The effect of repeated, lethal sampling on wild bee abundance and diversity

Abstract

Summary1. Bee pollinators provide a critical ecosystem service to wild and agricultural plants but are reported to be declining world-wide due to anthropogenic change. Long-term data on bee abundance and diversity are scarce, and the need for additional quantitative sampling using repeatable methods has been emphasized. Recently, monitoring programmes have begun using a standardized method that employs a combination of pan traps and sweep netting, resulting in lethal sampling of bees. This standardized method can remove a large number of bees from sites during each sampling day, raising concern that the sampling itself could have a negative effect on bee populations. 2. We conducted an experiment to assess whether lethal sampling for bees using pan traps and netting affected bee abundance and diversity when done every two weeks throughout a season and over multiple years. We compared bee abundance, richness, evenness and functional group composition between sites that had been sampled every two weeks from 2009 to 2012 to similar sites not previously sampled. 3. We found that the standardized method for sampling bees, with specimens from 132 morphospecies, did not affect bee communities in terms of abundance, rarefied richness, evenness, or functional group composition. Thus, our results indicate that the bee communities we sampled are robust to such sampling efforts, despite removing an average of 2862 bees per season. 4. We discuss several explanations for why sampling did not affect bee abundance or community structure, including a density-dependent response to reduced competition for resources. 5. These results suggest that bee monitoring programmes sampling once every two weeks with pan traps and netting will not affect bee community structure. We urge researchers monitoring bees to utilize standardized protocols so that results can be compared across space and time.

Phenological change in a spring ephemeral: implications for pollination and plant reproduction

“…An alternative explanation for why our late phenology treatment received such low visitation rates is that the bees were present but choosing to forage outside of the experimental array. Bees in our study system have an average flight period of less than 2 weeks (Eickwort et al ., ; Gezon et al ., ), however, which is shorter than the time period between placement of the early and late plants in the array. Furthermore, we did not observe Lasioglossum ( Dialictus ) foraging outside the array while the late phenology plants were present (Z. J. Gezon, pers obs ).…”

“…Changes in pollinator visitation that accompanied advanced flowering time affected pollen limitation and plant reproduction. C. lanceolata was primarily visited by the small sweat bees Lasioglossum ( Dialictus ), which are most abundant early in the season (Gezon et al ., ). We found no difference in pollinator visitation rates between the early and control treatment plants in the array experiment.…”

Wild bee diversity is enhanced by experimental removal of timber harvest residue within intensively managed conifer forest

“…We note that although our lethal sampling approach had the potential to reduce local bee populations, this was unlikely for two reasons. First, a recent study that assessed the impacts of lethal removal did not detect an effect of regular and repeated lethal trapping on bee communities (Gezon, Wyman, Ascher, Inouye, & Irwin, ). Of note, that study collected bees over a longer time period (5 years) and more frequently (5–9 times per season) than the sampling we undertook in our study (Gezon et al, ).…”

Interpreting insect declines: seven challenges and a way forward

“…We stress that it is unlikely for standardised quantitative monitoring programmes to cause population‐wide decline due to over‐collection (e.g. Gezon et al ., ), although the potential risks to rare or localised species should always be evaluated carefully. In the sense that local depletion effects are unrepresentative of wider regional population changes, then this is a detection bias issue that arises due to low recruitment rates into the sampled population prior to the next sampling interval.…”