Agriculture Reference
In-Depth Information
A
B
Ecosystem biomass
1
1
1
3
3
3
Species diversity
1 species per box =
low alpha diversity
3 types of boxes =
high beta diversity
3 species per box =
high alpha diversity
1 type of box =
low beta diversity
Disturbance
Maturity
Time
FIGURE 16.2
Alpha diversity vs. beta diversity in an
agroecosystem context.
For the sake of simplicity, each shape
represents a crop plant and each box a locality. This scale is
somewhat arbitrary in that a locality could comprise many more
crop plants; the point of the diagram is to show the contrast
between the two arrangements, which might represent (A) three
crops planted in strips, and (B) an intercrop of the three crops.
FIGURE 16.3
Changes in species diversity and biomass
during secondary succession.
fewer interactions occur. Following the disturbance,
the ecosystem begins the recovery process that is
called secondary succession (Chapter 17). During this
process, the system begins to restore the diversity of
species, interactions, and processes that existed before
disturbance.
Eventually the system reaches something called
maturity, which might be defined as the successional
condition in which the full potential for energy flow,
nutrient cycling, and population dynamics in that phys-
ical environment can be realized. The structural and
functional diversity of the ecosystem at maturity pro-
vides resistance to change in the face of further minor
disturbance.
Even though diversity tends to increase through the
stages of succession, recent research in ecology indi-
cates that maturity may not represent the stage with the
greatest diversity, at least in terms of species. Rather,
the greatest diversity is achieved as a system
approaches maturity, with diversity declining slightly
thereafter as full maturity is attained. Biomass contin-
ues to increase at maturity, though at a slower rate
(Figure 16.3).
diversity, but since the same species in the same relative
proportions are found at all locations over a wide area,
the grassland's beta and gamma diversity are relatively
low. As a contrasting example, consider a landscape
made up of a complex mosaic of simple communities,
such as nonnative grassland, a forest community dom-
inated by a single species, and a scrub community
growing on steep slopes. Alpha diversity is relatively
low in each of the communities, but any transect across
the area crosses a variety of species groupings, making
beta and gamma diversity relatively high.
The alpha and beta scales of diversity in particular
have useful application in agroecosystems. A cropping
system with high beta diversity, for example, can often
provide the same advantages as one with high alpha diver-
sity while offering greater ease of management
(Figure 16.2).
Successional Processes and Changes in Diversity
Diversity and Stability
Studies of natural ecosystems in early stages of develop-
ment or following disturbance have shown that all the
dimensions of diversity tend to increase over time. This
process takes place through niche diversification, habitat
modification, competitive displacement, resource parti-
tioning, and the development of coexistence, mutualisms,
and other forms of interference. Variability and fluctuation
in ecosystem processes are damped by this diversification,
giving the system the appearance of greater stability as
diversity increases.
When an ecosystem is disturbed, each of the dimen-
sions of its ecological diversity is simplified, or set back
to an earlier stage of development. The number of
species is reduced, vertical stratification decreases,
There has been considerable discussion in ecology about
the relationship between diversity and stability. There
appears to be some correlation between the two — that
is, the greater the diversity of an ecosystem, the more
resistant it is to change, and the better able it is to recover
from disturbance — but there is disagreement over the
degree and strength of the correlation.
Much of the problem arises from the restricted nature
of the accepted definition of stability. “Stability” usually
refers to the relative absence of fluctuations in the
populations of organisms in the system, implying a
steady-state condition, or a lack of change. This notion
of stability is inadequate, especially in relation to
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