Environmental Engineering Reference
In-Depth Information
diversity of arthropods, and far more arthropod predators including many taxa entirely absent from
temperate forests. For this reason, the conversion of native habitats nearer the equator is often more
destructive to biodiversity than habitat loss in the temperate zone. Bioenergy cropping systems that
include a diversity of species as feedstocks or where the target species are grown in concert with other
plants are almost always preferable to monocultures. The cultivation of native species is preferable over
the use of exotics, and it is also beneficial for biodiversity when numerous native species are growing
along with the feedstock. The greater structural diversity provided by the varying plant architecture
(i.e., forest canopy, subcanopy, shrubs, and herbs) provides more niches to allow coexistence of many
more species. For woody (e.g., trees and shrubs) and grass-based feedstocks, polycultures will usually
support more species of birds, mammals, amphibians, reptiles, and arthropods than monocultures.
6.2.1.4 species-area relationships
One of the most consistent relationships in ecology is the finding that as area increases, the number
of species found in this area also increases. The rate of increase as one moves from a square meter to
a square kilometer obviously differs for a patch of arctic tundra compared with a tropical rainforest,
but the general positive relationship is virtually universal. This simple empirically verified pattern
leads to some surprisingly profound insights. Generally speaking, from a species-area perspective,
larger patches of undisturbed or less disturbed habitats support more native species. If more intensive
systems for growing bioenergy feedstocks allowed larger areas to be set aside for conservation, this
would benefit species dependent on native habitats; the less land that is disturbed, the greater the mean
and median size of remaining habitat blocks. Less intensive systems sometimes require more area to
produce equal yields so that production systems and the amount of area under production are linked.
Many species, such as large predators and herbivores, require large areas to persist. Additionally,
some biomass production systems, especially those involving re-establishment of native warm season
grasses or trees where appropriate, may, if purposefully placed on the landscape, create corridors,
increase native habitat patch sizes, and reduce fragmentation. Nevertheless, habitat loss resulting from
conversion of diverse native habitats into intensive biomass cropping systems would result in a reduc-
tion in patch size and increasing fragmentation and isolation of native habitats. Simply put, along
with pressures to convert land for more traditional uses such as agriculture, housing, and commercial
development, an expanding bioenergy market will likely be a mixed bag for biodiversity.
6.3
BIoenerGy croPPInG systems and sPecIes selectIon
6.3.1 r
ESiduES
Cellulosic “residues” and “waste” have been touted as bioenergy feedstocks with little to no eco-
logical downside and high compatibility with traditional agricultural and forestry practices. In the
context of municipal organic waste (e.g., grass clippings, trimmings, leaves, scrap wood, and paper
products), this generalization is quite tenable (Fargione et al. 2008; Koh et al. 2008). However, in the
context of agricultural crops and forest residues left after traditional management, this generalization
is much more context specific (Harmon 2001; Lal and Pimentel 2007; Robertson et al. 2008).
Organic residues are an important source of soil organic matter; food for soil organisms (Burger
2002); and, in minimum tillage and no-till row crop agricultural systems, provide dormant season
cover to the soil surface and are an important driver of soil organic matter dynamics (Six et al. 1999).
Soil organic matter content influences soil tilth, erodibility, and water-holding and cation-exchange
capacity (Fisher and Binkley 2000; Vance 2000; Blanco-Canqui et al. 2005). Accumulation of
carbon from organic residues into the soil also provides an important sink for atmospheric CO
2
(Batjes 1998; Harmon 2001; Post et al. 2004; Omonode and Vyn 2006). Cover of organic material
on the soil surface reduces erosive potential by dampening the effect of rain drops and decelerating
and decentralizing surface flow (Dabney et al. 2004; Sayer 2006; Montgomery 2007). The quan-
tity of organic residue required to maintain soil health and reduce erosion risk may vary greatly
