Domesticating the World: Conversion of Natural Ecosystems
Since the dawn of settled agriculture, humans have been altering
the landscape to secure food, create settlements, and pursue commerce
and industry. Croplands, pastures, urban and suburban areas, industrial
zones, and the area taken up by roads, reservoirs, and other major
infrastructure all represent conversion of natural ecosystems.
These transformations of the landscape are the defining mark of
humans on Earth's ecosystems, yielding most of the food, energy,
water, and wealth we enjoy, but they also represent a major source
of ecosystem pressure.
Conversion alters the structure of natural ecosystems, and how they
function, by modifying their basic physical properties—their
hydrology, soil structure, and topography—and their predominant
vegetation. This basic restructuring changes the complement of species
that inhabits the ecosystem and disrupts the complex interactions
that typified the original ecosystem. In many cases, the converted
ecosystem is simpler in structure and less biologically diverse.
In fact, habitat loss from conversion of natural ecosystems represents
the primary driving force in the loss of biological diversity worldwide
(Vitousek et al. 1997:495).
Historically, expansion of agriculture into forests, grasslands,
and wetlands has been the greatest source of ecosystem conversion.
Within the last century, however, expansion of urban areas with
their associated roads, power grids, and other infrastructure, has
also become a potent source of land transformation.
- Worldwide, humans have converted approximately 29 percent of
the land area—almost 3.8 billion ha—to agriculture and
urban or built-up areas (WRR calculations).
- Agricultural conversion to croplands and managed pastures has
affected some 3.3 billion ha—roughly 26 percent of the land
area. All totaled, agriculture has displaced one-third of temperate
and tropical forests and one-quarter of natural grasslands. Agricultural
conversion is still an important pressure on natural ecosystems
in many developing nations; however, in some developed nations
agricultural lands themselves are being converted to urban and
industrial uses (WRR calculations).
- Urban and built-up areas now occupy more than 471 million ha—about
4 percent of land area. Almost half the world's population—some
3 billion people—live in cities. Urban populations increase
by another 160,000 people daily, adding pressure to expand urban
boundaries (UNEP 1999:47). Suburban sprawl magnifies the effect
of urban population growth, particularly in North America and
Europe. In the United States, the percentage of people living
in urban areas increased from 65 percent of the nation's population
in 1950 to 75 percent in 1990, but the area covered by cities
roughly doubled in size during the same period (PRB 1998).
- Future trends in land conversion are difficult to predict, but
projections based on the United Nations' intermediate-range population
growth model suggest that an additional one-third of the existing
global land cover could be converted over the next 100 years (Walker
et al. 1999:369).
