Sweeping beauty: is grassland arthropod community composition effectively estimated by sweep netting?

Abstract

Arthropods are critical ecosystem components due to their high diversity and sensitivity to perturbation. Furthermore, due to their ease of capture they are often the focus of environmental health surveys. There is much debate regarding the best sampling method to use in these surveys. Sweep netting and pan trapping are two sampling methods commonly used in agricultural arthropod surveys, but have not been contrasted in natural grassland systems at the community level. The purpose of this study was to determine whether sweep netting was effective at estimating arthropod diversity at the community level in grasslands or if supplemental pan trapping was needed. Arthropods were collected from grassland sites in Montana, USA, in the summer of 2011. The following three standardized evaluation criteria (consistency, reliability, and precision) were developed to assess the efficacy of sweep netting and pan trapping, based on analyses of variations in arthropod abundances, species richness, evenness, capture frequency, and community composition. Neither sampling method was sufficient in any criteria to be used alone for community-level arthropod surveys. On a taxa-specific basis, however, sweep netting was consistent, reliable, and precise for Thysanoptera, infrequently collected (i.e., rare) insects, and Arachnida, whereas pan trapping was consistent, reliable, and precise for Collembola and bees, which is especially significant given current threats to the latter's populations worldwide. Species-level identifications increase the detected dissimilarity between sweep netting and pan trapping. We recommend that community-level arthropod surveys use both sampling methods concurrently, at least in grasslands, but likely in most nonagricultural systems. Target surveys, such as monitoring bee communities in fragmented grassland habitat or where detailed information on behavior of the target arthropod groups is available can in some instances employ singular methods. As a general ecological principle, consistency, reliability, and precision are appropriate criteria to evaluate the applicability of a given sampling method for both community-level and taxa-specific arthropod surveys in any ecosystem.

Flower resource and land management drives hoverfly communities and bee abundance in seminatural and agricultural grasslands

“…Pan traps can also oversample pollinators in resource-poor environments by "sucking in" pollinators and can undersample in flower-rich sites where there are many competing stimuli (Hickman, Wratten, Jepson, & Frampton, 2001;Roulston, Smith, & Brewster, 2007;Wilson, Griswold, & Messinger, 2008). However, they do reduce the sampling bias associated with hand netting (Spafford & Lortie, 2013). Ideally, any site pollinator assessment should use a combination of trapping and net sweeping to collect data.…”

Prescribed patch burnings increase thrips species richness and body size in grassland communities

“…In each sampling date, thrips were sampled from the vegetation with a sweep net in four small Ó 2017 The Royal Entomological Society, Insect Conservation and Diversity,11,[204][205][206][207][208][209][210][211][212] Fire effects on grassland thrips 205 independent equidistant transects per plot (about ten sweeps per transect), once in the morning and once in the afternoon. This method is considered consistent, reliable and precise for Thysanoptera (Spafford & Lortie, 2013). For soil thrips samples, five pitfall traps per plot were used during 4 days.…”

NDVI as a predictor of canopy arthropod biomass in the Alaskan arctic tundra

“…Canopy-dwelling arthropod biomass was measured via sweep netting using a standard insect net (Robel et al 1995). Although it is possible that sweep netting captures arthropods differently within and across vegetation communities (Southwood 1978), we chose this method to collect canopy-dwelling arthropods for several reasons, including that (1) all canopy arthropod sampling methods introduce some bias (Doxon et al 2011), but sweep netting may introduce less sampling bias compared to other techniques (Sørensen et al 2002), and (2) samples associated with sweep netting tend to overlap in abundance and composition with other collection methods (Noyes 1989, Spafford andLortie 2013). Samples were collected weekly from late May to mid-to-late July over four years (2010)(2011)(2012)(2013).…”