The Overarching Role of Spatial and Temporal Scale

The study of the vulnerability of ecosystems to global change requires explicit consideration of spatial and temporal scale. Agents of global change simultaneously operate over a range of scales; some operate mainly in the short term at local scales while others are more pervasive (Table 5.1). Further, ecosystem services are delivered at different scales of time and space from the immediate release of nutrients in the vicinity of a root tip to infiltration of water and storage in capillary porosity for periods of months to years.

Ecological processes that sustain provision of these services also operate at a variety of scales. The diversity of scales and the match between scaling of processes and their outputs may significantly influence their vulnerability.

Soil ecosystem services depend either on the direct and immediate outputs of organism activities (e.g., nutrient release from digestive processes) or their longer-term effects on soil physical properties (e.g., water retention in soil pores built by bioturbators). They may also depend on the direct effect of abiotic processes that operate at large scales (e.g., porosity created by alternations of drying and rewetting cycles). Biological processes that sustain ecosystem services may operate at four different scales of time and space depending on their nature and location (Lavelle 1997):

1. Short-term digestion-associated processes. Digestion occurs in the immediate vicinity of microorganisms where exoenzymes are active, in the guts of invertebrates or in the rhizosphere soil close to active root tips. Such microsites are a few cubic microns to millimeters in volume and processes develop during periods of hours to a few days.

2. Intermediate phase in fresh biogenic structures. Microbial activation triggered during gut transit or mechanical mixing of organic materials with soil culminates in fresh biogenic structures, such as fresh earthworm casts or termite fecal pellets. Activity then progressively decreases during the few days or weeks following deposition.

3. Longer-term scale ofstabilized biogenic structures. Some structures created by invertebrates or roots are highly compact. These structures are the components of stable macroaggregate structures that determine soil hydraulic properties and resistance to erosion (Blanchart et al. 1999; Chauvel et al. 1999). Their life span may extend over periods of months to years depending on their composition and the dynamics of soil structural features (Decaens 2000).

4. Soil profiles. Biogenic structures combine with other structures and elements of soil to form soil horizons. In some cases, creeping of soil along slopes may be triggered

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