Effects of habitat size and patch isolation on reproductive success of the serpentine morning glory

We examined the effects of habitat area and patch isolation on reproductive success in serpentine morning glory (Calystegia collina [Convolvulaceae]), a primarily self-incompatible clonal plant endemic to serpentine outcrops in northern California's coast ranges. Within a 4000-km(2) region, we compared the reproductive success of C. collina on 16 small (<5 ha) and 7 large (>300 ha) outcrops. Flower and fruit production were significantly higher on large serpentine outcrops than on small outcrops. Fruit production also was positively correlated with the soil's ratio of calcium to magnesium. Successful pollination was positively affected by flower density and the number of other flowering patches within 100 m of a C. collina patch. The number of nearby flowering patches was considerably higher on large than on small outcrops. Flowers on large outcrops did not receive significantly more bee visitors than flowers on small outcrops, suggesting that pollination success is related to the quality rather than the quantity of pollen deposited. Fruit production by plants on both small and large outcrops was enhanced by the experimental addition of pollen from other patches, but not by the addition of pollen from the same patch. These findings demonstrate that the size of habitat may have strong effects on the reproductive success of locally endemic plants by enhancing opportunities for successful sexual reproduction. They also warn against the presumption that naturally patchy plant species are invulnerable to the effects of habitat fragmentation.