Shifting effects of rock roughness across a benthic food web
Habitat heterogeneity affects the spatial pattern of stream organisms, but it is unclear how broadly heterogeneity affects the distribution of organisms within a food web. Specifically, rougher rocks have greater algal biomass than smoother rocks, and we hypothesized bottom-up food web control of food web structure, in which rougher rocks would also have higher grazer and predator abundance. We surveyed algal biomass and macroinvertebrates on rocks of differing roughness. We also conducted a field experiment to separately examine rock roughness and algal biomass effects by manipulating algal biomass by raking or scrubbing rocks within created rock clusters. Neither the survey nor the experiment strongly supported a bottom-up scenario. Algal biomass increased with rock roughness. Grazing mayfly abundance was distributed evenly among geologic rock types, except for a higher abundance of baetids on rocks with large cavities, where predatory stoneflies were also abundant. In the rock cluster experiment, the moderate raking disturbance produced higher grazer abundance and reduced algal biomass relative to unmanipulated controls. We concluded that fine-scale roughness directly promoted algal biomass, whereas larger-grain roughness (crevices) affected the distribution of the food web components by forming clumped distributions of grazing baetid mayflies and predatory stoneflies.
KeywordsSpatial heterogeneity Rock roughness Surface texture Habitat heterogeneity Stream food webs
We appreciate the use of facilities at the Rocky Mountain Biological Laboratory in Gothic, CO. Barbara Peckarsky advised on sampling sites and Emilee Tarver, Barrett Philips, Sarah Hobson, and Russell Hobson helped with field and laboratory work. Funding was provided by NSF CAREER grant DEB-0447449 and Oklahoma EPSCoR to EAB.
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