Generalist species are the linchpins of networks, as they are important for maintaining network structure and function. Previous studies have shown that interactions between generalists tend to occur consistently across years and sites. However, the link between temporal and spatial interaction persistence across scales remains unclear. To address this gap, we collected data on plant-pollinator interactions throughout the flowering period for five years across six plots in a subalpine meadow in the Rocky Mountains. We found that interactions between generalists tended to persist more in time and space such that interactions near the network core were more frequently recorded across years, within seasons, and among plots. We posit that species’ tolerance of environmental variation across time and space plays a key role in generalization by regulating spatiotemporal overlap with interaction partners. Our results imply a role of spatiotemporal environmental variation in organizing species interactions, marrying niche concepts that emphasize species environmental constraints and their community role.
We conducted this study in a subalpine meadow at the University of Colorado’s Mountain Research Station (40°01'48" N, 105°32'26" W), located at 2900 m of elevation 22 km west of Boulder, CO, USA. The meadow faces east and is surrounded by aspen and spruce-fir forest. We collected interaction data weekly during the entire flowering period from 2015 to 2019. The flowering season at the study site typically starts after snowmelt in late May to early June and extends to late September. Interactions were recorded on 16–18 weeks per year. We sampled on average 6.95 (±1.17 SD) days apart. On each sampling date, we sampled plant-pollinator interactions in six 20-30 m × 2 m plots (five 30 m × 2 m and one 20 m × 2 m) by observing flowers (plant-centered sampling). Plots were at a similar elevation, 2962 - 2978m. Sampling was conducted in fair weather during mornings between 8:00 am and 12:00 pm, a time range when pollinator activity is high and before the onset of thunderstorms that often occur at mid-day during the summer in the Rocky Mountains. We sampled plant-pollinator interactions within each plot (in random order) by doing 15-min surveys in which we carefully observed all flowers for visitors while walking the periphery of plots to minimize trampling. When we observed an interaction, defined as a pollinator contacting the reproductive structures of a flower, we recorded the identity of the plant and pollinator species. Insect pollinators were collected with aspirators or aerial nets for later identification in the laboratory. Expert entomologists (see Acknowledgements) assisted with insects that are difficult to identify. We compiled all the observations from the study into a species x species plant-pollinator interaction matrix, sorting rows and columns to maximize binary nestedness. This sorting organizes plant species (top-to-bottom in rows) and pollinator species (left-to-right in columns) from most to least generalist according to their degree (number of partner species) such that generalist species are packed into the top left corner of the matrix. Using this matrix organization we created three matrices with cell values representing each variable of temporal or spatial persistence: the number of years, the span of days (phenophase, the maximum date minus the minimum date in which an interaction was recorded), and the number of plots in which interactions were recorded.