Abstract
In many contexts, nutrient excretion by consumers can impact ecosystems by altering the availability of limiting nutrients. Variation in nutrient excretion can be predicted by mass balance models, most of which are premised on two key ideas: (1) consumers maintain fixed whole-body nutrient content (i.e., %N and %P), so-called fixed homeostasis; (2) if dietary nutrients are not matched to whole-body nutrients, excesses of any nutrient are released as excretion to maintain fixed homeostasis. Mass balance models thus predict that consumer excretion should be positively correlated with diet nutrients and negatively correlated with whole-body nutrients. Recent meta-analyses and field studies, however, have often failed to find these expected patterns, potentially because of a confounding influence—flexibility in whole-body nutrient content with diet quality (flexible homeostasis). Here, we explore the impact of flexible homeostasis on nutrient excretion by comparing the N and P excretion of four genetically diverged Trinidadian guppy (Poecilia reticulata) populations when reared on diets of variable P content. As predicted by mass balance, P excretion increased on the high-P diet, but, contrary to the notion of fixed homeostasis, guppy whole-body %P also increased on the high-P diet. While there was no overall correlation between excretion nutrients and whole-body nutrients, when the effect of diet on both whole-body and excretion nutrients was included, we detected the expected negative correlation between whole-body N:P and excretion N:P. This last result suggests that mass balance can predict excretion rates within species, but only if dietary effects on whole-body nutrient content are controlled. Flexible homeostasis can obscure patterns predicted by mass balance, creating an imperative to accurately capture an organism’s diet quality in predicting its excretion rate.
Highlights
Consumers are a critical component of nutrient cycles, and variation in the rate at which consumers excrete dissolved nutrients, like nitrogen (N) and phosphorus (P), can alter nutrient limitation and ecosystem function [1,2,3]
Intraspecific variation in nutrient stoichiometry in 2009 from an high-predation risk (HPred) locality where guppies coexist with a diversity of large piscivorous fish, and an low predation risk (LPred) locality without piscivorous fish
Our results show flexible homeostasis in guppy whole-body nutrients under variable diet quality, and this flexible homeostasis obscured the negative correlation between whole-body and excretion nutrients that is predicted by mass balance models
Summary
Consumers are a critical component of nutrient cycles, and variation in the rate at which consumers excrete dissolved nutrients, like nitrogen (N) and phosphorus (P), can alter nutrient limitation and ecosystem function [1,2,3]. Because variation in excretion can affect ecosystems, ecologists have sought to understand the causes of variation in excretion rates, often using the principle of mass-balance [4,5]. Most models employing mass balance assume that each consumer has an immutable whole-body nutrient content, determined by its genetics, sex, and ontogeny [6,7,8]. By these models, all consumers maintain homeostasis by precisely regulating their whole-body %N and %P–retaining the nutrients needed to build tissue and releasing excess nutrients as waste, often as excretion. While some field studies have found evidence for these patterns [9,10,11], many, especially those within a single species, have failed to do so [9,12,13,14]
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