DOI: 10.5061/dryad.qnk98sfkc
title: Rare and declining bee species are key to consistent pollination of wildflowers and crops across large spatial scales

description: 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.
description: <em>Data collection.</em> 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, <em>Vaccinium
 corymbosum</em>;<em> </em>and cranberry,
 <em>Vaccinium macrocarpon</em>) crops and one non-native crop
 (watermelon, <em>Citrullus lanatus</em>)<em>
 </em>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 <em>P. reptans </em>in April-May, <em>P.
 tanacetifolia </em>in June-July, and <em>M. fistulosa
 </em>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 m<sup>2</sup> transects.
 Collection effort was 60 minutes site<sup>-1</sup>
 day<sup>-1</sup> for blueberry and watermelon, and 120 minutes
 site<sup>-1</sup> day<sup>-1</sup> 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.
 <em>Data processing.</em> 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.
description: Analyses were run in R version 4.0.5 (2021-03-31) "Shake and
 Throw"