Disturbance, Succession, and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 239

Another aspect of successional development that has
important agroecological implications is the increase in
biomass and the standing crop of organic matter with time,
especially in the early stages of succession. Since biomass
is eventually converted to detritus and humus as it passes
through the decomposers, this increase in biomass results
indirectly in an increase in soil organic matter.
During the early stages of recovery, nutrient availabil-
ity is usually high and nutrient conservation relatively
inefficient. Fast-growing, ruderal plant species quickly
become dominant, and population interaction is limited to
the few species present. As succession progresses, nutrient
retention improves, colonizing species begin to occupy a
greater diversity of niches in the system, population inter-
action intensifies (especially interactions that involve
resource partitioning and mutualistic interference), and the
structure of the ecosystem becomes more complex and
interconnected.
If enough time is allowed to pass after a disturbance,
an ecosystem eventually reaches a point (formerly referred
to as the climax stage) at which most of the characteristics
presented in Table 17.1 cease to change significantly in
rate or character. In terms of species diversity, for example,
new colonizing species equal the number of emigrating
species or those going extinct. Nutrient losses from the
system are balanced by inputs from outside. The popula-
tion levels of species fluctuate seasonally, but do so around
a fairly constant mean number. At this stage, the system
is once again in a tenuous equilibrium with the regional
climate and local conditions of soil, topography, and mois-
ture availability. Change still occurs, but it is no longer
directional, developmental change, but change oriented
around an equilibrium point. In chapter 2, we described
such a condition as one of dynamic equilibrium , a concept
that takes into account the fact that all environments are
constantly changing and evolving, with new disturbances
occurring frequently on at least a small scale.
In the typical mature ecosystem, then, localized sites
may be undergoing disturbance on a regular basis, but the
characteristics listed in Table 17.1 are developed suffi-
ciently enough for energy and nutrient utilization to be
highly efficient, food webs complex, and mutualistic rela-
tionships prevalent. The system is relatively stable, in the
double sense of being able to resist change and to be
resilient when disturbance occurs. Thus, the disturbance
events that do occur do not result in drastic change, but
neither do they allow a steady-state condition.

example, these high-intensity disturbance events — as
long as they are low in frequency — tend to generate forest
systems with both high species diversity and high biomass
(Vandermeer et al., 2000; Mascaro et al., 2005). Ecologists
studying these systems have posited the intermediate dis-
turbance hypothesis , which states that in natural ecosys-
tems where environmental disturbances are neither too
frequent nor too seldom (at some intermediate frequency)
both diversity and productivity can be high (Connell,
1978; Connell and Slayter, 1977). The disturbance in these
systems retains the early-successional characteristic of
high productivity, while the system's overall stability
allows the high species diversity more characteristic of
mature ecosystems.
Some natural ecosystems for which the intermediate
disturbance hypothesis may apply are presented in
Table 17.2. An examination of these systems reveals that
intermediate disturbance can come about through a great
variety of different combinations of disturbance fre-
quency, disturbance intensity, and disturbance scale. At an
ecosystem level, relatively intense and frequent distur-
bance on a small scale, for example, can have an effect
similar to that of low-intensity, low-frequency disturbance
on a larger scale.
In many intermediate-disturbance situations, distur-
bance distributed irregularly over the landscape in time and
space creates what is known as a patchy landscape , in
which numerous stages of succession occur in a relatively
small area. The variation in developmental stage from patch
to patch contributes to the maintenance of considerable
diversity at the ecosystem level. Successional patchiness
can therefore be seen as an important aspect of the ecolog-
ical dynamics of ecosystems. Patch size, variation in patch
development, and the nature of the interfaces between
patches all become important variables, and ecologists have
invested considerable study attempting to understand their
role in natural ecosystems (Pickett and White 1985; Hubbell
et al., 1999). The inherent patchiness of many agricultural
landscapes points out the potential application of interme-
diate disturbance and patchiness to agroecosystem man-
agement (Bruun, 2000). As we will see in more detail in
Chapter 22, the concept of patchiness has become especially
important in approaches that seek to conserve biodiversity
and ecosystem services in agricultural landscapes
(McIntyre and Hobbs, 1999; Swift et al., 2004).

APPLICATIONS TO AGROECOSYSTEM
MANAGEMENT

I NTERMEDIATE D ISTURBANCE

Modern agriculture has developed practices, technologies,
and inputs that allow farmers to ignore most successional
processes. In place of natural recovery, farmers use inputs
and materials that replace what is removed at harvest or
altered with cultivation. Constant disturbance keeps the

In some ecosystems, the frequency, intensity, and scale of
disturbance is such that the system never reaches full
maturity, but is nevertheless able to maintain the species
diversity, stability, and energy-use efficiency characteristic
of a mature ecosystem. Where hurricanes occur, for