150
S. Harrison
outcrops were separated by 10--3200 m; distances among the 4 clusters ranged from
16-45 km. The four large outcrops chosen were approximately 6, 16, 30 and 55
km 2 • Each large outcrop was matched with one of the four clusters of small ones,
and within the large outcrop, a set of sampling sites was chosen with the same
spatial configuration as that cluster of small outcrops. The plant community was
sampled at all sites in April and May of 1996 and 1997, using three 5x50-m belt
transects at each site.
From this data, I compared small and large outcrop sites with respect to
Whittaker's (1960) components of diversity: local (alpha), regional (gamma), and
among-site differentiation (beta diversity). Beta or differentiation diversity was
measured by a metric proposed by Colwell and Coddington (1994): the total number of unshared species between two species lists divided by the number of species
in the two lists. For each site, I calculated its (1) "total differentiation", the proportion of species unshared between that site and all other sites of its kind (small or
large outcrop), and (2) "within-cluster differentiation", the proportion of species
unshared between that site and all other sites in its cluster. I separately considered
the woody flora and three nested sets of the herbaceous flora on serpentine: (1) all
herb species, (2) herbs that are native versus alien to the study region, and (3) herbs
that are strictly endemic to serpentine within the study region.
For serpentine-endemic herbs and woody species (most of which show high
fidelity to serpentine), I found that small outcrops had roughly equal total diversity,
lower local diversity, and higher differentiation diversity than sites within large
outcrops (Table 1). The higher differentiation diversity on small outcrops prevailed
at the "within-cluster" as well as the "total" level, supporting the interpretation that
patchiness per se and not environmental gradients were responsible for the higher
beta diversity among small outcrops. This means that for habitat specialists, communities on small patches consist of smaller samples drawn from the same total
regional pool as communities on large continuous sites. This result is consistent
with the idea that for habitat specialists, community structure is shaped by random
colonization and local extinction.
For all herbaceous species together (85% of which are not endemic to serpentine), the only difference between the small outcrops and the sites within large
continuous outcrops was higher local diversity on the small outcrops. Even this
difference disappeared when only the native herbaceous species were considered
(Table 1). These results indicate that, not surprisingly, generalist (non-endemic)
species are far less affected by the spatial structure of serpentine patches than are
the specialist ( endemic) species. These results also indicate that small outcrops had
a higher local diversity of non-native species; in fact, there were an average of 6.04
± 3.88 (s.d.) alien species on small outcrops, versus 1.33 ± 1.76 on sites within
large outcrops (MANOV A [multivariate analysis of variance], p > 0.001).
Environmental variables created additional gradients in diversity. For example,
local diversity of all herbs decreased significantly with increasing elevation, and
alien herb diversity decreased with increasing distance inland and decreasing levels of calcium. However, none of these variables explained away the differences in
diversity between the small and large outcrops (Harrison 1997, 1999). Overall, the
S. Harrison
outcrops were separated by 10--3200 m; distances among the 4 clusters ranged from
16-45 km. The four large outcrops chosen were approximately 6, 16, 30 and 55
km 2 • Each large outcrop was matched with one of the four clusters of small ones,
and within the large outcrop, a set of sampling sites was chosen with the same
spatial configuration as that cluster of small outcrops. The plant community was
sampled at all sites in April and May of 1996 and 1997, using three 5x50-m belt
transects at each site.
From this data, I compared small and large outcrop sites with respect to
Whittaker's (1960) components of diversity: local (alpha), regional (gamma), and
among-site differentiation (beta diversity). Beta or differentiation diversity was
measured by a metric proposed by Colwell and Coddington (1994): the total number of unshared species between two species lists divided by the number of species
in the two lists. For each site, I calculated its (1) "total differentiation", the proportion of species unshared between that site and all other sites of its kind (small or
large outcrop), and (2) "within-cluster differentiation", the proportion of species
unshared between that site and all other sites in its cluster. I separately considered
the woody flora and three nested sets of the herbaceous flora on serpentine: (1) all
herb species, (2) herbs that are native versus alien to the study region, and (3) herbs
that are strictly endemic to serpentine within the study region.
For serpentine-endemic herbs and woody species (most of which show high
fidelity to serpentine), I found that small outcrops had roughly equal total diversity,
lower local diversity, and higher differentiation diversity than sites within large
outcrops (Table 1). The higher differentiation diversity on small outcrops prevailed
at the "within-cluster" as well as the "total" level, supporting the interpretation that
patchiness per se and not environmental gradients were responsible for the higher
beta diversity among small outcrops. This means that for habitat specialists, communities on small patches consist of smaller samples drawn from the same total
regional pool as communities on large continuous sites. This result is consistent
with the idea that for habitat specialists, community structure is shaped by random
colonization and local extinction.
For all herbaceous species together (85% of which are not endemic to serpentine), the only difference between the small outcrops and the sites within large
continuous outcrops was higher local diversity on the small outcrops. Even this
difference disappeared when only the native herbaceous species were considered
(Table 1). These results indicate that, not surprisingly, generalist (non-endemic)
species are far less affected by the spatial structure of serpentine patches than are
the specialist ( endemic) species. These results also indicate that small outcrops had
a higher local diversity of non-native species; in fact, there were an average of 6.04
± 3.88 (s.d.) alien species on small outcrops, versus 1.33 ± 1.76 on sites within
large outcrops (MANOV A [multivariate analysis of variance], p > 0.001).
Environmental variables created additional gradients in diversity. For example,
local diversity of all herbs decreased significantly with increasing elevation, and
alien herb diversity decreased with increasing distance inland and decreasing levels of calcium. However, none of these variables explained away the differences in
diversity between the small and large outcrops (Harrison 1997, 1999). Overall, the