Angélica L. González, Régis Céréghino, Olivier Dézerald, Vinicius F. Farjalla, Céline Leroy, Barbara A. Richardson, Michael J. Richardson, Gustavo Q. Romero & Diane S. Srivastava
Chemical elements such as carbon (C), nitrogen (N) and phosphorus (P) are the building blocks of living organisms. Differences in chemical composition between individuals and species play fundamental roles in key ecological processes such as predation, decomposition and nutrient cycling. Although organisms differ widely in the amount of these chemical elements in their bodies, we still know little about the underlying causes of these elemental differences. Moreover, no studies have addressed this question in ecological communities that span broad geographic extents.
We analyzed elemental (%C, %N, %P as a fraction of total mass) composition and element ratios (expressed as C:N, C:P and N:P) of 872 aquatic insects and other invertebrates (71 species) inhabiting tank bromeliads from five distant sites across Central and South America (Costa Rica, Puerto Rico, French Guiana, and two locations in Brazil). Leaves of tank bromeliads form compartments that fill with rainwater and capture leaf litter, supplying the energy to fuel a food web of invertebrates (mainly insect larvae), providing highly-replicated but miniature ecosystems. We tested (1) if differences between organisms in their elemental composition depend on their evolutionary history, diet (consumer of animals vs. detritus) and body size, and (2) if patterns in elemental composition are consistent across broad geographical scales.
The taxonomic family that invertebrates belonged to was most important in explaining differences in elemental composition, at least at geographic scales. Species that consumed other animals, rather than detritus, had higher elemental contents, especially of N and P as compared to C. This pattern was found in all sites, and occurs because carnivores optimize growth by choosing the most nutrient-rich prey to consume, and then preferentially store these nutrients in their bodies. Smaller invertebrates also have higher nutrient content than large invertebrates, owing to their greater need for nutrients to grow proportionally faster than large organisms. Differences between individuals of the same species were mainly due to differences in body size.
So, differences between invertebrate species in their elemental composition depend on taxonomy and diet, whereas differences between individuals within a species depend on body size. As these patterns were largely consistent across a broad geographic region, there may be universal mechanisms underlying the elemental content of invertebrates.