Environmental Engineering Reference
In-Depth Information
survival of these forests and did not alter the successional sequence of
forest development.
Recent developments on the functional complexity of ecosystems show
that small disturbances may actually enhance ecosystem function and
increase resilience. Holling (1986) and Holling et al.
(1994) suggest that
some natural disturbances initiated by i re, wind and herbivores are an
inherent part of the internal dynamics of ecosystems and in many cases
set the timing of successional cycles.These natural perturbations are part
of ecosystem development and evolution, and seem to be crucial for main-
taining ecosystem resilience and integrity (Costanza et al., 1993). In the
absence of such shocks, the system will become highly connected and this
will provoke even larger perturbations that are more destructive to the
ecosystem because they reduce the ability of the system to survive similar
shocks in the future (Scholes and Walker, 1993).
Some empirical studies reveal that herbivores may enhance a system's
ability to resist environmental perturbation. For example, in their work
on Florida mangroves, Simberlof et al.
(1978) reported that the action of
isopod and other invertebrate root borers resulted in new growth of roots
at the point of attack. More extensive root systems in mangroves result in
greater stability and resistance to storms and therefore benei t the plant.
In Britain, Hulme et al.
(1999) carried out a study to evaluate the ef ects
of sheep grazing on the productivity of upland
Agrostis- Festuca
grassland.
The experiment controlled sheep grazing at light, heavy and moderate
levels. Both low and high levels of grazing resulted in the spread of less
desirable species such as
Nardus stricta
and
Molinia caerulea.
Moderate
levels of grazing maintained preferred species such as
Festuca rubra
and
Agrostis capillaris
and prevented the spread of
Nardus stricta
and
Molinia
caerulea
.
Non-equilibrium rangeland systems as practised by nomadic pastoral-
ists in parts of Africa have evolved opportunistic management regimes
that employ moderate levels of grazing intensity that do not eliminate key-
stone elements but instead maintain the resilience of these components.
Scholes and Walker (1993) have suggested that events such as i re and
herbivory may play an important role in maintaining the diversity and
resilience of such systems. Nutrient release following small i res supports
a l ush of new growth without destroying all of the old growth. Rangeland
patches are af ected but the forage resource remains intact. Small i res
prevent the accumulation of forest biomass, which fuels very large i res
that can decimate large areas of rangeland (Scholes and Walker, 1993) or
whole forests (Holling et al., 1994). Such events may af ect the parameters
of the system and cause it to cross a threshold into an alternative state,
which may alter the system's capacity to provide ecological services. For
