Flowering, Fruit Set, and Seed Predation in Arctostaphylos Morroensis, a Rare Obligate-Seeding Shrub

Obligate seeding plants—those that are killed by fire and whose persistence depends on soil-stored seedbanks—may be particularly susceptible to indirect effects of habitat loss and fragmentation. Especially for rare species, fragmentation can create conditions that reduce the likelihood of achieving sufficient seed bank stores due to changes in the interactions of the remaining plants with their pollinators or their natural enemies such as herbivores or seed predators. Arctostaphylos morroensis (Morro Manzanita) is an endemic obligate seeder whose distribution has been reduced greatly to a small portion of coastal California in habitat that is fragmented by development. We examined the reproductive ecology of A. morroensis to determine the factors that affect seed input to the soil seed bank. In stands of different ages and percent cover of Morro Manzanita, we observed insect pollinators, and measured flower and fruit production, and rates of fruit predation over two years, hypothesizing that, based on previous literature on obligate-seeders, fruit set should be relatively high across all stands, fruit set should be resource-limited, and seed input should be relatively high. Our observations suggest that A. morroensis reproduction is dependent on pollinators, which were primarily bees. Contrary to our predictions, we found that fruit set was relatively low (averaging 10–18% over both years) at all stands and appears to be pollinator-limited. Fruit predation rates were high, with the majority of fruits in experimental trays removed in a matter of weeks. We suspect that seed input in this rare species is strongly limited by low fruit set and high seed predation.