Contests between web spiders: patience as a strategy for successful invasion

Abstract

When a species moves into a new area, direct contests with functionally similar residents may determine whether they can establish themselves. In recent years, the web-building cellar spider, Pholcus manueli, has largely displaced its larger well-established congener, Pholcus phalangioides, in SW Ohio. The relationship between these two species is interesting both because they are congeners and because both are invasive; the resident, P. phalangioides, had robust populations in the area for more than 100 years. We hypothesized that this example of over-invasion was related to interactions over web sites or webs. We first staged interspecific contests between adult females over empty web sites and existing webs in the laboratory. The smaller P. manueli never won but many encounters ended as ties with little active engagement between the spiders. We then staged contests with juvenile spiders that were close in size. We introduced well-fed or food-limited P. manueli into webs of P. phalangioides. In these matches, the relative size difference, a proxy for resource holding potential, determined the outcome. However, fights involving well-fed P. manueli were shorter and more aggressive than contests with hungry individuals. Additionally, well-fed invaders were more likely to take over webs whilst hungry P. manueli were less likely to engage in contests, thus allowing coexistence. We posited that newly introduced individuals, immersed in a population of P. phalangioides, persisted on the sidelines of the resident’s webs, and engaged in interspecific aggression only when the dynamics of the interaction were in their favor.