Do stream logjams influence riparian web spider density and distribution?

Background/Question/Methods

The exchange of energy at ecotones is influential in shaping the communities of adjacent ecosystems. The riparian-stream boundary is an ecotone in which emergent aquatic prey provide an important subsidy to riparian consumers, such as web-building spiders. Terrestrial spiders have been shown to be positively correlated with aquatic insect emergence, and emergence is linked to benthic productivity. Logjams, or accumulations of wood in streams, can increase aquatic macroinvertebrate production through their retention of organic materials and diversification of habitat. Because of this, we hypothesized that there would be an increase in spider density situated near logjams. To test this, spider webs were counted, classified and mapped within 6-m of the stream bank of a 40-m reach in a northern Minnesota stream.  In addition, a Web Space Index (WSI) was developed to estimate availability of web-building substrate and structure. Data were collected during two periods, late May and late August of 2011. Web locations were organized into categories for analysis and visual representations were created using ArcGIS software. Additionally, 50 or 60 wooden emergence traps were positioned randomly throughout the study reach and captured emerged insects were counted after 72-h of deployment.

Results/Conclusions

We found 106 spider webs in May and 422 in August. Most were either vertically oriented orb webs of the family Tetragnathidae or horizontally oriented sheet webs of the family Linyphiidae. Both May and August showed a steep decline in web number with increasing distance from the stream (P < 0.001). However, webs in May were heavily concentrated around the logjam, whereas the August webs were dispersed throughout the reach. Neither the abundance of emergent insects nor the WSI showed a strong correlation with web density in May, but WSI had a significant effect in August. Although there was a significant aggregation of spider webs around in-stream wood in May, no link was found between emergence and web number. These results suggest that prey cluster around logjams and are limiting in May, but become more abundant and/or dispersed in August, in which case available web-space becomes the limiting factor. Alternatively, as spider populations increase through the summer, more spiders choose sub-optimal habitats located further from the logjam.