Environmental Engineering Reference
In-Depth Information
What does “stability of the ecosystem” Mean?
By definition, stability is the ability of an ecosystem to return to an
equilibrium state after a temporary disturbance (Holling 1973). MacCillivray
and Grime (1995) updated this defi nition considering both the community's
ability to (1) resist change in order to maintain an ecosystem function
(
resistance
), and (2) recover to normal levels of function after disturbance
(
resilience
).
Many marine ecosystems of the world share a similar confi guration of
their biological community structure, characterized by a crucial intermediate
trophic level often occupied by a small plankton-feeding pelagic species
(Bakun 1996). The major control of trophic dynamics in these
wasp-waist
ecosystems (sensu Rice 1995) is neither “bottom-up” nor “top-down” but
rather “both up and down from the middle”, as variations in size of these
populations may have major effects on the trophic levels above, which
depend on the wasp-waist species as their major food source, and also
on the trophic levels below, which are fed upon by massive wasp-waist
populations (Bakun 1996).
In the ecosystem development theory of Odum (1969), stability is
viewed as one property of mature ecosystems, which tend, over time, to
increase in size and diversity within the constraints of available resources.
Hence, along with other characteristics, mature ecosystems, according to
Odum (1969), should incorporate a high information content, attain high
biomass, and have a high capacity to entrap and hold nutrients for cycling
within the system.
System recovery time, defi ned as the time required for all elements of a
system to return to their baseline biomass values following a perturbation,
is used here as a measure of ecosystem internal stability, or resistance to
external changes. This approach seeks to identify the ecosystem attributes
directly involved in the stability and to address their relation to ecosystem
maturity (Christensen 1995, Ludovisi et al. 2005). A comparative analysis
of systems behavior was carried out using a set of ecosystem goal functions
previously identifi ed as representative of Odum's attributes of ecosystem
maturity (Christensen 1995). The attributes were chosen to represent three
different aspects of ecosystem development: (
i
) complexity in community
structure; (
ii
) community energetics; and, (
iii
) overall community
homeostasis.
According to Holling et al. (1995) the structure of biological communities
is therefore controlled through the balance of destabilizing and stabilizing
forces. While destabilizing forces are important in maintaining diversity,
resilience and opportunity, stabilizing forces, such as nutrient recycle, are
important in maintaining productivity and biogeochemical cycles. The role
of destabilizing forces may be particularly important in pelagic marine
