News |
Energy & Sustainability
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 |
BIODIVERSITY: Native wildflowers add diversity to this prairie-like California grassland.
Image: © David Hooper
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 outcomes.
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 Group.)
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 years."
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.
