Ecosystems - How do ecosystems and associated services respond to climatic change? [Present]

Figure 1: Type of vegetation in relation to rainfall in the Amazon region. The overlaid arrows show the trajectories of change as simulated by the 19 general circulation models used in the IPCC AR4 forced to start from the averaged observed climate over the period 1970-1999 AD (red star). The tips of the arrows represent the simulated late 21st-century (2070-2099 AD) rainfall regime. The purple arrow shows the mean of all model trajectories and the blue arrow the mean of all models that simulate the late 20th century adequately. Figure modified from Mahli et al. (2009).

Ecosystem services, the benefits humans derive from the biodiversity and functioning of ecosystems, provide a direct link between society and the modifications to the biosphere in response to climate change. Examples of services include crop and forest products, climate regulation through carbon fixation, crop and wild plant pollination by native insects, and recreational, esthetic or religious values. In some regions, the prospect of a warming climate portends a change for the positive: it will allow the production of new crops including cereals or wines of high market value, increased production of some forest species or more enjoyable weather for tourism. However, such positive changes and associated opportunities are not the rule: abrupt changes in ecosystem services associated with climate change are already being observed, and many more are expected (Mooney et al. 2009).

The destruction of entire ecosystems is the most extreme manifestation of the effect of a changing climate. Consider the case of coral reefs, which serve as nurseries for many fish species. As water temperatures rise, bleaching of reefs deprives local populations of important resources from fishing (Hoegh-Guldberg et al. 2007). Coral reef loss also exposes local populations to increased risks from storm damage. Furthermore, income from tourism is lost and thereby an important incentive for sustainable coastal management. Finally, we lose an irreplaceable cultural asset at a global level.

Another example comes from the southwestern United States. A regional-scale tree die-off in semiarid woodlands following the drought in the year 2000 has been referred to as an ecosystem crash (Breshears et al. 2011). The death of trees cascaded to widespread mortality of other species, from pinyon to juniper woodlands. This abrupt event likely altered most ecosystem services fundamentally, with both positive and negative effects. There were short-term effects on grass availability for ranchers (positive), culturally important products such as pinyon nuts (negative) and overall cultural landscape value (negative). Longer-term effects concerned soil erosion and regional climate through changed albedo.

At the planetary scale, although model projections remain conflicting, the shrinking of the Amazon rainforest due to climate change and ensuing land-atmosphere feedbacks has been shown to have potential dramatic consequences for global climate (Mahli et al. 2009). Seemingly less striking changes can entail equally dramatic consequences. Because biotas are the providers of ecosystem services, shifts in the distribution of functionally important species have the potential to disrupt ecosystem services. The distributions of plants and their pollinators can be modified independently from each other, either because of different response speeds or because they are driven by different climatic variables.

Even before the changes in distributions, the subtle matching in phenologies between plants and pollinators is lost and so is the service of pollination, with costly consequences for food production and for culturally important rare species. Conversely, climate change is a golden opportunity for some pest species when their phenology or their distribution synchronizes with those of host plants. Several such cases have already been observed in forest species, such as the altitudinal expansion of the common mistletoe and of the pine processionary moth in the European Alps.

A spectacular case is that of the mountain pine beetle in North America (Kurz et al. 2008). With warming climate this species has been expanding northwards, affecting millions of hectares of coniferous forest. Compounded with increasing fire risk during warmer and drier summers, highly flammable beetle damaged forests have contributed to dramatic increase in burned areas, with considerable effects on regional carbon budgets (expected average emissions for western Canada: 36 g C m-2 yr-1) and potential positive climate feedbacks. The same type of dynamics applies to invasive species, when the climate-driven expansion of exotics such as C4 grasses into shrubby ecosystems (Australia, Cape Region of South Africa) profoundly modifies long-term fire regimes.

Such abrupt changes in ecosystem services are serious challenges to adaptive capacity. Learning from past events, detecting early warning signals and fostering resilience of socio-ecosystems will be essential.

selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_1.pdf

Breshears DD, Lopez-Hoffman L and Graumlich LJ (2011) Ambio 40: 256-263

Hoegh-Guldberg O et al. (2007) Science 318(5857): 1737-1742

Kurz WA et al. (2008) Nature 452: 987-990

Mahli Y et al. (2009) PNAS 106(49): 20610-20615

Mooney H et al. (2009) Current Opinion in Environmental Sustainability 1: 46-54