Assessing the effects of biological invasions and reduced flows on a spring-fed stream food web in San Felipe Creek (Del Rio, Texas, USA)

Invasive riparian vegetation can greatly alter the structure and function of ecological communities by changing habitat and resource availability. I conducted a field study in San Felipe Creek, a spring-fed tributary of the Rio Grande, to examine the effects of an invasive riparian plant, Giant Reed (Arundo donax), on the stream food web. I hypothesized that shading by Giant Reed alters the structure of the food web via bottom-up pathways. I also hypothesized that invasive Suckermouth Catfish would benefit from the invasion of Giant Reed via inputs of detritus. Alternatively, I hypothesized that the invasion of Giant Reed would lead to a reduction in benthic periphyton, an important food resource for Suckermouth Catfish, having negative effects on their overall population. To test these hypotheses, I sampled periphyton, benthic macroinvertebrates, and fishes at reaches with (invaded; n = 3 reaches) Giant Reed and without (control; n = 3 reaches) during the summer, fall, and winter of 2021 and 2022. I also sampled diets of 4 abundant species within San Felipe Creek to determine if diets differed between control and invaded reaches. At the base of the food web, I found that periphyton biomass was 46% lower in invaded reaches, when compared to control reaches. Total macroinvertebrate densities were similar between control and invaded reaches. However, a subset of Families and functional feeding guilds were lower in invaded reaches. Fish relative abundance was 75% lower in invaded reaches, when compared to control reaches. This pattern was driven by Lepomis spp. and functional feeding guilds (invertivores and omnivores). Overall diet composition was similar across all species in both invaded and control reaches. Overall, my findings support my hypothesis and indicate that invasive Giant Reed does have bottom-up effects that extend to multiple trophic levels. I found no evidence of a facilitative relationship between Suckermouth Catfish and Giant Reed. I did find that Suckermouth Catfish fed primarily on benthic periphyton (70%) which may be negatively affected by the invasion of Giant Reed. Based on my findings, the removal of Giant Reed should help improve the stream food web by increasing productivity from the bottom up.