Effects of contrasting stream geomorphology on riparian habitat structure and prey subsidies for web-weaving spider assemblages in southeastern Piedmont streams

Background/Question/Methods

Prey subsidies and habitat structure have the potential to shape riparian web-weaving spider abundance and distribution in small streams. Structure, as overhanging riparian vegetation, is prime habitat for web-weaving spiders, especially those assemblages relying on emerging adult stream insects as prey (e.g., Tetragnathidae). Human-altered stream geomorphology (i.e., channel incision and widening) is potentially capable of affecting spider assemblages directly, by altering riparian habitat, and/or indirectly, by affecting abundance and diversity of emergent prey. We studied this interaction in 7 Alabama Piedmont streams by addressing 2 main questions: (1) Do differences in habitat structure among streams of contrasting geomorphology affect spider distributions, and (2) Do among-stream differences in abundance of stream insect emergence affect riparian web-building spider abundance? We measured several measures of stream geomorphology to quantify channel condition and developed a semi-quantitative survey to assess suitable spider habitat structure. We also quantified emerging adult stream insects (using replicated instream emergence traps) and spider diversity and abundance (using nighttime searches for active webs) in each study stream over a three-month period from July-September 2012.

Results/Conclusions

Bank height ratio, a measurement of channel incision, indicated that channels spanned a wide range of conditions from stable, unincised to highly incised channels (1.74 m to 6.74 m, respectively). Potential habitat structure, in the form of small overhanging branches and foliage, was present and abundant at all streams. Preliminary data showed that emerging insect abundances varied among streams, with generally high emergence from streams with stable unincised channels and lower emergence from streams with heavily incised channels. Tetragnathid spiders numerically dominated web-weaving spider assemblages (80-90%) at all study streams, but showed higher densities (no./ m²) alongside streams with stable channels (e.g. 5.7 no./m²) and lower densities alongside streams with incised channels (e.g. 0.78 no./m²). Results also showed higher emerging insect abundance at streams with stable unincised (vs. incised) channels. These preliminary results suggest that stream geomorphology indirectly affects riparian web-weaving spider assemblages, potentially by reducing availability of emerging aquatic prey for terrestrial spiders.