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1. Diet of a threatened endemic fox reveals variation in sandy beach resource use on California Channel Islands

   https://doi.org/10.1371/journal.pone.0258919  

   Page, Henry M. ; Schamel, Juliann ; Emery, Kyle A. ; Schooler, Nicholas K. ; Dugan, Jenifer E. ; Guglielmino, Angela ; Schroeder, Donna M. ; Palmstrom, Linnea ; Hubbard, David M. ; Miller, Robert J. ( October 2021 , PLOS ONE)

   Hyrenbach, David (Ed.)

   The coastal zone provides foraging opportunities for insular populations of terrestrial mammals, allowing for expanded habitat use, increased dietary breadth, and locally higher population densities. We examined the use of sandy beach resources by the threatened island fox ( Urocyon littoralis ) on the California Channel Islands using scat analysis, surveys of potential prey, beach habitat attributes, and stable isotope analysis. Consumption of beach invertebrates, primarily intertidal talitrid amphipods ( Megalorchestia spp.) by island fox varied with abundance of these prey across sites. Distance-based linear modeling revealed that abundance of giant kelp ( Macrocystis pyrifera ) wrack, rather than beach physical attributes, explained the largest amount of variation in talitrid amphipod abundance and biomass across beaches. δ 13 C and δ 15 N values of fox whisker (vibrissae) segments suggested individualism in diet, with generally low δ 13 C and δ 15 N values of some foxes consistent with specializing on primarily terrestrial foods, contrasting with the higher isotope values of other individuals that suggested a sustained use of sandy beach resources, the importance of which varied over time. Abundant allochthonous marine resources on beaches, including inputs of giant kelp, may expand habitat use and diet breadth of the island fox, increasing population resilience during declines in terrestrial resources associated with climate variability and long-term climate change. 

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2. Fatty acid stable isotopes add clarity, but also complexity, to tracing energy pathways in aquatic food webs

   https://doi.org/10.1002/ecs2.3360  

   Chiapella, Ariana M. ; Kainz, Martin J. ; Strecker, Angela L. ( February 2021 , Ecosphere)

   Tracing the flow of dietary energy sources, especially in systems with a high degree of omnivory, is an ongoing challenge in ecology. In aquatic systems, one of the persistent challenges is in differentiating between autochthonous and allochthonous energy sources to top consumers. Bulk carbon stable isotope values of aquatic and terrestrial prey often overlap, making it difficult to delineate dietary energy pathways in food webs with high allochthonous prey subsidies, such as in many northern temperate waterbodies. We conducted a feeding experiment to explore how fatty acid stable isotopes may overcome the challenge of partitioning autochthonous and allochthonous energy pathways in aquatic consumers. We fed hatchery‐reared Arctic Char (Salvelinus alpinus) diets of either benthic invertebrates, terrestrial earthworms, or a mixture of both. We then compared how the stable carbon isotopes of fatty acids (δ¹³C_FA) distinguished between diet items and respective treatments inS. alpinusliver and muscle tissues, relative to bulk stable isotopes and fatty acid profiles. Although a high degree of variability of fatty acid stable carbon isotope values was present in all three measures, our results suggest that the ability of this method to overcome the challenges of bulk stable isotopes may be overstated. Finally, our study highlights the importance of further experimental investigation, and consideration of physiological and biochemical processes when employing this emerging method.

    

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3. Head shape predicts isotopic diet in anoles and day geckos

   https://doi.org/10.1111/1365-2435.14331  

   Figueira, Tyler J. ; Kennedy‐Gold, Stevie ; Piantoni, Carla ; Screen, Robyn M. ; Wright, Amber N. ( April 2023 , Functional Ecology)

   Trophic morphology affects resource acquisition; therefore, species differences in such traits may be informative for inferring resource use overlap and potential species interactions.

   In lizards, head size and shape determine the size and hardness of prey that can be consumed. Lizards with large differences in head morphology are expected to overlap less in prey use than lizards with more similar traits.

   Stable isotopes are increasingly being used to describe diet, yet how traditional functional traits affect isotopic diet is often not clear a priori.

   We measured head size, head shape, 𝛿¹⁵N, and 𝛿¹³C under controlled resource availability in an enclosure experiment using introduced lizards in Hawaiʻi to test whether functional traits predict isotopic diet.

   Brown anolesAnolis sagreihad the tallest and narrowest heads, the highest values of 𝛿¹³C, and the lowest values of 𝛿¹⁵N. Gold dust day geckosPhelsuma laticaudahad the shortest and widest heads, the lowest values of 𝛿¹³C, and the highest values of 𝛿¹⁵N. Green anolesAnolis carolinensiswere intermediate in both diet and morphology.

   As a result of isotopic diet overlap, green anoles have reduced competitor‐free resource space in the presence of both of the other lizard species.

   Head shape was the best predictor of diet and the only trait that explained variation within as well as among species. Head size was sexually dimorphic, and therefore the weaker diet correlations with this trait may be explained by sexual selection.

   Breadth in morphospace did not correlate with isotopic diet breadth, nor did the amount of overlap in morphospace predict the amount of overlap in isotopic diet space.

   While lizards were able to locally depress prey in experimental enclosures, no shifts in diet were detected in response to the presence of heterospecifics.

   The generality of head shape in predicting isotopic diet, and whether it does so independent of habitat use, warrants additional study. Head shape provides a potentially fruitful avenue for trait‐based approaches to studying ecology and evolution in lizards.

   Read the freePlain Language Summaryfor this article on the Journal blog.

    

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4. Stable isotopes reveal limited Eltonian niche conservatism across carnivore populations

   https://doi.org/10.1111/1365-2435.13266  

   Manlick, Philip J. ; Petersen, Shelby M. ; Moriarty, Katie M. ; Pauli, Jonathan N. ; McArthur, ed., Clare ( January 2019 , Functional Ecology)

   Niche conservatism—the retention of ecological traits across space and time—is an emerging topic of interest because it can predict responses to global change. The conservation of Grinnellian niche characteristics, like species‐habitat associations, has received widespread attention, but the conservation of Eltonian traits such as consumer–resource interactions remains poorly understood.

   The inability to quantify Eltonian niches through space and time has historically limited the assessment of Eltonian niche conservatism and the dynamics of foraging across populations. Consequently, the relative influence of endogenous factors like phylogeny versus exogenous features like environmental context has rarely been addressed.

   We tested Eltonian niche conservatism using a paired design to compare foraging among four populations of American martensMartes americanaand Pacific martensMartes caurina, morphologically and ecologically similar sister taxa that are allopatrically distributed throughout western North America. We developed a three‐stage isotopic framework and then quantified dietary niche overlap between the sister species and paired island‐mainland sites to assess the relative influence of endogenous (i.e., species) versus exogenous (i.e., environment) factors on Eltonian niches. First, we calculated pairwise dietary overlap in scaled δ‐space using standard ellipses. We then estimated proportional diets (“p‐space”) for individuals using isotopic mixing models and developed a novel utilization distribution overlap approach to quantify proportional dietary overlap. Lastly, we estimated population‐level proportional diets and quantified the differential use of functional prey groups across sites.

   We detected no pairwise overlap of dietary niches in δ‐space, and distributions of individual diets in p‐space revealed little overlap in core diets across populations. All pairwise comparisons of individuals revealed significant differences in diet, and population‐level comparisons detected contrasting use of functional prey groups.

   We developed a multi‐faceted isotopic framework to quantify Eltonian niches and found limited evidence of Eltonian niche conservatism across carnivore populations. Our findings are consistent with the growing recognition of dietary plasticity in consumers and suggest that consumer–resource dynamics are largely driven by exogenous environmental factors like land cover and community composition. These results illustrate the context‐dependent nature of foraging and indicate consumer functionality can be dynamic.

   Aplain language summaryis available for this article.

    

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5. Reindeer turning maritime: Ice‐locked tundra triggers changes in dietary niche utilization

   https://doi.org/10.1002/ecs2.2672  

   Hansen, Brage Bremset ; Lorentzen, Jon Runar ; Welker, Jeffrey M. ; Varpe, Øystein ; Aanes, Ronny ; Beumer, Larissa Teresa ; Pedersen, Åshild Ønvik ( April 2019 , Ecosphere)

   The rapid warming of the Arctic may not only alter species’ abundances and distributions, but likely also the trophic interactions within and between ecosystems. On the high‐arctic tundra, extreme warm spells and associated rain‐on‐snow events in winter can encapsulate the vegetation entirely in ground‐ice (i.e., basal ice) and directly or indirectly affect plants, herbivores, and carnivores. However, the implications of such extreme events for trophic interactions and food‐web ecology are generally far from understood. Here, we show that wild Svalbard reindeer populations increasingly isolated by lack of sea‐ice respond to rain‐on‐snow and ice‐locked pastures by increased kelp consumption. Based on annual population surveys in late winters 2006–2015, the proportion of individual reindeer feeding along the shoreline increased the icier the winter. Stable isotope values (δ³⁴S, δ¹³C, δ¹⁵N) of plants, washed‐ashore kelp, and fresh reindeer feces collected along coast‐inland gradients, confirmed ingestion of marine biomass by the reindeer in the shoreline habitat. Thus, even on remote islands and peninsulas increasingly isolated by sea‐ice loss, effects of climate change may be buffered in part by behavioral plasticity and increased use of resource subsidies. This marine dimension of a terrestrial herbivore's realized foraging niche adds to evidence that global warming significantly alters trophic interactions as well as meta‐ecosystem processes.

    

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