Eric Seabloom, The role of seed limitation, resource competition,
and community complementarity in invasions and restoration
we look around the grasslands on California’s hillsides, almost
all the plants we see are exotic species from the Mediterranean
region of Europe. In the 200 years since Europeans arrived, California’s
native perennial bunchgrasses have been almost completely replaced
by exotic grasses such as wild oats, soft brome, and medusahead,
and exotic forbs such as yellow starthistle. This is one of the
most dramatic conversions of a native plant community that has ever
occurred in the world, and the reasons for it are still not well
understood. The conventional wisdom blames some combination of livestock
grazing and the intrinsically greater competitive ability of the
exotic species over the natives.
Since most of the exotic species have an annual
life-cycle, unlike the native perennials, this grassland conversion
has had potentially major ecological effects. Not only California’s
native plant and animal biodiversity, but also ecosystem processes
such as nutrient cycling and fire regimes, have almost certainly
been greatly altered. Ongoing grassland invasions continue to erode
the value of grasslands for livestock grazing. So far, attempts
to use managed fire or grazing to control invasions and restore
grasslands have met with limited success.
To better understand grassland invasions, we initiated
a study in 1999 at UC Santa Barbara’s Sedgwick Reserve. By
tilling the soil and replanting it, we were able to create pure
stands of either native bunchgrasses or exotic annual grasses. Then
we “invaded” stands of full-grown native grasses with
seeds of exotics, and vice versa. The results were a great surprise.
We found that native grasses could invade and establish themselves
in exotic grasslands, but exotics could not invade and establish
in stands of natives. This was true even under a full range of additional
treatments we tried, such as disturbance and fertilization. This
result seemed to contradict the conventional wisdom about the competitive
superiority of exotic species over the natives they displace.
interpretation is that native bunchgrasses are actually competitively
superior to exotic annual grasses, but that the recovery of native
grasslands is strongly limited by the lack of sufficient seeds.
If this is true, it implies that the decline of native grasslands
was caused by past events, such as drought and overgrazing in the
nineteenth century. It also implies that restoration may not be
as forbidding a task as has often been thought. However, we need
to expand and strengthen our findings using parallel experiments
at multiple sites.
We have recently set up field experiments at the
Quail Ridge, McLaughlin, and Hastings (UC Berkeley) Reserves, as
well as Sedgwick, where we will test the ability of communities
of various mixes of native species to re-invade exotic annual grasslands.
The reserves offer us the opportunity to assess the robustness of
our results across a 700 km latitudinal gradient. They also offer
us places where we can conduct long-term monitoring of naturally
occurring stands of perennial bunchgrasses. Our hope is that through
combining the rigor of controlled field experiments with geographic
breadth, long-term observations and modeling, we can contribute
a better conceptual and practical understanding of Californian grassland
invasions. We could not do this without the secure highquality field
sites provided by the UC Natural Reserves.
Photo Credits: Title, Research (Mike Benard), Survey
and Grass pictures courtesy of Eric Seabloom