Crop visitation by wild bees declines over an 8‐year time series: A dramatic trend, or just dramatic between‐year variation?

Abstract

1. Despite widespread recognition of the need for long-term monitoring of pollinator abundances and pollination service provision, such studies are exceedingly rare.2. In this study, we assess changes in bee visitation and net capture rates for 73 species visiting watermelon crop flowers at 19 farms in the mid-Atlantic region of the United States from 2005 to 2012.3. Over the 8 years, we found a 58% decline in wild bee visitation to crop flowers, but no significant change in honey bee visitation rate. Most types of wild bees showed similar declines in both the visitation and the net capture data; bumble bees, however, declined by 56% in the visitation data but showed no change in net capture rates. Trends in pollination services, that is, estimated pollen deposition, largely followed the trends in visitation and net capture rates.4. While we detected large and significant declines in wild bees when using generalised linear mixed models (GLMMs), permutation analyses that account for non-directional variation in abundance were non-significant, demonstrating the challenge of identifying and describing trends in highly variable populations. 5. As far as we are aware, this article represents one of fewer than 10 published time-series (defined as >5 years of data) studies of changes in bee abundance, and one of only two such studies conducted in an agricultural setting. More such studies are needed in order to understand the magnitude of bee decline and its ramifications for crop pollination.

Six years of wild bee monitoring shows changes in biodiversity within and across years and declines in abundance

“…Surveys of bumble bees for 8 years in the Rocky Mountains showed no directional trends in abundance (Ogilvie et al, 2017 ) and in Coastal California, there were declines in abundance across 10 years, potentially caused by competition with honey bees and climate‐driven changes in floral resources (Thomson, 2016 ). Finally, Aldercotte et al ( 2022 ) found declines in abundance of visitors to watermelon flowers over 8 years. In some studies, natural history traits helped explain changes in bee abundance over time or responses to environmental fluctuations (Bartomeus et al, 2013 ; Hamblin et al, 2018 ).…”

“…Insect monitoring efforts can find evidence for directional increases and decreases in abundance over time that are actually the result of sampling starting or ending at high or low abundance years (Aldercotte et al, 2022 ; Didham et al, 2020 ; Fournier et al, 2019 ). Because of the complicating effects of year‐to‐year variability, 10 or more years of monitoring may be needed to come to strong conclusions about trends in population dynamics (Didham et al, 2020 ; Fournier et al, 2019 ).…”

“…Insect monitoring efforts can find evidence for directional increases and decreases in abundance over time that are actually the result of sampling starting or ending at high or low abundance years (Aldercotte et al, 2022;Didham et al, 2020;Fournier et al, 2019).…”

“…Bee abundance and richness can also change greatly across years and there are pros and cons to different methods of assessing changes over time (Aldercotte et al, 2022 ; Graham et al, 2021 ; Ogilvie et al, 2017 ). One method for studying changes in bee species over time is to compare historical records with more recent collections (Bartomeus et al, 2013 ; Burkle et al, 2013 ; Cameron et al, 2011 ; Mathiasson & Rehan, 2019 ; Wood et al, 2019 ).…”

“…Also, it can be logistically difficult to continue these types of standardized sampling for a long enough period of time or with enough sample sites to have the statistical power to detect larger population trends (Didham et al, 2020 ; Lebuhn et al, 2013 ; Tronstad et al, 2022 ). Not surprising, standardized longitudinal studies of bee communities are rare but the existing ones have reported high year‐to‐year variation in abundance and a mix of species that are stable, increasing, and decreasing in abundance over time (Aldercotte et al, 2022 ; Graham et al, 2021 ; Herrera, 2019 ; Ogilvie et al, 2017 ; Onuferko et al, 2018 ; Roubik et al, 2021 ). Jointly, these studies have increased the need and interest in formal monitoring of bee biodiversity to assess if there are declines and potential links to decreasing pollination services (Woodard et al, 2020 ).…”