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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]

Understanding how intraspecific variation in the use of prey resources impacts energy metabolism has strong implications for predicting long-term fitness and is critical for predicting population-to-community level responses to environmental change. Here, we examine the energetic consequences of variable prey resource use in a widely distributed marine carnivore, juvenile sand tiger sharks (Carcharias taurus). We used carbon and nitrogen isotope analysis to identify three primary prey resource pools—demersal omnivores, pelagic forage, and benthic detritivores and estimated the proportional assimilation of each resource using Bayesian mixing models. We then quantified how the utilization of these resource pools impacted the concentrations of six plasma lipids and how this varied by ontogeny. Sharks exhibited variable reliance on two of three predominant prey resource pools: demersal omnivores and pelagic forage. Resource use variation was a strong predictor of energetic condition, whereby individuals more reliant upon pelagic forage exhibited higher blood plasma concentrations of very low-density lipoproteins, cholesterol, and triglycerides. These findings underscore how intraspecific variation in resource use may impact the energy metabolism of animals, and more broadly, that natural and anthropogenically driven fluctuations in prey resources could have longer term energetic consequences. [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]

Isle Royale National Park is generally considered a pristine ecosystem, but the island archipelago has a long history of human impacts that have altered the island's mammal communities through extirpations and introductions. The origin of red foxes (Vulpes vulpes) on the islands are of particular interest given their ubiquity and uncertain colonization history. Red foxes were first reported on Isle Royale in 1925, shortly after the foundation of a small fox farm that began on Isle Royale in 1922. We sequenced two mitochondrial haplotypes from red fox scats collected on Isle Royale and compared them to haplotypes from the mainland surrounding Lake Superior, the predominant source of the island's other native mammals. Some Isle Royale foxes matched widespread haplotypes commonly found across Canada, but over half of our samples matched haplotypes previously detected only in Newfoundland. While we cannot conclude a singular origin, we offer a working hypothesis red foxes on Isle Royale are derived from a combination of natural colonization and human introduction. Specifically, we propose native red foxes may be admixed with fur-farmed foxes from an introduction in the early 20th century. [ABSTRACT FROM AUTHOR]

Animal foraging and competition are defined by the partitioning of three primary niche axes: space, time, and resources. Human disturbance is rapidly altering the spatial and temporal niches of animals, but the impact of humans on resource consumption and partitioning--arguably the most important niche axis--is poorly understood. We assessed resource consumption and trophic niche partitioning as a function of human disturbance at the individual, population, and community levels using stable isotope analysis of 684 carnivores from seven communities in North America. We detected significant responses to human disturbance at all three levels of biological organization: individual carnivores consumed more human food subsidies in disturbed landscapes, leading to significant increases in trophic niche width and trophic niche overlap among species ranging from mesocarnivores to apex predators. Trophic niche partitioning is the primary mechanism regulating coexistence in many communities, and our results indicate that humans fundamentally alter resource niches and competitive interactions among terrestrial consumers. Among carnivores, niche overlap can trigger interspecific competition and intraguild predation, while the consumption of human foods significantly increases human-carnivore conflict. Our results suggest that human disturbances, especially in the form of food subsidies, may threaten carnivores by increasing the probability of both interspecific competition and human-carnivore conflict. Ultimately, these findings illustrate a potential decoupling of predator-prey dynamics, with impacts likely cascading to populations, communities, and ecosystems. [ABSTRACT FROM AUTHOR]

Isle Royale (western Lake Superior, USA) is a geographically isolated archipelago with the potential to sustain endemic populations. The island has served as an ecological model for predator–prey interactions between wolves and moose, but little is known about the 17 other mammalian inhabitants. Previous studies of locally abundant red squirrels (Tamiasciurus hudsonicus) described phenotypic differences in pelage colour and skull size, resulting in the classification of an endemic Isle Royale subspecies (T. h. regalis). However, the phylogenetics of this putative subspecies have not been explored. We sequenced mitochondrial DNA to compare haplotypes and genetic variation of seven red squirrels from Isle Royale to 42 other individuals sampled across North America. We detected 25 haplotypes in total, and four haplotypes within Isle Royale. Sequences obtained from red squirrels inhabiting Isle Royale were not unique and were shared with individuals from across the Great Lakes Region and north-eastern North America. Our results indicate that extant T. hudsonicus on Isle Royale are polyphyletic and should not be considered a unique subspecies. These findings suggest that Isle Royale is less insular and more connected than previously assumed and highlights the need to investigate the origins of other taxa in putatively isolated systems, including Isle Royale. [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]

Carnivores exhibit strong interspecific competition and partition niche axes to minimize agonistic interactions. Niche partitioning, though, is contingent upon resource heterogeneity, and recent landscape homogenization may limit the abilities of carnivores to partition niche space. The negative fitness consequences associated with niche overlap may be particularly problematic for repatriating carnivores, and could delay the recovery of rare or endangered species. American martens (Martes americana) and fishers (Pekania pennanti) are the most commonly translocated carnivores in North America and both were reintroduced to a highly modified landscape in Wisconsin, United States. To date, fishers have flourished while martens remain endangered. To assess the role of competition in marten recovery, we used a combination of occupancy modeling, point pattern analyses, and stable isotope analyses to assess 5 coexistence mechanisms: spatial segregation, dietary segregation, temporal avoidance, and differential use of habitat and snow features. Over 7 years, we observed consistently high fisher occupancy and consistently low marten occupancy. Moreover, martens and fishers overlapped in their use of space and time, and neither exhibited habitat preferences. Isotopic analyses revealed complete dietary overlap, with martens falling entirely within the isotopic niche of fishers. Deep, uncompressed snow, however, had a negative effect on fisher activity. We propose that extensive landscape homogenization has resulted in niche compression and that marten recovery has been limited by increased competition with fishers. Restoration programs often overlook competitive interactions, but our results emphasize the importance of interspecific competition for recovering carnivore populations and highlight the challenge of reconstructing carnivore communities in increasingly homogenized landscapes. [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]

Species reintroductions are successful when established populations maintain both demographic stability and genetic diversity. Such a result may be obtained by ensuring both structural habitat connectivity and genetic connectivity among reintroduced and remnant populations. Nevertheless, prezygotic barriers such as assortative mating can prevent the flow of genetic material between populations, even when migration between populations is high. Limited gene flow may be particularly relevant for reintroductions that were sourced either from captive-bred populations or from disparate locations in the wild. American martens (Martes americana) have been reintroduced repeatedly in the Upper Midwestern United States in an effort to establish self-sustaining populations. We quantified levels of genetic diversity within and spatial genetic variance among four marten populations during two time periods separated by 10 years. Spatially informed and naïve discriminant analysis of principal components were used to assign individuals to populations. Results indicate that heterozygosity declined and inbreeding coefficients increased between the two collection periods, while genetic structure among populations also increased. Data are consistent with assortative mating contributing to reapportioning of genetic variation. Population assignment tests show that migration among populations is apparent, but admixture (based on cluster membership probabilities) is low and declined over time. Specifically, martens may be successfully dispersing between populations but a lack of admixture indicates a lack of reproductive contributions to genetic diversity by migrants. Because marten reintroductions in this region are well-documented and well-monitored, lessons can be derived from results to inform future reintroductions. We encourage a careful balance of supplementing genetic diversity via augmentation while avoiding translocation of animals from disparate populations that may result in reproductive isolation of migrants. In combination with the maintenance of a functionally connected landscape, this strategy would maximize the likelihood of a successful reintroduction in terms both of demography and genetics. [ABSTRACT FROM AUTHOR]

Island ecosystems are globally threatened, and efforts to restore historical communities are widespread. Such conservation efforts should be informed by accurate assessments of historical community composition to establish appropriate restoration targets. Isle Royale National Park is one of the most researched island ecosystems in the world, yet little is actually known about the biogeographic history of most Isle Royale taxa. To address this uncertainty and inform restoration targets, we determined the phylogeographic history of American martens (Martes americana), a species rediscovered on Isle Royale 76 years after presumed extirpation. We characterized the genetic composition of martens throughout the Great Lakes region using nuclear and mitochondrial markers, identified the source of Isle Royale martens using genetic structure analyses, and used demographic bottleneck tests to evaluate (eliminate redundancy of test). 3 competing colonization scenarios. Martens exhibited significant structure regionally, including a distinct Isle Royale cluster, but mitochondrial sequences revealed no monophyletic clades or evolutionarily significant units. Rather, martens were historically extirpated and recolonized Isle Royale from neighbouring Ontario, Canada in the late 20th century. These findings illustrate the underappreciated dynamics of island communities, underscore the importance of historical biogeography for establishing restoration baselines, and provide optimism for extirpated and declining Isle Royale vertebrates whose reintroductions have been widely debated. [ABSTRACT FROM AUTHOR]

Conservation biologists are generally united in efforts to curtail the spread of non-native species globally. However, the colonization history of a species is not always certain, and whether a species is considered non-native or native depends on the conservation benchmark. Such ambiguities have led to inconsistent management. Within the Tongass National Forest of Alaska, the status of American marten ( Martes americana) on the largest, most biologically diverse and deforested island, Prince of Wales (POW), is unclear. Ten martens were released to POW in the early 1930s, and it was generally believed to be the founding event, although this has been questioned. The uncertainty surrounding when and how martens colonized POW complicates management, especially because martens were selected as a design species for the Tongass. To explore the history of martens of POW we reviewed other plausible routes of colonization; genetically and isotopically analyzed putative marten fossils deposited in the late Pleistocene and early Holocene to verify marten occupancy of POW; and used contemporary genetic data from martens on POW and the mainland in coalescent simulations to identify the probable source of the present-day marten population on POW. We found evidence for multiple routes of colonization by forest-associated mammals beginning in the Holocene, which were likely used by American martens to naturally colonize POW. Although we cannot rule out human-assisted movement of martens by Alaskan Natives or fur trappers, we suggest that martens be managed for persistence on POW. More generally, our findings illustrate the difficulty of labeling species as non-native or native, even when genetic and paleo-ecological data are available, and support the notion that community resilience or species invasiveness should be prioritized when making management decisions rather than more subjective and less certain conservation benchmarks. [ABSTRACT FROM AUTHOR]

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]