COS 263-3 - Invasive plants affect native plant pollination through pollinator-mediated cross-boundary effects

Abstract: Cross-boundary effects occur when components of one ecosystem affect the functions, processes or composition of another neighboring ecosystem. While trophic cross-boundary effects have been well-studied in aquatic-terrestrial interfaces, investigating cross-boundary effects at ecological boundaries between terrestrial ecosystems remains an emerging area of research. Moreover, the spillover of mutualistic interactions across ecological boundaries remains understudied. Here, we examine whether an invaded plant community has cross-boundary effects on the pollination and seed set of a neighboring native plant community via the spillover of pollinators across an ecological boundary. We hypothesize that invasive plants may negatively impact the pollination and seed set of native plants in a neighboring ecosystem through competitive indirect effects via shared pollinators and by altering plant-pollinator network structure. To address this question, we leverage a mosaic of native-dominated serpentine grasslands that co-occur with invaded non-serpentine grasslands in the California Coast Range. We collected data on pollinator visitation, heterospecific pollen transfer, plant-pollinator network structure and native plant seed set for six sites that contained ecological boundaries between serpentine and non-serpentine grasslands. Along with our transect pollinator surveys, we mapped the spatial proximity of native and invasive plants using ground-truthing and drone imaging. The meadows spanned an invasion gradient. We show that invasive plants have context-dependent cross-boundary effects on native plant pollination. At low and high levels of invasion, invasive plants decreased the diversity and abundance pollinators, while at medium levels of invasion, invasion plants had positive indirect effects on native plants through increasing visitation from shared pollinators. Cross-boundary effects appeared stronger at the landscape scale than the within-meadow scale. The movement of pollinators across boundaries influenced network structure and seed set. This study suggests that the spillover of pollinators across ecological boundaries can extend the effects of plant invasions into neighboring, uninvaded ecosystems. Via indirect effects from shared pollinators, invasive plants may alter the biodiversity and pollination of a neighboring ecosystem.