Environmental Engineering Reference
In-Depth Information
FIGURE 10.9
Cattle corrals in Tanzania, East Africa. The corrals are the circular and irregular forms outlined
by shrubs and small trees. Active corrals contain structures as well. An abandoned corral with a degenerating
woody boundary appears near the center of the photograph. These systems are similar to those investigated by
Porensky (2011)
.
(Photo copyright S.T.A. Pickett.)
the associated buildings and infrastructure generate heterogeneity within or adjacent to
ecosystems. Forest harvesting, whether by clear-cutting or by different patterns of thin-
ning, generates patches that differ from those that are not harvested, or that were har-
vested at different times in the past. Similarly, the infrastructure that people insert into
managed ecosystems is a source of heterogeneity. In managed forests, even small, one-
lane unpaved roads alter surface water flow, local light availability, and access of new
invaders (
Gascon et al. 2000
). A more extreme example is the heterogeneity that results
from urban development and management of different properties or neighborhoods
in cities and suburbs. Such heterogeneity combines biological elements, buildings and
infrastructure, and a variety of surfaces, ranging from native to paved (
Cadenasso et al.
2007
). Urban spatial heterogeneity is conspicuously mosaic in form (
Figure 10.2
;
McGrath
et al. 2007
). The question of how urban ecosystems function remains an important frontier
for ecology (see Chapter 17). Metropolitan areas, consisting of cities, suburbs, and exurbs,
are in fact ecosystems and their unique heterogeneous structures and associated functions
require further elucidation (
McGrath and Pickett 2011; Pickett et al. 2011
).
These examples point to some basic ways of thinking about heterogeneity and eco-
systems. Some point to parallels with population and evolutionary ecology, such as
concern with biodiversity and genetic structure of populations. Some reflect the widely
recognized distributions of organisms along local and regional gradients (
Fox et al. 2011
).
Others connect with the continental and global gradients of diversity (
Colwell 2011
).
Attention to heterogeneity, such as that of plant community composition, has been part of
the core of ecology for a long time (
Hutchings et al. 2000
), but only recently has been
applied in the ecosystem context. Other examples have relied on the newer disciplines of
landscape ecology (
Lovett et al. 2005; Dutilleul 2011
) or ecological engineering (
Jones et al.
1994
). For all aspects of heterogeneity, however, empirical evidence about how heterogene-
ity affects ecosystems is still modest, and this ecological specialty is still too new to sup-
port confirmed generalizations (
Turner and Cardille 2007
). We can, however, provide a
framework that suggests what to look for in understanding the relationships between
