November 15, 2004
Order of species loss has important biodiversity
consequences, grassland study reveals
By Jennifer McNulty
In a study that mimicked the natural order of species loss
in a grassland ecosystem, researchers found that declining biodiversity
greatly reduced resistance to invasive species and that the
presence of even small numbers of rare species had profound
Erika S. Zavaleta Photo:
The results, which appeared in the November 12 issue of Science,
have important implications for understanding the biodiversity
crisis, said researcher Erika S. Zavaleta, assistant professor
of environmental studies at UCSC.
Previous experiments relied on random species removal rather
than realistic patterns of loss, which turn out to be quite
We replicated natural patterns and processes and found
that both patterns of abundance and the order of species loss
matter a great deal, said Zavaleta, who conducted the
five-year study with Kristin B. Hulvey, a doctoral student in
environmental studies at UCSC. This defines a new direction
In the realistic loss scenario, entire groups of plants with
unique functions disappeared faster than expected by chance,
and invader resistance declined dramatically. The results suggest
that biodiversity losses in natural systems can have far greater
impacts than indicated by randomized-loss experiments.
The five-year study is among the first to combine conservation
biologys focus on the order of species loss with experimental
scrutiny of the consequences of those losses. It was conducted
at the Jasper Ridge Biological Preserve at Stanford University.
After four years of observation to understand natural patterns
of species loss, 70 research plots were planted with common
annual grasses, rare summer-flowering forbs, and rare native
bunch grasses. Plots reflected six levels of species diversity.
Half of the plots were then invaded with Centaurea soltitialis
L. (yellow starthistle), one of the most ecologically and
economically damaging weeds in California.
Then we watched how able the starthistle was to invade
the various communities, said Zavaleta. Starthistle
was much less able to invade the more diverse communities.
In fact, from the most diverse to the least diverse plots,
starthistle biomass increased by more than 100 percent. Even
in plots where the total biomass of five rare species made up
as little as 3 percent of the grassland, the extra diversity
thwarted invaders, said Zavaleta. Plots with 15 species
had 75 percent more starthistle than plots with 20 species,
Each species, no matter how few in number, potentially provides
different services within the ecosystem, explained Zavaleta.
Those extra species in our study are functionally unique.
They are alive at different times of the year, and their roots
are at different depths, she said. It turns out
that very rare things can matter a lot.
Other research has shown that more diverse communities use
resources more efficiently, from water and nutrients to light,
said Zavaleta. By monopolizing resources, theyre
keeping invaders away, she added.
With fewer species, there are holes in the system and
invaders can come in. Just a couple of members of certain species
can help plug those holes. There may be only a few sprinkled
around, but they can still be exerting a big effect on the way
the system works.
Zavaleta likens the ecosystem benefits of retaining rare species
to the role of a child plugging a dike with his thumb. Its
a small difference that can have huge consequences, she
said. Most ecologists would like to see natural systems
able to do that--resist accidental invasion.
Underscoring the importance of the experiments design,
Zavaleta and Hulvey note that their findings differ from the
results of studies at the same site that tested the effects
of randomized changes in species richness on starthistle. That
work found little difference in invader success depending on
the number of species in each plot.
The incorporation of realistic species loss order into
our experimental design profoundly altered the observed relation
between diversity and invasibility, they write.
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