Geoscience Reference
In-Depth Information
have recognisable patterns and organisation, but are still diffi cult to understand and
predict (e.g., Pahl-Wostl, 1995; Arrow et al., 2000; Gaichas, 2008). Such complexity
also means that whole ecosystem investigations are problematic and resource inten-
sive. Consequently, research and management usually focus on a small subset of
interactions. Basing management on investigations of these subsets in isolation,
without an understanding of broader ecosystem processes and linkages, can severely
compromise management success. Chaotic, complex and/or non-linear systems com-
monly exhibit processes where effects are not proportional to causes, and where
output is not equal to input, so that small changes in component structure and
process can potentially have substantial effects on the wider ecosystem (Pahl-Wostl,
1995; Manson, this volume). Non-linearity in ecosystems has been shown in, for
example, biogeochemical cycles (Qi et al., 2002), population dynamics (e.g., Turchin,
1993) hydrological cycles (Rodriguez-Iturbe et al., 1991), and climate (e.g., Ghil
et al., 1991).
A theoretical example of non-linear response in an ecosystem process can be seen
in fi gure 25.1. In this model, it is assumed that the structure and organisation of a
system are related to its stability and resilience, and therefore its capacity to repair
damage caused to biota (measured in biomass). In fi gure 25.1a, it is simply assumed
that the various factors which together stress an ecosystem cause a reduction in
growth rate of biota and a decline in biomass, but have little effect on the structure
and organisation of the ecosystem (i.e., stability and resilience), creating a linear
response to stress (fi gure 25.1c). In fi gure 25.1b, the negative effects of stress also
change the structure and organisation of the ecosystem (reducing stability and resil-
a)
MODEL (1)
b)
MODEL (2)
sum of stress factors
sum of stress factors
(-)
(-)
(-)
(+)
(+)
structure-
organization
structure-
organization
biomass
biomass
(+)
(+)
effect
(2) nonlinear
c)
(1) linear
stress
Figure 25.1
Examples of simple ecosystem interactions that can lead to either linear
(Model 1) or non-linear (Model 2) responses (see text for explanation). Modifi ed from
Pahl-Wostl (1995).
