From brown to green: how does caddisfly detritus processing affect the growth of invertebrate algal grazers
Abstract
Wetland nutrient cycling occurs through complex feedback mechanisms and dynamic inter- and intra-species interactions. In the shallow, oligotrophic ponds at the Mexican Cut Nature Preserve, near the Rocky Mountain Biological Laboratory in the Elk Mountains of Western Colorado, detritovore caddisfly density is negatively coupled with cyclic fluctuations in tiger salamander populations. The caddisfly Limnephilus externus is among the most biomass abundant invertebrates in these ponds and plays a vital role in wetland nutrient cycling by: 1) liberating N and P stored in detritus and 2) stimulating algal growth. In order to test whether the indirect effect of caddisfly detritus processing on algal growth affects species that consume algae, I manipulated caddisfly density in microcosms to create a gradient of detritus processing and compared the effects of nutrient release under ambient light (with stimulation of algal growth) and low light (no stimulation of algal growth). I predicted that caddisfly (detritovore) and mayfly (algal grazer) growth and development would be higher in light vs. low-light conditions, and that the positive feedback on caddisflies to be strongest at high densities. Preliminary results show that: 1) increasing caddisfly density increases the concentration of ammonium available for benthic primary production, 2) chlorophyll-a (proxy for algal biomass) increases along with increased nutrient availability, and 3) caddisfly algae grazing increases at high densities. Further analyses remain to be conducted to determine how consuming algae benefits the growth and development of Limnephilus caddisflies and Callabaetis mayflies. In a broader context, these results provide evidence that long-term changes in the abundance of a top predator can have cascading and indirect effects on ecosystem processes like nutrient cycling.
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