Conserved grasslands support similar pollinator diversity as pollinator-specific practice regardless of proximal cropland and pesticide exposure

Abstract

Pollinator diversity and abundance are declining globally. Cropland agriculture and the corresponding use of agricultural pesticides may contribute to these declines, while increased pollinator habitat (flowering plants) can help mitigate them. Here we tested whether the relative effect of wildflower plantings on pollinator diversity and counts were modified by proportion of nearby agricultural land cover and pesticide exposure in 24 conserved grasslands in Iowa, USA. Compared with general grassland conservation practices, wildflower plantings led to only a 5% increase in pollinator diversity and no change in counts regardless of the proportion of cropland agriculture within a 1 km radius. Pollinator diversity increased earlier in the growing season and with per cent flower cover. Unexpectedly, neither insecticide nor total pesticide concentrations on above-ground passive samplers were related to pollinator diversity. However, pollinator community composition was most strongly related to date of sampling, total pesticide concentration, and forb or flower cover. Our results indicate very little difference in pollinator diversity between grassland conservation practices with and without wildflower plantings. Given the relatively high economic costs of wildflower plantings, our research provides initial evidence that investment in general grassland conservation may efficiently conserve pollinator diversity in temperate regions of intensive cropland agriculture.

Comparing modern identification methods for wild bees: Metabarcoding and image-based morphological taxonomic assignment

“…Due to the objectives of the larger study of which this work was a part (i.e., the need to preserve the bee tissues for additional analyses; [ 28 , 29 ]), the specimens had to be frozen immediately after field collection and could not be pinned or examined under a microscope. Photographing and assigning the lowest taxonomic unit using dichotomous keys and expert advice was moderately time intensive (~4 months for this study); however, the processing steps (e.g., photographing and sorting by potential taxa) completed by an amateur taxonomist greatly reduced the amount of time needed from an expert taxonomist compared to the traditional pinning identification method.…”

“…Additional information, including the intertegular distance (which is correlated to mass and foraging distance; [ 50 , 51 ]) and bee gender for sexually dimorphic species, could be obtained from the images, and the images provided a publicly available reference database [ 31 ]. Richness and abundance data from the morphological identification method can also be used to calculate diversity indices (see [ 29 ]), which would not be possible with the richness-only results from metabarcoding.…”

“…Pollinators collected with sweep nets were placed in clean plastic bags and immediately frozen. Sampling details are further described in Kraus et al [ 29 ].…”

“…Here we compared image-based bee identification (hereafter referred to as morphological identification) and metabarcoding approaches to identifying wild bee taxa while preserving the tissues for other analyses (e.g., Hladik et al [ 28 ]), summarizing the relative strengths and weaknesses of both methods. Wild bees were collected from Iowa as part of a study designed to understand the effects of land use and pesticide exposure on bee health and diversity (see [ 28 , 29 ]). The hypotheses addressed in this study included: (1) there would be general agreement among the two identification methods, (2) the morphological identification would provide definitive information on the presence of specific taxa, and (3) the molecular data would resolve some cryptic species that were indiscernible from the images using the photographic approach.…”