Responses of riparian tetragnathid spiders to wildfire in forested ecosystems of the California Mediterranean climate region, USA

Mediterranean ecosystems of California are characterized by high interannual variability in precipitation and susceptibility to frequent high-severity wildfires. Variability in precipitation and fire severity are likely to become more pronounced because of climate change, but their relative effects on linked aquatic-terrestrial components of Mediterranean ecosystems have received limited attention. We investigated the effects of wildfire on riparian spiders of the family Tetragnathidae, which are common shoreline consumers that can be highly reliant on aquatic food resources in stream ecosystems. From 2011-2012, we assessed stream geomorphology; density and community composition of aquatic benthic macroinvertebrates; and density, Hg body loads, trophic position (TP), and reliance on aquatically derived energy (based on naturally abundant C and N isotopes) of tetragnathid spiders in study sections of 12 paired stream reaches in Yosemite National Park. The riparian zone of one member of each pair had experienced a high-severity wildfire, whereas the riparian zone of its counterpart had experienced a low-severity fire. After the 2013 Rim Fire, we resurveyed a subset of these variables in study sections of 4 reaches by means of a paired before-after-control-impact design. We also explored how reach-and catchment-scale variability might affect spider density and trophic dynamics. Differences in spider responses between paired sections were not statistically significant, but model-selection results suggested that variability in benthic invertebrate density, catchment-scale fire frequency, and precipitation were important drivers of spider density and TP. The consistent signal of precipitation across multiple spider responses suggests that climate variability could have greater effects on aquatic-terrestrial ecological linkages than the influence of fire alone.