Valentina Franco-Trecu

Mercury (Hg) is a non-essential, globally distributed toxic metal that bioaccumulates in aquatic organisms and biomagnifies across trophic levels, posing a threat to top predators such as marine mammals. Our aim was to evaluate total mercury (THg) relationships in mother-pup pairs of two pinniped species, South American sea lions (Otaria flavescens) and South American fur seals (Arctocephalus australis), from Isla de Lobos, Uruguay. We quantified THg in the hair of 11 O. flavescens and 46 A. australis mother-pup pairs. THg concentrations were significantly higher in O. flavescens than in A. australis for both adult females (30.5 ± 9.3 μg/g vs. 5.7 ± 3.4 μg/g, dry weight) and pups (25.3 ± 13.2 μg/g vs. 4.4 ± 2.5 μg/g, dry weight). Hg levels in O. flavescens pups are among the highest recorded for pinnipeds. A significant positive relationship between female and pup THg (O. flavescens p=0.003; A. australis p=0.002) supported transplacental Hg transfer. In O. flavescens, pup sex significantly influenced this relationship, with males showing lower THg for a given maternal value, whereas no sex effect was detected in A. australis. These finding support the use of pups as effective minimally invasive proxies for maternal Hg levels. This study highlights the relevance of early-life exposure in environmental toxicology and supports integrating pup-based sampling in long-term contaminant monitoring programs.

Double‐hierarchical generalized linear models (DHGLMs) offer a powerful yet overlooked quantitative approach for analysing ecological data with hierarchical structures. DHGLMs account for unit‐specific residual variances, allowing the estimation of variance components that expand our ability to quantify biodiversity patterns across levels of biological organization—from individuals to communities. A particularly promising application of these models lies in isotopic ecology, where stable isotopes of carbon (𝛿 ¹³ C) and nitrogen (𝛿 ¹⁵ N) are used to quantify continuous trophic (co)variation across individuals, populations and communities. Here, we show how DHGLMs can be a powerful tool for ecologists interested in trophic patterns and demonstrate the application of these models to isotopic data, showcasing study systems across three levels of biological organization. These examples illustrate how DHGLMs decompose (co)variance components within and across biological units of interest, uncover patterns of diet variation and ecological interactions. We also discuss the advantages, limitations and potential of DHGLMs for advancing research on different questions related to variation in diet and specialization at different biological levels.

Mercury (Hg) is a non-essential, globally distributed toxic metal that bioaccumulates in aquatic organisms and biomagnifies across trophic levels, posing a threat to top predators such as marine mammals. Our aim was to evaluate maternal-pup total mercury (THg) relationships in mother-pup pairs of two pinniped species, South American sea lions (Otaria flavescens) and South American fur seals (Arctocephalus australis), from Isla de Lobos, Uruguay. We quantified THg in the hair of 11 O. flavescens and 46 A. australis mother-pup pairs. THg concentrations were significantly higher in O. flavescens than in A. australis for both adult females (30.5 ± 9.3 μg/g vs. 5.7 ± 3.4 μg/g, dry weight) and pups (25.3 ± 13.2 μg/g vs. 4.4 ± 2.5 μg/g, dry weight). A positive relationship between female and pup THg supported transplacental Hg transfer. In O. flavescens, pup sex significantly influenced this relationship, with males showing lower THg for a given maternal value, whereas no sex effect was detected in A. australis. These results confirm that pups serve as effective, minimally invasive proxies for maternal contamination. Hg levels in O. flavescens pups are among the highest recorded for pinnipeds and may have immunological and developmental consequences. This study highlights the relevance of early-life exposure in environmental toxicology and supports integrating pup-based sampling in long-term contaminant monitoring programs.

Individual diet specialization is prevalent in wild populations and scales up to drive ecological processes at the population and community levels. As the population trophic niche emerges from the combination of foraging decisions made by individuals, specific individuals with particular niches can disproportionately contribute to the niche of their population, i.e. keystone niche individuals. We investigated whether individuals of South American fur seals and sea lions differ in their contribution to the trophic niche width of their populations. To quantify trophic variation within and between individuals over long time frames, we analyzed stable isotopes (δ¹³C and δ¹⁵N) of serially sampled whiskers (18 individuals of O. flavescens and 20 of A. australis females; average of 33 measures per individual). We used these repeated observations per individual to model population‐level trophic niches in the isotopic bivariate space accounting for between and within‐individual variation. Then, we estimated niche width, position and orientation for each individual in the population. Finally, we quantified the relative contribution of each individual to the niche of its population by estimating how niche width changes when a given individual is removed from its population. While most conspecifics make negligible contributions, a few keystone niche individuals contribute disproportionately to population niche width. Individuals with niches located close to the population centroid promote the contraction of the population trophic niche, while individuals with niches located far from the population centroid promote its expansion. Further, wide individual niches with a divergent orientation compared to the population niche tend to expand the population niche width. Our findings reveal how intraspecific niche variation can shape population niches from the inside out, highlighting that individuals differ substantially in their contribution to patterns at higher levels of biological organization.

Studying trophic feeding habits and their variation over time is crucial for understanding individual and population success. Although many generalist feeders’ populations appear to use diverse trophic resources, they are often composed of individuals that specialize in only a subset of these resources, a phenomenon known as individual trophic specialization (ITS). The South American sea lion (SASL) (Otaria flavescens), widely distributed along the South American coast, is an opportunistic and generalist predator with a diverse diet, making it a good model for studying ITS. We aimed to estimate the diet composition and the degree of ITS among SASL females on Isla Arce, Argentina, over a period of three years (2012 to 2014). For that, we analysed stable isotopes (δ15N and δ13C) in vibrissae from 12 females. Over the three years studied, females from this colony primarily fed on pelagic prey. At the population level, the most commonly consumed prey varied between the Argentine anchovy (Engraulis anchoita) in 2012 and 2014 and the Patagonian squid (Doryteuthis gahi) in 2013. Consuming mostly pelagic resources throughout the year, unlike previous studies in nearby colonies, suggests the existence of different foraging strategies in lactating females. Although the prey contribution varied between years, the analysis indicated that SASL females breeding on Isla Arce show some degree of ITS, which could be a mechanism adopted by individuals to mitigate the potential competition given by sustained population increases.

Disentangling the determinants of trophic structure is central to ecology. The capacity to capture subjugate and consume a prey (i.e. gape limitation) is a relevant limitation to acquire energy for most organisms, especially those in smaller size ranges. This generates a size hierarchy of trophic positions in which large organisms consume small ones. Body size is tightly correlated to gape limitation and explains a large fraction of variance in the body size‐trophic position relationship. However, a considerable fraction of variance still remains to be explained. Consumer search space dimensionality (2D or 3D) and feeding strategies, temperature and the size structure of primary producers can alter the trophic structure, but tests based on information from natural food webs are scarce. We generated specific predictions about the body size trophic position relationship and evaluated them using information from a subtropical South Atlantic coastal marine ecosystem: benthic realm (2D, rocky shore and sandy beach) and the pelagic realm (3D). We characterized this marine coastal food web based on stable isotopes of carbon and nitrogen from 256 samples from primary producers (macroalgae and phytoplankton) to large predators (sand shark) in summer and winter. Consumer body size encompassed six orders of magnitude in weight from 10⁻² to 6 × 10⁴ g. Isotopic signal corresponded to an integration of carbon sources from basal consumers to top predators. The body size‐trophic position relationship showed a linear positive association with different slopes for the benthic and pelagic environments. This implies a smaller predator prey size ratio for pelagic (3D) with respect to benthic consumers (2D) as theoretically expected. No seasonal differences were found in slopes and most of the overall variance in benthic environments was largely explained by feeding strategies of the different taxonomic groups. We provide an integrated evaluation on the role of body size, consumer search space and feeding strategy to understand the determinants of trophic position. Results demonstrate that integrating gape limitation hypothesis, the dimensionality of consumer search space and feeding strategies into a formal robust framework to understand trophic structure is feasible even in complex natural ecosystems.