Resilience in aquatic ecosystems - hysteresis, homeostasis, and ...

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Reynolds/ Aquatic Ecosystem Health and Management 5 (2002) 3–17
stress (Costanza and Mageau, 1999). Is this not just
another way of asking how much structural and
resourcing resilience the system has
In the same way that systems emerge only through
the aggregate of the behaviors of individual organisms
of discrete populations, so system health should be
manifest in the representation of key indicator species
and functional types. Thus, the indices of ecosystem
sustainability promoted by Costanza and Mageau
(1999) provide a ready framework of dimensions for
distinguishing “good” from less desirable resilience.
The lines in Figure 5a represent the axes of a threedimensional
plot, which share a common origin at
“0”. The z-dimension corresponds to specific processing
rate (representing gross primary production,
foraging rate, oxidative capacity); the y-dimension
corresponds to structure (representing greater species
richness and interconnectance, more information
organized in more biological complexity). These two
axes define a (y-z) plane analogous to that used by
Reynolds (1999) to distinguish lakes on the basis of
their metabolic properties. The third (x-) dimension
corresponds to resilience as a separate measure of
ecosystem behavior. It was not qualified by Costanza
and Mageau; their intention is the “structural” context
but it holds for “resourcing” too. In Figure 5b, two trajectories
of ecosystem development are inserted into
the three-dimensional space created by the axis
boundaries. For simplicity, they refer strictly to the
early development and they do not show symptoms of
disturbance. Both trajectories commence close to the
origin and, quite properly, demonstrate high biomass
production. Both trajectories acquire resilience, also
as anticipated but one of them also rises with respect
to the y axis, as it gains more species, develops more
network connectances and becomes more complex.
This is the trajectory of Ulanowicz’ (1986) ascendancy
and the resilience it builds is of the structural kind.
The other trajectory advances in productivity and processing
but the biomass investment is in a few specialist
species, not in structural diversity. Managers
will attest to its high resilience to therapy. The second
trajectory acquires resourcing resilience but high
diversity is delayed. It is productive but has low information.
Presently judgement resides with the view that
high-diversity, low-productivity systems are healthier
than low-diversity ones, though high productivity is a
characteristic of early successions and it is what
makes them so attractive for exploitation. For
exploitation to be sustainable, systems must always be
allowed the resilience to recover their ascendancy.
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