31° South: Dietary niche of an arid‐zone endemic passerine

Understanding species interactions among top marine predators and interactions with their prey can provide important insight into community-level responses to changing prey availability and the role of apex predators as indicators of ecosystem change. On the northeast Newfoundland coast, marine predators rely on capelin (Mallotus villosus), a dominant forage fish, as a food source. Capelin migrate into coastal regions to spawn during July, essentially transforming food supply from low during early summer (i.e. pre-spawning) to high later in the summer (i.e. spawning). During July-August, 2016, we used stable isotopic ratios of nitrogen (δ15N) and carbon (δ13C) to investigate shifts in dietary niche metrics at the predator group-level (trophic position, dietary niche breadth) and community-level (niche overlap, trophic diversity) for multiple marine predators under varying capelin availability. Predator groups included non-breeding shearwaters (great shearwater Ardenna gravis, sooty shearwater A. grisea), humpback whales (Megaptera novaeangliae), and gull chicks (herring gull Larus argentatus, great black-backed gull L. marinus). We also tested the sensitivity of community-level dietary metrics to a variety of published trophic discrimination factors. Tissue samples from shearwaters (blood cellular component), gull chicks (whole blood), and whales (skin), representing average diets over 2-3 weeks, were collected during three periods (early, mid, late) corresponding to increasing capelin availability. Isotopic niche breadth (Standard Ellipse Area, SEAb) narrowed and trophic position shifted toward higher δ15N for all predator groups as capelin availability increased, suggesting a higher reliance on capelin. Trophic diversity (distance to centroid) decreased with increased capelin availability, while pairwise niche overlap between predator groups was highly variable and sensitive to varying trophic discrimination factors. Findings suggest that although capelin is the dominant forage fish during the summer, predators rely on capelin as prey to varying degrees. Combining species- and community-level metrics of dietary niche and trophic diversity may provide a more complete picture of predator responses to prey availability and, thus, may be important monitoring tools to indicate changes in the food supply of marine predators.

The endangered northern quoll (Dasyurus hallucatus) is a predatory marsupial with a wide and disjointed distribution across northern Australia. The disjunct Pilbara population occurs in a uniquely arid area, and faces different threatening processes to populations elsewhere. To better understand the ecology of this small carnivore, we undertook a dietary analysis of 498 scats collected across ∼100,000 km2. We calculated dietary composition and niche breadth and modeled these against biogeophysical factors (latitude, longitude, rainfall, elevation, and distance to coast) for 10 study landscapes. We also conducted pairwise comparisons of diet groups to evaluate regional dietary differences. Quolls were highly omnivorous, consuming at least 23 species of vertebrates (mammals, birds, reptiles, frogs), as well as arthropods, molluscs, fruit, and carrion. Diet varied widely across the region, with up to 3-fold differences in dietary niche breadth between study landscapes. We found few clear environmental drivers of the diet of D. hallucatus. The most frequently consumed food type was insects, but their occurrence in diets decreased as that of rodents and vegetation increased, indicating potential dietary preferences. The broad and variable diet of D. hallucatus indicates opportunism similar to that of other small carnivores. Given this broad dietary niche, conservation managers will need a priori knowledge of local prey abundance if they are to accurately predict the composition of D. hallucatus diets.

Simple SummaryIn this study, we performed DNA metabarcoding to examine prey composition and pest diversity in the diets of four insectivorous species of bats. Then, we evaluated the correlation between bat activity and insect resources and assessed dietary niche similarity and niche breadth among species and factors that influence prey consumption in bats. Additionally, we explored the functional response between these predators and their prey, understanding how bat feeding behavior adapts to the availability of different pest species. We proved that bats provide vital pest consumption services in agricultural ecosystems and their diet included arthropods from 23 orders and 200 families. Moreover, bats responded to the availability of insects. For example, a higher abundance of insects, especially Lepidoptera, and a higher insect diversity led to an increase in the duration of bat activity. In areas with more abundant insects, the number of bat passes also increased. Our results suggested that dietary niche differentiation promotes the coexistence of different bat species and increases the ability of bats to consume insect prey and agricultural pests. Our findings provide greater insights into the role of bats that prey on agricultural pests and highlight the importance of combining bat conservation with integrated pest management.Insectivorous bats are generalist predators and can flexibly respond to fluctuations in the distribution and abundance of insect prey. To better understand the effects of bats on arthropod pests, the types of pests eaten by bats and the response of bats to insect prey need to be determined. In this study, we performed DNA metabarcoding to examine prey composition and pest diversity in the diets of four insectivorous species of bats (Hipposideros armiger, Taphozous melanopogon, Aselliscus stoliczkanus, and Miniopterus fuliginosus). We evaluated the correlation between bat activity and insect resources and assessed dietary niche similarity and niche breadth among species and factors that influence prey consumption in bats. We found that the diets of these bats included arthropods from 23 orders and 200 families, dominated by Lepidoptera, Coleoptera, and Diptera. The proportion of agricultural pests in the diet of each of the four species of bats exceeded 40% and comprised 713 agricultural pests, including those that caused severe economic losses. Bats responded to the availability of insects. For example, a higher abundance of insects, especially Lepidoptera, and a higher insect diversity led to an increase in the duration of bat activity. In areas with more abundant insects, the number of bat passes also increased. The dietary composition, diversity, and niches differed among species and were particularly significant between H. armiger and T. melanopogon; the dietary niche width was the greatest in A. stoliczkanus and the narrowest in H. armiger. The diet of bats was correlated with their morphological and echolocation traits. Larger bats preyed more on insects in the order Coleoptera, whereas the proportion of bats consuming insects in the order Lepidoptera increased as the body size decreased. Bats that emitted echolocation calls with a high peak frequency and duration preyed more on insects in the order Mantodea. Our results suggest that dietary niche differentiation promotes the coexistence of different bat species and increases the ability of bats to consume insect prey and agricultural pests. Our findings provide greater insights into the role of bats that prey on agricultural pests and highlight the importance of combining bat conservation with integrated pest management.

Insular species are predicted to broaden their niches, in response to having fewer competitors. They can thus exploit a greater proportion of the resource spectrum. In turn, broader niches are hypothesized to facilitate (or be a consequence of) increased population densities. We tested whether insular lizards have broader dietary niches than mainland species, how it relates to competitor and predator richness, and the nature of the relationship between population density and dietary niche breadth. We collected population density and dietary niche breadth data for 36 insular and 59 mainland lizard species, and estimated competitor and predator richness at the localities where diet data were collected. We estimated dietary niche shift by comparing island species to their mainland relatives. We controlled for phylogenetic relatedness, body mass and the size of the plots over which densities were estimated. We found that island and mainland species had similar niche breadths. Dietary niche breadth was unrelated to competitor and predator richness, on both islands and the mainland. Population density was unrelated to dietary niche breadth across island and mainland populations. Our results indicate that dietary generalism is not an effective way of increasing population density nor is it result of lower competitive pressure. A lower variety of resources on islands may prevent insular animals from increasing their niche breadths even in the face of few competitors.

Effects of population density of mule deer Odocoileus hemionus on forage selection were investigated by comparing diet characteristics of two subpopulations of deer in southern California, USA, that differed in population density during winter. Quality of diet for deer, as indexed by faecal crude protein, was higher at the low‐density site than at the high‐density site in winter, when deer densities were different. Quality of diet was similar in summer when both areas had comparable densities of deer. Both outcomes are consistent with predictions from density‐dependent selection of diets by deer. Dietary niche breadth, however, differed in a manner opposite to predictions of niche theory based on diet selection under an ideal‐free distribution. During winter, when differences in density between the two study sites were pronounced, niche breadth along the dietary axis in the low‐density area was twice that of the high‐density site. Generalist herbivores feeding primarily on low‐quality browse at high population density in winter would be expected to increase their dietary breadth by feeding on additional species of plants as they depleted their food supply. Mule deer in our study, however, decreased the breadth of their dietary niche as population density increased. We hypothesize that by rapidly eliminating high‐quality forages from an area by heavy grazing, deer at higher population densities narrowed their dietary niche. Theoretical models for changes in niche dimensions, including the ideal‐free distribution, need to consider such empirical outcomes.

Context Climate change is having significant impacts on species worldwide. The endangered eastern quoll (Dasyurus viverrinus) has recently undergone rapid and severe population decline, with no sign of recovery. Spatially and temporally-explicit weather modelling suggests a prolonged period of unfavourable weather conditions during 2001–03 as the proximate cause of decline. However, the mechanisms of this weather-induced decline are not currently understood. Aims The aim of this study was to investigate the hypothesis that changing weather conditions have altered the availability of key prey species, potentially contributing to the species’ decline. Methods We analysed 229 scats collected from 125 individual wild quolls across four sites between July 2011 and May 2012. Variation in dietary composition and niche breadth was compared across sites and seasons. We also compared contemporary dietary composition and niche breadth to historic dietary studies performed before the species’ decline, to identify any key changes in dietary composition over time. Key results Dietary composition and niche breadth were similar across sites but differed between seasons. Dietary niche contracted during winter (July) and early spring (September) when insect larvae formed the bulk of quoll diet, rendering the species vulnerable to weather-related fluctuations in food availability at that time. Large differences were also evident between current and historic dietary composition, with a marked shift from insect larvae to mammals, predominantly due to a reduction in corbie (Oncopera intricata) and southern armyworm (Persectania ewingii) moth larvae. Quoll abundance appears positively related to corbie larva abundance during winter, and both quoll and corbie larva abundance appear negatively related to winter rainfall. Conclusions The lower contribution of insects at sites with low quoll densities suggests that insects represent an important food item for eastern quolls during winter, when dietary niche is narrowest and energy demands are highest. Our findings suggest that weather-induced fluctuations in quoll abundance, including the significant statewide decline during 2001–03, are potentially driven by weather-induced fluctuations in corbie larva abundance. Implications Continued deterioration in climatic suitability with recent and predicted climate change could further threaten eastern quolls through reductions in the availability and stability of reliable food sources at critical life-history stages when dietary options are already limited.

The variation in niche breadth can affect how species respond to environmental and resource changes. However, there is still no clear understanding of how seasonal variability in food resources impacts the variation of individual dietary diversity, thereby affecting the dynamics of a population's dietary niche breadth. Optimal foraging theory (OFT) and the niche variation hypothesis (NVH) predict that when food resources are limited, the population niche breadth will widen or narrow due to increased within-individual dietary diversity and individual specialization or reduced within-individual dietary diversity, respectively. Here, we used DNA metabarcoding to examine the composition and seasonality of diets of the avivorous bat Ia io. Furthermore, we investigated how the dietary niches changed among seasons and how the population niche breadth changed when the availability of insect resources was reduced in autumn. We found that there was differentiation in dietary niches among seasons and a low degree of overlap, and the decrease of insect resource availability and the emergence of ecological opportunities of nocturnal migratory birds might drive dietary niche shifts toward birds in I. io. However, the population's dietary niche breadth did not broaden by increasing the within-individual dietary diversity or individual specialization, but rather became narrower by reducing dietary diversity via predation on bird resources that served as an ecological opportunity when insect resources were scarce in autumn. Our findings were consistent with the predictions of OFT, because birds as prey for bats provided extremely different resources from those of insects in size and nutritional value. Our work highlights the importance of size and quality of prey resources along with other factors (i.e., physiological, behavioral, and life-history traits) in dietary niche variation.

Legacy cities often encompass large numbers of vacant land parcels that can be managed as greenspace to promote biodiversity and provision ecosystem services. Understanding the forces that shape arthropod predator communities within urban greenspaces is key to advancing sustainable landscaping programs targeted at promoting biodiversity. Our aim was to understand how the frequency of mowing and the extent of landscape fragmentation surrounding a vacant lot habitat influenced predator diets and shaped predator communities. We focused on a group of small web‐building spiders, as their dispersal success can be influenced by landscape variables, and their establishment is sensitive to local management intensity. We hypothesized that reduced landscape fragmentation and mowing frequency would sustain a species‐rich and abundant community of these generalist predators by supporting a high dietary niche breadth and reducing exploitative competition, measured as dietary niche overlap. We found significant dietary niche partitioning among the spider species studied. We found support for our hypothesis that managing vacant lots with less frequent mowing results in a greater dietary niche breadth for web‐building spiders. Interestingly, at the landscape scale, dietary niche breadth was positively associated with impervious surface, which did not support our hypothesis. A higher spider dietary breadth and reduced dietary overlap within a sampled spider community were not positively associated with spider activity density or species richness within a habitat patch, as we predicted. Our findings suggest that reducing the mowing frequency of vacant lots could improve their habitat quality for spiders by supporting diverse and abundant prey. However, humans are unlikely to accept infrequently mown vacant lots, as these sites are often viewed as overgrown and messy by residents. Working with residents to green their occupied residential parcels is another option to add vegetation biomass and complexity for biodiversity in legacy cities. For instance, adding well‐maintained gardens, shrubs and trees can improve arthropod habitat quality while also meeting the expectations for urban landscape appearance held by many.

Identifying the ecological traits of threatened species may help to identify species vulnerable to exploitation and habitat loss, predict future declines in abundance and provide the basis for their conservation. We assessed the relationship of dietary and habitat niche breadth on conservation status of skate species (Chondrichthyes, Rajiformes). We hypothesized that skate species with a narrow niche breadth have a higher conservation concern than skate species with wide niche breadth. Conservation status (IUCN), dietary and depth range data on 57 skate species were collected. Generalized linear mixed effect models were fitted with taxonomic level (genera nested within family) included as a random effect term in each model. In each model, body size of skate species was included as a variable because this life history trait is a known predictor of vulnerability. A model selection approach to test the level of support for alternative models was applied. Our results support the hypothesis that a narrow niche breadth is related to high vulnerability. Skate species with high probability of being of conservation concern have a narrow trophic niche breadth and a large body size. We conclude that dietary niche breadth together with body size may be an important trait to identify potentially vulnerable skate species. Conservation biology faces the challenge of identifying what species are vulnerable to anthropogenic stressors and to determine why this is so. The predictive value of our results may allow to know a priori which species should receive prioritary attention.

Despite the shared prediction that the width of a population's dietary niche expands as food becomes limiting, the Niche Variation Hypothesis (NVH) and Optimal Foraging Theory (OFT) offer contrasting views about how individuals alter diet selection when food is limited. Classical OFT predicts that dietary preferences do not change as food becomes limiting, so individuals expand their diets as they compensate for a lack of preferred foods. In contrast, the NVH predicts that among-individual variation in cognition, physiology or morphology create functional trade-offs in foraging efficiency, thereby causing individuals to specialize on different subsets of food as food becomes limiting. To evaluate (a) the predictions of the NVH and OFT and (b) evidence for physiological and cognitive-based functional trade-offs, we used DNA microsatellites and metabarcoding to quantify the diet, microbiome and genetic relatedness (a proxy for social learning) of 218 moose Alces alces across six populations that varied in their degree of food limitation. Consistent with both the NVH and OFT, dietary niche breadth increased with food limitation. Increased diet breadth of individuals-rather than increased diet specialization-was strongly correlated with both food limitation and dietary niche breadth of populations, indicating that moose foraged in accordance with OFT. Diets were not constrained by inheritance of the microbiome or inheritance of diet selection, offering support for the little-tested hypothesis that functional trade-offs in food use (or lack thereof) determine whether populations adhere to the predictions of the NVH or OFT. Our results indicate that both the absence of strong functional trade-offs and the digestive physiology of ruminants provide contexts under which populations should forage in accordance with OFT rather than the NVH. Also, because dietary niche width increased with increased food limitation, OFT and the NVH provide theoretical support for the notion that plant-herbivore interaction networks are plastic rather than static, which has important implications for understanding interspecific niche partitioning. Lastly, because population-level dietary niche breadth and calf recruitment are correlated, and because calf recruitment can be a proxy for food limitation, our work demonstrates how diet data can be employed to understand a populations' proximity to carrying capacity.

Ecological studies investigating niche breadth and overlap often have limited spatial and temporal scale, preventing generalizations across varying environments and communities. For example, it is not clear whether species having restricted diets maintain such patterns relative to closely related species and across their geographic range of co-occurrence. We used stable isotope analysis of hair and fur samples collected from four regions of sympatry for Canada lynx (Lynx canadensis) and bobcat (Lynx rufus) spanning southern Canada and the northern United States, to test the prediction that the more generalist species (bobcat) exhibits a wider dietary niche than the more specialist species (Canada lynx) and that this pattern is consistent across different regions. We further predicted that Canada lynx diet would consistently exhibit greater overlap with that of bobcat compared to overlap of bobcat diet with Canada lynx. We found that Canada lynx had a narrower dietary niche than bobcat, with a high probability of overlap (85–95%) with bobcat, whereas the bobcat dietary niche had up to a 50% probability of overlap with Canada lynx. These patterns of dietary niche breadth and overlap were consistent across geographic regions despite some regional variation in diet breadth and position, for both species. Such consistent patterns could reflect a lack of plasticity in species dietary niches. Given the increasingly recognized importance of understanding dietary niche breadth and overlap across large spatial scales, further research is needed to investigate the mechanisms by which broad-scale patterns are maintained across species and systems.

Group living animals invariably risk resource competition. Cooperation in foraging, however, may benefit individuals in groups by facilitating an increase in dietary niche. To test this, we performed a comparative study of social and solitary spider species. Three independently derived social species of Stegodyphus (Eresidae) occupy semi-arid savannas and overlap with three solitary congeners. We estimated potential prey availability in the environment and prey acquisition by spiders in their capture webs. We calculated dietary niche width (prey size) and breadth (taxonomic range) to compare resource use for these six species, and investigated the relationships between group size and average individual capture web production, prey biomass intake rate and variance in biomass intake. Cooperative foraging increased dietary niche width and breadth by foraging opportunistically, including both larger prey and a wider taxonomic range of prey in the diet. Individual capture web production decreased with increasing group size, indicating energetic benefits of cooperation, and variance in individual intake rate was reduced. However, individual biomass intake also decreased with increasing group size. While cooperative foraging did not completely offset resource competition among group members, it may contribute to sustaining larger groups by reducing costs of web production, increasing the dietary niche and reducing the variance in prey capture.

Identifying the mechanisms that structure niche breadth and overlap between species is important for determining how species interact and assessing their functional role in an ecosystem. Without manipulative experiments, assessing the role of foraging ecology and interspecific competition in structuring diet is challenging. Systems with regular pulses of resources act as a natural experiment to investigate the factors that influence the dietary niches of consumers. We used natural pulses of mast‐fruiting of American beech (Fagus grandifolia) to test whether optimal foraging or competition structure the dietary niche breadth and overlap between two congener rodent species (Peromyscus leucopus and P. maniculatus), both of which are generalist consumers. We reconstructed diets seasonally over a 2‐year period using stable isotope analysis (δ13C, δ15N) of hair and of potential dietary items and measured niche dynamics using standard ellipse area calculated within a Bayesian framework. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beechnuts (a high‐quality food resource) and having a narrower niche breadth during masting seasons compared to nonmasting seasons when dietary niches expanded and more fungi (a low‐quality food source) were consumed. In contrast, changes in dietary niche overlap were consistent with competition theory, with higher diet overlap during masting seasons than during nonmasting seasons. Overall, dietary niche dynamics were closely tied to beech masting, underscoring that food availability influences competition. Diet plasticity and niche partitioning between the two Peromyscus species may reflect differences in foraging strategies, thereby reducing competition when food availability is low. Such dietary shifts may have important implications for changes in ecosystem function, including the dispersal of fungal spores.

Understanding species' dietary ecology and interspecific interactions is crucial for multi-species conservation planning. In Central Asia and the Himalayas, wolves have recolonized snow leopard habitats, raising considerable concern about resource competition between these apex predators. Using micro-histological analysis of prey species remains (e.g., hair) in their fecal samples, we determined the prey composition, dietary niche breadth, and the extent of diet overlap between these two apex predators in Shey Phoksundo National Park, Nepal. We analyzed 152 scat samples collected along 89 survey transects from April to June 2021. Our findings reveal a significant overlap in their diets (Pianka's index = 0.93), with snow leopard and wolf scats containing the remains of 11 and 10 prey species, respectively. However, the interspecific difference in prey selection was apparent, with significant deviations between observed and expected prey use indicating non-random prey selection relative to availability: Snow leopards exhibited a higher occurrence of wild prey items in their diet (55.28%), primarily blue sheep (Pseudois nayaur) (24.83%), whereas wolves relied predominantly on domestic livestock (67.89%), with goats (Capra hircus) accounting for over one-fourth of their diet (29.15%). Yaks (Bos grunniens) comprised a significant portion of the biomass consumed by both predators, with higher for wolves (43.68%) than snow leopards (36.47%). Overall, the narrow dietary niche breadth with high overlap indicates potential resource competition between snow leopards and wolves. However, a comprehensive understanding of resource competition will require further study on other axes of niche partitioning, including habitat and time. Nevertheless, the region's low prey richness means that, with increasing human influence, any reduction in wild prey or increase in livestock could intensify competition between snow leopards and wolves, which could have implications for livestock depredation.

A premise in ungulate foraging theory is that animals become less selective and expand the breadth of their dietary niche as the availability of palatable forage declines with increasing herbivore population density or drought. Increased niche variation resulting from intraspecific competition is thought to create less similar diet composition and decreased diet overlap between individuals within a population at higher densities than between individuals within less dense populations. These ideas were largely developed in relatively mesic environments and their applicability to ungulate foraging in semiarid environments is unclear. We tested the idea that white‐tailed deer (Odocoileus virginianus) contract dietary niche breadth; reduce dietary plant species diversity, richness, and evenness; and become more individualistic in forage choices in response to a fourfold difference in population density (12 deer/km2 versus 50 deer/km2) in semiarid shrubland in Texas, USA. We used the bite count method to determine diet composition of tame female white‐tailed deer seasonally during summer 2009 to spring 2011. We were able to determine impacts of drought on foraging dynamics a posteriori because sampling during each season fortuitously occurred under both drought and non‐drought conditions. Population density did not affect diet richness, diversity, breadth, evenness, overlap, and similarity. Diet richness, diversity, breadth, and evenness tended to be greater in non‐drought conditions. For white‐tailed deer, the idea that dietary niches expand in response to increasing population density is not robust across environments. In semiarid environments, variation in precipitation has a much stronger influence on dietary niche breath and intraspecific diet overlap of deer than population density does.