May 2012, Week 1


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Fri, 4 May 2012 22:34:03 -0400
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How Biodiversity Keeps Earth Alive

     Species loss lessens the total amount of biomass on
     a given parcel, suggesting that the degree of
     diversity directly impacts the amount of life the
     planet can support

By David Biello  |
May 3, 2012 | 4

In 1994 biologists seeded patches of grassland in Cedar
Creek, Minn. Some plots got as many as 16 species of
grasses and other plants-and some as few as one. In the
first few years plots with eight or more species fared
about as well as those with fewer species, suggesting
that a complex mix of species-what is known as
biodiversity-didn't affect the amount of a plot's leaf,
blade, stem and root (or biomass, as scientists call
it). But when measured over a longer span-more than a
decade-those plots with the most species produced the
greatest abundance of plant life.

"Different species differ in how, when and where they
acquire water, nutrients and carbon, and maintain them
in the ecosystem. Thus, when many species grow together,
they have a wider set of traits that allow them to gain
the resources needed," explains ecologist Peter Reich of
the University of Minnesota, who led this research to be
published in Science on May 4. This result suggests "no
level of diversity loss can occur without adverse
effects on ecosystem functioning." That is the reverse
of what numerous studies had previously found, largely
because those studies only looked at short-term

The planet as a whole is on the cusp of what some
researchers have termed the sixth mass extinction event
in the planet's history: the wiping out of plants,
animals and all other forms of life due to human
activity. The global impact of such biodiversity loss is
detailed in a meta-analysis led by biologist David
Hooper of Western Washington University. His team
examined 192 studies that looked at species richness and
its effect on ecosystems. "The primary drivers of
biodiversity loss are, in rough order of impact to date:
habitat loss, overharvesting, invasive species,
pollution and climate change," Hooper explains. Perhaps
unsurprisingly, "biodiversity loss in the 21st century
could rank among the major drivers of ecosystem change,"
Hooper and his colleagues wrote in Nature on May 3.
(Scientific American is part of Nature Publishing

Losing just 21 percent of the species in a given
ecosystem can reduce the total amount of biomass in that
ecosystem by as much as 10 percent-and that's likely to
be a conservative estimate. And when more than 40
percent of an ecosystem's species disappear-whether
plant, animal, insect, fungi or microbe-the effects can
be as significant as those caused by a major drought.
Nor does this analysis take into account how species
extinction can both be driven by and act in concert with
other changes-whether warmer average temperatures or
nitrogen pollution. In the real world environmental and
biological changes "are likely to be happening at the
same time," Hooper admits. "This is a critical need for
future research."

The major driver of human impacts on the rest of life on
this planet-whether through clearing forests or dumping
excess fertilizer on fields-is our need for food.
Maintaining high biomass from farming ecosystems, which
often emphasize monocultures (single species) while also
preserving biodiversity-some species now appear only on
farmland-has become a "key issue for sustainability,"
Hooper notes, "if we're going to grow food for nine
billion people on the planet in the next 40 to 50

Over the long term, maintaining soil fertility may
require nurturing, creating and sparing plant and
microbial diversity. After all, biodiversity itself
appears to control the elemental cycles-carbon,
nitrogen, water-that allow the planet to support life.
Only by acting in conjunction with one another, for
example, can a set of grassland plant species maintain
healthy levels of nitrogen in both soil and leaf. "As
soil fertility increases, this directly boosts biomass
production," just as in agriculture, Reich notes. "When
we reduce diversity in the landscape-think of a
cornfield or a pine plantation or a suburban lawn-we are
failing to capitalize on the valuable natural services
that biodiversity provides."

At least one of those services is largely unaffected,
however, according to Hooper's study-decomposition.
Which means the bacteria and fungi will still happily
break down whatever plants are left after this sixth
extinction. But thousands of unique species have already
been lost, most unknown even to science-a rate that
could halve the total number of species on the planet by
2100, according to entomologist E. O. Wilson of Harvard
University. Ghosts of species past haunt ecosystems
worldwide, which have already lost not just one or
another type of grass or roundworm but also some of
their strength at sustaining life as a whole.


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