A50: Effect of Humidity on Foraging Success and Web Structure in Two Invasive Species of Cellar Spiders (Araneae: Pholcidae)

The coexistence of two ecologically similar species could be explained by differential use of microhabitats. This study investigated factors that influence foraging and web building behavior of two invasive species of arthropod predators in the family Pholcidae. Both are well-adapted to human dwellings, with Pholcus phalangioides, the larger species, undergoing displacement in Southwest Ohio by Pholcus manueli. We predicted P. phalangioides would kill more prey and invest more in web structure at low humidity, while P. manueli would use more webbing and forage more successfully at high humidity. Spiders were kept in either an environmental room at 70% humidity or an environmental chamber at 44% humidity. For foraging trials, 10 fruit flies were added and the number of dead prey was recorded after 24 hours. For web structure trials, spiders were given 24 hours to build a web, which was collected on a glass slide and photographed against a grid on a dark background; ImageJ was used to measure web area and density (as percent cover). Body condition (estimated by mass and tibia-patella length) did not differ between the species, indicating they were at similar hunger levels before the foraging trials. Percent mass gain was greater for P. manueli, which had more dead prey than P. phalangioides and both species had more dead prey in low humidity than in high humidity. P. manueli webs covered more area and had higher density than P. phalangioides webs.  In particular, P. manueli increased web density comparatively more in low humidity than P. phalangioides. Future studies should explore the costs and benefits of this difference and whether it is important to the successful range expansion of this species. Investigating the interaction between invasive species and their habitat is important to my career as a scientist and communicator in ecology and conservation.

Author(s): Cynthia A. Fischer, Applied Biology Major

Advisor(s): Ann Rypstra, Department of Biology

Alex Salazar, Department of Biology