Terrestrial ecosystems consist of plants, animals, and soil biota and their environment. The distribution of biota within and across ecosystems is constrained by the physical and chemical conditions of the atmosphere, the availability of nutrients and/or pollutants, and disturbances from natural origin (fire, wind-throw, etc.) or human land use. Global change affects all of these factors, but through widely differing pathways and at different scales. Global climate change, for example, affects local weather and climate in ways that are strongly dependent on location. Increased atmospheric CO₂ concentration, on the other hand, is geographically more uniform.

At the global scale, the sum of all ecosystems (including the marine biosphere) exerts a significant role on the balance of carbon and water in the atmosphere. Feedbacks exist between climate-driven changes in biospheric functioning, such as enhanced or reduced primary productivity, and the amount of greenhouse gas (GHG)-related radiative forcing. In addition, changes in land surface characteristics affect the atmosphere by altering the radiation balance at the reflective surface, creating further feedbacks. These biospheric feedbacks are discussed in TAR WGI Chapter 3.

In trying to understand the effects of global change on the biosphere as a whole, scientists often focus on higher level entities such as ecosystems or biomes (the collection of ecosystems within a particular climatic zone with similar structure but differing species—e.g., the temperate forest biome). However, all factors of change act on individual organisms that are part of a complex web of interactions within and between species and their ecosystem and within landscapes that contain mosaics of different ecosystems. It is not feasible to model the impacts of global change at global, or even regional or landscape, scales at an individual-by-individual level. Thus, most models of global change have dealt with impacts at the ecosystem or biome level. This is in contrast with limitations to observational and experimental studies in which only a selection of individuals and species of a few ecosystems can be included.

Table 5-1: Ecosystems function with links to good/services and possible societal value (modified from Ewel et al., 1998).
Function	Goods/Service	Valuea
Production	Food Fiber (timber and non-wood products) Fuel Fodder	Direct
Biogeochemical cycling	Nutrient cycling (especially N and P absorption/deposition) Carbon sinks	Mostly indirect, although future values have to be considered
Soil and water conservation	Flood and storm control Erosion control Clean water Clean air Water for irrigation Organic matter or sediment export Pollution control Biodiversity	Mostly indirect, although future values have to be considered
Animal-plant interactions	Pollination Animal migration Biodiversity	Mostly indirect, future, bequest, and existence values have to be considered
Carrier	Lansdcape connectivity Animal migration Biodiversity Aesthetic/spiritual/cultural service	Mostly indirect and existence, but bequest may have to be considered
a Value definitions are from Pearce and Moran (1994); see Table 5-2.

Table 5-2: Examples of goods and services with possible uses and values (adapted from Pearce and Moran, 1994).
Value	Examples of Goods and Services
Direct use	Food, fiber, fuel, fodder, water supply, recreation, non-wood forest products
Indirect use	Biodiversity, biogeochemical cycles, tourism, flood and storm control, clean water supply, pollution control
Option	Future discoveries (i.e., pharmacological and biotechnological), future recreation
Bequest	Intergenerational and sustainable development
Existence	Mostly conservation, aesthetic, spiritual

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