Environmental Engineering Reference
In-Depth Information
Agricultural, Urban and Human-Related Ecotones
The early literature discussing ecotones largely dealt with natural ecotones that are
generated by environmental factors such as soils, geology, and climate. More recent
research is increasingly including human-related boundaries. Human activity is
generating boundaries that did not exist before, changing their steepness and shifting
ecotonal location. These boundaries include a diversity of ecotone types, such as forest
clear-cut edges, margins between built-up and natural landscapes, and human-
generated features, such as lakes and plantations. Research on these human-generated
ecotones and their effects on biodiversity are related to a study area that is sometimes
termed “countryside biogeography,” examining biodiversity in human-dominated
landscapes.
Another recently developing research area that is relevant is that of urban ecology.
Sharp, human-caused transitions may result from human activities such as urbaniza-
tion, land-use changes, agriculture, grazing, or burning (see Fig. 9.1 ). These
boundaries occur at multiple spatial scales, ranging from local ecotones between
agricultural plots, urban areas, and roads, and their neighboring native habitat, to
large-scale ecotones such as shifting desert borders owing to desertification processes
and river divergence ( Fig. 9.1 ). These ecotones may be either static and fixed in space
or dynamic and shifting in location over space and time.
A substantial amount of work in both natural and human-related landscapes has
focused on what has been called the “edge effect.” This is the effect of the
juxtaposition of contrasting environments on an ecosystem. It refers to how the
local environment changes along some type of boundary, or edge and how biodi-
versity is affected by such edges. This idea is attributed by animal ecologists to
Aldo Leopold and his 1933 topic Game Management . It encompasses a wide range
of both biotic and abiotic trends associated with boundaries between adjacent
habitat types, natural or anthropogenic. Much of the reference to edge effects in
the recent landscape ecology literature has been related to human-caused
boundaries, and especially to boundaries between forest fragments and neighboring
patches of habitat that have been cleared. Again, there are no clear-cut conclusions
as to the effect of human-generated ecotones on biodiversity. The response largely
depends on the type of edge and its history as well the species in focus.
Its conservation implications are therefore complex and deserve further scien-
tific attention. A vast amount of research has been done on the effect of forest edges,
especially the effect of human-made forest fragments and their edges on biodiver-
sity. A review of edges [ 28 ] suggested there are many discrepancies in the litera-
ture, and a better understanding and search for general patterns requires a much
more mechanistic approach to examine the processes underlying such patterns.
In this context, there has been much effort to understand the effect of forest edges
on predation, brood parasitism, and the breeding success of birds. Paton [ 31 ] found
that in the majority of studies, nest success varied near edges with an increase in
both depredation and brood parasitism rates. The most conclusive studies suggest
that edge effects in birds usually occur within 50 m of an edge [ 31 ]. Since these
reviews, multiple studies on dozens of species and regions have been conducted.
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