A stream insect detritivore violates common assumptions of threshold elemental ratio bioenergetics models

Abstract

Ecologists increasingly use threshold elemental ratios (TERs) to explain and predict organism responses to altered resource C∶P or C∶N. TER calculations are grounded in diet-dependent growth, but growth data are limited for most taxa. Thus, TERs are derived instead from bioenergetics models that rely on simplifying assumptions, such as fixed organism C∶P and no P excretion at peak growth. We examined stoichiometric regulation of the stream insect detritivore Pycnopsyche lepida to assess bioenergetics model assumptions and compared bioenergetics TERC∶P estimates to those based on growth. We fed P. lepida maple and oak leaf diets along a dietary C∶P gradient (molar C∶P range = 950–4180) and measured consumption, growth, stoichiometric homeostasis (H), and elemental assimilation and growth efficiencies over a 5-wk period in the laboratory. Pycnopsyche lepida responses to varying resource C∶P depended on litter identity and were strongest among oak diets, on which growth peaked at diet C∶P = 1620. Pycnopsyche lepida fed oak litter exhibited flexible body C∶P during growth and in response to altered diet C∶P (non-strict homeostasis; H = 4.74), low P use efficiencies, and P excretion at peak growth. These trends violated common bioenergetics model assumptions and caused deviation of estimated TERC∶P from C∶P = 1620. Bioenergetics TERC∶P further varied among P. lepida of differing growth status on varying diet C∶P (overall TERC∶P range = 1030–9540). Our study identifies novel effects of nutrient enrichment and litter identity on detritivore stoichiometric regulation and supports growth-based approaches for future TER calculations.