Selective feeding determines patterns of nutrient release by stream invertebrates

Abstract

One common stoichiometric approach to predicting patterns of nutrient release (excretion + egestion) by animals in aquatic ecosystems is to base predictions on elemental mass-balance constrained by homeostatic maintenance. An easily measured resource composite (i.e., seston, epilithon, or leaf litter) often is used to represent ingested stoichiometry, but whether such a composite is a good indicator of food actually ingested is a relatively unexplored assumption. We examined the application of a stoichiometric model to the diets of 4 generalist stream invertebrates. We fed 3 trichopteran and 1 amphipod taxa rations consisting of cultured algae, stream epilithon, and several species of conditioned leaf litter. The rations ranged widely in C ∶ N from 10 to 69 (molar) and in C ∶ P from 165 to 3500. After a 2-d feeding period, we measured NH 4 + and PO 4 3− excretion, and C, N, and P egestion rates. The relationships observed between the stoichiometries of release and ration were unexpected. Total N ∶ P release rates conformed to stoichiometric predictions for only 1 taxon. Excretion and egestion rates and ratios were generally similar across diets and rarely varied with ration stoichiometry. These patterns were the result of smaller-than-expected responses to leaf-litter rations, which were the most imbalanced relative to body stoichiometry. Analysis of the C ∶ N stoichiometry of foregut material for 2 taxa showed selective ingestion of an N-rich fraction of leaf litter, in 1 case reducing an apparent 8.4 ∶ 1 C ∶ N imbalance between diet and body composition to 1.5 ∶ 1. Our results show that selective feeding can reduce potential stoichiometric imbalances, altering patterns of nutrient release relative to expectations based on bulk-diet stoichiometry. Assuming that stream invertebrates consume materials stoichiometrically similar to a resource composite can obscure understanding of stoichiometric imbalances and the role of invertebrates in nutrient cycles.

Long‐term stoichiometry and fates highlight animal egestion as nutrient repackaging, not recycling, in aquatic ecosystems

“…We provide robust support for previous findings and our first predictions (P1 and P2) regarding diet and taxonomic variation in egesta nutrient content (Patrick ; Hood et al . ; Halvorson et al . ).…”

“…) and shredder egesta are often more N and P rich than detrital food resources (Hood et al . ; Halvorson et al . ).…”

“…First, we hypothesized egesta elemental contents would vary across diets and shredder taxa, given findings of other studies (Hood et al . ; Halvorson et al . ).…”

Ecological stoichiometry of aquatic fungi: current knowledge and perspectives

“…Nevertheless, some studies found high consumption of low-nutrient leaf litter, indicating compensatory feeding on low-quality food (Danger et al, 2013a;Flores et al, 2014). When evaluating these discrepancies, we must keep in mind that leaf-shredding detritivores selectively feed on locally enriched patches of overall nutrient-impoverished litter (Arsuffi and Suberkropp, 1985;Lauridsen et al, 2014;Hood et al, 2014). An important implication is that apparent stoichiometric imbalances of bulk litter can be overcome, potentially leading to counterintuitive consumption rates, consumer-mediated nutrient recycling, and changes in elemental concentrations of the remaining litter (Fig 1).…”

“…Leaf litter conditioned by fungi is preferentially consumed by detritivores, with consumptions rates being influenced by leaf litter quality (Irons et al, 1988;Schlief and Mutz, 2006;B€ arlocher and Sridhar, 2014) and probably by stoichiometric requirements of the detritivores. Finally, leaf litter consumption can lead to changes in litter stoichiometry through selective feeding on nutrient-rich leaf patches (Lauridsen et al, 2014), which in turn influences rates of consumer-driven nutrient cycling (Hood et al, 2014) …”