154
S. Harrison
tween the size or isolation of Helianthus populations and either the frequency of
pollinator visits or the number of pollinator species (multiple regressions, p always
> 0.20). Rates of attack by seed predators were not related to either population size
or isolation (multiple regressions, p always> 0.20).
6 Local Extinction and Colonization in Five
Serpentine Seep Plants
The studies reported above all hint at the importance of local extinction and
recolonization in shaping the patterns of plant diversity on patchy serpentine outcrops, but they lack direct evidence for this proposition. However, for the specialized microhabitat just described, there is direct evidence on population turnover.
Serpentine seeps are typically isolated by hundreds to thousands of meters from one
another, and support a specialized flora; thus they form patches within large patches
of serpentine. In the study region there are 5 serpentine-seep specialist plants, all of
which are considered uncommon to rare by the California Native Plant Society
(Skinner and Pavlik 1995). These are Helianthus exilis (Asteraceae), Senecio
cLevelandii (Asteraceae), Astragalus clevelandii (Fabaceae), Delphinium uLiginosum
(Ranunculaceae), and Mimulus nudatus (Scrophulariaceae). With collaborators John
Maron and Gary Huxel, I asked how isolation affected the chances of extinction
and recolonization in these species.
In 1981-82, the five species' distributions were surveyed in a 4200-hectare area
as part of an environmental assessment for a mine (0' Appolonia 1982). This survey found a total of 218 populations on 87 seeps. In 1997 and 1998, we resurveyed
all these localities; 17 had been destroyed by construction, while at another 14 sites
we were unable to find any seep habitat. On the remaining 56 seeps, there were 32
presences in 1981-82 followed by absences in 1997-98, i.e. local extinctions, and
100 presences in 1981-82 followed by presences in 1997-98, or non-extinctions.
This subset of the data was used in logistic regressions to determine the correlates
of local extinction. There were also 64 absences in 1981-82 followed by presences
in 1997-98, or putative colonizations, and 79 absences in 1981-82 followed by
absences in 1997-98, or non-colonizations. This subset of the data was used in
logistic regressions to examine the correlates of colonization.
For each seep that was found in both sets of years, and for each of the five
species, we measured the distances to the nearest three conspecific populations
found in 1997-98 and calculated their harmonic mean as our measure of population
isolation. We measured the downstream length of each seep on topographic maps
as an index of seep size. We also measured the distance from each seep to the
nearest major human-caused disturbance (e.g. new roads and mining activities).
Chances of local extinction increased with increasing isolation (Fig. 1) and
decreased with increasing distance from disturbance (isolation, t = 2.19, P = 0.029;
distance from disturbance, t = -1.82, P = 0.068; overall model, -2 log likelihood =
7.68,2 df, P = 0.021). Chances of colonization were affected by species (Mimulus,
S. Harrison
tween the size or isolation of Helianthus populations and either the frequency of
pollinator visits or the number of pollinator species (multiple regressions, p always
> 0.20). Rates of attack by seed predators were not related to either population size
or isolation (multiple regressions, p always> 0.20).
6 Local Extinction and Colonization in Five
Serpentine Seep Plants
The studies reported above all hint at the importance of local extinction and
recolonization in shaping the patterns of plant diversity on patchy serpentine outcrops, but they lack direct evidence for this proposition. However, for the specialized microhabitat just described, there is direct evidence on population turnover.
Serpentine seeps are typically isolated by hundreds to thousands of meters from one
another, and support a specialized flora; thus they form patches within large patches
of serpentine. In the study region there are 5 serpentine-seep specialist plants, all of
which are considered uncommon to rare by the California Native Plant Society
(Skinner and Pavlik 1995). These are Helianthus exilis (Asteraceae), Senecio
cLevelandii (Asteraceae), Astragalus clevelandii (Fabaceae), Delphinium uLiginosum
(Ranunculaceae), and Mimulus nudatus (Scrophulariaceae). With collaborators John
Maron and Gary Huxel, I asked how isolation affected the chances of extinction
and recolonization in these species.
In 1981-82, the five species' distributions were surveyed in a 4200-hectare area
as part of an environmental assessment for a mine (0' Appolonia 1982). This survey found a total of 218 populations on 87 seeps. In 1997 and 1998, we resurveyed
all these localities; 17 had been destroyed by construction, while at another 14 sites
we were unable to find any seep habitat. On the remaining 56 seeps, there were 32
presences in 1981-82 followed by absences in 1997-98, i.e. local extinctions, and
100 presences in 1981-82 followed by presences in 1997-98, or non-extinctions.
This subset of the data was used in logistic regressions to determine the correlates
of local extinction. There were also 64 absences in 1981-82 followed by presences
in 1997-98, or putative colonizations, and 79 absences in 1981-82 followed by
absences in 1997-98, or non-colonizations. This subset of the data was used in
logistic regressions to examine the correlates of colonization.
For each seep that was found in both sets of years, and for each of the five
species, we measured the distances to the nearest three conspecific populations
found in 1997-98 and calculated their harmonic mean as our measure of population
isolation. We measured the downstream length of each seep on topographic maps
as an index of seep size. We also measured the distance from each seep to the
nearest major human-caused disturbance (e.g. new roads and mining activities).
Chances of local extinction increased with increasing isolation (Fig. 1) and
decreased with increasing distance from disturbance (isolation, t = 2.19, P = 0.029;
distance from disturbance, t = -1.82, P = 0.068; overall model, -2 log likelihood =
7.68,2 df, P = 0.021). Chances of colonization were affected by species (Mimulus,