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1. 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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2. Reconstructing hyperdiverse food webs: Gut content metabarcoding as a tool to disentangle trophic interactions on coral reefs

   https://doi.org/10.1111/2041-210X.13206  

   Casey, Jordan M. ; Meyer, Christopher P. ; Morat, Fabien ; Brandl, Simon J. ; Planes, Serge ; Parravicini, Valeriano ; Mahon, ed., Andrew ( June 2019 , Methods in Ecology and Evolution)

   Anthropogenic stressors have strong impacts on ecosystems. To understand their influence, detailed knowledge about trophic relationships among species is critical. However, this requires both exceptional resolution in dietary assessments and sampling breadth within communities, especially for highly diverse, tropical ecosystems.

   We used gut content metabarcoding across a broad range of coral reef fishes (8 families, 22 species) in Mo'orea, French Polynesia, to test whether this technique has the potential to capture the structure of a hyperdiverse marine food web. Moreover, we explored whether taxonomic groups (families) and traditional, broad‐scale trophic assignments explained fish diet across four different metrics of quantifying predator–prey interactions.

   Metabarcoding yielded a large number (4,341) of unique operational taxonomic units (i.e. prey) with high‐resolution taxonomic assignments (i.e. often to the level of genus or species). We demonstrate that across multiple metrics, taxonomic group at the family level is a consistently better, albeit still weak, predictor of empirical trophic relationships than frequently used, broad‐scale functional assignments. Our method also reveals a complex trophic network with fine‐scale partitioning among species, further emphasizing the importance of examining fish diets beyond broad trophic categories.

   We demonstrate the capacity of metabarcoding to reconstruct diverse and complex food webs with exceptional resolution, a significant advancement from traditional food web reconstruction. Furthermore, this method allows us to pinpoint the trophic niche of species with niche‐based modelling, even across hyperdiverse species assemblages such as coral reefs. In conjunction with complementary techniques such as stable isotope analysis, applying metabarcoding to whole communities will provide unparalleled information about energy and nutrient fluxes and inform their susceptibility to disturbances even in the world's most diverse ecosystems.

    

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3. Harvest of female moose at high density: modelling the impacts of harvest on population size and biomass yield

   https://doi.org/10.2981/wlb.00163  

   Doak, Patricia ; Carroll, Cameron J. ; Kielland, Knut ( July 2016 , Wildlife Biology)

   Management of harvested mooseAlces alcespopulations at or above ecological carry capacity risks habitat degradation, nutritional limitation, and increased population vulnerability during severe winters. Selective female harvests have the potential to curb population growth while providing hunting opportunities. Using a female‐only, stage‐structured population model parameterized from an Interior Alaska moose population, we examined numbers of harvested individuals and biomass yield associated with reducing a population from 14 500 to 10 000 individuals over 3, 5 and 8 years. We compared harvest of cow—calf pairs versus unaccompanied females. The higher potential for adult female survival compared with calf survival to impact population growth rate resulted in higher yields from cow—calf harvests. Achieving the population objective required the mean annual harvest of 889, 626 and 477 cow—calf pairs or 1161, 805 and 605 unaccompanied females, for the three harvest durations, respectively. Over a five‐year period, cow—calf harvests yielded approximately 56% more individuals and 17% greater biomass, an estimated difference of 130 metric tonnes. The two harvest scenarios resulted in similar stage distributions and population growth rates following the termination of harvest. While the cow—calf harvests can provide higher yields, they also require substantially higher hunter effort to achieve population objectives. The harvest of unaccompanied females will result in greater population reduction per individual harvested and will therefore be the preferable strategy when hunter effort is limited. In addition, the large harvest numbers necessary to achieve the modelled management goal, suggest that some moose populations may escape the range where they can be easily be controlled through female harvest, especially when harvest is limited by hunter interest or access.

    

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4. Trophic ecology of large herbivores in a reassembling African ecosystem

   https://doi.org/10.1111/1365-2745.13113  

   Pansu, Johan ; Guyton, Jennifer A. ; Potter, Arjun B. ; Atkins, Justine L. ; Daskin, Joshua H. ; Wursten, Bart ; Kartzinel, Tyler R. ; Pringle, Robert M. ; Chapman, ed., Samantha ( December 2018 , Journal of Ecology)

   Megafauna assemblages have declined or disappeared throughout much of the world, and many efforts are underway to restore them. Understanding the trophic ecology of such reassembling systems is necessary for predicting recovery dynamics, guiding management, and testing general theory. Yet, there are few studies of recovering large‐mammal communities, and fewer still that have characterized food‐web structure with high taxonomic resolution.

   In Gorongosa National Park, large herbivores have rebounded from near‐extirpation following the Mozambican Civil War (1977–1992). However, contemporary community structure differs radically from the prewar baseline: medium‐sized ungulates now outnumber larger bodied species, and several apex carnivores remain locally extinct.

   We used DNA metabarcoding to quantify diet composition of Gorongosa’s 14 most abundant large‐mammal populations. We tested five hypotheses: (i) the most abundant populations exhibit greatest individual‐level dietary variability; (ii) these populations also have the greatest total niche width (dietary diversity); (iii) interspecific niche overlap is high, with the diets of less‐abundant species nested within those of more‐abundant species; (iv) partitioning of forage species is stronger in more structurally heterogeneous habitats; and (v) selectivity for plant taxa converges within guilds and digestive types, but diverges across them.

   Abundant (and narrow‐mouthed) populations exhibited higher among‐individual dietary variation, but not necessarily the greatest dietary diversity. Interspecific dietary overlap was high, especially among grazers and in structurally homogenous habitat, whereas niche separation was more pronounced among browsers and in heterogeneous habitat. Patterns of selectivity were similar for ruminants—grazers and browsers alike—but differed between ruminants and non‐ruminants.

   Synthesis. The structure of this recovering food web was consistent with several hypotheses predicated on competition, habitat complexity, and herbivore traits, but it differed from patterns observed in more intact assemblages. We propose that intraspecific competition in the fastest‐recovering populations has promoted individual variation and a more nested food web, wherein rare species use subsets of foods eaten by abundant species, and that this scenario is reinforced by weak predation pressure. Future work should test these conjectures and analyse how the taxonomic dietary niche axis studied here interacts with other mechanisms of diet partitioning to affect community reassembly following wildlife declines.

    

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5. Bioenergy landscapes drive trophic shifts in generalist ants

   https://doi.org/10.1111/1365-2656.13407  

   Helms, IV, Jackson A. ; Roeder, Karl A. ; Ijelu, Selassie E. ; Ratcliff, Ian ; Haddad, Nick M. ; Shik, ed., Jonathan Zvi ( December 2020 , Journal of Animal Ecology)

   Changes in trophic niche—the pathways through which an organism obtains energy and nutrients—are a fundamental way in which organisms respond to environmental conditions. But the capacity for species to alter their trophic niches in response to global change, and the ways they do so when able, remain largely unknown.

   Here we examine food webs in three long‐term and large‐scale experiments to test how resource availability and nutritional requirements interact to determine an organism's trophic niche in the context of one of the largest global trends in land use—the rise in bioenergy production.

   We use carbon and nitrogen stable isotope analyses to characterize arthropod food webs across three biofuel crops representing a gradient in plant resource richness (corn monocultures, fields dominated by native switchgrass and restored prairie), and to quantify changes in the trophic niche of a widespread generalist ant species across habitats. In doing so, we measure the effects of basal resource richness on food chain length, niche breadth and trophic position. We frame our results in the context of two hypotheses that explain variation in trophic niche—the niche variation hypothesis which emphasizes the importance of resource diversity and ecological opportunity, and the optimal diet hypothesis which emphasizes dietary constraints and the availability of optimal resources.

   Increasing plant richness lengthened food chains by 10%–20% compared to monocultures. Niche breadths of generalist ants did not vary with resource richness, suggesting they were limited by optimal diet requirements and constraints rather than by ecological opportunity. The ants instead responded to changes in plant richness by shifting their estimated trophic position. In resource‐poor monocultures, the ants were top predators, sharing a trophic position with predatory spiders. In resource‐rich environments, in contrast, the ants were omnivores, relying on a mix of animal prey and plant‐based resources.

   In addition to highlighting novel ecosystem impacts of alternate bioenergy landscapes, our results suggest that niche breadth and trophic diversification depend more on the presence of optimal resources than on ecological opportunity alone.

    

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