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 TER C ∶ 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 TER C ∶ P from C ∶ P = 1620. Bioenergetics TER C ∶ P further varied among P. lepida of differing growth status on varying diet C ∶ P (overall TER C ∶ 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.

Optimal metabolic regulation along resource stoichiometry gradients

“…; Halvorson et al . ), suggesting that other constraints might have masked a CUE‐food C:P relation. Moreover, when food C‐to‐nutrient (especially C:P) ratios are low, growth rates can be inhibited, but due to excessive rather than limiting nutrients (Boersma & Elser ; Elser et al .…”

Quantity and quality limit detritivore growth: mechanisms revealed by ecological stoichiometry and co‐limitation theory

“…Each tub was flushed every 2–3 d and amended with 1,000 μg/L N as KNO 3 and 50, 100, or 500 μg/L P (low, med, or high P treatments, respectively) as Na 2 HPO 4 (Halvorson et al. ).…”

“…, Halvorson et al. ). Reduced consumption should increase C relative to P assimilation efficiencies, because animals prioritize C acquisition to support metabolism but exhibit lower P demands due to reduced growth (Sterner ).…”

Light and dissolved phosphorus interactively affect microbial metabolism, stoichiometry and decomposition of leaf litter

“…Interestingly, litter C:N did not respond strongly to light, indicating similar total microbial biomass or N content within each level of dissolved P (Fig. Light and dissolved P availability may ultimately affect growth rates of detritivorous animals by changing litter N and P content (Fuller, Evans-White & Entrekin, 2015;Halvorson et al, 2015) or leaf litter biochemistry (Franken et al, 2005;Albariño, Villanueva & Canhoto, 2008). Light and dissolved P availability may ultimately affect growth rates of detritivorous animals by changing litter N and P content (Fuller, Evans-White & Entrekin, 2015;Halvorson et al, 2015) or leaf litter biochemistry (Franken et al, 2005;Albariño, Villanueva & Canhoto, 2008).…”