18.104.22.168 Terrestrial and marine ecosystems
Many polar species are particularly vulnerable to climate change because they are specialised and have adapted to harsh conditions in ways that are likely to make them poor competitors with potential immigrants from environmentally more benign regions (e.g., Callaghan et al., 2005; Peck et al., 2006). Other species require specific conditions, for example winter snow cover or a particular timing of food availability (Mehlum, 1999; Peck et al., 2006). In addition, many species face multiple, concurrent human-induced stresses (including increased ultraviolet-B radiation, increasing contaminant loads, habitat loss and fragmentation) that will add to the impacts of climate change (Walther et al., 2002; McCarthy et al., 2005).
Plants and animals in the polar regions are vulnerable to attacks from pests (Juday et al., 2005) and parasites (Albon et al., 2002; Kutz et al., 2002) that develop faster and are more prolific in warmer and moister conditions. Many terrestrial polar ecosystems are vulnerable because species richness is low in general, and redundancy within particular levels of food chains and some species groups is particularly low (Matveyeva and Chernov, 2000). Loss of a keystone species (e.g., lemmings, Turchin and Batzli, 2001) could have cascading effects on entire ecosystems.
In Arctic ecosystems, adaptive capacity varies across species groups from plants that reproduce by cloning, which have relatively low adaptive potential, through some insects (e.g., Strathdee et al., 1993) that can adapt their life cycles, to micro-organisms that have great adaptive potential because of rapid turnover and universal dispersal. The adaptive capacity of current Arctic ecosystems is small because their extent is likely to be reduced substantially by compression between the general northwards expansion of forest, the current coastline and longer-term flooding of northern coastal wetlands as the sea level rises, and also as habitat is lost to land use (see Figure 15.3). General vulnerability to warming and lack of adaptive capacity of Arctic species and ecosystems are likely, as in the past, to lead to relocation rather than rapid adaptation to new climates (see Figure 15.3).
Figure 15.3. Present and projected vegetation and minimum sea-ice extent for Arctic and neighbouring regions. Vegetation maps based on floristic surveys (top) and projected vegetation for 2090-2100, predicted by the LPJ Dynamic Vegetation Model driven by the HadCM2 climate model (bottom) modified from Kaplan et al. (2003) in Callaghan et al. (2005). The original vegetation classes have been condensed as follows: grassland = temperate grassland and xerophytic scrubland; temperate forest = cool mixed forest, cool-temperate evergreen needle-leaved and mixed forest, temperate evergreen needle-leaved forest, temperate deciduous broadleaved forest; boreal forest = cool evergreen needle-leaved forest, cold deciduous forest, cold evergreen needle-leaved forest; tundra = low- and high-shrub tundra, erect dwarf-shrub tundra, prostrate dwarf-shrub tundra; polar desert/semi-desert = cushion forb, lichen and moss tundra. Also shown are observed minimum sea-ice extent for September 2002, and projected sea-ice minimum extent, together with potential new/improved sea routes (redrawn from Instanes et al., 2005; Walsh et al., 2005).
As air and sea water temperatures have increased in the Bering Sea, there have been associated changes in sea-ice cover, water-column properties and processes including primary production and sedimentation, and coupling with the bottom layer (Grebmeier et al., 2006). A change from Arctic to sub-Arctic conditions is happening with a northward movement of the pelagic-dominated marine ecosystem that was previously confined to the south-eastern Bering Sea. Thus communities that consist of organisms such as bottom-feeding birds and marine mammals are being replaced by communities dominated by pelagic fish. Changes in sea ice conditions have also affected subsistence and commercial harvests (Grebmeier et al., 2006).
Many Arctic and sub-Arctic seas (e.g., parts of the Bering and Barents Seas) are among the most productive in the world (Sakshaug, 2003), and yield about 7 Mt of fish per year, provide about US$15 billion in earnings (Vilhjálmsson et al., 2005), and employ 0.6 to 1 million people (Agnarsson and Arnason, 2003). In addition, Arctic marine ecosystems are important to indigenous peoples and rural communities following traditional and subsistence lifestyles (Vilhjálmsson et al., 2005).
Recent studies reveal that sea surface warming in the north-east Atlantic is accompanied by increasing abundance of the largest phytoplankton in cooler regions and their decreasing abundance in warmer regions (Richardson and Schoeman, 2004). In addition, the seasonal cycles of activities of marine micro-organisms and invertebrates and differences in the way components of pelagic communities respond to change, are leading to the activities of prey species and their predators becoming out of step. Continued warming is therefore likely to impact on the community composition and the numbers of primary and secondary producers, with consequential stresses on higher trophic levels. This will impact economically important species, primarily fish, and dependent predators such as marine mammals and sea birds (Edwards and Richardson, 2004).
Substantial evidence indicates major regional changes in Antarctic terrestrial and marine ecosystems in areas that have experienced warming. Increasing abundance of shallow-water sponges and their predators, declining abundances of krill, Adelie and Emperor penguins, and Weddell seals have all been recorded (Ainley et al., 2005). Only two species of native flowering plant, the Antarctic pearlwort (Colobanthus quitensis) and the Antarctic hair grass (Deschampsia antarctica) currently occur in small and isolated ice-free habitats on the Antarctic continent. Their increased abundance and distribution was ascribed to the increasing summer temperatures (Fowbert and Smith, 1994). Elsewhere on continental Antarctica, climate change is also affecting the vegetation, which is largely composed of algae, lichens and mosses, and changes are expected in future, as temperature, and water and nutrient availability, change (Robinson et al., 2003).
The marked reduction reported in the biomass of Antarctic krill (Euphausia superba) and an increase in the abundance of salps (principally Salpa thompsoni), a pelagic tunicate, may be related to regional changes in sea ice conditions (Atkinson et al., 2004). This change may also underlie the late-20th century changes in the demography of krill predators (marine mammals and sea birds) reported from the south-west Atlantic (Fraser and Hoffmann, 2003), and this connection indicates a potential vulnerability to climate change whose importance cannot yet be determined.
Recent studies on sub-Antarctic islands have shown increases in the abundance of alien species and negative impacts on the local biota such as a decline in the number and size of Sphagnum moss beds (Whinam and Copson, 2006). On these islands, increasing human activities and increasing temperatures are combining to promote successful invasions of non-indigenous species (Bergstrom and Chown, 1999).