Humans have modified ecosystems around the globe. Due to such modifications, freshwater streams are experiencing shifts in thermal and hydrologic regimes. These changes are also linked to declines in the commonness and diversity of freshwater vertebrates. In particular, communities that occupy headwater streams are particularly sensitive to changes in hydrologic regimes that affect habitat availability and suitability for obligate (e.g., fishes) and facultatively aquatic organisms (e.g., salamanders). Ultimately, shifts in habitat suitability could affect interactions between these species to synergistically affect their ability to persist in headwater streams. This study used artificial stream mesocosms to investigate the interactive effects of flow variability and interspecific interactions of two common, headwater stream vertebrates: Black-bellied Salamanders (Desmognathus quadramaculatus) and Common Creek Chubs (Semotilus atromaculatus). Individuals were either paired with a conspecific or heterospecific (e.g., salamander or Common Creek Chub) and then crossed with one of three flow treatments: (1) high variability, (2) low variability, or (3) no variability. These results suggest that flow variability had no effect on intraspecific or interspecific competition. However, there was an effect of interspecific competition as salamanders decreased in body condition in the presence of Common Creek Chubs. Our findings suggest that these two common species have potentially adapted to variability in flow in their natural environment. Additionally, our results reinforce previous findings that Common Creek Chubs could play a role in the distribution of stream salamanders.

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