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

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Reynolds/ Aquatic Ecosystem Health and Management 5 (2002) 3–17
external sources are truncated, owes principally to
Sas (1989). In this instance, the restorative therapy,
the reduction in external nutrient loading, is the forcing
agent but the anticipated response of a reduced
algal biomass response is neither immediate nor even
linear. Several mechanisms are known to contribute to
this behavior, including the buffer provided by unused
supportive capacity and the recyclability of nutrient
resources within the system (‘internal loading’). This
buffering capacity is not inexhaustible but, in terms of
management and of returns on investment, it can seem
stubbornly persistent (Søndergaard et al., 1993).
Supposing the capacity of the impacted system to
have once supported lower algal and plant biomasses,
the eutrophication and the protracted recovery are
clearly parts of a strongly hysteretic behavior pattern.
During the delay period, at least, the resilient system
fails to regain a state of improved health.
In almost every other ecological context in which
the word has been used, resilience refers to the collective
capacity for homeostasis that is acquired by
developing ecosystems. Before the advent of a subdiscipline
dealing with the large-scale properties of
systems (‘macroecology’: Brown and Maurer, 1989),
the manifestation of a functionally mature ecosystem
was considered to be its stability. The notion of a sort
of self-maintaining equilibrium condition, with very
little fluctuation, was memorably challenged by
Holling (1973). He recognized the scales and amplitude
of normal, natural environmental variability that
surviving systems have successfully to absorb. Their
ability to do so is a measure of their resilience. Thus,
resilience is the property of the system that keeps it at
some recognizable steady state and permits it to return
there following disturbances wrought by external
forcing. It conforms to the role of a Lorenzian attractor
of Chaos Theory in suppressing random behavior
(Gleick, 1988). Thus, the supposed stability of a system
is the manifestation either of its accommodation
of mild environmental fluctuations or of the speed
with which it is able to return to an erstwhile steady
state. Provided the system retains the inocula of component
species from which new populations may
develop, it may well recover its former structure
(Harrison, 1979). In other words, and in contrast with
the earlier definition, the resilient system is one that is
able to regain quickly a state of perceived health.
It would be relatively simple to declare that these
contrasted and seemingly opposed usages of
resilience are unnecessarily confusing and that one of
them should be rejected. Neither would it be too
difficult to suggest that, because of the narrowness of
its application and because the property it described
is, in any case, closer to the understanding of resistance
than to resilience, Sas’ (1989) usage should be
the one that is discarded. Before that happens, however,
the opportunity should be taken to explore the conceptual
commonality of both usages to the adverse
anthropogenic impacts on the health of aquatic
ecosystems. More to the point, we need to assure ourselves
of the mechanisms of ecosystem resilience, the
factors that contribute to and achieve the so-called
steady states and the critical levels of forcing that
might move the system to some other steady state.
Here, I seek to formulate a model that incorporates
the assembly of aquatic systems, their robustness and
the levels of forcing they may tolerate: the objective is
a single view of ecosystem function that subsumes
both understandings of system resilience.
The structure of aquatic ecosystems
Functional ecosystems comprise complex consortia
of organisms with closely interrelated activities
that together process the resources of geographicallydistinct
segments of the planet into living biomass.
Most of the driving energy for this processing comes
from sunlight which has to be captured, stored chemically,
and then released in a regulated manner to sustain
the work involved in gathering and assembling
resources into biological structures. At their most fundamental
level, these exchanges are biochemical and
they occur at the level of cytological structures—light
harvesting centres, ribosomes, and mitochondria—the
assembly of which is coordinated within cells, according
to instructions copied in their DNA, each time it is
replicated. Cells form tissues, the specialist functions
of which contribute to the physiological workings of
whole organisms, whose nature is to grow and reproduce.
Organisms are, in turn, functionally differentiated
among energy-capturing primary producers and
resource-cycling heterotrophs. Their populations contribute
to the ecological makeup of communities and
their mutual interactions, most significantly in governing
the trophic linkages of the food web, determine
the eventual structure of the ecosystems.
This much is well understood but there are two
aspects that require emphasis. One is the continuous
nesting of the relevant processes, from molecule
and photon up to the material and energy budgets
of whole ecosystems. In order to comprehend and
quantify large-scale functions, it is justifiable to
view ecosystems as fractal series, each level being