Principles of terrestrial ecosystem ecology.pdf

310 14. Landscape Heterogeneity and Ecosystem Dynamics
Disturbance interval / Recovery interval [log scale]
10.0
5.0
1.0
0.5
0.1
0.05
0.01
Equilibrium
or steady
state
Figure 14.2. Effect of disturbance size (relative to
the size of the landscape) and disturbance frequency
(relative to the time required for the ecosystem to
recover) on the stability of landscape processes.
Landscapes are close to equilibrium when disturbances
are small (relative to the size of the study
area) and when they are infrequent (relative to the
time required for ecosystem recovery). As distur-
Fire can also create large patches of a single
successional stage on the landscape (Johnson
1992). The 1988 fires in Yellowstone National
Park that burned 3200km 2 of old pine forest
altered large areas of the landscape. Fires of
this magnitude and intensity recur every few
centuries (Romme and Knight 1982). Longterm
human fire suppression has increased the
proportion of late-successional communities in
many areas. This results in a more homogeneous
and spatially continuous, fuel-rich environment
in which fires can burn large areas.
Even large disturbed areas, however, are often
internally quite patchy. Fires, for example,
generate islands of unburned vegetation
and patches of varying burn severity. These
unburned islands act as seed sources for postfire
succession and protective cover for wildlife,
greatly reducing the effective size of
the disturbance (Turner et al. 1997). In many
cases, these patches become less distinct as
Stable, low
variance
Stable
high
variance
Stable,
very high
variance
Stable
low
variance
0.25
0.50
Disturbance extent / Landscape extent [log scale]
Unstable
system,
malfunction
or crash
0.75
bances become more frequent or larger, the landscape
becomes more heterogeneous and there is
increasing probability that the individual patches
may undergo a different successional trajectory.
(Redrawn with permission from Landscape Ecology,
Vol. 8 © 1993 Kluwer Academic Publishers; Turner
et al. 1993.)
succession proceeds, so spatial heterogeneity
may decline with time in non–steady state
mosaics.
Human-induced disturbances alter the
natural patterns and magnitude of landscape
heterogeneity. Half of the ice-free terrestrial
surface has been transformed by human activities
(Turner et al. 1990). We have cleared or
selectively harvested forests, converted grasslands
and savannas to pastures or agricultural
systems, drained wetlands, flooded uplands, and
irrigated dry lands. Isolated land use changes
may augment landscape heterogeneity by creating
small patches within a matrix of largely
natural vegetation. As land use change
becomes more extensive, however, the humandominated
patches become the matrix in which
isolated fragments of natural ecosystems are
embedded, causing a reduction in landscape
heterogeneity. These contrasting impacts of
human actions on landscape heterogeneity