Environmental Engineering Reference
In-Depth Information
grasslands that are lightly grazed by well-managed numbers of domestic animals
(alpine meadows with cattle and sheep) or natural forests where harvests remove
only annually renewable fruits, seeds, or nuts (such as the collection of Brazil nuts
in an old-growth tropical rain forest) or are limited to a few valuable animals (such
as the trapping of small mammals for their pelts in Canada's boreal forest): the
primary production of these areas remains virtually (or largely) intact.
In between these two extremes is a wide continuum of human interventions.
Many urban and most suburban residential areas have retained some of their site's
potential primary productivity thanks to lawns, parks, and street trees: these new
anthropogenic ecosystems are highly fragmented and have a low biodiversity and
low productivity when compared with their natural predecessors, but on a small
scale they might approach or even surpass the performance of the ecosystems they
replaced. Selective logging (including its extreme mode, which uses helicopters to
remove large tree trunks from steep slopes) takes only some targeted trees, preserves
much of the site's productivity, and does not change its regenerative potential, while
harvesting timber by forest clear-cutting is analogous in its destructive impact to
clearing land for i eld crops.
This most extensive of all anthropogenic land conversions is a unique hybrid
of destruction and high productivity. The cultivation of annual or perennial crops
is usually predicated on a near total elimination of a climax natural ecosystem, and
most of the i elds have lower primary productivities than the plants they replaced.
But in many cases the difference in productivity is not that large, and good agro-
nomic practices (multicropping with rotations that include high-yielding leguminous
cover crops) may actually result in higher yields. The conversion of a short-grass
Canadian prairie to an alfalfa i eld may have a minimal impact on overall primary
productivity, and it will maintain such important ecosystem services as protecting
soil against erosion, retaining moisture, and adding bacterially i xed nitrogen.
Similarly, conversions of tropical forests to rubber or cocoa plantations maintain
a semblance of an arboreal ecosystem (indeed, the FAO classii es these plantations
in a forest category), support a relatively rich complement of heterotrophs, and
continue to provide protection against excessive soil erosion. But in both cases, a
crucial difference remains: before conversion, the primary productivity of a natural
ecosystem was entirely available for consumption by a variety of wild heterotrophs,
and carbon and nutrients in the unconsumed organic matter were recycled (largely
in situ) through bacterial and fungal metabolism.
The different consequences of these interventions and a continuum of invasive
practices that makes it impossible to dei ne clear intervention categories are the best
