Human Dimensions That Drive Soil Degradation - Principles of Sustainable Soil Management in Agroecosystems - page 174

and migration to even less productive and more fragile soils, for example, drier land
or steeper slopes (Warren et al. 2001; El-Swaify 1997). Moreover, as populations
grow, people are increasingly concentrating in urban areas and sprawling ex-urban
settlements (Alig et al. 2004). These clusters of people fragment the landscapes
of large areas, and in doing so, they threaten ecosystem processes. In fact, human
demographic shifts are said to be the leading source of soil degradation (Tolba and
El-Kholy 1992). This is due to the expansion of populations into what was previously
prime agricultural land, thereby triggering both real and perceived needs for intensi-
fied agricultural production (Meyer and Turner 1992). This chain of processes driven
by human demographic shifts affects the ability of biological systems to support
human needs (Vitousek et al. 1997).
Between 1980 and 2000, the population of the United States increased by more
than 50 million people, about a 24% increase (Mackun and Wilson 2011). During
the same time frame, the amount of land used for urban and built uses increased by
roughly 34% (USDA Natural Resources Conservation Service 2009). This growth is
not projected to slow in the future; the United Nations estimates that the world's popu-
lation will surpass 9 billion people by 2050 and exceed 10 billion in 2100 (United
Nations 2011). All of these extra people will require expanded infrastructure and
development, and in the past, increases in developed land have predominantly come
from the conversion of croplands and forests (Alig et al. 2004).
In the United States, the quantity of land in agricultural production peaked
around 1950 at 1161 million acres. Agricultural land has been declining since then;
there were 922 million acres in 2007, and the current rate of decline is estimated to
be 5.39 million acres per year (US Department of Agriculture 2007). The conversion
of agricultural land to urban uses is caused by increasing urban demand for land,
especially low-density residential and commercial development at the urban fringe
(Heimlich and Anderson 2001). In addition to conversion, this leads to adaptation
of the agricultural system to increase yields on smaller areas of land (Dasgupta et
al. 2000). More generally, agricultural intensification is often triggered by growth
in population and density, whether caused by natural increase or migration (Ostrom
et al. 1999). The harmful effects of such intensification were described above (see
Section 7.3).
Human demographic shifts are at the core of land use and land cover changes. As
the population grows, so does the amount of land needed for infrastructure and other
development. This increased amount of land oftentimes comes at the expense of land
previously allocated to agricultural uses. Furthermore, larger populations require
more food, which forces agricultural producers to increase yields on smaller tracts
of land. This chain of events leads to soil degradation.
7.4.2 P overty and a ccess to L and and c redIt
Causal links have been established between differential access to land and credit
among rural populations in developing regions, and the abandonment of degraded
agricultural lands for new agricultural and forest land frontiers (Barbier 1997). This
process of exploitation and abandonment has been found to perpetuate a cycle of
land degradation among the rural poor (Munasinghe and Cruz 1995).