Web-building Spider Response to a Logjam in a Northern Minnesota Stream

Abstract

Logjams, or accumulations of wood in streams, can increase aquatic macroinvertebrate production through organic material retention and habitat diversification. Past studies showed a positive correlation between web-building spiders near streams and aquatic insect emergence. We hypothesized there would be an increase in terrestrial web-building spider density near logjams. To test this, we counted webs within 6 m of the stream bank along a 40 m reach in a northern Minnesota stream centered on a spanning logjam. We then estimated the availability of web-building substrate during late May and late Aug. of 2011. Webs in May heavily concentrated around the logjam, whereas webs in Aug. appeared dispersed throughout the reach. The web-building substrate did not show a significant correlation with web density in May, but it had a significant effect in Aug. These results cautiously suggest logjams have a positive effect on spider web density, but that effect varies through time. Further studies may explicitly link logjam-mediated prey to spider web distributions.