The impact of long-term pan trapping on native bee abundance, richness and diversity

Nearly 90% of flowering plants, including many food crops, are pollinated by insects or other animals, making pollination a critical ecosystem service. Bees provide the majority of pollination services in many areas; therefore, assessing trends in bee populations over time is necessary for understanding and interpreting how plant reproduction responds to climate change and other disturbances. A standardized method for sampling bees using a combination of pan traps and hand netting bi-monthly has been developed and is widely used, allowing researchers to collect and compare data on bee population trends across habits and over time. Bees are challenging to identify and therefore must be sacrificed for species-level identification. Because bee sampling requires removing individuals from populations, we explored whether long-term bee monitoring using the standardized method had any effect on bee abundance and diversity over time, which could have important conservation implications for regions wanting to adopt bee monitoring programs To determine whether repeated bi-monthly sampling of bees affected bee abundance, richness and diversity, we performed long-term and repeated sampling of bees using standardized methods. Beginning in 2009, we began sampling nine sites once every two weeks. Bee abundance, richness or diversity did not decrease over the four years of sampling, indicating that sampling one year does not decrease bee abundance in subsequent years. Within each year, we did not see a steady decrease in bee abundance, richness or diversity over the course of the season, indicating that repeated sampling does not affect bee abundance from one sample period to the next. In addition, we performed two control experiments, where we sampled bees in meadows that had never previously been sampled. We sampled one new site each week for the duration of the 2010 and 2012 summer. We found no significant difference in bee abundance, richness or diversity between the repeatedly sampled sites and the sites that had never been sampled before. Our results indicate that the standardized methods for collecting bees do not affect bee abundance or diversity in our study region (Rocky Mountain West); thus, the bi-monthly standardized sampling methods will have little impact on bee populations and ecosystem health.