Ecosystems in a Heterogeneous World - Fundamentals of Ecosystem Science - page 190

and the sediment concentration of runoff water. Movement of plant seeds from pit to pit,
and from surface to pits was also increased by the proportion of disturbed surface.
Plant Neighborhoods
A common feature in many ecosystems, ranging from deserts through savannas to
moist forests, is the spatial heterogeneity created by nonuniform arrangement of individ-
ual plants. This is especially conspicuous when trees are the dominant growth forms. The
conditions near or beneath various species often differ considerably. For example, in
deserts, the area beneath shrubs may contain higher amounts of organic matter, and lack a
soil microphytic crust that elsewhere generates an impervious surface ( Schlesinger et al.
1996 ). In this case, heterogeneity reflects the presence or absence of a woody plant of any
species ( Figure 10.6 ). In other cases, the growth forms may be key differentials in hetero-
geneity. For example, in savannas, the understory grasses and grazing pressures beneath
trees can differ substantially ( Vetaas 1992 ). In other cases, the identity of the species is the
key driver of heterogeneity. For example, in moist forests, soil nutrients, moisture, and lit-
ter dynamics can differ beneath different species of trees ( Lavery et al. 2004 ). Thus, in
many ecosystems, the heterogeneity of higher plant species can affect several ecosystem
processes based on differences in nutrients, light, water, and microbes.
Stream Patchiness
Heterogeneity is not restricted to terrestrial ecosystems; streams exhibit great hetero-
geneity as well ( Fisher et al. 1998 ). Heterogeneity results when pools and riffles alternate
with one another along a stream channel, and have contrasting sediments and current veloc-
ities. Heterogeneity in streams can also be created by natural dams formed by tangles of
branches or large logs that accumulate fine sediments and organic matter on their upstream
sides ( Figure 10.7 ). Heterogeneity through time in stream channels also exists. The migration
FIGURE 10.6 An arid landscape near Adelaide, Australia, showing the contrast between shrub-dominated
patches and the mosaic dominated by microphytic soil crust. Shade, sediment, seeds, and leaf litter are among the
factors that differ beneath the shrub-dominated patches compared to the crusted areas among shrubs. (Photo copy-
right S.T.A. Pickett.)