Exploring mechanisms explaining coexistence patterns of <i> Rhyacophila </i> species (Trichoptera) in streams near the Rocky Mountain Biological Laboratory

Abstract

Coexistence of closely related species can be explained by a number of biotic and abiotic factors. In Trichoptera (caddisflies), the patterns of coexistence of the species Rhyacophila in streams near the Rocky Mountain Biological Laboratory (RMBL) has been tracked for 43 years. This study is a continuation from previous student projects carried out in 2017 and 2018 to explore the mechanisms explaining patterns of coexistence of these caddisflies. In addition to coexistence patterns, data from previous studies include species preferences for streams of different altitude, size, water temperature, and the coverage of the nuisance diatom Didymosphenia geminata. The coverage of D. geminata varies among years (favored in low-flow years), and streams (favored in streams with lower levels of phosphorus). Previous studies considered those factors only at the scale of stream reaches, but microhabitat preferences within stream reaches by different species of Rhyacophila remain to be investigated. Another previously unexplored factor is resource partitioning of prey by these predatory caddisflies. Questions asked in this study include: 1) Do certain pairs of species coexist in stream reaches by partitioning microhabitats (specifically, current velocities, water depths, substrate particle sizes or coverage of Didymo?) 2) Do Rhyacophila species shift microhabitats in the presence of potential competitors? 3) Does partitioning of food (prey) resources explain coexistence of pairs of species of Rhyacophila? We addressed those questions by continuing surveys in seven out of eight long- term stream sites near RMBL and adding measurements of microhabitat and diet preferences of coexisting Rhyacophila species. We also surveyed eight small streams in the Upper East River drainage while waiting for water levels to be safe in the larger, long-term sites. Results showed that two species that typically coexist at warmer water sites (R. brunnea and R. valuma) did not partition microhabitats, while the two species that coexist in colder water sites (R. alberta and R. hyalinata) preferred different water depth and substrate microhabitats. We were unable to test effects of Didymo during this very high-water year. In addition, while we could not distinguish specific prey species in the gut contents of these caddisflies, one of each pair of coexisting species had significantly larger mouth gapes than the other, suggesting that they may partition prey species (larger: Baetis vs smaller: chironomids).

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