Abstract
Recent evidence of widespread declines in insect pollinator populations in the U.S. has raised extraordinary concern and has ushered the immediate need for more research on plant-pollinator interactions to better assess the magnitude of potential ecological and societal impacts. In California, vernal pools are seasonal wetlands host to several endemic annual plant species, many of which depend on pollination by native specialist bees for reproduction. Pollen limitation, or inadequate pollen receipt in plants, reduces viable seed production and consequently has the potential to impair the long-term persistence of annual plant populations in vernal pools. Across California, the construction of vernal pools is a common habitat mitigation strategy to offset impacts from urban and agricultural developments. Yet, few studies have examined pollen limitation and pollinator foraging frequency in plant populations of constructed vernal pools. Based on previous studies, I hypothesized that there would be greater pollen limitation in Fremont's goldfields (Lasthenia fremontii), a common endemic annual, in constructed versus natural vernal pools in Placer County, California. I tested this hypothesis by I) measuring pollen limitation and seed viability in L. fremontii in constructed and natural vernal pools; 2) for individuals in which pollen limitation was measured, I recorded plant height and the number of inflorescences as well as local site characteristics including population abundance of L. fremontii, the timing of flowering, and elevation within the pool; and 3) collected observations of pollinator visits to L. fremontii in constructed and natural vernal pools. Pollen limitation indices were calculated for plant pairs from both pool types by comparing two treatments: openpollinated (control) L. fremontii plants with those that received pollen supplementation via hand pollination. I found no support for my hypothesis. Mean pollen limitation per pool was low in both constructed and natural vernal pools and no statistically significant difference was found between groups. This finding suggests that pollen delivery to L. fremontii by pollinators was similarly effective in both the constructed and natural vernal pools during the study period. Furthermore, observations of pollinator foraging revealed that generalist, opportunistic insects comprised the majority of visitors to L. fremontii; no native specialist bees were observed visiting L. fremontii. Plant species richness in vernal pools was positively associated with the percent viable L. fremontii seeds in the constructed and natural pools; while, plot elevation and maximum percent cover of L. fremontii had negative effects on the percent of viableseeds. The relationship between species diversity and increased biomass production, as shown for L.fremontii, and previously demonstrated by others (Cardinale et al. 2007, Hector et al. 1999), is valuable to the field of vernal pool restoration ecology, especially considering the loss of vernal pool habitat seen in recent decades throughout California. Thirty-nine percent of the L. fremontii plants monitored for this study were wholly or partially damaged by cocooning and/or herbivory caused by the sunflower moth caterpillar (Homoeosoma sp.). This invasive moth has the propensity to cause ecological damage to fragile vernal pool ecosystems in California. Further research is needed to understand and attempt to quantify the impacts of Homoeosoma sp. on non-cultivated Asteraceae populations where found in vernal pools, open grasslands, and roadside areas in the Central Valley.