Summary1. In the face of human-induced declines in the abundance of common species, ecologists have become interested in quantifying how changes in density affect rates of biophysical processes, hence ecosystem function. We manipulated the density of a dominant detritivore (the cased caddisfly, Limnephilus externus) in subalpine ponds to measure effects on the release of detritus-bound nutrients and energy. 2. Detritus decay rates (k, mass loss) increased threefold, and the loss of nitrogen (N) and phosphorus (P) from detrital substrates doubled across a range of historically observed caddisfly densities. Ammonium and total soluble phosphorus concentrations in the water column also increased with caddisfly density on some dates. Decay rates, nutrient release and the change in total detritivore biomass all exhibited threshold or declining responses at the highest densities. 3. We attributed these threshold responses in biophysical processes to intraspecific competition for limiting resources manifested at the population level, as density-dependent per-capita consumption, growth, development and case : body size in caddisflies was observed. Moreover, caddisflies increasingly grazed on algae at high densities, presumably in response to limiting detrital resources. 4. These results provide evidence that changes in population size of a common species will have nonlinear, threshold effects on the rates of biophysical processes at the ecosystem level. Given the ubiquity of negative density dependence in nature, nonlinear consumer density-ecosystem function relationships should be common across species and ecosystems.
Evidence varies on how subsidies affect trophic cascades within recipient food webs. This could be due to complex nonlinearities being masked by single-level manipulations (presence/absence) of subsidies in past studies. We predicted that trophic cascade strength would increase nonlinearly across a gradient of subsidies. We set out to reveal these complex, nonlinear relationships through manipulating a quantitative gradient of detrital subsidies to lake benthic food webs along with the presence/absence of trout. Contrary to our prediction, we found that trophic cascades only occurred at low subsidy levels, disappearing as subsidies increased. This threshold in trophic cascade strength may be due to an increase in intermediate predators in the absence of top predators, as well as changes in the proportion of armored vs. un-armored primary consumers. Future studies on the effect of subsidies on trophic cascade strength need to incorporate naturally occurring gradients to reveal the complex direct and indirect interactions within food webs.
Increasing concentrations of dissolved organic carbon (DOC) in the northeastern U.S. have been attributed to two potential mechanisms: recovery from acidification and changing climate. Maine's high‐elevation lakes (>600m) could potentially provide unique insight into the response of surface water chemistry to declining acidic deposition and interannual climate variability. The geochemical response in 29 lakes was analyzed during 30 years of change in sulfate ( SO42−) deposition and climate. All 29 lakes exhibited positive trends in DOC from 1986 to 2015, and 19 of 29 lakes had statistically significant increases in DOC throughout the study period. These results illustrate a region‐wide change from low‐DOC lakes (<5mg/L) to moderate DOC lakes (5–30mg/L). Increasing DOC trends for these high‐elevation lakes were more consistent than for lower elevation lakes in the northeastern U.S. A linear mixed effects model demonstrated that lake water SO42− and climate variables describe most of the variability in DOC concentrations (r2 = 0.78), and the strongest predictor of DOC concentration was an inverse relationship with SO42−. Due to SO42− concentrations trending toward preacidification levels and projections of a warmer, wetter, and more variable climate, there is uncertainty for the future trajectory of DOC trends in surface waters. Long‐term monitoring of Maine's high‐elevation lakes is critical to understand the recovery and response in surface water chemistry to a changing chemical and physical environment in the decades ahead.
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