Plant–Pollinator Interactions in a Northern California Coastal Habitat, San Bruno Mountain, San Mateo County, California, USA

Animal pollination of plants is a crucial ecosystem service for maintaining global biodiversity and ecosystem function. High pollinator abundance and diversity can likewise improve the reproductive success of the plant community. Plant–pollinator interaction networks have the potential to identify dominant, specialist, and generalist pollinator species within a system, and their host plant counterparts. Understanding these relationships is paramount for buffering natural systems from biodiversity loss in a world where pollinator abundances continue to decline rapidly. San Bruno Mountain (SBM) in California, USA is one of the last natural, open spaces in the urban landscape of the northern San Francisco Peninsula. We conducted a series of timed meanders and vegetation surveys at eight sample sites within SBM (four grassland and four coastal scrub sites) to identify plant species prevalence and pollinator species visitation of flowering plants. We used nonmetric multidimensional scaling (NMDS), similarity percentage analysis, and bipartite networks to examine plant and pollinator species richness, community composition, and trophic interactions across the SBM landscape in grassland and coastal scrub habitats. We encountered a total of 59 pollinator and 135 plant species over the course of the study. While species richness did not vary significantly between vegetation types, the NMDS results revealed significant differences between grassland and coastal scrubland plant and pollinator community composition. The bipartite analyses identified generalist pollinators and plant host species as important contributors to the biodiversity of SBM due to the high numbers of interactions between these pollinator and plant taxa across the landscape. These results also highlight the conservation importance of specialist pollinators and their plant host plant taxa for maintaining high diversity and ecosystem integrity. In the future, adaptive restoration activities could be used at SBM and other similar open land habitats to bolster the abundance of native herbaceous flowering pollinator host plants in the area.