Experimental calibration of trapping methods for addressing bias in arthropod biodiversity monitoring

Background/Question/Methods

Our understanding of ecosystem processes is filtered through the perspective of the researchers making the observations, through their constraints, biases, and choices. Ultimately, observed biodiversity trends are heavily context dependent. For example, in a 2019 field study that paired urban and natural ecosystem arthropod monitoring in the Great Lakes Region we found that the sampling methodology deployed failed to capture several key taxa predicted to be important in these ecosystems, leading to a new research question: how do different approaches to biodiversity sampling affect our ultimate conclusions? In this case, what is the community composition of arthropods caught in each trap type and how do they compare to one another? To directly examine how choice of trap affects how observations are made, we developed a sampling design study in which four types of arthropod traps were compared on managed grassland sites in Northeast Ohio. We used three ground trapping methods: traditional pitfalls, yellow ramp traps, and a newly designed jar ramp trap, as well as sticky cards which target flying insects. Traps were placed at three sites, every other week from June – September 2020.

Results/Conclusions

Over 13,600 specimens were caught over seven sampling periods. We used univariate and multivariate approaches to examine how the observed community differed between trap types. Trapping efficiency varied by trap type: overall, yellow ramp traps caught the greatest number of individuals (7,771), followed by sticky cards (4,199), then jar ramp trap (1108), with pitfalls catching the least (585). However, the capture of functional groups of arthropod (flying, ground crawling, etc.) and individual taxonomic groups varied dramatically by trap type, suggesting that both targeted and broad-spectrum methods for sampling are necessary, depending on the ecological question being asked. Future biodiversity monitoring schemes should conduct calibration experiments to maximize the chances of capturing target taxa, while minimizing disturbance to their habitat, and thus activity patterns of taxa. Because the ground insect guild is both less mobile and associated with important ecosystem services, they are useful bioindicators, and it may be desirable to develop a calibration study that looks at relative trap efficiencies for this guild to facilitate ecological synthesis. This study illustrates the implications for biodiversity sampling of arthropods in environments with physical constraints on trapping, such as green roofs, alvars, rocky shorelines or cliff edges, or anywhere soil is too shallow or cannot be disturbed.