Grazer Traits, Competition, and Carbon Sources to a Headwater-Stream Food Web

Abstract

We investigated the effect of grazing by a dominant invertebrate grazer (the caddisfly Glossosoma penitum) on the energy sources used by other consumers in a headwater-stream food web. Stable isotope studies in small, forested streams in northern California have shown that G. penitum larvae derive most of their carbon from algae, despite low algal standing crops. We hypothesized that the caddisfly competes with other primary consumers (including mayflies) for algal food and increases their reliance on terrestrial detritus. Because Glossosoma are abundant and defended from predators by stone cases, their consumption of algal energy may reduce its transfer up the food chain. We removed Glossosoma (natural densities >1000 caddisflies/m2) from five approximately 4 m2) stream sections during the summer of 2000 and measured responses of algae, invertebrate primary consumers, and invertebrate predators. The treatment reduced Glossosoma biomass by 80-90%. We observed a doubling in chlorophyll a per area in sections with reduced Glossosoma abundance and aggregative increases in the biomass of undefended primary consumers. Heptageniid mayfly larvae consumed more algae (as measured by stable carbon isotope ratios and gut content analysis) in caddisfly removal plots at the end of the 60-day experiment, although not after one month. We did not see isotopic evidence of increased algal carbon in invertebrate predators, however. Patterns of caddisfly and mayfly diets in the surrounding watershed suggested that mayfly diets are variable and include algae and detrital carbon in variable proportions, but scraping caddisflies consume primarily algae. Caddisfly and mayfly diets are more similar in larger, more productive streams where the mayflies assimilate more algae. Isotopic analysis, in combination with measurements of macroinvertebrate abundance and biomass in unmanipulated plots, suggested that a substantial portion of the invertebrate community (>50% of biomass) was supported, at least partially, by local algal carbon during midsummer. These data suggest that algae may be more important to community dynamics in headwater streams than their relatively low productivity would suggest. Through their high densities and relative invulnerability to predation, armored grazers may also affect community structure and flow of algal and detrital carbon in headwater streams.

Benthic community structure and ecosystem functions in above- and below-waterfall pools in Borneo

“…Specifically, taxa were photographed and uploaded onto the Flickr website (http://flickr.com/photos/tropical-streams/ sets/), where interested experts commented or requested to see the actual specimens. Ash-free dry mass (AFDM) of macroinvertebrates was estimated using length-mass regressions (Benke et al, 1999;Sabo et al, 2002;McNeely et al, 2007). In cases in which no taxon-specific equations were available, estimates were made using equations from taxa with similar body shapes .…”

Dependence of stream predators on terrestrial prey fluxes: food web responses to subsidized predation

“…While this finding suggests high potential for apparent competition between terrestrial prey and in situ prey species, the topdown effect of steelhead on the biomass of benthic invertebrates was limited to a small subset of the aquatic community (vulnerable herbivores), and was not strengthened in the absence of terrestrial prey. This weak top-down control is likely due to the abundance of morphologically predator-resistant invertebrates in this tributary stream food web (McNeely et al 2007), with approximately 65% of the total benthic invertebrate biomass and 82% of herbivore biomass classified as armored and invulnerable to predation. Though predatory fishes have also been shown to induce trophic cascades on detrital processing rates (Konishi et al 2001, Boyero et al 2007,, but see Ruetz et al 2002), we did not estimate changes in the processing of terrestrial detritus as part of this study.…”

“…This assumption was based on previous work demonstrating that members of this functional group disproportionately rely on in situ production in our study system (McNeely et al 2007). Invertebrates were further partitioned into groups based on their relative vulnerability to predation, with armored invertebrates such as cased caddisflies classified as invulnerable, and soft-bodied invertebrates such as mayflies classified as vulnerable.…”

“…Within covered pools sticky traps were assumed to have equal capture efficiency, allowing us to compare the effects of our predator treatments on aquatic insect emergence. The biomass and composition of benthic aquatic invertebrates in our experimental pools were estimated by sampling 6 randomly selected benthic rocks from each unit at the end of the experiment as in McNeely et al (2007). Each rock was moved quickly from the stream bottom into a net positioned downstream and placed in a tray to minimize the escape of more mobile aquatic insects.…”

Glossossoma nigrior (Trichoptera: Glossosomatidae) respiration in moving fluid

“…Glossosoma nigrior construct cases of rock, fastened together by a silk that they secrete. This rock armor may contribute to the success of G. nigrior, which outcompete other, more vulnerable soft-bodied macroinvertebrates that utilize the same resources but do not build cases (McNeely et al, 2007). Therefore, G. nigrior can have a large impact on energy and nutrient flow through aquatic ecosystems (Kohler and Wiley, 1997).…”