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Herbivory is a dominant feeding strategy among animals, yet herbivores are often protein limited. The gut microbiome is hypothesized to help maintain host protein balance by provisioning essential macromolecules, but this has never been tested in wild consumers. Using amino acid carbon (δ13C) and nitrogen (δ15N) isotope analysis, we estimated the proportional contributions of essential amino acids (AAESS) synthesized by gut microbes to five co‐occurring desert rodents representing herbivorous, omnivorous and insectivorous functional groups. We found that herbivorous rodents occupying lower trophic positions (Dipodomys spp.) routed a substantial proportion (~40%–50%) of their AAESS from gut microbes, while higher trophic level omnivores (Peromyscus spp.) and insectivores (Onychomys arenicola) obtained most of their AAESS (~58%) from plant‐based energy channels but still received ~20% of their AAESS from gut microbes. These findings empirically demonstrate that gut microbes play a key functional role in host protein metabolism in wild animals. [ABSTRACT FROM AUTHOR]

Food web ecology has revolutionized our understanding of ecological processes, but the drivers of food web properties like trophic position (TP) and food chain length are notoriously enigmatic. In terrestrial ecosystems, above‐ and belowground systems were historically compartmentalized into "green" and "brown" food webs, but the coupling of these systems by animal consumers is increasingly recognized, with potential consequences for trophic structure. We used stable isotope analysis (δ13C, δ15N) of individual amino acids to trace the flow of essential biomolecules and jointly measure multichannel feeding, food web coupling, and TP in a guild of small mammals. We then tested the hypothesis that brown energy fluxes to aboveground consumers increase terrestrial food chain length via cryptic trophic transfers during microbial decomposition. We found that the average small mammal consumer acquired nearly 70% of their essential amino acids (69.0% ± 7.6%) from brown food webs, leading to significant increases in TP across species and functional groups. Fungi were the primary conduit of brown energy to aboveground consumers, providing nearly half the amino acid budget for small mammals on average (44.3% ± 12.0%). These findings illustrate the tightly coupled nature of green and brown food webs and show that microbially mediated energy flow ultimately regulates food web structure in aboveground consumers. Consequently, we propose that the integration of green and brown energy channels is a cryptic driver of food chain length in terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Animals often consume resources from multiple energy channels, thereby connecting food webs and driving trophic structure. Such 'multichannel feeding' can dictate ecosystem function and stability, but tools to quantify this process are lacking. Stable isotope 'fingerprints' are unique patterns in essential amino acid (EAA) δ13C values that vary consistently between energy channels like primary production and detritus, and they have emerged as a tool to trace energy flow in wild systems. Because animals cannot synthesize EAAs de novo and must acquire them from dietary proteins, ecologists often assume δ13C fingerprints travel through food webs unaltered. Numerous studies have used this approach to quantify energy flow and multichannel feeding in animals, but δ13C fingerprinting has never been experimentally tested in a vertebrate consumer.We tested the efficacy of δ13C fingerprinting using captive deer mice Peromyscus maniculatus raised on diets containing bacterial, fungal and plant protein, as well as a combination of all three sources. We measured the transfer of δ13C fingerprints from diet to consumer liver, muscle and bone collagen, and we used linear discriminant analysis (LDA) and isotopic mixing models to estimate dietary proportions compared to known contributions. Lastly, we tested the use of published δ13C source fingerprints previously used to estimate energy flow and multichannel feeding by consumers.We found that EAA δ13C values exhibit significant isotopic (i.e. trophic) fractionation between consumer tissues and diets. Nevertheless, LDA revealed that δ13C fingerprints are consistently routed and assimilated into consumer tissues, regardless of isotopic incorporation rate. Isotopic mixing models accurately estimated the proportional diets of consumers, but all models overestimated plant‐based protein contributions, likely due to the digestive efficiencies of protein sources. Lastly, we found that δ13C source fingerprints from published literature can lead to erroneous diet reconstruction.We show that δ13C fingerprints accurately measure energy flow to vertebrate consumers across tissues with different isotopic incorporation rates, thereby enabling the estimation of multichannel feeding at various temporal scales. Our findings illustrate the power of δ13C fingerprinting for quantifying food web dynamics, but also reveal that careful selection of dietary source data is critical to the accuracy of this emerging technique. [ABSTRACT FROM AUTHOR]

Competition is a major determinant of where species occur and how species interact. Among carnivorans, interspecific competition is particularly apparent, as many of these species have evolved to be efficient killers. Theoretically, phylogenetically related carnivorans that occupy seasonal habitats, share common resources, and differ in body size by a factor of 2.5–10× should exhibit the most interference competition.Fishers Pekania pennanti and martens Martes americana and Martes caurina are members of the subfamily Guloninae (Mustelidae, Carnivora) that occupy forests throughout northern North America. These taxa occur sympatrically throughout much of their range, utilise similar habitats, and consume similar prey; fishers and martens also differ in body size by a factor 2–5×. Consequently, these two taxa appear to be locked in particularly strong interspecific competition and should attempt to limit competitive overlap.We review the current knowledge of this dyadic interaction in the framework of ecological niches and niche partitioning. In particular, we explore the three critical niche axes of diet, space, and time.We found that, in contrast to the traditional view of them being highly specialised, both martens and fishers are dietary generalists; however, they also appear to be specialists in complexity, at least in space and habitats. Collectively, martens and fishers exhibit high degrees of diet and habitat niche overlap across their ranges, and this overlap is likely to have the greatest fitness consequences for the smaller and subordinate martens. Nevertheless, fine‐scale habitat and prey partitioning, and especially partitioning along snow clines, seem to be the mechanisms by which these two taxa can coexist.We predict that rapid ecological change – especially from increasingly homogenised forests and prey communities, as well as from declining snow cover and snowpack due to climate warming – is likely to destabilise marten–fisher coexistence. As the climate continues to change, fishers and martens are likely to experience distributional and numerical shifts and increased isolation at their southern range boundaries, and vulnerable populations – especially of martens – will be driven to local extirpation. [ABSTRACT FROM AUTHOR]

1. Intraspecific variation, including individual diet variation, can structure populations and communities, but the causes and consequences of individual foraging strategies are often unclear. 2. Interactions between competition and resources are thought to dictate foraging strategies (e.g. specialization vs. generalization), but classical paradigms such as optimal foraging and niche theory offer contrasting predictions for individual consumers. Furthermore, both paradigms assume that individual foraging strategies maximize fitness, yet this prediction is rarely tested. 3. We used repeated stable isotope measurements (d13C, d15N; N = 3,509) and 6 years of capture-mark-recapture data to quantify the relationship between environmental variation, individual foraging and consumer fitness among four species of desert rodents. We tested the relative effects of intraspecific competition, interspecific competition, resource abundance and resource diversity on the foraging strategies of 349 individual animals, and then quantified apparent survival as function of individual foraging strategies. 4. Consistent with niche theory, individuals contracted their trophic niches and increased foraging specialization in response to both intraspecific and interspecific competition, but this effect was offset by resource availability and individuals generalized when plant biomass was high. Nevertheless, individual specialists obtained no apparent fitness benefit from trophic niche contractions as the most specialized individuals exhibited a 10% reduction in monthly survival compared to the most generalized individuals. Ultimately, this resulted in annual survival probabilities nearly 4× higher for generalists compared to specialists. 5. These results indicate that competition is the proximate driver of individual foraging strategies, and that diet-mediated fitness variation regulates population and community dynamics in stochastic resource environments. Furthermore, our findings show dietary generalism is a fitness maximizing strategy, suggesting that plastic foraging strategies may play a key role in species' ability to cope with environmental change. [ABSTRACT FROM AUTHOR]

Biotic homogenization is a ubiquitous consequence of human disturbance. Through a mix of local extinctions and invasions, diverse communities of specialists are often replaced by or inundated with generalist species, resulting in uncertain consequences for ecological functions. While concern about biotic homogenization is growing, intraspecific variation and individual diet specialization (IS) have also emerged as key drivers of ecological functions. The niche variation hypothesis predicts that when a population is released from interspecific competition (ie "ecological release"), intraspecific competition will promote resource niche expansion and IS. It then follows that if biotic homogenization reduces taxonomic diversity and interspecific competition, intraspecific competition within the remaining taxa should expand population resource niches and increase the prevalence of IS. Here, we integrate the rapidly growing fields of biotic homogenization and intraspecific variation and assess the potential for generalist species to compensate for multiple forms of biotic homogenization via adaptive foraging, resource niche expansion, and IS. [ABSTRACT FROM AUTHOR]

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 martens Martes americana and Pacific martens Martes 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. A plain language summary is available for this article. Plain Language Summary [ABSTRACT FROM AUTHOR]

Augmentations are used frequently to restore populations of conservation concern. Carnivores, in particular, are often targeted for augmentations, yet quantitative assessments of these efforts are rare. In Wisconsin, USA, American martens were reintroduced and subsequently augmented, yet remain state endangered. To evaluate this restoration effort, we sampled martens before and after augmentation, used genetic mark-recapture and parentage analyses to quantify stage-specific vital rates for each step of the augmentation, and assessed demographic viability with and without augmentation. Surprisingly, augmentation provided minimal genetic and demographic contributions, and persistence was instead driven by intrinsic population attributes such as recruitment. Our findings question augmentation as a primary restoration strategy for carnivores, and we urge conservation practitioners to focus on identifying and enhancing limiting population processes such as immigration and juvenile survival prior to using costly and controversial measures like augmentation. [ABSTRACT FROM AUTHOR]

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ABSTRACT The American marten ( Martes americana), the only state-listed endangered mammal in Wisconsin, has undergone serial reintroductions within the state. Recovery has been slower than anticipated and the limiting factors remain unidentified. The lack of basic knowledge on marten foraging in the Great Lakes Region led us to quantify the diet of martens inhabiting the Chequamegon-Nicolet National Forest (CNNF) in northern Wisconsin using a dual approach of scat and stable isotope analyses. We collected marten scat at winter rest sites within the CNNF from 2000-2011. We identified prey items based on morphological characteristics of indigestible prey remains and calculated percent occurrence for each prey item. We sampled marten hair from museum specimens, and opportunistically from carcasses within the CNNF for isotopic analysis. We collected hair and feather samples from potential prey species in the CNNF in 2010-2013. Our concurrent analyses revealed that shrews and deer were most important to the diet of martens in Wisconsin. These findings contrast with studies conducted elsewhere that report voles as the principle diet items, and shrews as one of the least used prey items. Consequently, such a strong reliance on secondary, less profitable or high-risk prey could be contributing to the delayed recovery of martens in Wisconsin. © 2014 The Wildlife Society. [ABSTRACT FROM AUTHOR]