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A specialist bee and its host plants experience phenological shifts at different rates in response to climate change

Citation

Weaver, Sarah; Mallinger, Rachel (2022), A specialist bee and its host plants experience phenological shifts at different rates in response to climate change, Dryad, Dataset, https://doi.org/10.5061/dryad.zcrjdfndn

Abstract

Changes in climate can alter the phenology of organisms, potentially decoupling partners within mutualisms. Previous studies have shown that plant and pollinator phenologies are shifting over time, but these shifts have primarily been documented for generalists and within small geographic regions, and the specific climatic cues regulating these shifts are not well-understood. We examined phenological shifts in a specialist pollinator and its host plant species over a 117-year study period using a digitized dataset of over 4,000 unique collection records. We assess how climatic cues regulate these organisms’ phenologies using PRISM weather data associated with each record. We tested the hypothesis that rates of phenological change would be greater at northern latitudes. We found that the phenology of the specialist bee pollinator Habropoda laboriosa is changing over time, but at different rates across its range. Specifically, phenology is advancing to a greater degree in more northern populations, with increasing phenological advances of 0.04 days/year with each degree of latitude, and with a delay in phenology in more southern populations. In contrast, only one species in the host plant genus Vaccinium is experiencing phenological change over time. For this plant, rates of change are also variable across latitudes, but in a pattern opposite that of the bee; while phenology is advancing across its range, rates of advance are highest in more southern populations, with decreasing phenological advances of 0.01 days/year with each degree of latitude. The phenologies of both the bee and three of four Vaccinium spp. were regulated primarily by spring temperature, with phenologies overall advancing with increasing temperature, and with the strongest responses shown by the bee in northern populations. Our study provides partial support for the hypothesis that phenologies advance most at northern latitudes, but demonstrates that pollinators and plants do not adhere similarly to this prediction. Additionally, we illustrate the potential for phenological mismatch between a specialist pollinator and its host plants by showing that plants and pollinators are advancing their phenologies at different rates across space and time and with differing responses to changing climatic cues.

Methods

Habropoda laboriosa records

We used Julian date of collection from each H. laboriosa specimen as a proxy for adult bee foraging phenology. Records of H. laboriosa were gathered online from the Symbiota Collections of Arthropods Network (SCAN). We also databased specimens housed in the Florida State Collection of Arthropods (FSCA). Only records that contained a clearly interpretable date were included for analysis. We removed one specimen that was georeferenced to the west coast, where no other H. laboriosa specimens have been collected or recorded. To prevent bias introduced by varying collection efforts and techniques, only one specimen representing a collection event was retained for analysis. Specimens were considered to be from the same collection event if they had both the same date of collection and location of collection. Collection techniques using bee bowls or other traps are more likely to collect multiple individuals on a given date than collection events using hand netting, and these differences may affect our analyses especially if collection methods changed in popularity and relative use over time. All H. laboriosa specimens represent the adult life stage when bees are foraging from flowers; this life stage is brief (3-7 weeks per year) and both males and females are foraging from flowers during these few weeks. We used the latitude, longitude, and date of collection from these specimen records in our analysis.

Vaccinium spp. records

We used Julian date of collection from each flowering Vaccinium spp. specimen as a proxy for flowering phenology. We collected herbaria specimen records of wild Vaccinium spp. with ranges that overlap with H. laboriosa. Vaccinium spp. that do not occur in H. laboriosa’s range were not included as we were primarily interested in the phenologies of these two taxa as they interact. We only used Vaccinium spp. that included 400 or more recorded specimens in the flowering stage. Plant species with fewer than 400 flowering records (n = 72) were excluded as data were too limiting to draw conclusions about flowering phenology over time and space. Our final data set included the following species: V. corymbosum, V. arboreum, V. stamineum, and V. pallidum. Herbarium records of wild Vaccinium spp. were gathered online from the SEINet North American Plant Network (SEINet). Only those specimens with a clearly interpretable date of collection, a link to a reference photograph of the specimen, and geographic coordinates or county-level location of collection were included for analysis. Specimens without geographic coordinates but with a county-level location of collection were georeferenced to the center of the county that they were collected within. To prevent bias from different sampling or observation protocols over time, only one specimen per collection event was retained for analysis. Specimens with the same date of collection and location of collection were considered to be from the same collection event.

To assess phenological shifts in plant flowering over time, as well as to determine how weather variables affect plant flowering, we only included Vaccinium specimens that were flowering at the time of collection or observation. Plants included in this study were designated to be flowering if their reference images contained one or more open flowers. Specimens with highly distorted images and those that were highly damaged, such that we were unable to determine whether or not the plant was flowering at the time of collection, were not included for analysis.

Weather data

We collected all weather data from PRISM using the “Data Explorer: Download Time Series Values in Bulk” feature. This feature provides time series data by location and date as values collected from within the PRISM standard 4 km grid cells. We collected data from PRISM for each cell that contained the collection coordinates of a specimen included in the study, and for each year corresponding to a collection event. For each cell, PRISM calculates annual, monthly, and seasonal (average of three months) variables including average temperature, maximum mean temperature, minimum mean temperature, total number of freezing days (days with any amount of time below 0°C), total precipitation as rainfall, and average solar radiation. We focused primarily on spring averages (average of Feb, March, and April) as these averaged variables allowed us to examine weather over the time period of interest (spring) while avoiding correlated predictor variables from multiple individual months. This period also overlaps well with the phenology of our study organisms as 96% of specimens were collected from February to May. For all analyses, we only included specimens and associated climate data from 1901 – 2018 because PRISM climate estimates were only available for these years at the time of data collection. This 1901 – 2018 range included over 89% of all specimens that met all other criteria for inclusion in the study as previously described.

Usage Notes

1) For all datasets, variable names for columns 1:28 are Darwin Core terms. We provide the definition for each term in the README document. Theses definitions come directly from the Darwin Core website.

2) For all datasets, variable names for columns 29:275 are abbreviations for climate variables collected from PRISM using the “Data Explorer: Download Time Series Values in Bulk” feature. We provide a definition and units for each abbreviation in the README document. All definitions come directly from the Climate NA help page.