Environmental Engineering Reference
In-Depth Information
Although the methodology and i ndings of these studies were questioned (Caddy
et al. 1998), slowly declining trends in mean trophic levels (the phenomenon known
as i shing down marine food webs) have been subsequently coni rmed by many
studies using detailed local data, including i shing around Iceland, in the Gulf of
Thailand, in both eastern and western Canadian waters, in the Chesapeake Bay, and
in the Celtic Sea (Zeller et al. 2009). Even if the continuing declines were to amount
to just 0.1 of trophic level per decade, such an apparently small decrease would lead
inevitably to more i shery collapses, and within a century, there would be no car-
nivorous i shes available for large-scale commercial capture.
But such conclusions were questioned once again by Branch et al. (2010) whose
comparison of catch data, trawl surveys, and stock assessments showed no discern-
ible i shing-down the food web on the global scale and concluded that marine
organisms at all trophic levels (from mollusks to tunas) are being harvested in ever-
higher quantities. But this is hardly good news: it means that intensifying i sheries
can collapse even as the mean trophic levels are stable or increasing. In turn, these
conclusions were questioned by the authors of original i shing-down reports, offer-
ing yet another illustration of our uncertain understanding of human impacts
on the biosphere. Moreover, Smith at al. (2011) argue that even the i shing of low-
trophic-level species at conventional maximum sustainable yield levels can have
large impacts on other parts of marine ecosystems and that reducing such rates
would be desirable.
Harvesting Woody Phytomass
Reconstructing past wood harvests (at any scale) is to engage in guesses and crude
approximations. As already shown (in chapter 2), historical data on wood consump-
tion as household and industrial fuel and as a raw material in construction are
scarce, highly discontinuous, and hard to interpret. Modern statistics, some going
back to the late nineteenth century, offer fairly good accounts of commercial wood
uses for lumber and pulp, but as far as the use of wood as household fuel in low-
income countries is concerned, they too are largely estimates. In this section I i rst
present the global estimates of fuelwood combustion and then review the best avail-
able data on the use of wood as a raw material.
Estimates of aggregate fuelwood consumption during the medieval period or in
antiquity would be nothing but guesses, not only because of our poor knowledge
of what constituted typical usage rates but also because a complete lack of informa-
tion about the actual extent of seasonal heating and the frequency and duration of
