Environmental Engineering Reference
In-Depth Information
simply not feasible for ecosystems in regions of the world that are dominated
by humans (Vitousek et al.
1997
; Palmer et al.
2004
). Therefore, pristine, or even
pre-industrial, reference states can be an exclusive management goal only for a
relatively small part of the world, where human activity is restricted either
naturally (e.g., inhospitable terrain) or by legislation (e.g., National Parks).
Ecosystem health and resilience
A healthy ecosystem is defined as 'being stable and sustainable, maintaining its
organization and autonomy over time and its resilience to stress' (Rapport et al.
1998
). Rapport et al. (
1998
) and Constanza and Mageau (
1999
) proposed that
vigour (metabolism or productivity), organisation (biodiversity, number of
trophic interactions) and resilience to stress (the ability to resist or recover
from stress) are key attributes of ecosystem health. The key issue that emerges
here is that ecosystem health is a time-dependent concept. All integral parts of
the definition ('stable', 'sustainable', 'maintaining autonomy over time' and
'resilience') have a time component, and satisfying these criteria requires
limited change over time. Thus, health is essentially defined by the lack of
change in system organisation and functioning. Such a definition might seem
to avoid the problem of needing definition of a reference state, but this is not
entirely so, since under the definition of health above any change in state could
be interpreted as a loss of health and, even adding in the more directional
markers of 'key attributes', the health of an already degraded system is essen-
tially undefined unless its history is known. In other words, either the direction
of change needs to be determined or a normal state/target needs to be defined.
One important difference from integrity, however, is that this target can be
aspirational and does not necessarily have to have existed.
A key component in the definition of ecosystem health is the concept of
resilience. Resilience has been defined as the rate at which a system approaches
steady state after perturbation (Pimm
1984
), or more recently for systems that
operate in a non-linear fashion 'as the capacity of a system to absorb disturb-
ance and reorganize while undergoing change so as to retain essentially the
same function, structure, identity and feedbacks' (Folke et al.
2004
). Ideally,
resilience must be assessed in the context of a specific disturbance, i.e., a
system that is highly resilient to one type of disturbance might not be resilient
to another type of disturbance (Walker et al.
2004
). The problem of using
resilience as a measure, when defined as the capacity to absorb/recover from
a perturbation, is that this is only known for the largest disturbance yet
experienced for specific ecosystems. Because of our limited ability to predict
the dynamics of an ecosystem under stress, it is in practice extremely difficult
to quantify the resilience of an ecosystem (Constanza & Mageau
1999
;
Karr
1999
).
