Agriculture Reference
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2003). This alternative view, which is more consistent with the majority of empirical
observations, is depicted in Figure 5.4b. Thus, with continued degradation caused
by local stressors, an altered soil system becomes resistant, and the tipping point in
response to external stressors and climate change will shift to the right (Figure 5.4b),
making the ecosystem more resilient to external disturbance (Norström et al. 2009).
Management that seeks to control local anthropogenic disturbances and reverse deg-
radation (Figure 5.4b, dark block arrows) moves the tipping point back to the left,
toward lower resilience to anthropogenic and climate disturbance.
Noteworthy, is the fact that the alternative states depicted in Figures 5.4a and 5.4b
are not assumed to be stable. Moreover, the conceptual model works with or without
thresholds. If the ecosystem's state declines linearly with external disturbance, it is
expected that the slope of this relationship will decrease as degradation increases
(i.e., as the intercept decreases). The general view held that reducing local stress-
ors will mitigate the impacts of external stressors, such as degradative forces and
climate change, may be fundamentally flawed, at least in terms of one facet of resil-
ience, namely, the ability of soil system to resist externally induced stress (Hughes
et al. 2003). The other facet of resilience is recovery. There is growing evidence that
protected or less degraded systems return more quickly to their original state fol-
lowing a range of disturbances (including external stress) than unprotected or more
degraded ecosystems. Thus, the alleviation of local stressors can potentially enhance
soil recovery from degradative forces and climate change impacts (Knowlton and
Jackson 2008). If resilience to degradative forces varies in relation to soil system
state as depicted in Figure 5.4a, then two general postulations arise:
1. Soil systems exposed to local or persistent degradative forces are more sus-
ceptible to external forces than less degraded communities.
2. Soil systems that are conserved/protected from degradative forces are less
susceptible to anthropogenic perturbations than those in areas without simi-
lar management.
These impacts stem from a multiplicity of local stressors, such as soil erosion, con-
tinuous cultivation, and sedimentation. It is therefore not surprising that the concept
of resilience—to climate change in particular—is perhaps more strongly advocated
as an underpinning of management for tropical soils than for any other ecosystem.
This conceptual model implies that soil resistance (i.e., the extent to which the tip-
ping point is shifted to the right; Figure 5.4b) should covary with increasing degrada-
tion. This is true only up to a point. Beyond a threshold level of degradation, changes
in species composition and interactions may become irreversible, impairing ecosys-
tem function and (both aspects of) resilience (Srivastava and Vellend 2005). Here,
soil with a high probability of experiencing heavy degradation appears to degrade at
lower threshold values of stressors (e.g., temperatures) than soil ecosystems in more
permanently conserved locations (Graham et al. 2008), leading to the suggestion that
management to improve soil quality could increase soil resistance (Seybold 1999).
On severely degraded soils such as these, managing for resistance may be unsuccess-
ful, and removing local stressors could offer the only hope for recovery in between
disturbances. The challenge for managers will be to identify the levels of local stress
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