Competition between spiders and pitcher plants? Prey availability and trophic interactions in bogs

Abstract

Web-building spiders (e.g. Frontinella communis and Neriene radiata) and northern pitcher plants (Sarracenia purpurea) are co-occurring sit-and-wait predators that may compete for shared arthropod prey. We evaluated prey availability and capture rates by web-weaving spiders and pitcher plants in a headwater stream box complex (Harvard Pond) in central Massachusetts. We established 40 plots with spider and pitcher plant removal treatments, and collected arthropod prey captured by open pitcher plants, pitfall traps, and sticky traps in those plots. The removal of web-building spiders significantly changed the distribution of prey captured by the pitcher plants (Chi-square test: P = 2.2 × 10-16 at Harvard Pond). Plugging pitcher plants yielded no difference in prey capture by sticky traps or pitfall traps. In pitcher plants, the total number of Odonata, Diptera, and Hemiptera captured more than doubled when web-weaving spiders were removed from the plots. In pitfall traps in web-weaving spider removal plots, the total number of ants decreased by more than 50%. Hunting spider capture by sticky traps showed moderate increase (P = 0.08) when web-weaving spiders were removed, suggesting an unforeseen interaction between these groups of spiders. The increased predation risk of hunting spiders on ants may explain the marked decrease in ant activity when web-weaving spiders were removed.