Jacob Edward Allgeier, Seth J. Wenger, Amy D. Rosemond, Daniel E. Schindler, and Craig A. Layman

ECOLOGY

Significance

A fundamental dilemma in ecology is to reconcile the degree to which ecological processes are generalizable among taxa and ecosystems or determined primarily by taxonomic identity. We apply a unique dataset of organisms from a diverse marine community to test the applicability of two theories, metabolic theory of ecology (MTE) and ecological stoichiometry (EST), and the role of taxonomic identity for predicting nutrient excretion rates by fishes and macroinvertebrates. Excretion rates were principally explained by body mass and taxonomic identity, providing strong support for MTE, but also highlighting the intrinsic importance of taxonomic identity. Little support for basic predictions of EST was found. This research reveals animal-mediated nutrient cycling is largely generalizable by metabolic processes, but refined predictions require taxa-specific understanding.

Abstract

Reconciling the degree to which ecological processes are generalizable among taxa and ecosystems, or contingent on the identity of interacting species, remains a critical challenge in ecology. Ecological stoichiometry (EST) and metabolic theory of ecology (MTE) are theoretical approaches used to evaluate how consumers mediate nutrient dynamics and energy flow through ecosystems. Recent theoretical work has explored the utility of these theories, but empirical tests in species-rich ecological communities remain scarce. Here we use an unprecedented dataset collected from fishes and dominant invertebrates (n = 900) in a diverse subtropical coastal marine community (50 families, 72 genera, 102 species; body mass range: 0.04–2,597 g) to test the utility of EST and MTE in predicting excretion rates of nitrogen (E_N), phosphorus (E_P), and their ratio (E_NP). Body mass explained a large amount of the variation in E_N and E_P but not E_NP. Strong evidence in support of the MTE 3/4 allometric scaling coefficient was found for E_P, and for E_N only after accounting for variation in excretion rates among taxa. In all cases, including taxonomy in models substantially improved model performance, highlighting the importance of species identity for this ecosystem function. Body nutrient content and trophic position explained little of the variation in E_N, E_P, or E_NP, indicating limited applicability of basic predictors of EST. These results highlight the overriding importance of MTE for predicting nutrient flow through organisms, but emphasize that these relationships still fall short of explaining the unique effects certain species can have on ecological processes.

See: http://www.pnas.org/content/112/20/E2640.abstract.html?etoc

PNAS May 19, 2015 vol. 112 no. 20 E2640-E2647

Fig. 2. Linear regression models for excretion rates of N (E_N), P (E_P) (µg/h), and N:P (E_NP) and wet body mass of individual organisms (n = 675, 372, and 367 for N, P, and N:P, respectively). Subsequent regressions are between excretion-mass model residuals and either δ¹⁵N or body nutrient content (for N, P, or N:P). *Slope significantly differs from zero (α < 0.05). Families are indicated by a unique combination of color and symbol.