Plant-insect interactions and dioecy influence early successional detrital systems

Terrestrially derived organic matter inputs to streams strongly influence aquatic food webs. Aquatic detritivores (microbes and invertebrates) rely on leaf litter from terrestrial vegetation that falls into streams as substrate, a carbon source, and a nutrient source. Sitka willow (Salix sitchensis), a dioecious shrub that dominates the Pumice Plain at Mount St. Helens (Lawetlat'la, WA, USA) is a primary source of organic matter for early successional streams on the flanks of the volcano. Male and female Sitka willows are differentially attacked by a terrestrial stem-boring weevil, Cryptorhynchus lapathi, which induces chemical litter changes and causes branch death and summer leaf litter fall. In this study, we collected summer litter fall due to weevil attack as well as natural autumn litter fall from both male and female willows, then incubated all litter types during both the summer and autumn seasons. We hypothesized that willow sex (male, female), litter type (weevil-induced, naturally abscised), and incubation season (summer, fall) would differentially influence microbial and invertebrate communities colonizing leaf litterbags as well as the decomposition rate of the litter itself. This study provides insights into how sex-based interactions with herbivores might influence detrital in-stream community dynamics and ecosystem processes.