Environmental Engineering Reference
In-Depth Information
forest in a small watershed. Few such kinds of heterogeneity have been examined for their
functional significance ( Turner and Cardille 2007 ). However, examples do exist. An experi-
ment in the high arctic showed that heterogeneity in depths of moss cover determined the
soil temperature and duration of favorable thermal conditions for microbial activity.
Deeper moss patches reduced the temperature and active season ( Gornall et al. 2007 ). The
thinner moss layers had higher levels of soil moisture and of plant-available nitrogen.
A second example of the role of internal spatial heterogeneity, which also interacts with
time, involves large woody debris left by extreme floods in the savanna landscape of
Kruger National Park, South Africa ( Pettit and Naiman 2007 ). Large woody debris, such as
accumulations of uprooted trunks, large branches, and tangles of twigs, had two effects in
the riparian zones. First, it permitted fire to occur closer to the river than usual, and
resulted in unusually high mortality of large trees that were within 5 m of piles of debris.
Second, plots supporting debris had higher levels of plant-available phosphorus in the
soils, and higher levels of vegetation cover. Over 36 months of observation, the large
woody debris piles and the patchy fire in the riparian zone resulted in alternative patterns
of succession. The altered heterogeneity in the riparian ecosystems has the potential to
cause long-lasting or permanent changes in the ecosystem functioning. Increased phospho-
rus availability can shift grazing patterns in the phosphorus-limited African savanna sys-
tem. Increased nutrient availability is likely to favor exotic and weedy species ( Foxcroft
et al. 2009 ). Increased plant cover in the riparian zones, particularly of densely branched,
upland shrubs, may enhance flammability and hence susceptibility to future fires.
Together these changes may well increase mortality of the crucial large riparian trees, or
inhibit their germination and establishment. Thus, the entire ecology of the riparian corri-
dor may be sensitive to the pulsed input of coarse woody debris and new thresholds of
resources, disturbance, and species composition.
External heterogeneity refers to the spatial pattern outside the boundaries selected for a
particular ecosystem study. Traditionally ecologists were silent about even such contextual
facts as what kinds of ecosystems were on the borders of a study site. Issues such as the
identity of adjacent and nearby habitat types, and their arrangement on the landscape
have not been much addressed in ecosystem ecology.
External heterogeneity may act on the focal ecosystem in several ways. Exactly how
external heterogeneity functions depends on the nature of flows from outside the eco-
system ( Cadenasso et al. 2003; Strayer et al. 2003 ). Flows may be by diffusion, by the mass
movement of biota or advection of organisms and other materials, or movement along spe-
cific paths or networks. For organisms or propagules, or certain chemicals, mortality rate
or decay rate will interact with the velocity of movement to determine the amount that
arrives at a focal ecosystem from a distant source. Which pathway supports movement of
a flow of interest determines how the focal ecosystem and its surrounding landscape inter-
act. In the case of diffusion or mass movements, the mosaic structure of the surrounding
environment will be crucial. How permeable the patches and their boundaries are to the
mass flow is the determining factor, and this is modified by the spatial arrangement of
patches.
Flows of water are an excellent example of how an altered external mosaic and imposi-
tion of a novel network affect a target ecosystem. Consider a stream in an unmanaged, for-
ested watershed. The movement of water can be overland, in superficial layers, or in deep
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