Bony traits and genetics drive intraspecific variation in vertebrate elemental composition

Elemental content of a host-parasite relationship in the threespine stickleback

Parasite infections are ubiquitous and their effects on hosts could play a role in ecosystem processes. Ecological stoichiometry provides a framework to study linkages between consumers and their resource, such as parasites and their host, and ecosystem process; however, the stoichiometric traits of host-parasite associations are rarely quantified. Specifically, it is unclear whether parasites' elemental ratios closely resemble those of their host or if infection is related to host stoichiometry, especially in vertebrate hosts. To answer such questions, we measured the elemental content (%C, %N, and %P) and molar ratios (C:N, C:P, and N:P) of parasitized and unparasitized Gasterosteus aculeatus (three-spined stickleback) and their cestode parasite, Schistocephalus solidus. Host and parasite elemental content were distinct from each other, and parasites were generally higher in %C and lower in %N and %P. Parasite infections were related to host C:N, with infected hosts being lower in C:N. Parasite elemental content was independent of their host, but parasite body mass and parasite density were important drivers of parasite stoichiometry. Overall, these potential effects of parasite infections on host stoichiometry along with parasites' distinct elemental compositions suggest parasites may further contribute to differences in how individual hosts store and recycle nutrients.

SEED: A framework for integrating ecological stoichiometry and eco-evolutionary dynamics

Characterising the extent and sources of intraspecific variation and their ecological consequences is a central challenge in the study of eco-evolutionary dynamics. Ecological stoichiometry, which uses elemental variation of organisms and their environment to understand ecosystem patterns and processes, can be a powerful framework for characterising eco-evolutionary dynamics. However, the current emphasis on the relative content of elements in the body (i.e. organismal stoichiometry) has constrained its application. Intraspecific variation in the rates at which elements are acquired, assimilated, allocated or lost is often greater than the variation in organismal stoichiometry. There is much to gain from studying these traits together as components of an 'elemental phenotype'. Furthermore, each of these traits can have distinct ecological effects that are underappreciated in the current literature. We propose a conceptual framework that explores how microevolutionary change in the elemental phenotype occurs, how its components interact with each other and with other traits, and how its changes can affect a wide range of ecological processes. We demonstrate how the framework can be used to generate novel hypotheses and outline pathways for future research that enhance our ability to explain, analyse and predict eco-evolutionary dynamics.

Environmental-related variation of stoichiometric traits in body and organs of non-native sailfin catfishes Pterygoplichthys spp

Intraspecific variation in stoichiometric traits was thought to be an adaptive response to reduce the elemental imbalance between organism and diet in the habitat. Studying the spatial variation of stoichiometric traits of non-native species and the factors contributing to the variation could help to better understand the invasion mechanism of non-native fish. In this study, stoichiometric traits (i.e. carbon [C], phosphorus [P], calcium [Ca] and their ratios) variation in the body and organs of non-native sailfin catfishes Pterygoplichthys spp. were investigated across 13 river sections in the main river basins of Guangdong province. The relationships between environmental factors and stoichiometric traits were analyzed using a general linear model and an information-theoretic approach. A manipulated feeding experiment was conducted to investigate the impact of food quality on the stoichiometry of sailfin catfishes in a greenhouse. Sailfin catfishes exhibited considerable variability in body and organ elemental composition. Site identity was the main factor contributing to the variation, which could be explained by a combination of environmental factors including climate, diet quality, fish species richness and trophic status in the invaded rivers. Water chemistry (i.e. total nitrogen and phosphorus, ammonia nitrogen and soluble reactive phosphorus) contributed to the most variation of stoichiometric traits. Imbalances of P and Ca between sailfin catfishes and food resources varied among sampling sites, reflecting the spatial heterogeneity of nutrients limitation. Juvenile sailfin catfishes exhibited stoichiometric homeostasis (0 < 1/H < 0.25) for all elemental contents and ratios in the feeding experiment. These findings suggested variation in stoichiometric traits of sailfin catfishes might be attributed to the changes in elemental metabolism to cope with context-specific environments. This study provided heuristic knowledge about environmental-related variation in stoichiometric traits, which could enhance the understanding of the non-native species' adaptation to resource fluctuation in the invaded ecosystems.

Consumer trait responses track change in resource supply along replicated thermal gradients

Rising temperatures may alter consumer diets through increased metabolic demand and altered resource availability. However, current theories assessing dietary shifts with warming do not account for a change in resource availability. It is unknown whether consumers will increase consumption rates or consume different resources to meet increased energy requirements and whether the dietary change will lead to associated variation in morphology and nutrient utilization. Here, we used populations of Gambusia affinis across parallel thermal gradients in New Zealand (NZ) and California (CA) to understand the influence of temperature on diets, morphology and stoichiometric phenotypes. Our results show that with increasing temperature in NZ, mosquitofish consumed more plant material, whereas in CA mosquitofish shifted towards increased consumption of invertebrate prey. In both regions, populations with plant-based diets had fuller guts, longer relative gut lengths, better-orientated mouths and reduced body elemental %C and N/P. Together, our results show multiple pathways by which consumers may alter their feeding patterns with rising temperatures, and they suggest that warming-induced changes to resource availability may be the principal determinant of which pathway is taken.

Phosphorus limitation does not drive loss of bony lateral plates in freshwater stickleback (Gasterosteus aculeatus)

Connecting the selective forces that drive the evolution of phenotypes to their underlying genotypes is key to understanding adaptation, but such connections are rarely tested experimentally. Threespine stickleback (Gasterosteus aculeatus) are a powerful model for such tests because genotypes that underlie putatively adaptive traits have been identified. For example, a regulatory mutation in the Ectodysplasin (Eda) gene causes a reduction in the number of bony armor plates, which occurs rapidly and repeatedly when marine sticklebacks invade freshwater. However, the source of selection on plate loss in freshwater is unknown. Here, we tested whether dietary reduction of phosphorus can account for selection on plate loss due to a growth advantage of low-plated fish in freshwater. We crossed marine fish heterozygous for the 16 kilobase freshwater Eda haplotype and compared the growth of offspring with different genotypes under contrasting levels of dietary phosphorus in both saltwater and freshwater. Eda genotype was not associated with growth differences in any treatment, or with mechanisms that could mitigate the impacts of phosphorus limitation, such as differential phosphorus deposition, phosphorus excretion, or intestine length. This study highlights the importance of experimentally testing the putative selective forces acting on phenotypes and their underlying genotypes in the wild.

Stoichiometric consequences of size-selective mortality: An experimental test using the Japanese medaka (Oryzias latipes)

The determinants of intraspecific stoichiometric variation remain difficult to elucidate due to their multiple origins (e.g. genetic vs. environmental) and potential interactive effects. We evaluated whether two size-selected lines of medaka (Oryzias latipes) with contrasted life-history strategies (small- and large-breeder lines with slow growth and early maturity vs. fast growth and late maturity) differed in their organismal stoichiometry (percentage and ratios of carbon [C], nitrogen [N] and phosphorus [P]) in a mesocosm experiment. We also tested how size-selection interacted with environmental conditions (i.e. two levels of fish density and light intensity), body condition and sex. Results showed that large-breeder fish were significantly N-enriched compared to small-breeders, while the two size-selected lines did not differ in body P composition. Size-selection interacted with density - high density only affected small-breeders leading to decreasing %C and C: N - and with sex - large-breeder females had higher %C and C:N values than large-breeder males. Finally, C:P and N:P ratios increased with body condition due to decreasing %P. Overall, our results show that the ecological consequences of size-selective mortality extend to organismal stoichiometry and may, from there, change nutrient cycling and ecosystem functioning.

Patterns and potential drivers of intraspecific variability in the body C, N, and P composition of a terrestrial consumer, the snowshoe hare (Lepus americanus)

Intraspecific variability in ecological traits is widespread in nature. Recent evidence, mostly from aquatic ecosystems, shows individuals differing at the most fundamental level, that of their chemical composition. Age, sex, or body size and condition may be key drivers of intraspecific variability in the body concentrations of carbon (C), nitrogen (N), and phosphorus (P). However, we still have a rudimentary understanding of the patterns and drivers of intraspecific variability in chemical composition of terrestrial consumers, particularly vertebrates.Here, we investigate the elemental composition of the snowshoe hare Lepus americanus. Based on snowshoe hare ecology, we predicted older, larger individuals to have higher concentration of N or P and lower C content compared with younger, smaller individuals. We also predicted females to have higher concentrations of N, P, and lower C than males due to the higher reproductive costs they incur. Finally, we predicted that individuals in better body condition would have higher N and P than those in worse condition, irrespective of age.We obtained C, N, and P concentrations and ratios from a sample of 50 snowshoe hares. We then used general linear models to test our predictions on the relationship between age, sex, body size or condition and stoichiometric variability in hares.We found considerable variation in C, N, and P stoichiometry within our sample. Contrary to our predictions, we found weak evidence of N content decreasing with age. As well, sex appeared to have no relationship with hare body elemental composition. Conversely, as expected, P content increased with body size and condition. Finally, we found no relationship between variability in C content and any of our predictor variables.Snowshoe hare stoichiometry does not appear to vary with individual age, sex, body size, or condition. However, the weak relationship between body N concentration and age may suggest varying nutritional requirements of individuals at different ages. Conversely, body P's weak relationship to body size and condition appears in line with this limiting element's importance in terrestrial ecosystems. Snowshoe hares are keystone herbivores in the boreal forest of North America, and the substantial stoichiometric variability we find in our sample could have important implications for nutrient dynamics, in both boreal and adjacent ecosystems.