-- A clever experiment conducted along the Eel River in northern
California sheds new light on a seeming contradiction in the
field of ecology about whether a diverse ecosystem is inherently
more resistant to invasion by alien species.
tussock in the study supporting seven native species,
including three species of monkey flower (yellow flowers).
has become more important in recent years as scientists campaign
to preserve the planet's biodiversity, focusing on areas with
the largest concentration of species, or attempt to restore
native habitats and native species in the face of dramatic
increases in the number of non-native invaders.
while many studies have shown that more diverse communities
are better at resisting invaders, surveys of diverse ecosystems
show they contain the highest number of exotic, non-native
In a series
of experiments reported in this week's issue of Science, Jonathan
Levine, a graduate student in integrative biology at the University
of California, Berkeley, shows clearly that both are true.
Small communities with high diversity are better at resisting
invading species, but at larger scales, invaders are more
likely to get established in diverse ecosystems.
are diverse for a reason. They are rich in resources, they
lack predators, they have a moderate amount of natural disturbance
- in general, they are hospitable to a broad range of species.
What's good for native species is good for invading species,"
Levine said. "So, you can't rely on biodiversity alone to
prevent invasion. You have to keep exotic species from getting
a foothold in the first place."
and one of his faculty advisors, Carla D'Antonio, associate
professor of integrative biology at UC Berkeley, wrote a review
article for the ecological journal Oikos last year that questioned
the theoretical foundation and scientific support for the
claim that diversity reduces susceptibility to invasion.
an interesting and potentially scary finding," D'Antonio said
of Levine's study. "People want to believe that, if we have
more diversity in the ecosystem, it will be more resistant
to invasion. Restoration ecologists want to create invasion-resistant
communities. But Jonathan's research suggests that even if
you get diversity-induced resistance on a small scale, it
won't necessarily translate to the scale of the whole ecosystem.
of native diversity are often going to be hotspots of invader
diversity if invader propagules are abundant in a region."
are seeds or pieces of plants capable of developing into mature
experiment was designed to distinguish between the effects
of diversity and the effects of other factors in the ecosystem
that are associated with or encourage diversity.
on a seven-kilometer section of the South Fork of the Eel
River in northern California where a type of sedge (Carex
nudata) grows in small tussocks that create tiny island communities
he could easily manipulate. These tussocks, most only a half-foot
across, are submerged and dormant most of the year, but when
the waters subside between May and September they blossom
with some 60 separate native plant species.
site at a riffle along the South Fork of the Eel River.
The yellow flags indicate individual tussocks, each a
mini-ecosystem unto itself.
65 tussocks at a single 20-meter riffle of the river, he removed
all species except for the sedge itself, then planted the
tussocks with one to nine different native species, creating
artificial communities of varying diversity. The following
season, after the natives had become established, he seeded
each tussock with three invasive non-native species commonly
found in the area - Canadian thistle, creeping bent grass
and common plantain, all invaders from Europe.
experiments have shown, the more diverse tussocks resisted
the invasions better than the less diverse. The commonly accepted
reason, he said, is that more species exploit more fully the
resources of the habitat, leaving fewer resources and, thus,
less opportunity for invaders.
when Levine looked at tussocks along the entire stretch of
river, he found invading species to be more abundant on the
most diverse tussocks. The fact that the intrinsic protective
effects of diversity did not agree with the observed patterns
of diversity along the river suggested that something other
than diversity was involved in driving the ecosystem-wide
noticed that downstream tussocks were both more diverse and
more likely to be invaded than upstream tussocks. Perhaps,
he thought, this is because more seeds are dumped on downstream
tussocks as a result of the natural stream flow.
out other alternatives, he conducted a second experiment in
which he planted the same number of seeds on all tussocks
under study. Choosing 190 natural tussocks of a range of diversities
all along the seven-kilometer study site, he added large numbers
of seeds on each of them - enough seeds to swamp the numbers
delivered naturally by the stream. The invaders colonized
all tussocks about equally, suggesting that the observed pattern
was due to the supply of seeds and not resource availability
or some other characteristic of the tussock.
this suggests that the number of seeds dumped on the downstream
tussocks overpowers the intrinsic protective effect of diversity,"
an important conclusion for wildland managers to consider,
in this particular ecosystem, seed supply was the important
factor affecting the number of invasions, other factors probably
operate in other systems. These could range from the availability
of nutrients or light to the abundance of herbivores or frequency
long time biologists have been repeating the idea that diversity
offers protection against invasion without really understanding
the theoretical underpinnings or support for it," D'Antonio
said. "Many of the confirming experiments have been one-on-one
competition-type studies, some of them even in laboratory
flasks. But on the ecosystem scale, there are many other factors
that come into the equation besides competition."
is supported by a National Science Foundation Graduate Research
Fellowship and Dissertation Improvement Grant.