Riparian hunting spiders do not rely on aquatic subsidies from intermittent alpine streams

Drying in alpine streams might decrease aquatic-terrestrial trophic linkages by reducing terrestrial predation on aquatic prey. We tested this hypothesis by investigating whether a common riparian predator (hunting spiders) in alpine environments assimilated a lower proportion of aquatic prey with increasing stream intermittency. We used high temporal-resolution data from electrical resistance sensors to map patterns of naturally-occurring flow intermittency across 30 headwater streams of Val Roseg, a glacierized catchment in the Swiss Alps. We collected riparian hunting spiders, as well as potential terrestrial and aquatic macroinvertebrate prey, from streams and their associated riparian zones across two seasons (Alpine spring and summer). We estimated aquatic contributions to spider diets (p(A)) using (i) a gradient approach with aquatic invertebrate and spider carbon stable isotope ratio values (delta C-13), and (ii) Bayesian carbon and nitrogen (delta N-15) isotope mixing models. Spider p(A) from the gradient method were not statistically different from zero in spring (0.08 +/- 0.10) and low in summer (0.16 +/- 0.04). Mixing models also estimated low dependence on aquatic prey in both seasons, although with potentially higher contributions in summer. Spider diet did not vary with increasing flow intermittency in either season. Our results suggested that alpine hunting spiders obtain most of their carbon from terrestrial prey. The slight increase in spider p(A) during summer may correlate with peak emergence periods for aquatic insects, indicating opportunistic feeding by this riparian predator.

Automated summary

Investigating the impact of stream intermittency on the assimilation of aquatic prey by alpine riparian predators, it was found that alpine hunting spiders primarily consume terrestrial prey, with potentially higher contributions from aquatic prey during summer. This indicates opportunistic feeding behavior by these predators.