania and Australia will be at risk. These data indicate that biodiversity loss due to secondary effects will be most pronounced in Indo-Malaysia and primary effects will dominate elsewhere.
Secondary effects are especially important when considering biodiversity because areas with large coastal urban populations and agricultural development, such as Indo-Malaysia, will experience large amounts of human migration and land conversion. Range loss due to secondary effects was greater than loss due to primary effects in Indo-Malaysia for 22–46% of species in maximum range loss scenarios, while 9% of species are only vulnerable to secondary effects. The authors also note that different species in similar island scenarios respond very differently to primary and secondary effects. The Smokey Flying Squirrel has a projected range loss of 1–3% due to primary effects, but a loss of 2–60% from secondary effects. Comparatively, the Rajah Sundiac Maxomys may experience primary loss of 1–15%, while secondary effects play a lesser role in range area loss. Thus, in some instances secondary effects will dominate species range loss, while secondary effects will not contribute significantly to that of other species. The effects are dependent on both region and species.
This study makes some key assumptions that could contribute significantly to a change in the aforementioned estimates. It ignores the expected temperature and precipitation changes due to climate change, and it also only analyzes the secondary effects on mammals on the SEAP islands. The model may also be overly conservative because it only considered the most intensive farming types, did not consider other ecological interactions such as interspecific competition due to coastal species migration, and did not include the worst-case and more liberal estimates of SLR. However, the study still provides further evidence that SLR anticipated from global warming will have many major consequences for biodiversity, and it expands our understanding of biodiversity loss to include the impacts of secondary effects due to SLR.