Nonlinear effects of consumer density on multiple ecosystem processes

Abstract

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.

When does diversity matter? Species functional diversity and ecosystem functioning across habitats and seasons in a field experiment

“…; Klemmer et al . ). Indeed, the only significant density effect on decomposition in our study was a positive relationship in spring pools, which suggests that even if predacious behaviour by late‐instar caddisflies weakened the linkage between consumer biomass and leaf decomposition, the addition of more individuals nevertheless increased process rates.…”

High interspecific variation in nutrient excretion within a guild of closely related caddisfly species

“…This guild has been shown to increase detritus decay rates and ambient water‐column N and P concentrations, which have been posited to increase algal productivity (Klemmer et al. ). Therefore, if these excretion rates scale to the whole‐pond level and supply a substantial percentage of nutrient demand, community reassembly associated with species gains or losses within this guild could change inorganic nutrient availability during the short growing season and consequently affect ecosystem productivity.…”

“…, ), (2) previous work has linked their detritus processing with algal biomass (Klemmer et al. ), and (3) in other systems, the biomass‐dominant aquatic invertebrates have been shown to supply a significant proportion of an ecosystem's nutrient demand (e.g., Grimm , Vanni , Hall et al. ).…”

“…Experimental studies that relate detritus processing to nutrient regeneration in this system have demonstrated that nutrients released during detritus processing by this caddisfly guild can stimulate algal growth (Klemmer et al. ). Further, because long‐term survey data demonstrate climate change‐related shifts in species composition along both a hydrologic permanence gradient and an elevational gradient (Wissinger et al.…”

Keratin subsidies promote feather decomposition via an increase in keratin-consuming arthropods and microorganisms in bird breeding colonies

“…The decomposition of subsidised resources can be strongly affected by resource consumers (Tank et al 2010;Klemmer et al 2012); e.g. leaf decomposition rates are proportional to increases in detritivore abundance in headwater streams (Benfield and Webster 1985).…”