Forage selection by mule deer: does niche breadth increase with population density?

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.

The parallel niche release hypothesis (PNR) indicates that reduced competition with dominant competitors results in greater density and niche breadth of subordinate competitors and which may support an adaptive advantage.We assessed support for the PNR by evaluating relationships between variation in niche breadth and intra‐ and interspecific density (an index of competition) of wolves (Canis lupus) coyotes (C. latrans), and bobcats (Lynx rufus).We estimated population density (wolf track surveys, coyote howl surveys, and bobcat hair snare surveys) and variability in space use (50% core autocorrelated kernel density home range estimators), temporal activity (hourly and overnight speed), and dietary (isotopic δ13C and δ15N) niche breadth of each species across three areas of varying wolf density in the Upper Peninsula of Michigan, USA, 2010–2019.Densities of wolves and coyotes were inversely related, and increased variability in space use, temporal activity, and dietary niche breadth of coyotes was associated with increased coyote density and decreased wolf density supporting the PNR. Variability in space use and temporal activity of wolves and dietary niche breadth of bobcats also increased with increased intraspecific density supporting the PNR.Through demonstrating decreased competition between wolves and coyotes and increased coyote niche breadth and density, our study provides multidimensional support for the PNR. Knowledge of the relationship between niche breadth and population density can inform our understanding of the role of competition in shaping the realized niche of species.

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.

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.

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.

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.

Ecological theory predicts that species that can utilise a greater diversity of resources and, therefore, have wider niche breadths should also occupy larger geographic areas (the 'niche breadth-range size hypothesis'). Here, we tested this hypothesis for a blood-sucking group of insects of medical significance: the Triatominae (aka 'kissing bugs') (Hemiptera: Reduviidae). Given that niches can be viewed from different perspectives, we tested this hypothesis based on both dietary and climatic niches. We assembled the most complete dataset of triatomine feeding patterns to date by reviewing 143 studies from the literature up to 2021 and tested whether the niche breadth-range size hypothesis held for this group for both dietary and climatic components of the niche. Temperature and precipitation niche breadths were estimated from macro-environmental variables, while diet breadth was calculated based on literature data that used PCR and/or ELISA to identify different types of hosts as blood sources per triatomine species. Our results showed that temperature and precipitation niche breadths, but not dietary breadth, were positively correlated with range sizes, independent of evolutionary history among species. These findings support the predictions from the range size-niche breadth hypothesis concerning climate but not diet, in Triatominae. It also shows that support for the niche breadth-range size hypothesis is dependent upon the niche axis under consideration, which can explain the mixed support for this hypothesis in the ecological literature.

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.

Although ungulates are the main prey of wolves (Canis lupus) throughout their range, substantial dietary diversity may allow wolves to persist even when ungulates are declining or rare. Alexander Archipelago wolves (Canis lupus ligoni) inhabit distinct mainland and island biogeographic units, each with a unique assemblage of available prey. We quantified biogeographic variability in wolf diets across the archipelago using DNA metabarcoding of prey in 860 wolf scats collected during 2010–2018 in 12 study sites. We hypothesized that wolves would increase their dietary diversity and niche breadth as the proportion of ungulate species in their diets decreased, but that this could be mediated by the availability of coastal resources. Application of DNA metabarcoding achieved fine taxonomic resolution of prey remains and identified 55 diet items representing species from 42 genera and 29 families, many previously undetected in coastal wolf diets. Overall, ungulates made up the largest proportion of wolf diets but were also most variable between study sites (occurrence per item index [O/I] = 0.130–0.851). On islands, Sitka black‐tailed deer (Odocoileus hemionus) were the most consumed ungulate species, whereas moose (Alces alces) and mountain goats (Oreamnos americanus) contributed more to mainland wolf diets. Wolves responded to biogeographical variation in availability of their primary prey by altering their foraging patterns. Wolves increased the number and diversity of species consumed and widened their dietary niche as the proportion of ungulates in their diet declined rather than prey switch to one or few individual diet items. Across all study sites combined, beaver (Castor canadensis; O/I = 0.125), marine mammals (O/I = 0.113), and black bears (Ursus americanus; O/I = 0.067) were important alternate prey. In areas where ungulates had become scarce, sea otters (Enhydra lutris) were particularly important, in one case even becoming the primary diet item suggesting that the ongoing expansion of sea otter populations postreintroduction restores an important food source for these cryptic predators. Here, we show extensive variation in the diet of wolves and elucidate regional consumer–resource interactions across an archipelagic landscape.

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.

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.

Effect of population density on diet composition and dietary niche breadth of guanaco (Lama guanicoe, Müller 1776) in northeastern Patagonian steppes

AimDietary niche breadth has long been hypothesized to decrease towards lower latitudes as the numbers of competitors increase. Geographical variation in niche breadth is also hypothesized to be linked to high ambient energy levels, water availability, productivity and climate stability – reflecting an increased number of available prey taxa. Range size and body size are also hypothesized to be strongly and positively associated with niche breadth. We sought to determine which of these factors is associated with geographical variation in niche breadth across broad spatial scales and thus potentially drive the latitudinal diversity gradient.LocationGlobal.MethodsWe collated volumetric dietary data for 308 lizard species. For each species, we gathered data on number of sympatric lizard species (a proxy for the number of competitors), annual temperature and precipitation, net primary productivity, seasonality, range size and body size. We examined the relationship between dietary niche breadth and focal parameters using both ordinary and phylogenetic generalized least squares regressions.ResultsNiche breadth was positively related to annual precipitation, temperature seasonality and range size, and negatively related to body size. Lizard species richness increased towards lower latitudes. Dietary niche breadth, however, was unrelated to parameters reflecting diversity gradients, such as primary productivity, annual temperature, precipitation seasonality and, crucially, the number of potential competitors.Main conclusionsContrary to prevailing ecological theory, competition is unrelated to dietary niche breadth. We found no support for interspecific competition driving the latitudinal diversity gradient. Rather, we found variation in niche breadth to be associated with water availability, climate stability, range size and body size. Our study casts doubt on the common assumption that tropical species are specialists, promoting greater alpha diversity, and on the assumption that the number of sympatric species is reflected in the intensity of interspecific competition.

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.