Extrinsic and Intrinsic Determinants of Vulnerability

Soil organisms vary considerably in their susceptibility to global change, and even the same taxonomic and/or functional group may vary in its response according to the nature, extent, and frequency and intensity of perturbation (Wall et al. 2001). Thus, the vulnerability of individual components of the soil fauna is context dependent.

Determinants of vulnerability are wide ranging, encompassing both intrinsic and extrinsic factors. Many of these are intuitive, and empirical assessments of vulnerability to a particular driver of global change are surprisingly rare (but see Ruess et al. 1999). One conspicuous gap in knowledge lies in the significance of species interactions in determining the vulnerability of specific organisms or the assemblages they constitute.

Among the intrinsic determinants of vulnerability is a suite of life-history traits including body size, life-cycle longevity, and host plant and habitat specificity. Soil organisms are more vulnerable to perturbations if they are large bodied (Wardle 1995; Eggleton et al. 1998). Within this domain, surface-dwelling and relatively sedentary taxa are particularly prone to changes in soil moisture and texture (e.g., Brown et al. 2001). The life-history traits of high dispersal and migratory ability tend to counteract local extinctions, though the efficacy of these traits in stemming population decline is dependent on local source pools and inherent landscape diversity (Warren et al. 2001).

Longer-lived taxa are particularly vulnerable to perturbation, since they have less chance to recover than multivoltine (producing several generations in one year) species. Within the invertebrates, soil-dwelling species are characterized by having longer generation times than their aboveground counterparts (Andersen 1987; Brown & Gange

1990), a trait that undoubtedly increases their susceptibility to change. The implications of the reduced mobility and dispersal and longer life spans of soil organisms are very apparent when trying to restore the lost diversity of a system. In the restoration of exarable land, it has been shown that the colonization and establishment of soil biodiversity lags behind that of aboveground organisms (Korthals et al. 2001). Organisms that are either food or habitat specialists are more susceptible to even minor changes in their resources. Strict host plant or vegetation structure specialist species may respond negatively to changes in the occurrence (Cherrill & Brown 1990) or quality of their resource (Masters et al. 1993), whereas generalist species may switch hosts or move to adjacent habitats with global change (Bale et al. 2002). Both spatial and temporal synchrony with their required resource is a pivotal requirement of specialist species, with even slight asynchrony potentially causing extinction. Specialism is therefore a key indicator of the extent and spatial scale of vulnerability to global change.

An organism's vulnerability is also related to its ability to withstand change, in terms of "flexibility" (resilience) or "rigidity" (resistance). These attributes will vary intrinsically, but also in response to local conditions. Species may be unchanged, susceptible and become locally extinct (depending on recruitment), opportunistic and increase their performance in the new environment, or elastic and change in the short term but then recover to their former level (Brown et al. 2001).

Extrinsic determinants of vulnerability reflect habitat characteristics, such as the diversity of habitat and landscape. It is a common assumption that mosaic landscapes (roughly defined as more than one land use or land cover per hectare) and the species they support are less vulnerable to perturbation and more likely to recover quickly from disturbance or depletion, but this is unproven. Small fragments of pristine ecosystems (for example patches of primary tropical forest as small as 0.25 ha) may retain high biodiversity, at least over the medium term, after isolation (de Souza & Brown 1994; Eggleton et al. 2002), but there are few measurements of associated processes and services, and long-term effects are largely unknown.

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