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. This study reports the results from two studies in which we evaluated prey availability and capture rates by web-weaving spiders and pitcher plants in two bogs of the Quabbin Reservoir watershed of central Massachusetts. At both Swift River Bog (a kettlehole bog) and on bog islands in Harvard Pond (a headwater-stream bog complex), we removed all web-building spiders and their webs from fixed areas around pitcher plants. Arthropod prey captured by these pitcher plants was collected twice a week, identified to Order (with ants [Formicidae] separated from other Hymenoptera), and compared with prey captured by pitcher plants in control plots with naturally occurring densities of spiders. At Harvard Pond, we added a treatment to restrict the ability of pitcher plants to capture prey, and we also used pitfall traps and sticky traps to determine the background availability of cursorial prey across the treatments.
Results/Conclusions
In both bogs, the removal of web-building spiders significantly changed the distribution of prey captured by the pitcher plants (Chi-square test: P = 0.001 at Swift River, P = 2.2 × 10-16 at Harvard Pond). At Harvard Pond, the total number of Coleoptera, Diptera, and Hemiptera captured by pitcher plants more than doubled when spiders were removed, whereas the total number of Formicidae and Acarina captured by pitcher plants decreased by half. At Swift River Bog, the total number of Diptera captured by pitcher plants increased slightly when spiders were removed, whereas the number of Coleoptera, Collembola, and Formicidae captured decreased. At Harvard Pond, where we simultaneously measured background prey availability with pitfall traps and sticky traps, there were strong differences in prey capture among all treatment comparisons (P < 0.002, all six pairwise comparisons among controls, spider removals, pitchers occluded, and spiders removed + pitchers occluded). Differences in prey capture among treatments suggest intra-guild interaction between spiders and pitcher plants in bogs, including intraguild predation (pitcher plants preying on spiders) and kleptoparisitism of pitcher-plant prey by web-weaving and/or cursorial spiders.
