Litter consumption by macrodetritivores depends more on mechanical than on nutritional constraints

Abstract

Ecosystem functions greatly depend on trophic interactions between consumers and their resources. Resource consumption depends on ingestion, digestion, and allocation processes. Mechanical constraints are expected to influence ingestion, while metabolic and nutritional constraints are expected to influence allocation. Leaf litter are resources presenting a high mechanical and nutritional heterogeneity that depends on plant identity and on physical and microbial processing over the course of decomposition. Litter consumption by detritivores is known to depend on metabolic and nutritional constraints but the importance of mechanical constraints is yet unknown. After accounting for metabolic constraints on consumption rate, we tested the relative importance of mechanical and nutritional constraints in explaining litter consumption rates by detritivores. For this, we exposed 16 leaf treatments (eight leaf species either just leached or leached and microbially conditioned) to four aquatic and five terrestrial detritivore taxa in laboratory no‐choice consumption experiments. We investigated two mechanical constraints: grabbing and fragmenting the resource, by measuring suitable couples of mechanical traits for both litter and detritivores. We also investigated four nutritional constraints related to N, P, K and Ca contents in both detritivores and litter. For each constraint, we also tested if trait matching significantly contribute to explain consumption. Our analysis revealed that both mechanical and nutritional constraints are influencing mass‐independent consumption rate but that mechanical constraints predominate over nutritional constraints. Litter fragmentation, studied through litter toughness and detritivore biting force, was especially important to explain consumption rate. Nutritional constraints were dominated by P constraints. Trait‐matching had very weak importance and was significant only for P constraints. Our findings highlight the importance of mechanical constraints for litter consumption by detritivores.