Environmental Engineering Reference
In-Depth Information
a simple claim that humans already “co-opt” about 40% of the planet's photosyn-
thetic production. If true, this i nding implies highly worrisome consequences for
the future of humanity. If the combination of further population growth and higher
per capita consumption were to double, the overall human demand for the products
of primary production (a development that could easily be foreseen to occur before
the middle of the twenty-i rst century), then we would have to harvest all but a
small share of the products of terrestrial photosynthesis, transforming every culti-
vable area into cropland or a tree plantation and leaving virtually nothing to sustain
natural ecosystems.
That would clearly be an impossible situation, a development incompatible with
the perpetuation of a well-functioning biosphere. To my surprise, there was no
immediate (or later) critical reexamination of that original claim; instead, the 40%
rate of “appropriation” or “co-optation” attracted considerable attention, and it
became a frequently cited indicator of the progressing human impact on the bio-
sphere. A comprehensive analysis of the concept and, even more so, a detailed
examination of all individual assumptions and approximations that went into con-
structing the i rst HANPP assessment and its successors, introduced in the previous
section, could easily i ll a long book chapter rather than a short subsection.
That is why I will focus on only a few key problems, some general and integral
to any construction of additive indicators or indices, others specii c to the HANPP
exercises. Questions and rel ections should start with the meaning and appropriate-
ness of a highly problematic denominator chosen to calculate the appropriation
ratio. As I explained in some detail in the i rst chapter, the NPP is a theoretical
concept, not a variable that is subject to direct measurement or a physical entity
that can be left alone or harvested—and hence it is incorrect to say that people can
use it, directly or indirectly. And while Imhoff et al. (2004a) estimated all below-
ground production, most of the other published NPP rates refer only to aboveg-
round production, but that exclusion is not always clarii ed either in the original
publication or in the references made to HANPP studies.
Some HANPP studies make the distinction explicit, and some even use the
acronym aHANPP. This restriction creates two obvious problems. First, grasslands
store more phytomass underground rather than aboveground, and their below-
ground NPP is in most cases considerably higher than their shoot productivity:
its share is roughly 50%-65% in tall grasses, 60%-80% in mixed-grass prairie,
and 70%-80% in short-grass ecosystems (Stanton 1988). Large herbivores only
rarely remove belowground phytomass, and hence calculating the HANPP of grass-
lands by considering only shoot NPP and shoot consumption by grazers misrepre-
Search WWH ::




Custom Search