Biodiversity promotes ecosystem function in experiments, but it remains uncertain how biodiversity loss affects function in larger-scale natural ecosystems, where rare and declining species which are likely to be lost and function needs to be maintained across space and time. Here we explore the importance of rare and declining bee species to the pollination of three wildflowers and three crops using large-scale (72 sites across 5,000 km2), multi-year datasets. Half (82/164) bee species were rare or declining, but these species provided ~15% of overall pollination. To determine the number of species important to ecosystem function, we used two methods of 'scaling up', both of which have previously been used for biodiversity-function analysis. First, we summed bee species’ contributions to pollination across space and time and then found the minimum set of species needed to provide a threshold level of function across all sites; according to this method, effectively no rare and declining bee species were important to pollination. Second, we account for the “insurance value” of biodiversity by finding the minimum set of bee species needed to simultaneously provide a threshold level of function at each site in each year. The second method leads to the conclusion that 25 rare and eight declining bee species (36% and 53% of all rare and declining bee species, respectively) are important. Our findings provide some of the strongest evidence yet for the importance of rare and declining species, thereby providing a more direct link between real-world biodiversity loss and ecosystem function.

Data collection.

The wildflower data were collected over two years (2017-2018) at 24 study sites in central New Jersey (i.e., 48 site-years per plant species). At each site we placed a single fenced array of potted plants. Arrays included three seven-gallon pots of each of three plant species. All arrays were in 10-20 meters from natural forests in open habitats. We also studied two native (blueberry, Vaccinium corymbosum; and cranberry, Vaccinium macrocarpon) crops and one non-native crop (watermelon, Citrullus lanatus) that rely on wild bees for pollination. Each crop was studied at 16 commercial farms (hereafter, sites) in a 100 km x 50 km region in central and southern New Jersey and eastern Pennsylvania over two years (i.e., 32 site-years per crop species). For this analysis, we focused on bees and excluded other pollinators

In 2017 and 2018, we vacuum-collected individual bees from wildflowers. Bees were collected form P. reptans in April-May, P. tanacetifolia in June-July, and M. fistulosa in July-August. All sampling rounds had temperatures of at least 17 C and wind speeds less than 4.5 m/s. For crops, bees were net-collected along fixed 50-200 m² transects. Collection effort was 60 minutes site^-1 day^-1 for blueberry and watermelon, and 120 minutes site^-1 day^-1 for cranberry, with two years of sampling for each crop. Blueberry was sampled 2010-2011, cranberry was sampled 2010-2011, and watermelon was sampled 2009-2010). Sampling corresponded with peak bloom of each crop: April-May for blueberry; May-July for cranberry; July for watermelon.

Data processing.

We performed “single-visit experiments”, in which we allowed a single bee to visit an unpollinated flower, to determine how many pollen grains each bee species deposits per visit. Stigmas were preserved them in ethanol, stained, and the number of pollen grains were counted. We assigned bees to morphogroups because we could not identify most bee species on the wing and to get enough sample size per morphogroup to reliably estimate pollen deposition. For each site-year combination, we multiplied flower visitation frequency by mean per-visit pollen deposition to estimate each bee species’ contribution to pollination to each plant species.

Analyses were run in R version 4.0.5 (2021-03-31) "Shake and Throw"