By Diana Parsell
Smithsonian.com, January 28, 2008
Smithsonian.com, January 28, 2008
Scientists estimate that 80 percent of Earth's land surface now bears
the marks of human activities, from roads to crops to cell phone towers.
Under present land-use practices, studies show, society is seizing an
ever-bigger share of the planet's biological resources to satisfy human
demands. There is growing concern that the resulting environmental
changes may seriously undermine the natural functions of terrestrial
ecosystems. This could threaten their long-term capacity to sustain life
on Earth by providing essential services such as food production, water
and air filtration, climate regulation, biodiversity protection,
erosion control and carbon storage.
"Ultimately, we need to question how much of the biosphere's
productivity we can appropriate before planetary systems break down,"
Jonathan Foley and a group of co-authors caution in a paper published
last July in the Proceedings of the National Academy of Sciences (PNAS).
Foley, director of the Center for Sustainability and the Global
Environment at the University of Wisconsin-Madison, uses
state-of-the-art computer models and satellite measurements to analyze
links between land-use changes and environmental conditions around the
world. This research has shown that agriculture is the dominant form of
human land use today, with about 35 percent of all ice-free land now
used to grow crops and raise livestock. That's up from only 7 percent in
1700.
The physical extent of land conversion for human activities is only
part of the story, however. The intensity of such activities also
matters a great deal: more intensive land use usually consumes more
resources.
One of the best pictures so far of humanity's collective impact on
terrestrial ecosystems comes from a new study, also in the July PNAS,
by a team of European researchers. They compiled spatially explicit
maps, in units of 6.2 square miles, indicating not only what types of
local land use predominate around the world, but roughly how much
biomass energy—or natural productivity—the various land-use practices
consume. (The remaining biomass energy is available to support
biological functions in all other trophic levels, or food webs, of
ecosystems).
"Our results show that humans, just one of 2 to 20 million species on
the planet, use up 25 percent of the trophic energy available in all
terrestrial ecosystems," says lead author Helmut Haberl of Klagenfurt
University in Vienna. "That's quite a dramatic inequality. "
Patterns of human land use vary widely around the world, influenced
by biophysical and socioeconomic conditions. Across large areas of Asia
and sub-Saharan Africa, for example, subsistence agriculture and
small-scale farms are still standard. But in general, there's a steady
shift toward more intensive land use today, driven by rising living
standards and population growth that fuel increasing demand for goods
and services.
