Tracing impacts of partner abundance in facultative pollination mutualisms: from individuals to populations

Abstract

Partner abundance affects costs and benefits in obligate mutualisms, but its role in facultative partnerships is less clear. We address this gap in a pollination web consisting of two clovers (Trifolium) that differ in specialization on a bumble bee pollinator Bombus balteatus. We examine how pollination niche breadth affects plant responses to pollinator abundance, comparing early-flowering (specialized) and late-flowering (generalized) cohorts of T. parryi and early T. parryi to T. dasyphyllum, a pollination generalist. Co-pollinators disrupt the link between B. halteatus visitation and pollination rate for both clovers. Only for early-flowering T. parryi do visitation, pollination, and seed set increase with density of B. balteatus. Bumble bee density also alters timing of seed germination in T. parryi, with seeds from plants receiving augmented B. balteatus germinating sooner than seeds of open-pollinated counterparts. Benefits saturate at intermediate bumble bee densities. Despite strong effects of B. balteatus density on individual plant fitness components, population models suggest little impact of B. balteatus density on lamda in T. parryi or T. dasyphyllum. Findings show that functional redundancy in a pollinator guild mediates host-plant responses to partner density. Unexpected effects of pollinator density on life history schedule have implications for recruitment under pollinator decline.