Food type and temperature constraints on the fitness of a dominant freshwater shredder

Abstract

Freshwaters receive a large amount of materials and biologically available energy from their surrounding catchments. In particular, small forested streams are dependent on allochthonous organic matter inputs. However, since the middle 20th century large areas of forests in the Iberian Peninsula have been converted to eucalypt plantations, impairing invertebrate communities and stream energy fluxes. In addition, the dependence of organismal metabolic rates on temperature may influence invertebrate life cycles and development, and ultimately the riparian-stream system. We tested the effects of two food resources (alder and eucalypt) under two different temperatures (9 vs. 14°C) on the life history parameters and elemental composition of Brillia bifida (Chironomidae), a numerically dominant shredder in forested streams of NW Spain. Our results showed: (i) warmer temperature accelerated the development of B. bifida larvae (i.e., three times faster at warm than at cold temperature), (ii) Brillia's growth rate when fed on eucalypt leaves was only 30% of what it was on alder leaves, and (iii) both factors together modified larvae stoichiometry and nutrient fluxes (i.e., through fine particulate organic matter production). The observed elemental imbalances suggest, to some extent, a deviation from the “strict homeostasis” assumption for heterotrophs. Differential effects observed on larval development, growth and elemental composition point out that food type and temperature are influencing B. bifida performance, and this fact may reach other trophic compartments through detritivore-mediated nutrient cycling.