Interaction strengths between trophic levels can be determined using laboratory functional response experiments, which measure foraging rates across prey densities. These functional responses are often extrapolated to the field and applied to a diverse set of ecological issues, including the development of biocontrol programs and the management of invasive species. However, recent work shows that experimental arena size largely determines estimates of the functional response parameter space clearance (attack) rate, more so than factors such as body size and temperature. To accurately apply functional response results, we must understand the mechanisms behind the association between arena size and space clearance rate. We predicted that this arena size effect is due to thigmotactic behavior within the artificial constraints of arenas. To test this hypothesis, we video-recorded predators (wolf spiders) and prey (fruit flies) in arenas of different sizes to assess movement within arenas. We then performed feeding trials in circular and annular (ring-shaped) arenas of different sizes. Circular arenas allowed us to confirm the presence of the arena size effect. Annular arenas forced all individuals to aggregate around the edges, allowing us to account for thigmotactic behavior mathematically and simulate a situation where predators and prey do not display this behavior.
Results/Conclusions
Our video-tracking results clearly revealed thigmotactic behavior by both predator and prey. As expected, space clearance rate estimates depended strongly on arena size in circular arenas, confirming the arena size effect in our predator-prey system. In the annular arenas, such an effect was not observed: spiders showed similar foraging rates and space clearance rates in all tested arena sizes. We show how these results complement the video-tracking trials in suggesting that aggregation around arena edges results in a discrepancy between the calculated prey density and the prey density experienced by the predator. This mismatch results in a systematic miscalculation of space clearance rate, inflating the parameter as arena size increases. Our findings provide crucial insights into the interactions between behavior and experimental design, and serve as an important reminder that both must be accounted for when interpreting functional response results.