Ecological mechanisms and phylogeny shape invertebrate stoichiometry: A test using detritus‐based communities across Central and South America

Abstract

Stoichiometric differences among organisms can affect trophic interactions and rates of nutrient cycling within ecosystems. However, we still know little about either the underlying causes of these stoichiometric differences or the consistency of these differences across large geographical extents. Here, we analyse elemental (carbon, nitrogen, phosphorus) composition of 872 aquatic macroinvertebrates (71 species) inhabiting tank bromeliads (n = 140) from five distantly located sites across Central and South America to (i) test phylogenetic, trophic and body size scaling explanations for why organisms differ in elemental composition and (ii) determine whether patterns in elemental composition are universal or context dependent. Taxonomy explained most variance in elemental composition, even though phylogenetic signals were weak and limited to regional spatial extents and to the family level. The highest elemental contents and lowest carbon:nutrient ratios were found in organisms at high trophic levels and with smaller body size, regardless of geographical location. Carnivores may have higher nutrient content and lower carbon:nutrient ratios than their prey, as organisms optimize growth by choosing the most nutrient‐rich resources to consume and then preferentially retain nutrients over carbon in their bodies. Smaller organisms grow proportionally faster than large organisms and so are predicted to have higher nutrient requirements to fuel RNA and protein synthesis. Geography influenced the magnitude, more than the direction, of the ecological and/or phylogenetic effects on elemental composition. Overall, our results show that both ecological (i.e. trophic group) and evolutionary drivers explain among‐taxa variation in the elemental content of invertebrates, whereas intraspecific variation is mainly a function of body size. Our findings also demonstrate that restricting analyses of macroinvertebrate stoichiometry solely to either the local scale or species level affects inferences of the patterns in invertebrate elemental content and their underlying mechanisms. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13197/suppinfo is available for this article.

The bioelements, the elementome, and the biogeochemical niche

“…, González et al. ). In microbial communities, genotype‐niches have also been described in function of the different availabilities of different nutrients (Pereira and Berry ) or as a function of the values of diverse functional traits (Lennon et al.…”

Ecological, evolutionary, and geographical correlates of variation in consumer elemental composition

“…Based on the Growth Rate Hypothesis (GRH), we expect small organisms to grow faster and to have higher nutrient content, particularly P, than large organisms to fuel RNA and protein synthesis (Elser, Dobberfuhl, MacKay, & Schampel, ; Hessen et al, ; Sterner & Elser, ). Although Gonzalez et al () is not a direct test of the GRH, they did observe the highest elemental contents (N, P) and lowest C:nutrient ratios in detrital consumers with small body sizes. There is a well‐known elemental bottleneck at the detritivore and herbivore trophic levels as these organisms feed on resources (i.e.…”

“…In this issue of Functional Ecology , Gonzalez et al () collect an impressive dataset of detritus‐based communities inhabiting tank bromeliads from 5 sites spanning >40° latitude across South and Central America to investigate multiple potential drivers of variation in consumer elemental composition. Specifically, they test for evidence of phylogenetic, trophic group and body size signatures across taxonomic resolutions at local (i.e.…”

“…lower C:nutrient ratio) relative to detritivores (or herbivores). Gonzalez et al () found that organisms at high trophic levels had higher N content and lower C:N ratio than their prey. But carnivores had lower P than their prey and shredders had higher N than filter feeders, gatherers and scrapers.…”

“…While Gonzalez et al () investigate the role of invertebrate trophic groups, a next step may be to explicitly build quantitative food webs to investigate the patterns and causes of variation in elemental composition along food chains (Boersma et al, ; Leroux et al, ). Two ways to overcome the immense challenge of collecting food web data across large spatial extents are to study relatively simple ecosystems (e.g.…”