Plant-pollinator network architecture does not impact intraspecific microbiome variability.

Variation in how individuals interact with food resources can directly impact, and be affected by, their microbial interactions due to the potential for transmission. The degree to which this transmission occurs, however, may depend on the structure of forager networks, which determine the community-scale transmission opportunities. In particular, how the community-scale opportunity for transfer balances individual-scale barriers to transmission is unclear. Examining the bee-flower and bee-microbial interactions of over 1000 individual bees, we tested (1) the degree to which individual floral visits predicted microbiome composition and (2) whether plant-bee networks with increased opportunity for microbial transmission homogenized the microbiomes of bees within that network. The pollen community composition carried by bees was associated with microbiome composition at some sites, suggesting that microbial transmission at flowers occurred. Contrary to our predictions, however, microbiome variability did not differ based on transfer opportunity: bee microbiomes in asymmetric networks with high opportunity for microbial transfer were similarly variable compared to microbiomes in networks with more evenly distributed links. These findings suggest that microbial transmission at flowers is frequent enough to be observed at the community level, but that community network structure did not substantially change the dynamics of this transmission, perhaps due to filtering processes in host guts.

Mass-flowering monoculture attracts bees, amplifying parasite prevalence.

As the global agricultural footprint expands, it is increasingly important to address the link between the resource pulses characteristic of monoculture farming and wildlife epidemiology. To understand how mass-flowering crops impact host communities and subsequently amplify or dilute parasitism, we surveyed wild and managed bees in a monoculture landscape with varying degrees of floral diversification. We screened 1509 bees from 16 genera in sunflower fields and in non-crop flowering habitat across 200 km2 of the California Central Valley. We found that mass-flowering crops increase bee abundance. Wild bee abundance was subsequently associated with higher parasite presence, but only in sites with a low abundance of non-crop flowers. Bee traits related to higher dispersal ability (body size) and diet breadth (pollen lecty) were also positively related to parasite presence. Our results highlight the importance of non-crop flowering habitat for supporting bee communities. We suggest monoculture alone cannot support healthy bees.

Sex differences in pollinator behavior: Patterns across species and consequences for the mutualism.

Intraspecific variation in floral visitor behaviour and pollination efficiency has been much less studied than interspecific variation. Nevertheless, it is clear that large differences in these traits exist within species, and in particular between sexes within species. With the exception of a few well-studied interactions, however, the consequences of these differences in the pollinators and visited plants remain to be investigated. In this review, we document large and consistent differences in the foraging patterns of male and female pollinators that have been demonstrated to directly affect plant reproduction or that have clear potential to do so. Males and females differ in visitation frequency, type of flowers visited, and per-visit pollen transfer. Females gather more and different resources from flowers compared to males, and males generally tend to show more mobile foraging patterns than females. We argue that these sex-associated patterns have broad generality across pollinators, and that sex-associated differences can in some cases be larger than differences between species. We offer predictions about how these patterns will influence pollinator preference, specialization, and fidelity, as well as the cost, quality and quantity of pollination service to plants. In the face of increasing threats to plant-pollinator interactions, understanding their basic functioning and the variation inherent in their component parts is critical. We advocate for more attention to sex-based differences among pollinators in particular, and the consequences of intraspecific variation more broadly.

Sex differences in the foraging behavior of a generalist hawkmoth.

Within-species variation in pollinator behavior is widely observed, but its causes have been minimally investigated. Pollinator sex is associated with large differences in behavior that may lead to predictable differences in flower foraging, but this expectation has not been explicitly tested. We investigate sex-associated differences in nectar-foraging behavior of the hawkmoth Hyles lineata, using pollen on the proboscis as a proxy for flower visitation. We tested two predictions emerging from the literature: (1) the sexes differ in the flower species they visit, (2) females are more specialized in flower choice. We also examined potential drivers underlying these predictions by performing field and laboratory experiments to test whether males (3) switch among flower species more frequently, or (4) fly farther and therefore encounter more species than females. Consistent with prediction (1), pollen load composition differed between the sexes, indicative of visitation differences. Contrary to prediction (2), females consistently carried more species-rich pollen loads than males. (3) Both sexes switched between flower species at similar rates, suggesting that differences in floral fidelity are unlikely to explain why females are less specialized than males. (4) Males flew longer distances than females; coupled with larger between-site differences in pollen composition for females, this result suggests that sex differences in mobility influence foraging, and that females may forage more frequently and in smaller areas than males. Together, our results demonstrate that sex-associated foraging differences can be large and consistent over time, and highlight the importance of sex as a driver of variation in pollinator behavior.

Coevolutionary transitions from antagonism to mutualism explained by the Co-Opted Antagonist Hypothesis.

There is now good evidence that many mutualisms evolved from antagonism; why or how, however, remains unclear. We advance the Co-Opted Antagonist (COA) Hypothesis as a general mechanism explaining evolutionary transitions from antagonism to mutualism. COA involves an eco-coevolutionary process whereby natural selection favors co-option of an antagonist to perform a beneficial function and the interacting species coevolve a suite of phenotypic traits that drive the interaction from antagonism to mutualism. To evaluate the COA hypothesis, we present a generalized eco-coevolutionary framework of evolutionary transitions from antagonism to mutualism and develop a data-based, fully ecologically-parameterized model of a small community in which a lepidopteran insect pollinates some of its larval host plant species. More generally, our theory helps to reconcile several major challenges concerning the mechanisms of mutualism evolution, such as how mutualisms evolve without extremely tight host fidelity (vertical transmission) and how ecological context influences evolutionary outcomes, and vice-versa.