Cellar spider Pholcus phalangioides has been a successful invasive species in North America for over 100 years, residing in human dwellings and subsisting on a polyphagous insect diet. The arrival of a congener, Pholus manueli has led to competition between the two spiders for a very similar niche. While P. phalangioides is much larger in size, its population has diminished rapidly in the face of the P. manueli take over. We posit that the success of P. manueli may lie in the difference in interspecies interactions between the spiders, specifically their different levels of aggression. Experiments were set up with two males per female at high density of six and low density of three spiders. Mature, virgin spiders were selected and marked on their abdomen with acrylic paint. Spiders were introduced into clear plexiglass containers (43 x 31 x 26 cm) and the first 8 hours of spider interaction was filmed, and the number of agonistic interactions was determined. An aggressive act was identified as an active approach or lunge at another spider. When there was retaliation for an aggressive act and two individuals actually grappled with one another, it was classified as a fight. The number of aggressive acts and fights were analyzed separately using Generalized Linear Models with the Poisson distribution in JMP Pro 14. There were significantly higher observed levels of aggression in P. phalangioides than in P. manueli trials, though both spiders were more likely to engage in aggressive acts in high densities. Surprisingly, both species of pholcids had significantly higher amounts of fights in low density experiments. These results supported our hypothesis that higher levels of interspecies aggression occur in P. phalangiodes populations.
Authors: Sarah Strack
Faculty Advisor: Alexander Berry and Ann Rypstra, Department of Biology
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