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1. Inter- and intrapopulation resource use variation of marine subsidized western fence lizards

   https://doi.org/10.1007/s00442-023-05496-6  

   Ebersole, Alexi ; Bunker, Marie E. ; Weiss, Stacey L. ; Fox-Dobbs, Kena ( January 2024 , Oecologia)

   Marine resource subsidies alter consumer dynamics of recipient populations in coastal systems. The response to these subsidies by generalist consumers is often not uniform, creating inter- and intrapopulation diet variation and niche diversification that may be intensified across heterogeneous landscapes. We sampled western fence lizards,Sceloporus occidentalis, from Puget Sound beaches and coastal and inland forest habitats, in addition to the lizards’ marine and terrestrial prey items to quantify marine and terrestrial resource use with stable isotope analysis and mixing models. Beach lizards had higher average δ¹³C and δ¹⁵N values compared to coastal and inland forest lizards, exhibiting a strong mixing line between marine and terrestrial prey items. Across five beach sites, lizard populations received 20–51% of their diet from marine resources, on average, with individual lizards ranging between 7 and 86% marine diet. The hillslope of the transition zone between marine and terrestrial environments at beach sites was positively associated with marine-based diets, as the steepest sloped beach sites had the highest percent marine diets. Within-beach variation in transition zone slope was positively correlated with the isotopic niche space of beach lizard populations. These results demonstrate that physiography of transitional landscapes can mediate resource flow between environments, and variable habitat topography promotes niche diversification within lizard populations. Marine resource subsidization of Puget Sound beachS. occidentalispopulations may facilitate occupation of the northwesternmost edge of the species range. Shoreline restoration and driftwood beach habitat conservation are important to support the unique ecology of Puget SoundS. occidentalis.

    

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2. Isotopic niche partitioning in two sympatric howler monkey species

   https://doi.org/10.1002/ajpa.24028  

   Flores‐Escobar, Elizabeth ; Sanpera, Carolina ; Jover, Lluís ; Cortés‐Ortiz, Liliana ; Rangel‐Negrín, Ariadna ; Canales‐Espinosa, Domingo ; Dias, Pedro Américo D. ( February 2020 , American Journal of Physical Anthropology)

   Ecological similarity between species can lead to interspecific trophic competition. However, when ecologically similar species coexist, they may differ in foraging strategies and habitat use, which can lead to niche partitioning. As the body tissues of consumers contain a stable isotope signature that reflects the isotopic composition of their diet, stable isotope analysis is a useful tool to study feeding behavior. We measured the isotopic niche width, which is a proxy for trophic niche width, of mantled (Alouatta palliata) and black (A. pigra) howler monkeys. Specifically, studied populations in allopatry and sympatry to assess whether these species showed niche partitioning.

   Between 2008 and 2012, we collected hair samples from 200 subjects (113 black and 87 mantled howler monkeys) and used continuous flow isotope ratio mass spectrometry to estimateδ¹³C andδ¹⁵N. We described the isotopic niche width of each species in allopatry and sympatry with the Bayesian estimation of the standard ellipse areas.

   In allopatry, isotopic niche width and isotopic variation were similar in both species. In sympatry, black howler monkeys had a significantly broader isotopic niche, which was mainly determined by highδ¹⁵N values, and included the majority of mantled howler monkeys' isotopic niche. The isotopic niche of mantled howler monkeys did not differ between sympatry and allopatry.

   The coexistence of these ecologically similar species may be linked to trophic niche adjustments by one species, although the particular features of such adjustments (e.g., dietary, spatial, or sensory partitioning) remain to be addressed.

    

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3. Trophic position of Otodus megalodon and great white sharks through time revealed by zinc isotopes

   https://doi.org/10.1038/s41467-022-30528-9  

   McCormack, Jeremy ; Griffiths, Michael L. ; Kim, Sora L. ; Shimada, Kenshu ; Karnes, Molly ; Maisch, Harry ; Pederzani, Sarah ; Bourgon, Nicolas ; Jaouen, Klervia ; Becker, Martin A. ; et al ( December 2022 , Nature Communications)

   Abstract Diet is a crucial trait of an animal’s lifestyle and ecology. The trophic level of an organism indicates its functional position within an ecosystem and holds significance for its ecology and evolution. Here, we demonstrate the use of zinc isotopes (δ 66 Zn) to geochemically assess the trophic level in diverse extant and extinct sharks, including the Neogene megatooth shark ( Otodus megalodon ) and the great white shark ( Carcharodon carcharias ). We reveal that dietary δ 66 Zn signatures are preserved in fossil shark tooth enameloid over deep geologic time and are robust recorders of each species’ trophic level. We observe significant δ 66 Zn differences among the Otodus and Carcharodon populations implying dietary shifts throughout the Neogene in both genera. Notably, Early Pliocene sympatric C. carcharias and O. megalodon appear to have occupied a similar mean trophic level, a finding that may hold clues to the extinction of the gigantic Neogene megatooth shark. 

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4. Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator, the gentoo penguin ( Pygoscelis papua )

   https://doi.org/10.1002/ece3.1437  

   McMahon, Kelton W. ; Polito, Michael J. ; Abel, Stephanie ; McCarthy, Matthew D. ; Thorrold, Simon R. ( February 2015 , Ecology and Evolution)

   Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns inAAisotope fractionation in birds. We conducted a controlledCSIAfeeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individualAAcarbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ¹³C_C‐Dand Δ¹⁵N_C‐D, respectively). We found that essentialAAδ¹³C values and sourceAAδ¹⁵N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessentialAAΔ¹³C_C‐Dvalues reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDF_Glu‐Phe= 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ¹⁵N_C‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDF_Glu‐Pheequation with the avian‐specificTDF_Glu‐Phevalue from our experiment provided estimates that were more ecologically realistic than estimates using a singleTDF_Glu‐Pheof 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use ofCSIAin nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.

    

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5. Meta‐analysis of primary producer amino acid δ 15 N values and their influence on trophic position estimation

   https://doi.org/10.1111/2041-210X.13678  

   Ramirez, Matthew D. ; Besser, Alexi C. ; Newsome, Seth D. ; McMahon, Kelton W. ( August 2021 , Methods in Ecology and Evolution)

   Compound‐specific stable isotope analysis of individual amino acids (CSIA‐AA) has emerged as a transformative approach to estimate consumer trophic positions (TP_CSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TP_CSIAestimation is an understanding of beta (β) values—the differences between trophic and source AA δ¹⁵N values in the primary producers at the base of a consumers’ food web. Growing evidence suggests higher taxonomic and tissue‐specificβvalue variability than typically appreciated.

   This meta‐analysis fulfils a pressing need to comprehensively evaluate relevant sources ofβvalue variability and its contribution to TP_CSIAuncertainty. We first synthesized all published primary producer AA δ¹⁵N data to investigate ecologically relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning AA δ¹⁵N andβvalue variability. Lastly, we evaluated the sensitivity of TP_CSIAestimates to uncertainty in meanβ_Glx‐Phevalues and Glx‐Phe trophic discrimination factors (TDF_Glx‐Phe).

   We show that variation inβ_Glx‐Phevalues is two times greater than previously considered, with degree of vascularization, not habitat type (terrestrial vs. aquatic), providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰; non‐vascular autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily contributed toβ_Glx‐Phevalue variability, but we found no clear distinction among C₃, C₄and CAM plantβ_Glx‐Phevalues. Notably, we found that vascular plantβ_Glx‐Lysvalues (+2.5 ± 1.6‰) are considerably less variable thanβ_Glx‐Phevalues, making Lys a useful AA tracer of primary production sources in terrestrial systems. Our multi‐trophic level sensitivity analyses demonstrate that TP_CSIAestimates are highly sensitive to changes in bothβ_Glx‐Pheand TDF_Glx‐Phevalues but that the relative influence ofβvalues dissipates at higher trophic levels.

   Our results highlight that primary producerβvalues are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting forβvalue variability to improve TP_CSIAestimation accuracy and precision moving forward. We must ultimately expand libraries of primary producer AA δ¹⁵N values to better understand the mechanistic drivers ofβvalue variation.

    

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