Geoscience Reference
In-Depth Information
(and indeed desirable in some instances for their renewal) that ecosys-
tems will undergo change.
22
This is not necessarily problematic for the
biotic communities that form part of the system if it occurs in a way
that does not cause a qualitative shift in its structure and functionality.
However, if forces impact on an ecosystem which is already weakened
because, for example, of a decline in its biodiversity, major change may
occur from which a fundamentally different system emerges.
23
The ability of an ecosystem to remain in a particular state or domain of
attraction is related to the slow variables that con
gure the system and
ne the boundaries of the domain that it occupies.
24
Variables have
thresholds which, if exceeded, can result in the system entering a differ-
ent domain in which its determining variables, structures and functions
differ substantially from those of the preceding state.
25
Walker and Salt
use the example of the capacity of sediments in a lake to absorb levels of
phosphorus to illustrate how going beyond the threshold of a variable
can affect a system
de
s condition.
26
The lake ecosystem is at risk of entering
a new domain in which a clear water environment is replaced by the
domination of algal growth where levels of phosphorus exceed the
capacity of sediments to absorb them.
The phenomenon of ecological regime shifts is of major concern for
human well-being because it can result in the replacement of systems
that have hitherto provided humanity with essential services with
'
'
systems that are in an impoverished state and are therefore
unable to function as they did in their previous states or to support a
diversity of species.
27
Norse refers to changes of this nature having
occurred in marine ecosystems that once supported a rich biodiversity,
but which, as a result of over
poverty trap
'
shing, have become dominated by
22
C. S. Holling and L. H. Gunderson,
in L. H. Gunderson
and C. S. Holling (eds) Panarchy: Understanding Transformations in Human and
Natural Systems (Washington, DC: Island Press, 2002), p. 26.
'
Resilience and Adaptive Cycles
'
23
C. S. Holling,
'
Understanding the Complexity of Economic, Ecological and Social Systems
'
(2001) 4 Ecosystems,400;Schefferetal.,
'
Catastrophic Shifts
'
,591
-
2; Chapin III et al.,
'
Terrestrial Ecosystem Ecology
'
, pp. 342
-
6.
24
Walker and Salt,
'
Resilience Thinking
'
,pp.53
-
5, 59
-
62;S.R.CarpenterandC.Folke,
'
Ecology for Transformation
'
(2006) 21 Trends in Ecology and Evolution,311.
25
Ibid
.; B. Walker and D. Salt, Resilience Practice: Building Capacity to Absorb Disturbance
and Maintain Function (Washington, DC: Island Press, 2012), pp. 4
-
9; Chapin III et al.,
'
Terrestrial Ecosystem Ecology
'
,pp.343
-
6.
26
Walker and Salt,
'
Resilience Thinking
'
,pp.55
-
8.
27
Holling,
'
Understanding the Complexity
'
,400;C.Folke,
'
Resilience: The Emergence of a
Perspective for Social-Ecological Systems Analyses
'
(2006) 16 Global Environmental
Change, 257. See also Chapter 2,
Section 2.3
.
