Pollination Ecology of Island Endemic Plants: A Case Study on the California Channel Islands

Understanding the pollination requirements of threatened species is critical for the success of long-term conservation strategies. In a series of greenhouse experiments, we assessed the effects of different pollination treatments, including autogamy, facilitated autogamy, and cross-pollination on fruit and seed set for 2 generations of a rare island endemic plant. Santa Cruz Island rockcress, Sibara filifolia (Brassicaceae), is a rare, endangered annual herb that is endemic to the California Channel Islands. While efforts are underway to monitor and conserve this rare plant species, little is known about its pollination biology. We quantified the index of self-compatibility and multiplicative fitness. We also assessed how maternal plant treatment influenced seed germination and seedling survival rates. While S. filifolia is fully self-compatible, we demonstrate that facilitated pollination and outcrossing treatments positively affected all measures of reproductive output and fitness in first-generation plants. Metrics of predispersal fitness (e.g., fruit set and seed production) in the F2 generation also increased significantly in response to facilitated pollination and outcrossing. While our data suggest that S. filifolia fecundity increases with outcrossing, we observed lower fitness benefits of facilitated autogamy and outcrossing on the production of F2 seeds. This highlights the difficulty in providing specific recommendations when the consequences of outbreeding may not be detectable until later generations. More study is required to confirm that multiple generations of interpopulation gene flow can effectively improve population viability and decrease genetic erosion.