“facing a high risk of extinction in the
wild.” Evaluators said some of them met
more dire criteria, facing “very high” or
“extremely high” extinction risks. That
troubled group included one in eight of
the bird species, one in five mammals, one
in four corals and one in three amphibians. (Scientists have formally described
some 1. 7 million species, and estimates
of total richness run from 3 million to
10 million.)
Another indicator, the Living Planet
Index, averages changes in the sizes of
populations of 1,686 vertebrate species.
The index, put out by the World Wildlife
Fund, the Zoological Society of London
and their partners, slid almost 30 percent
from 1970 to 2005.
In this deadline year, “biodiversity is
still declining — there’s no doubt about
it,” Vié says.
Though species losses are only one
measure of diversity, if a species is crashing, so is any genetic variety within it.
And taking more and more species from
an ecosystem raises concerns that the
swamp, woods or pond will lose its distinctive traits, becoming something else,
in a form of system-level extinction. So,
as crude as they are, tallies of species’
statuses let conservationists take the
pulse of life on the planet.
The meaning of loss
As for the impact of these declines, Vié
says, “I don’t think people get it.” Too
often biodiversity loss has come to mean
extinction of some creature a continent
away. “It’s not because one beetle or one
frog is going extinct that we are worried,”
he says. “It’s that the losses are massive.”
So just what’s going to happen when
so much biodiversity disappears has
become a pressing question. Plenty of
experiments, albeit accidental ones,
have already demonstrated that subtract-
ing even one species can change an eco-
system. The Millennium Assessment
report lists 21 such “experiments,” car-
ried out by fishing fleets, overenthusiastic
gardeners or even wildlife managers.
Year
Changes in living planet indexes, 1970–2005
1. 6
Index (set at 1 in 1970)
1. 4
1. 2
1.0
0.8
0.6
0.4
0.2
�Terrestrial
�Freshwater
�Marine
0
1970 1980 1990
Drivers of change The Millennium Ecosystem Assessment lists the deposition of
reactive nitrogen, due in part to increased fertilizer use, as an important cause of ecosystem
change. Other drivers include habitat fragmentation, overexploitation and climate change.
SOURCE: MILLENNIUM ECOSYSTEM ASSESSMENT
mg/m2/year
5 25 50 100 250 500 750 1,000 5,000
2,000
with one or two produced only one-tenth
of the biomass of a normal year.
“We actually didn’t believe the results
when we first saw them,” Tilman says.
Tweaking the analysis this way and
that still produced the same findings. So
Tilman set up an experiment as a deliberate test of the effects of species number on
biomass. With 168 plots of one to 16 species, the experiment has been running for
16 years. In the early years it led to a paper
presenting evidence that yes, under the
same conditions, plots with more species
of plants eventually tend to yield more
biomass than plots with fewer species.
A 2006 paper in Nature by Bradley
Cardinale of the University of California, Santa Barbara and his colleagues
supports these findings. The team concludes that, overall, tests have shown
that greater diversity in systems from
grassland plants to rock-hugging marine
invertebrates increases the basic productivity of an ecosystem.
TREEFROG: BRAD WILSON; LIVING PLANET INDEX CHART: A. NANDY
www.sciencenews.org
