Biology Reference
In-Depth Information
been obtained in field situations subject to strong seasonal change, which
affects resource availability. Examples include Mediterranean annual grass-
land (Robinson et al. 1995), successional Great Plains grassland (Foster et
al. 2002), communities of streamside herbaceous plants (Levine 2000), and
high mountain forest (Wiser et al. 1998; Fornwalt et al. 2003). Invasibility
in these cases seems to reflect conditions that create abundant, under-
utilized resources. To test this idea about the influence of resource avail-
ability, Davis and Pelsor (2001) imposed a regime of watering, drying, and
physical disturbance on grassland plots at the Cedar Creek research area
in Minnesota. Their results showed that regimes that created short-term
increases in resource availability favored the establishment of invader
species. These observations suggest that the availability of unutilized
resources, rather than native species diversity per se, determines invasibil-
ity in many situations.
Considerable experimental evidence is now accumulating that, other
factors being equal, species-rich communities are less easily invaded.
Field experiments on the role of species diversity in preventing invasion
of alien plants have been carried out in grassland communities at Cedar
Creek, Minnesota, over a number of years. On plots planted with differ-
ent numbers of grassland plants, Knops et al. (1999) and Kennedy et al.
(2002) tested the role of diversity in affecting the number of spontaneous
invaders. In one experiment, plots 3 by 3 m in size were seeded with
equal total amounts of seed of 1 to 24 herbaceous plant species. Plots
were weeded during the first two years to maintain the desired diversity
levels. In the third and fourth years, the richness and biomass of invaders
were determined, along with measures of soil nitrate level and light pen-
etration into the vegetation. The results were that both the number and
biomass of invaders declined with increases in plot diversity. More
diverse plots also had lower levels of nitrate and light, suggesting that
resource availability was the mechanism inhibiting potential invaders.
Other studies at the same location suggest that the presence of diverse
functional groups of plants, such as legumes, nonlegumes, C
3
grasses, and
C
4
grasses, contributes to the resistance of grassland communities to
invasion (Symstad 2000).
Studies focused on individual invader species, in which disturbance
and other external factors could be controlled, also suggest that commu-
nity diversity impedes alien invasion. Naeem et al. (2000) analyzed the
growth of narrowleaf hawksbeard (
Crepis tectorum
), an alien Eurasian
annual, in field plots and greenhouse containers in relation to diversity of
