The invasion of Linyphia triangularis (Araneae: Linyphiidae) in Maine: Ecological and behavioral interactions with native species
The European sheet-web spider Linyphia triangularis (Araneae: Linyphiidae) has recently become established at Acadia National Park and other areas of coastal Maine, where it reaches very high densities. Surveys indicate that three large native sheet-web spiders, Pityohyphantes sp., Frontinella communis, and Neriene radiata, are relatively scarce in both forest and coastal habitat at Acadia. Anecdotal evidence suggests that these species were more abundant at Acadia prior to the invasion, and that they are currently more abundant elsewhere in Maine (i.e., Cobscook Bay State Park) where L. triangularis populations are smaller. These native species vary in morphology and web structure; in general, Pityohyphantes sp. is most similar to L. triangularis, whereas F. communis is least similar. Despite their similar size as adults, the annual phenology of natives differs considerably from that of L. triangularis, such that natives and the invader are rarely of similar size classes.
This dissertation documents the first experimental studies of interactions between L. triangularis and native spiders in Maine. Experiments conducted in the field and laboratory demonstrate negative interactions between L. triangularis and native linyphiids. Competition for habitat and aggressive web invasion appear to be more important than food competition in driving these interactions. Densities of L. triangularis comparable to those found at Acadia prevent released Pityohyphantes sp. from establishing webs on experimental plots (Chapter 1). I find no evidence that L. triangularis substantially reduces prey availability, or that natives (Pityohyphantes sp. and F. communis) remain at web sites longer if given additional prey (Chapter 2). Experiments conducted in the field (Chapter 3) and lab (Chapter 4) demonstrate that web invasion by L. triangularis is likely an important mechanism of its success at Acadia. Linyphia triangularis are more likely to usurp the webs of native spiders than vice-versa, and are also more willing to share webs. Phenological differences may affect native species differently (Chapter 4). Relative size predicts contest outcome between N. radiata and L. triangularis, whereas L. triangularis consistently wins contests with F. communis, independent of size. There is evidence that both species and size may predict outcomes of L. triangularis - Pityohyphantes sp. contests.