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

Reynolds/Aquatic Ecosystem Health and Management 5 (2002) 3–17 9
the maximum sustainable biomass is about 10.4 mol
(125 g) carbon m –2 . Some of the assumptions in that
calculation may now be viewed as inappropriate and
employing more realistic assembly costs of ~ 470 kJ
per mol organic carbon, a supportable maximum biomass
of about 25 mol carbon (300 g) m –2 may be proposed.
Both figures accommodate comfortably
reported natural maximal densities of phytoplankton
(~ 50 g C m –2 ) and the active parts of terrestrial plant
biomass (~ 75 g C m –2 : see Margalef, 1997). It is a
good deal less than the 20 to 30 kg C m –2 standing
crops of tropical forest stands, though this is, of
course, dominated by the accumulated necromass that
is wood.
The numbers used to construct Figure 1 are less
important to the present argument than the shape that
they generate. In systems supporting low biomass and
incurring low maintenance costs, there is a large
buffer of spare harvesting capacity which is available
to fund the provision of new harvesting biomass. It
will also provide a cushion against exploitation by the
consumer organisms of the phagotroph tier, without
loss of standing crop or growth potential, though it is
sustained at the cost of reduced investment. It is also
a cushion against submaximal solar energy income,
which, at this scale, is consequential upon variable
cloud cover, day length and declination. Wind mixing
and plankton entrainment to depth in lakes and seas
also exacts a heavy toll in light absorption which can
be quantified and set to subtract from the potential
energy income in Figure 1. However, so long as the
energy harvesting can satisfy the maintenance cost,
the existing structure can endure and recover quickly
from the setbacks of environmental variability.
The model can be adapted to represent the balance
between almost any set of anabolic assembly and
catabolic destruction, so long as they are amenable to
rendering in energy fluxes in comparable units. At the
scale of the individual cell, it summarizes the ability
to increase mass and divide; at the level of the population,
it represents the potential to increase the crop.
It does not yet explain the interactions of different
populations with each other, but the supposition is
advanced that the individuals and the species that are
able to invest most in new or additional structure are
those most likely to contribute to the assembly at the
next level (strongest-growing individuals contribute
most to the populations; strongest-growing populations
are, at least starting from equal abundance, most
likely to dominate communities). In other words,
rapid structural anabolism also demands high exergy
margins. This is a decisive mechanism about how the
dynamics of individuals— the only organizational
level under direct genetic control—eventually feed
upwards to determine the development of communities.
The emergent behavior, expressed in the assembly of
functional ecosystems, relates to the exergy arising from
the activities of the most efficient individuals.
This, in turn, establishes the pattern of development
in the building ecosystem. The apparent ability
of communities to organize themselves (autopoesis)
around the fluxes of usable energy has, for long, promoted
speculation and debate about its mechanisms
and controls, and about what Straškraba (1980) called
“the goal-seeking behavior” of systems. The principle
of maximum exergy permitting maximal structural
dynamism (Jørgensen, 1982) is but one of several
thermodynamic explanations for the high-order patterns
of ecosystem. That of Odum (1982) argued for
the selection of organisms and structures that make
the best use of the available power, while Ulanowicz
(1986) regarded system growth and development as
the simultaneous increase in system throughput, using
increasingly complex structural networks accumulating
more information. These theories are not mutually
exclusive (Salomonsen, 1992), indeed, they are
mutually supportive. Their merits and demerits are not
the concern here, save that it is important to emphasize
that each relates the strong and consistent patterns
of community organization to the sum of the
behaviors of individual organisms. Analogizing them
to the flow of energy and to the rate of its biological
dissipation that their activities represent, however,
recognizes openly the finite capacity to buffer contingent
variability in the energy sources. Some preliminary
estimates of their probable ranges are noted in
Table 1.
The effect of external forcing
on community assembly
It is rather obvious that few planetary ecosystems
carry either the biomass or the structural complexity
for which the foregoing passages argue, or achieve the
ultimate autopoetic goal of stability. The override of
resource scarcity as a causal impediment is generally
recognized and the systemic reaction to eutrophication
and its reversal make it an issue for consideration
here. The override caused by frequent exceedence of
buffer capacity is also well accepted but the relationship
between external forcing and internal exergy
needs to be clarified first.
We have already shown that the assembly of an
energy-harvesting network provides a capacity for