Science News
Week of Oct. 13, 2007; Vol. 172, No. 15
Invasive, Indeed
One species-Homo sapiens-consumes nearly a quarter of Earth’s natural
productivity
Sid Perkins
Some people live lightly on the land: Bedouin clans roam the deserts
of the Middle East and North Africa; small groups of indigenous
people follow reindeer herds across frigid Arctic terrain; and tribes
of hunter-gatherers forage the plains of southern Africa and the
forests of Amazonia and Papua New Guinea.
Then there’s the other 6.6 billion of us.
When we farm, clear forests, and build cities, dams, and roads, we
dramatically alter the landscape. In some places, we increase the
land’s productivity-measured as the amount of plant life at the base
of the food chain-by adding immense amounts of water and fertilizer.
New research indicates that on the whole, however, human presence
significantly decreases Earth’s biological productivity. For
instance, many of today’s cities occupy large patches of what had
been some of the world’s most fertile land.
Of the biological productivity that remains, people are gathering an
ever-increasing share, sometimes by boosting their quality of life,
but often merely by dint of their burgeoning numbers.
In some
regions, each spanning millions of square kilometers, human activity
consumes almost two-thirds of the biological productivity that would
otherwise be available.“We were surprised how intensively these regions were being affected”
by human presence, says K. Heinz Erb, an ecologist at Klagenfurt
University in Vienna. “
Only one-third of the natural productivity is
left for all the other species.”
Overall, nearly one-quarter of Earth’s land-based biological
productivity ends up in people’s hands and bellies, Erb and his
colleagues estimate. Other research suggests that people appropriate
a comparable, but slightly smaller, share of the ocean’s
productivity-defined as the mass of photosynthetic organisms at the
base of the sea’s food chain.
A projected 25 percent increase in the world’s population by 2030 is
bound to strain ecosystems even further. Increasing agricultural
efficiency by irrigating and fertilizing the land can add to the
strain by boosting erosion and the nutrient runoff that creates toxic
algal blooms and large anoxic zones in oceans. Adding insult to
injury, proposals to transition from fossil fuels to renewable
biofuels would place yet more of Earth’s productivity in people’s
hands.
Some scientists now wonder: At what point do the world’s ecosystems
begin to break down? Or, more frighteningly, has that process already
begun?
Reaping, sowing
Before people invented agriculture, they roamed the landscape in
search of sustenance. When resources became too scarce to nourish the
group, it was time to move on. When people began to farm the land,
however, their habits changed considerably, to the detriment of many
ecosystems. Settlers built year-round shelters and often cleared
acreage for their crops.
“The rise of modern agriculture and forestry has been one of the most
transformative events in human history,” says Jonathan A. Foley, an
environmental scientist at the University of Wisconsin-Madison.
Practices vary somewhat, but typically, people heavily farm the most
fertile land, use marginal lands for grazing domestic animals, and
plant single-species tree farms in areas where forests once stood.
Whatever the use, the production of forest or agricultural goods
comes at the expense of natural ecosystems, observes Foley.
Today, croplands and pastures are among the largest ecosystems on the
planet. People farm about 12 percent of the land outside of
Antarctica and Greenland and use about 23 percent for grazing, says
Foley. Together, land devoted to these uses equals the 35 percent of
Earth’s surface that natural forests occupy, he notes.
To estimate the effect that humans wreak on the world’s land-based
ecosystems, Erb and his colleagues used agricultural and forestry
statistics compiled for 161 nations that account for 97.4 percent of
Earth’s icefree land. Most of the remaining area is located on small,
uninhabited islands, Erb notes. In their computer model, the
researchers divided the planet’s land surface into grid squares no
larger than 10 kilometers per side.
The team estimates that if people weren’t around to alter the
landscape, the world’s natural vegetation would absorb enough carbon
dioxide from the atmosphere to lock away about 65.5 billion metric
tons of carbon each year. However, in 2000, the year for which the
data were compiled, Earth’s vegetation locked away only about 59.2
billion metric tons of carbon, or 9.6 percent less than it should
have, says Erb. Of that smaller carbon total, human activities
removed about 15.6 billion metric tons-a whopping 23.8 percent-from
the world’s ecosystems. A little more than half of the carbon that
people appropriated was harvested and used as food, forage, and wood,
Erb and his colleagues note in the July 31 Proceedings of the
National Academy of Sciences. Most of the rest was lost to
inefficiencies of agriculture, including the inability of crops to
store as much carbon as natural vegetation would have stored. A small
amount, about 7 percent of the carbon that people take out of the
system, went up in smoke produced primarily by slash-and-burn
agriculture, says Erb. All of this human-appropriated carbon became
unavailable to other species.
Human harvests don’t stop at the shoreline, either. The world’s most
productive fisheries typically lie in and near the shallow waters
that fringe the coasts of large islands and continents, says Daniel
Pauly, a fisheries biologist at the University of British Columbia in
Vancouver. Scientists have divided such coastal waters into 64 large
marine ecosystems. These areas can vary in character and inhabitants
as much as arctic tundra differs from an Amazonian rain forest.
About 95 percent of the world’s fish catch comes from large marine
ecosystems, says Pauly. For the past decade or so, that haul has
represented about 20 percent of the natural productivity of those
regions, as measured by the amount of carbon locked away by organisms
at the base of the ocean’s food chain.
Efficiency matters
While wilderness areas remain relatively unaffected by people, other
parts of the world are packed cheek by jowl with cities, farms, and
other human imprints.
Southern Asia, a 6.7-million-square-kilometer region that includes
India, is one of the most densely populated and heavily irrigated
regions on the planet, says Erb. There, human activity co-opts about
63 percent of the area’s natural productivity each year, he and his
colleagues estimate. In eastern and southeastern Europe, people
appropriate about 52 percent of the land’s productivity.
At the other extreme, in Australia, central Asia, and Latin America,
the percentage of productivity that ends up in human hands ranges
between 11 and 16 percent. Increasing the use of fertilizers and
irrigation could boost those percentages and help meet the needs of a
growing world population. However, long-term irrigation sometimes
renders the soil too salty for crops, and fertilizer, if used
unsparingly, runs off into rivers and streams and ends up in the
ocean, where it overfertilizes algae and thus creates huge zones
devoid of other life. “There’s no free biomass,” Erb cautions.
In the stampede to replace fossil fuels, some scientists have
proposed the large-scale cultivation of crops that can be transformed
into supposedly eco-friendly biofuels. That, too, might be
ecologically unwise.
“If the whole world begins to look like Iowa cornfields, we’ll have
to take an even larger share of global biological production into
human hands, and that leaves a lot less for other things,” says
Foley. “And those other things won’t be just pretty butterflies and
tigers and charismatic animals, they’ll be things that matter to us,
like the things that clean our water, preserve our soils, clean our
atmosphere, and pollinate our crops.”
“At what point does human activity begin to compromise a lot of our
environmental systems?” Foley continues. “At what point does this get
to be scary?”
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References:
Foley, J.A., et al. 2007. Our share of the planetary pie. Proceedings
of the National Academy of Sciences 104(July 31):12585-12586. Extract
available at http://www.pnas.org/cgi/content/extract/104/31/12585.
Haberl, H., K.H. Erb, et al. 2007. Quantifying and mapping the human
appropriation of net primary production in earth’s terrestrial
ecosystems. Proceedings of the National Academy of Sciences 104(July
31):12942-12947. Available at
http://www.pnas.org/cgi/content/full/104/31/12942.
Further Readings:
Harder, B. 2003. Catch zero. Science News 164(July 26):59-61.
Available at http://www.sciencenews.org/articles/20030726/bob10.asp.
Perkins, S. 2004. Paved paradise? Science News 166(Sept. 4):152-153.
Available at http://www.sciencenews.org/articles/20040904/bob8.asp>
Raloff, J. 2000. Sprawling over croplands. Science News 157(March
4):155. Available to subscribers at
http://www.sciencenews.org/articles/20000304/note12.asp.
Sources:
K. Heinz Erb
Institute of Social Ecology
Klagenfurt University
Schottenfeldgasse 29
1070, Vienna
Austria
Jonathan A. Foley
Center for Sustainablility and the Global Environment
University of Wisconsin, Madison
1710 University Avenue, Room 202A
Madison, WI 53726
Daniel Pauly
Fisheries Centre
Aquatic Ecosystems Research Laboratory (AERL), Room 333
2202 Main Mall
University of British Columbia
Vancouver, BC V6T 1Z4
Canada
