Comparing the Ecological Stoichiometry in Green and Brown Food Webs - A Review and Meta-analysis of Freshwater Food Webs.

Michelle A Evans-White,Halvor M Halvorson

doi:10.3389/fmicb.2017.01184

Abstract

The framework of ecological stoichiometry was developed primarily within the context of “green” autotroph-based food webs. While stoichiometric principles also apply in “brown” detritus-based systems, these systems have been historically understudied and differ from green ones in several important aspects including carbon (C) quality and the nutrient [nitrogen (N) and phosphorus (P)] contents of food resources for consumers. In this paper, we review work over the last decade that has advanced the application of ecological stoichiometry from green to brown food webs, focusing on freshwater ecosystems. We first review three focal areas where green and brown food webs differ: (1) bottom–up controls by light and nutrient availability, (2) stoichiometric constraints on consumer growth and nutritional regulation, and (3) patterns in consumer-driven nutrient dynamics. Our review highlights the need for further study of how light and nutrient availability affect autotroph–heterotroph interactions on detritus and the subsequent effects on consumer feeding and growth. To complement this conceptual review, we formally quantified differences in stoichiometric principles between green and brown food webs using a meta-analysis across feeding studies of freshwater benthic invertebrates. From 257 datasets collated across 46 publications and several unpublished studies, we compared effect sizes (Pearson’s r) of resource N:C and P:C on growth, consumption, excretion, and egestion between herbivorous and detritivorous consumers. The meta-analysis revealed that both herbivore and detritivore growth are limited by resource N:C and P:C contents, but effect sizes only among detritivores were significantly above zero. Consumption effect sizes were negative among herbivores but positive for detritivores in the case of both N:C and P:C, indicating distinct compensatory feeding responses across resource stoichiometry gradients. Herbivore P excretion rates responded significantly positively to resource P:C, whereas detritivore N and P excretion did not respond; detritivore N and P egestion responded positively to resource N:C and P:C, respectively. Our meta-analysis highlights resource N and P contents as broadly limiting in brown and green benthic food webs, but indicates contrasting mechanisms of limitation owing to differing consumer regulation. We suggest that green and brown food webs share fundamental stoichiometric principles, while identifying specific differences toward applying ecological stoichiometry across ecosystems.

Highlights

Ecological stoichiometry was developed and has been considered extensively within the context of autotroph-based, or “green” food webs (Sterner and Elser, 2002) that conform nicely to the classic trophic level concept of primary producers and upper level consumers (Lindeman, 1942; Hairston et al, 1960)
Throughout this paper we focus on plant litter as the basis of brown food webs, because it is a widespread form of detritus across inland ecosystem types
Our review and meta-analysis focusing on freshwater systems highlight current understanding of ecological stoichiometry in brown food webs, providing conceptual and quantitative comparison to green food webs

Summary

Introduction

Ecological stoichiometry was developed and has been considered extensively within the context of autotroph-based, or “green” food webs (Sterner and Elser, 2002) that conform nicely to the classic trophic level concept of primary producers and upper level consumers (Lindeman, 1942; Hairston et al, 1960). Unlike green food webs in which herbivores directly ingest but do not themselves contribute organic C and organic nutrients to the autotroph pool, detrital organic carbon and nutrients are repackaged and consumed several times in brown food webs, resulting in a “microbial loop” or “detrital processing chain.”. This and other inherent differences may result in distinct stoichiometric principles throughout green versus brown food webs Unlike green food webs in which herbivores directly ingest but do not themselves contribute organic C and organic nutrients to the autotroph pool, detrital organic carbon and nutrients are repackaged and consumed several times in brown food webs, resulting in a “microbial loop” or “detrital processing chain.” This and other inherent differences may result in distinct stoichiometric principles throughout green versus brown food webs

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