Hawkmoth and bee pollinators impact pollen dispersal at the landscape but not local scales in two species of Oenothera.

PREMISE: Animal pollinators play an important role in pollen dispersal. Here, we assessed differences in pollen and seed dispersal and the role of pollinator functional groups with different foraging behaviors in generating patterns of genetic diversity over similar geographic ranges for two closely related taxa. We focused on two members of Oenothera section Calylophus (Onagraceae) that co-occur on gypsum outcrops throughout the northern Chihuahuan Desert but differ in floral phenotype and primary pollinator: Oenothera gayleana (bee) and O. hartwegii subsp. filifolia (hawkmoth). METHODS: We measured breeding system and floral traits and studied gene flow and population differentiation at the local (<13 km; four populations) and landscape (60-440 km; five populations) scales using 10-11 nuclear (pollen dispersal) and three plastid (seed dispersal) microsatellite markers. RESULTS: Both taxa were self-incompatible and floral traits were consistent with expectations for different pollinators. Seed and pollen dispersal patterns were distinctly different for both species. We found no evidence of genetic structure at the local scale but did at the landscape scale; O. gayleana showed greater differentiation and significant isolation by distance than in O. hartwegii subsp. filifolia. The plastid data were consistent with gravity dispersal of seeds and suggest that pollen dispersal is the principal driver of genetic structure in both species. CONCLUSIONS: We demonstrated that pollinator functional groups can impact genetic differentiation in different and predictable ways. Hawkmoths, with larger foraging distances, can maintain gene flow across greater spatial scales than bees.

Pollinator identity and spatial isolation influence multiple paternity in an annual plant.

The occurrence and extent of multiple paternity is an important component of variation in plant mating dynamics. However, links between pollinator activity and multiple paternity are generally lacking, especially for plant species that attract functionally diverse floral visitors. In this study, we separated the influence of two functionally distinct floral visitors (hawkmoths and solitary bees) and characterized their impacts on multiple paternity in a self-incompatible, annual forb, Oenothera harringtonii (Onagraceae). We also situated pollinator-mediated effects in a spatial context by linking variation in multiple paternity to variation in plant spatial isolation. We documented pronounced differences in the number of paternal sires as function of pollinator identity: on average, the primary pollinator (hawkmoths) facilitated mating with nearly twice as many pollen donors relative to the secondary pollinator (solitary bees). This effect was consistent for both isolated and nonisolated individuals, but spatial isolation imposed pronounced reductions on multiple paternity regardless of pollinator identity. Considering that pollinator abundance and pollen dispersal distance did not vary significantly with pollinator identity, we attribute variation in realized mating dynamics primarily to differences in pollinator morphology and behaviour as opposed to pollinator abundance or mating incompatibility arising from underlying spatial genetic structure. Our findings demonstrate that functionally distinct pollinators can have strongly divergent effects on polyandry in plants and further suggest that both pollinator identity and spatial heterogeneity have important roles in plant mating dynamics.

Land-use change has no detectable effect on reproduction of a disturbance-adapted, hawkmoth-pollinated plant species.

PREMISE OF THE STUDY: Land-use change is cited as a primary driver of global biodiversity loss, with myriad consequences for species, populations, and ecosystems. However, few studies have examined its impact on species interactions, particularly pollination. Furthermore, when the effects of land-use change on pollination have been studied, the focus has largely been on species pollinated by diurnal pollinators, namely, bees and butterflies. Here, we focus on Oenothera harringtonii, a night-flowering, disturbance-adapted species that has experienced a range-wide gradient of land-use change. We tested the hypothesis that the negative impacts of land-use change are mitigated by long-distance pollination. METHODS: Our study included both temporal (4 yr) and spatial (19 populations range-wide, and 1, 2, and 5 km from the population center) data, providing a comprehensive understanding of the role of land-use change on pollination biology and reproduction. KEY RESULTS: We first confirmed that O. harringtonii is self-incompatible and reliant on pollinators for reproduction. We then showed that hawkmoths (primarily Hyles lineata) are highly reliable and effective pollinators in both space and time. Unlike other studies, we did not detect an effect of population size, increased isolation, or a reduction in suitable habitat in areas with evidence of land-use change on pollination (visitation, pollen removal and deposition). Furthermore, the proportion of suitable habitat and other fragmentation metrics examined were not associated with population size or density in this plant species. CONCLUSIONS: We conclude that nocturnal pollination of Oenothera harringtonii via hawkmoths is robust to the negative impacts of land-use change.

Nitrogen deposition, competition and the decline of a regionally threatened legume, Desmodium cuspidatum.

Increased nitrogen (N) deposition, resulting from the combustion of fossil fuels, production of synthetic fertilizers, growth of N(2)-fixing crops and high-intensity agriculture, is one of the anthropogenic factors most likely to cause global biodiversity changes over the next century. This influence may be especially large in temperate zone forests, which are highly N limited and occur in regions with the highest levels of N deposition. Within these ecosystems, N(2)-fixing plants, including legumes, may be more sensitive to N deposition than other plant species. Though it has long been recognized that the competitive edge conferred by N(2)-fixation diminishes with increasing soil N availability, the conservation implications of increased N deposition on native N(2)-fixers have received less attention. We focus on Desmodium cuspidatum, which has experienced dramatic population losses in the last 30-40 years in the northeastern United States. We explore competition between this regionally threatened legume and a common non-N(2)-fixing neighbor, Solidago canadensis, across a gradient of N deposition. Our data show that increased N deposition may be detrimental to N(2)-fixers such as D. cuspidatum in two ways: (1) biomass accumulation in the non-N(2)-fixer, S. canadensis, responds more strongly to increasing N deposition, and (2) S. canadensis competes strongly for available mineral nitrogen and can assimilate N previously fixed by D. cuspidatum, resulting in D. cuspidatum relying more heavily on energetically expensive N(2)-fixation when grown with S. canadensis. N deposition may thus reduce or eliminate the competitive advantage of N(2)-fixing species growing in N-limited ecosystems.