Environmental Engineering Reference
In-Depth Information
Three fundamental limitations make any large-scale, long-term accounting of human
claims on the biosphere's production challenging—and uncertain. First, these claims
belong to three different categories that might simply be labeled extraction, manage-
ment, and destruction, but a closer look shows that there are overlaps and blurred
boundaries. Second, no single measure can adequately express the increasing
extent and the overall magnitude of human claims on the biosphere. Third, although
a combination of several revealing variables provides a better assessment of these
claims, it does not bring a fully satisfactory appraisal because of the many uncer-
tainties in quantifying the natural baselines and assessing the true extent of
human interventions. Every one of these summary descriptions requires a closer
examination.
Fishing and whaling, the killing of elephants for ivory, the capture of wild animals
for the pet market, and the collecting of wild plants and mushrooms are common
examples of extractive activities. Agriculture is the dominant activity in the manage-
ment category (“constructive transformation” could be another term): natural eco-
systems are replaced by managed agroecosystems that produce annual crops or are
composed of perennial species, and although either the original vegetation has been
completely replaced or its makeup has been changed beyond recognition, the site
continues to produce phytomass and to support nonhuman heterotrophs. In con-
trast, the destructive transformation of natural ecosystems alters or entirely elimi-
nates a site's primary production capacity as it destroys natural vegetation either
to extract minerals or to expand settlements: industrial facilities and residential and
commercial buildings also need associated space for requisite transportation net-
works, the storage of raw materials, and waste disposal.
But complications and qualii cations are obvious. Extractive activities target par-
ticular species, and as long as they are well managed, they can continue with minimal
adverse effects. But even in such cases there may be a great deal of associated
damage, for the harvesting of a target species may kill or injure other species (even
those belonging to a different class of animals, as exemplii ed by dolphins caught
in tuna purse seines). And once the target species gets overexploited or harvested to
extinction, such actions will have changed not only the composition of affected
communities and ecosystems but also their long-term dynamics and overall produc-
tivity: an entire ecosystem can be affected by such degradation, sometime irrevers-
ibly. Quantifying the human intervention only in terms of the specii cally extracted
biomass captures only part of the actual ecosystem effect. At the same time, even a
complete removal of a key heterotroph may have no effect on the overall level of
primary productivity or may actually enhance it: the removal of elephants, whose
