16.4.4 Biodiversity

Oceanic islands often have a unique biodiversity through high endemism (i.e., with regionally restricted distribution) caused by ecological isolation. Moreover, human well-being on most small islands is heavily reliant on ecosystem services such as amenity value and fisheries (Wong et al., 2005). Historically, isolation – by its very nature – normally implies immunity from threats such as invasive species causing the extinction of endemics. However, it is possible that in mid- and high-latitude islands, higher temperature and the retreat and loss of snow cover could enhance conditions for the spread of invasive species and forest cover (Smith et al., 2003; see also Chapter 15, Section 15.6.3). For example, in species-poor, sub-Antarctic island ecosystems, alien microbes, fungi, plants and animals have been extensively documented as causing substantial loss of local biodiversity and changes to ecosystem function (Frenot et al., 2005). With rapid climate change, even greater numbers of introductions and enhanced colonisation by alien species are likely, with consequent increases in impacts on these island ecosystems. Climate-related ecosystem effects are also already evident in the mid-latitudes, such as on the island of Hokkaido, Japan, where a decrease in alpine flora has been reported (Kudo et al., 2004).

Under the SRES scenarios, small islands are shown to be particularly vulnerable to coastal flooding and decreased extent of coastal vegetated wetlands (Nicholls, 2004). There is also a detectable influence on marine and terrestrial pathogens, such as coral diseases and oyster pathogens, linked to ENSO events (Harvell et al., 2002). These changes are in addition to coral bleaching, which could become an annual or biannual event in the next 30 to 50 years or sooner without an increase in thermal tolerance of 0.2 to 1.0°C (Sheppard, 2003; Donner et al., 2005). Furthermore, in the Caribbean, a 0.5 m sea-level rise is projected to cause a decrease in turtle nesting habitat by up to 35% (Fish et al., 2005).

In islands with cloud forest or high elevations, such as the Hawaiian Islands, large volcanoes have created extreme vegetation gradients, ranging from nearly tropical to alpine (Foster, 2001; Daehler, 2005). In these ecosystems, anthropogenic climate change is likely to combine with past land-use changes and biological invasions to drive several species such as endemic birds to extinction (Benning et al., 2002). This trend among Hawaiian forest birds shows concordance with the spread of avian malaria, which has doubled over a decade at upper elevations and is associated with breeding of mosquitoes and warmer summertime air temperatures (Freed et al., 2005).

In the event of increasing extreme events such as cyclones (hurricanes) (see Section 16.3.1.3) forest biodiversity could be severely affected, as adaptation responses on small islands are expected to be slow, and impacts of storms may be cumulative. For example, Ostertag et al. (2005) examined long-term tropical moist forests on the island of Puerto Rico in the Caribbean. Hurricane-induced mortality of trees after 21 months was 5.2%/yr; more than seven times higher than background mortality levels during the non-hurricane periods. These authors show that resistance of trees to hurricane damage is not only correlated with individual and species characteristics, but also with past disturbance history, which suggests that individual storms cannot be treated as discrete, independent events when interpreting the effects of hurricanes on forest structure.