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S. Harrison
and 0 of 24 sites within large ones, while Bromus was found on 20 of 24 small
patches and 2 of 24 sites within large ones. Both species are extremely common in
the nonserpentine matrix. We examined two non-exclusive hypotheses. First, the
pattern might be a biological edge effect; small outcrops might receive a high influx of alien propagules from the nonserpentine matrix. Second, the pattern might
be a physical edge effect; small outcrops might have more favorable soil properties
than do the interiors of large outcrops.
We examined the edges of large outcrops to see whether, as the edge-effect idea
predicts, these resembled small outcrops in terms ofthe prevalence of alien grasses.
We found Avena at 3 of 24 large outcrop edges, intermediate between its prevalence in large outcrop interiors and small outcrops; neither difference was significant (X\1) = 3, 0.05 < P < 0.10). We found Bromus at 15 of 24 large patch edges,
significantly more than large outcrop interiors (X1(1) = 12.3, P < 0.001) but not
significantly different than on small outcrops (X2(I) = 0.004, P > 0.10). Average
abundances of both species were significantly higher on nonserpentine than on
serpentine, and on serpentine the abundance of Bromus decreased significantly
with increasing distance from the edge (r = -0.74; df = 1,9; P = 0.009). These
results suggest that as hypothesized, small patches are similar to the edges of large
patches in terms of their prevalence of aliens (especially Bromus).
We performed a growth experiment to determine whether soil conditions were
more favorable in small serpentine patches than the interiors of large ones, and also
to test for ecotype formation. There were three soil treatments (small outcrop, large
outcrop, and nonserpentine), and two seed source treatments (nonserpentine and
serpentine) for each species. Neither Avena nor Bromus performed significantly
better in any respect on the small outcrop soil than the large outcrop soil, thus
supporting the idea of biological edge effects. We also found some evidence for
ecotype formation; serpentine seeds outperformed nonserpentine seeds when grown
on small-outcrop serpentine soil. Our results suggest the importance of a landscape-level approach to invasion ecology. Even serpentine soils, with their strong
abiotic resistance to invasion, are more likely to be invaded when they are juxtaposed with other habitats that supply an abundance of alien propagules.
4 Reproductive Success in the Serpentine
Morning Glory
To investigate how patchiness might affect the persistence of serpentine endemic
species, collaborator Amy Wolf and I studied the serpentine morning glory Calystegia
collina (Convolvulaceae), an outcrossing clonal perennial found only on serpentines
in the north Coast Ranges of California (Wolf and Harrison, in press). We compared the reproductive success of Calystegia in 39 plant patches on 16 small outcrops and 7 large outcrops, using field measurements, pollen addition experiments,
and transplant experiments to compare: (1) flower, fruit and seed production, (2)
seed mass and seed germination, (3) pollinator abundance and visitation rate, (4)
S. Harrison
and 0 of 24 sites within large ones, while Bromus was found on 20 of 24 small
patches and 2 of 24 sites within large ones. Both species are extremely common in
the nonserpentine matrix. We examined two non-exclusive hypotheses. First, the
pattern might be a biological edge effect; small outcrops might receive a high influx of alien propagules from the nonserpentine matrix. Second, the pattern might
be a physical edge effect; small outcrops might have more favorable soil properties
than do the interiors of large outcrops.
We examined the edges of large outcrops to see whether, as the edge-effect idea
predicts, these resembled small outcrops in terms ofthe prevalence of alien grasses.
We found Avena at 3 of 24 large outcrop edges, intermediate between its prevalence in large outcrop interiors and small outcrops; neither difference was significant (X\1) = 3, 0.05 < P < 0.10). We found Bromus at 15 of 24 large patch edges,
significantly more than large outcrop interiors (X1(1) = 12.3, P < 0.001) but not
significantly different than on small outcrops (X2(I) = 0.004, P > 0.10). Average
abundances of both species were significantly higher on nonserpentine than on
serpentine, and on serpentine the abundance of Bromus decreased significantly
with increasing distance from the edge (r = -0.74; df = 1,9; P = 0.009). These
results suggest that as hypothesized, small patches are similar to the edges of large
patches in terms of their prevalence of aliens (especially Bromus).
We performed a growth experiment to determine whether soil conditions were
more favorable in small serpentine patches than the interiors of large ones, and also
to test for ecotype formation. There were three soil treatments (small outcrop, large
outcrop, and nonserpentine), and two seed source treatments (nonserpentine and
serpentine) for each species. Neither Avena nor Bromus performed significantly
better in any respect on the small outcrop soil than the large outcrop soil, thus
supporting the idea of biological edge effects. We also found some evidence for
ecotype formation; serpentine seeds outperformed nonserpentine seeds when grown
on small-outcrop serpentine soil. Our results suggest the importance of a landscape-level approach to invasion ecology. Even serpentine soils, with their strong
abiotic resistance to invasion, are more likely to be invaded when they are juxtaposed with other habitats that supply an abundance of alien propagules.
4 Reproductive Success in the Serpentine
Morning Glory
To investigate how patchiness might affect the persistence of serpentine endemic
species, collaborator Amy Wolf and I studied the serpentine morning glory Calystegia
collina (Convolvulaceae), an outcrossing clonal perennial found only on serpentines
in the north Coast Ranges of California (Wolf and Harrison, in press). We compared the reproductive success of Calystegia in 39 plant patches on 16 small outcrops and 7 large outcrops, using field measurements, pollen addition experiments,
and transplant experiments to compare: (1) flower, fruit and seed production, (2)
seed mass and seed germination, (3) pollinator abundance and visitation rate, (4)