The framework ecological stoichiometry uses elemental composition of species to make predictions about growth and competitive ability in defined elemental supply conditions. Although intraspecific differences in stoichiometry have been observed, we have yet to understand the mechanisms generating and maintaining such variation. We used variation in phosphorus (P) content within a Daphnia species to test the extent to which %P can explain variation in growth and competition. Further, we measured 33P kinetics (acquisition, assimilation, incorporation and retention) to understand the extent to which such variables improved predictions. Genotypes showed significant variation in P content, 33P kinetics and growth rate. P content alone was a poor predictor of growth rate and competitive ability. While most genotypes exhibited the typical growth penalty under P limitation, a few varied little in growth between P diets. These observations indicate that some genotypes can maintain growth under P-limited conditions by altering P use, suggesting that decomposing P content of an individual into physiological components of P kinetics will improve stoichiometric models. More generally, attention to the interplay between nutrient content and nutrient-use is required to make inferences regarding the success of genotypes in defined conditions of nutrient supply.

Ecological stoichiometry uses information on a few key biological elements (C, N and P) to explain complex ecological patterns. Although factors driving variation in these elements are well established, expanding stoichiometric principles to explore dynamics of the many other essential elements comprising biological tissues (i.e. the ionome) is needed to determine their metabolic relationships and better understand biological control of elemental flows through ecosystems. In this paper, we report observations of ionomic variation in two species of salamander (Ambystoma opacum and A. talpoideum) across ontogenic stages using specimens from biological collections of two wetlands sampled over a 30‐year period. This unique data set allowed us to explore the extent of ionomic variation between species, among ontogenic stages, between sites and through time. We found species‐ and, to a lesser extent, site‐specific differences in C, N and P along with 13 other elements forming salamander ionomes but saw no evidence of temporal changes. Salamander ionomic composition was most strongly related to ontogeny with relatively higher concentrations of many elements in adult males (i.e. Ca, P, S, Mg, Zn and Cu) compared to metamorphic juveniles, which had greater amounts of C, Fe and Mn. In addition to patterns of individual elements, covariance among elements was used to construct multi‐elemental nutrient balances, which revealed differences in salamander elemental composition between species and sites and changes in elemental proportions across ontogenic development. These multi‐elemental balances distinguished among species‐site‐ontogenic stage groups better than using only C, N and P. Overall, this study highlights the responsiveness of consumer ionomes to life‐history and environmental variation while reflecting underlying relationships among elements tied to biological function. As such, ionomic studies can provide important insights into factors shaping consumer elemental composition and for predicting how these changes might affect higher‐order ecological processes.

The growth rate hypothesis posits that the rate of protein synthesis is constrained by phosphorus (P) supply. P scarcity invokes differential expression of genes involved in processing of most if not all elements encompassing an individual (the ionome). Whether such ionome‐wide adjustments to P supply impact growth and trophic interactions remains unclear. We quantified the ionomes of a resource‐consumer pair in contrasting P supply conditions. Consumer growth penalty was driven by not only P imbalance between trophic levels but also imbalances in other elements, reflecting complex physiological adjustments made by both the resource and the consumer. Mitigating such imbalances requires energy and should impact the efficiency at which assimilated nutrients are converted to biomass. Correlated shifts in the handling of multiple elements, and variation in the supplies of such elements could underlie vast heterogeneity in the rates at which organisms and ecosystems accrue biomass as a function of P supply.

Speciation genomic studies have revealed that genomes of diverging lineages are shaped jointly by the actions of gene flow and selection. These evolutionary forces acting in concert with processes such as recombination and genome features such as gene density shape a mosaic landscape of divergence. We investigated the roles of recombination and gene density in shaping the patterns of differentiation and divergence between the cyclically parthenogenetic ecological sister‐taxa, Daphnia pulicaria and Daphnia pulex. First, we assembled a phased chromosome‐scale genome assembly using trio‐binning for D. pulicaria and constructed a genetic map using an F2‐intercross panel to understand sex‐specific recombination rate heterogeneity. Finally, we used a ddRADseq data set with broad geographic sampling of D. pulicaria, D. pulex, and their hybrids to understand the patterns of genome‐scale divergence and demographic parameters. Our study provides the first sex‐specific estimates of recombination rates for a cyclical parthenogen, and unlike other eukaryotic species, we observed male‐biased heterochiasmy in D. pulicaria, which may be related to this somewhat unique breeding mode. Additionally, regions of high gene density and recombination are generally more divergent than regions of suppressed recombination. Outlier analysis indicated that divergent genomic regions are probably driven by selection on D. pulicaria, the derived lineage colonizing a novel lake habitat. Together, our study supports a scenario of selection acting on genes related to local adaptation shaping genome‐wide patterns of differentiation despite high local recombination rates in this species complex. Finally, we discuss the limitations of our data in light of demographic uncertainty.