10
Population Persistence and
Community Diversity
in a Naturally Patchy Landscape:
Plants on Serpentine Soils
SUSAN HARRISON
Department of Environmental Science and Policy, University of California, Davis, One Shields
Avenue, Davis, CA 95616, USA
Abstract
Serpentine soils in California are patchily distributed at multiple spatial scales and
support a distinctive flora. In a region of 10 x 30 km in Northern California, I
compared plant species richness on 24 small «5 ha) serpentine outcrops and 24
equally-spaced sites within large (> 1 km2) outcrops. For serpentine endemic plants,
alpha (local) diversity was lower but beta (differentiation) diversity was higher on
small outcrops compared to sites within large outcrops. For alien plants, local diversity was higher on small outcrops than on sites within large outcrops. Experimental work examined mechanisms underlying these patterns. Two alien grasses
(Avena Jatua and Bromus hordeaceus) appeared to be more prevalent on small
outcrops because of edge effects rather than because of habitat quality. One serpentine endemic herb (Calystegif:l collin a) exhibited lower reproductive success on
small outcrops, evidently because of a shortage of compatible pollen. Another endemic herb (Helianthus exilis) was absent from small outcrops because of the absence of seeps, a specialized habitat found within large serpentine outcrops. Direct
evidence for local extinction and recolonization was found in five herbaceous species that inhabit serpentine seeps. These results illustrate the importance of largescale landscape structure for population persistence and community diversity in
plants.
Key words. diversity, species richness, fragmentation, patchy, landscape, serpentine, plants,
pollination, edge effects, extinction, colonization, spatial, isolation, natural rarity, invasion
147
Population Persistence and
Community Diversity
in a Naturally Patchy Landscape:
Plants on Serpentine Soils
SUSAN HARRISON
Department of Environmental Science and Policy, University of California, Davis, One Shields
Avenue, Davis, CA 95616, USA
Abstract
Serpentine soils in California are patchily distributed at multiple spatial scales and
support a distinctive flora. In a region of 10 x 30 km in Northern California, I
compared plant species richness on 24 small «5 ha) serpentine outcrops and 24
equally-spaced sites within large (> 1 km2) outcrops. For serpentine endemic plants,
alpha (local) diversity was lower but beta (differentiation) diversity was higher on
small outcrops compared to sites within large outcrops. For alien plants, local diversity was higher on small outcrops than on sites within large outcrops. Experimental work examined mechanisms underlying these patterns. Two alien grasses
(Avena Jatua and Bromus hordeaceus) appeared to be more prevalent on small
outcrops because of edge effects rather than because of habitat quality. One serpentine endemic herb (Calystegif:l collin a) exhibited lower reproductive success on
small outcrops, evidently because of a shortage of compatible pollen. Another endemic herb (Helianthus exilis) was absent from small outcrops because of the absence of seeps, a specialized habitat found within large serpentine outcrops. Direct
evidence for local extinction and recolonization was found in five herbaceous species that inhabit serpentine seeps. These results illustrate the importance of largescale landscape structure for population persistence and community diversity in
plants.
Key words. diversity, species richness, fragmentation, patchy, landscape, serpentine, plants,
pollination, edge effects, extinction, colonization, spatial, isolation, natural rarity, invasion
147