Ecologists have taken two distinct approaches in studying the distribution and diversity of communities: a species‐centric focus and an interaction‐network based approach. A current frontier in community‐level studies is the integration of these perspectives by investigating both simultaneously; one method for achieving this is evaluating the relative contributions of species turnover and host switching towards interaction turnover (i.e., the dissimilarity in interactions between two networks). We performed observations of plant‐pollinator interactions to investigate (1) patterns in interaction turnover across spatial, temporal, and environmental gradients and (2) the relative contribution of pollinator species turnover, floral turnover, simultaneous pollinator & floral turnover, and host switching towards interaction turnover. Field work was conducted on the Beartooth Plateau, an alpine ecosystem in Montana and Wyoming, with weekly observations of plant‐pollinator interactions across one growing season. Interaction turnover increased through time, with magnitudes consistently greater than 80%, even at time intervals as short as one week. Floral species turnover (41%) and simultaneous floral and pollinator species turnover (36%) accounted for almost all interaction turnover while host switching accounted for only 5%. Interaction turnover also significantly increased with spatial and elevational distance, albeit with lesser magnitudes than with temporal distance. The marginal spatial pattern was present for only some taxa (Bombus spp. and solitary bee species), potentially indicating variable habitat use by pollinators across the landscape. Weak environmental trends may be a consequence of unmeasured environmental variables, yet our finding that environmental gradients structure plant‐pollinator interaction partitions had not previously been tested with empirical data. Our observations suggest that host switching does not readily occur at the scales of alpine flowering phenology (i.e., ∼1 week); however, whether lack of host switching is indicative of inflexible pollinator foraging, or, more likely, a lack of opportunity or necessity to switch hosts, requires further investigation.

The study of mutualistic interaction networks has led to valuable insights into ecological and evolutionary processes. However, our understanding of network structure may depend upon the temporal scale at which we sample and analyze network data. To date, we lack a comprehensive assessment of the temporal scale‐dependence of network structure across a wide range of temporal scales and geographic locations. If network structure is temporally scale‐dependent, networks constructed over different temporal scales may provide very different perspectives on the structure and composition of species interactions. Furthermore, it remains unclear how various factors – including species richness, species turnover, link rewiring and sampling effort – act in concert to shape network structure across different temporal scales. To address these issues, we used a large database of temporally‐resolved plant–pollinator networks to investigate how temporal aggregation from the scale of one day to multiple years influences network structure. In addition, we used structural equation modeling to explore the direct and indirect effects of temporal scale, species richness, species turnover, link rewiring and sampling effort on network structural properties. We find that plant–pollinator network structure is strongly temporally‐scale dependent. This general pattern arises because the temporal scale determines the degree to which temporal dynamics (i.e. phenological turnover of species and links) are included in the network, in addition to how much sampling effort is put into constructing the network. Ultimately, the temporal scale‐dependence of our plant–pollinator networks appears to be mostly driven by species richness, which increases with sampling effort, and species turnover, which increases with temporal extent. In other words, after accounting for variation in species richness, network structure is increasingly shaped by its underlying temporal dynamics. Our results suggest that considering multiple temporal scales may be necessary to fully appreciate the causes and consequences of interaction network structure.

Wildfire regimes are changing in the western United States, yet the ways in which wildfires influence native bees, the resources they depend on for food and nesting, or the traits that influence their interactions with plants are poorly understood. In burned and unburned areas in Montana, USA, we investigated the abundance and diversity of native bees, floral and nesting resources, nesting success, and traits of flowers and bees. In two of the three localities studied, burned areas, including areas that burned with high-severity wildfires, supported higher density and diversity of native bees and the flowers they depend on for food and larval provisioning. Burned areas also had more bare ground for ground-nesting bees and more available coarse woody debris for cavity-nesting bees than unburned areas. Moreover, cavity-nesting bees were completely unsuccessful at nesting in artificial nesting boxes in unburned areas, while nesting success was 40% in burned areas. Mean bee intertegular distance (a trait strongly correlated with tongue length, foraging distance, and body size) was similar between burned and unburned areas. However, wildfires influenced both interspecific and intraspecific trait variation of bees and plants. Intraspecific variation in bee intertegular distance was higher in unburned than burned areas. Both interspecific and intraspecific variation in floral traits important for interactions with pollinators were generally higher in burned than unburned areas. Thus, wildfires generally increased the density and species diversity of bees and flowers as well as trait variation at both trophic levels. We conclude that wildfires-even large, high-severity wildfires-create conditions that support native bees and the resources they need to flourish, but that unburned areas maintain trait variation in landscape mosaics with heterogeneous fire conditions.

No abstract