Bottom-Up and Top-Down Controls on Food Webs in Headwater Streams

Author

Miriam O. BayerFollow

Year of Award

2018

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Wildlife Biology

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Dr. Winsor H. Lowe

Commitee Members

Dr. Lisa A. Eby, Dr. F. Richard Hauer, Dr. John Maron, Dr. Nicholas L. Rodenhouse

Keywords

intraguild predation, biofilm, headwater streams, benthic macroinvertebrates, Eurycea bislineata, Gyrinophilus porphyriticus

Subject Categories

Terrestrial and Aquatic Ecology

Abstract

Headwater streams account for 70% of stream channel length in the USA and are important as hotspots of nutrient uptake and native biodiversity. Biofilm, the mixed auto- and heterotrophic microbial community covering stream substrates, is where the majority of nutrient processing occurs, and forms the base of stream food webs, particularly in heavily shaded, oligotrophic streams. Both bottom-up (e.g., nutrients, light) and top-down (i.e., consumption) processes are known to affect periphyton, the autotrophic component of biofilm, but little is known about what controls the biofilm community as a whole. Top-down effects are common in streams, where fish are often the top predator and can cause cascading effects. However, salamanders – not fish – are the top predators in many small headwater streams, and the top-down effects of salamanders on stream food webs have received much less attention. I used experimental and observational approaches to investigate the role of top-down and bottom-up controls on headwater stream food webs. Specifically, I used stream mesocosms with two salamander species, Eurycea bislineata and Gyrinophilus porphyriticus, alone or in combination, to test the effects of salamander community composition on benthic and emergent macroinvertebrate density, biomass and community composition. To assess the relative importance of bottom-up and top-down determinants of biofilm biomass, I used a combination of stream surveys and pre-existing stream chemistry data from across the oligotrophic headwater streams of the Hubbard Brook Experimental Forest (HBEF). My experiment showed that stream salamanders alter benthic macroinvertebrate densities, but only when G. porphyriticus occurs alone, as it consumes E. bislineata, the smaller species, when it is present. This intraguild predation removes the top-down effects of G. porphyriticus on benthic macroinvertebrates. In my stream surveys, bottom-up variables (i.e., aspect, canopy cover, nutrients and pH) determined biofilm biomass, not salamander occupancy or benthic macroinvertebrate biomass. However, I did not encounter streams with the specific salamander community (i.e., G. porphyriticus present and E. bislineata absent) shown to produce top-down effects in my experimental study. My results demonstrate that salamanders can exert top-down control on benthic macroinvertebrate communities. This effect is, however, dependent on the salamander species present, and can be removed by intraguild interactions between salamander species. Furthermore, bottom-up factors, in particular light, appear to play the primary role in determining biofilm biomass. Despite prior evidence for the dominance of heterotrophic microbes in biofilms at the HBEF, the importance of light in controlling biofilm biomass suggests that the autotrophic component of these biofilms may be disproportionately important, and emphasizes the importance of understanding both the auto-and heterotrophic components of stream biofilms. My work links the rich history of research on stream salamander community ecology to broader studies of stream food webs, providing new avenues for future research.

Recommended Citation

Bayer, Miriam O., "Bottom-Up and Top-Down Controls on Food Webs in Headwater Streams" (2018). Graduate Student Theses, Dissertations, & Professional Papers.