The Quantification Exercise

Arable tilled ecosystems represent the most biologically simple of the three ecosystem types that we evaluated. The three perturbations that we considered were (1) invasive species: root parasites; (2) climate change: drought; and (3) land-use change: to forest. The quantification exercise (Table 5.A1 in the appendix on page 121) revealed high vulnerability of the primary ecosystem services provided by arable systems (food and fiber production) to both the invasive species and climate change scenarios, and also high vulnerability of fiber production to land-use change. However, there were also several instances in which secondary services (e.g., water quality, flood and erosion control, habitat provision, recreational values, carbon sequestration) were vulnerable to specific global change phenomena. For the majority of cases, negative impacts of both the invasive root pathogen and drought on the ecosystem good or service under consideration was predicted. This is because most of these goods and services are maximized by plant productivity, and both scenarios operate to reduce productivity, as well as reduce the ability of the soil biota to maximize productivity. In contrast, the land-use change scenario—that is, afforestation—had positive effects on many ecosystem services (Table 5.A1 in the appendix on page 121). This relates to forests representing a more complex, integrated ecosystem than arable tilled systems, and one in which the soil biota, rather than artificial inputs, plays a much greater role in the delivery of ecosystem services. Afforestation is therefore likely to enhance the role that soil biota plays in the delivery of such varied services as ecosystem carbon sequestration, nutrient retention, atmospheric regulation, habitat provision, and flood and erosion control.

Temperate grasslands are usually dominated by perennial plant species, and therefore typically represent a more complex, lower input ecosystem type than do arable ecosystems. The three perturbations that we considered were (1) invasive species: a generic invasive plant (weed) species; (2) climate change: drought; and (3) land-use change: to arable land. The quantification exercise (Table 5.A2 in the appendix on page 126) again predicted strong effects of global change phenomena on material goods provided by the ecosystem, such as food and fiber, and these effects were frequently negative. Global change effects on these goods arise through their influences on plant community composition and productivity, the ability of soil food web organisms to maximize this productivity, and linkages between the plant and soil community. This can involve important shifts from dominance by fungal based food webs and soil animals with large body sizes to bacterial based food webs and small bodied soil animals, especially in the case of the land-use change scenario. Global change phenomena also have wide ranging (and in the majority of cases, negative) effects on environmental services provided by the soil biota, including maintenance of water quality, habitat provision, bioremediation, recreation, carbon sequestration, and atmospheric regulation of gases (Table 5.A2 in the appendix on page 126). The effects of invasive plant and climate change on these types of services may arise through their adverse effects on plant productivity and soil biotic activity. Meanwhile, land-use change may affect these services through the grassland changing to a more biologically simplistic system in which the importance of the soil biota relative to that of artificial inputs in providing services diminishes.

Forest ecosystems represent the most complex and biologically organized of the three ecosystem types that we considered. The three perturbations evaluated in this case were (1) invasive species: exotic earthworms; (2) climate change: drought; and (3) land-use change: deforestation. The quantification exercise (Table 5.A3 in the appendix on page 132) revealed likely effects of all three agents on material goods provided by the forest, notably timber and wood-based products. Although deforestation directly and obviously impairs the ability of the forest to produce wood, there are also mechanisms through which drought and earthworm invasion may influence performance of both the above-ground and belowground subsystems that ultimately affect timber production (Table 5.A3 in the appendix on page 132). Forests, through virtue of being less disturbed by humans than grassland or arable systems, frequently have far greater recreation and biodiversity conservation values; the ability of the plant-soil system to provide these values are potentially indirectly responsive to both deforestation and drought (Table 5.A3 in the appendix on page 132). Environmental services provided by forests to which the soil biota contributes (e.g., water retention, erosion control, carbon sequestration, and regulation of atmospheric gases) are all maximized by forest stands with high biomass, which are in turn maintained by both decomposer food web activity and mycorrhizal associations. Ultimately, the type of global change agent operating will determine the direction of responses of ecosystem services; of the examples presented here, invasion of earthworms may increase plant productivity and the role of soil biota in providing these services, while drought and forest clearance may be expected to have generally detrimental effects.

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