Trophic niche partitioning between two prey and their incidental predators revealed various threats for an endangered species

Documenting trophic niche partitioning and resource use within a community is critical to evaluate underlying mechanisms of coexistence, competition, or predation. Detailed knowledge about foraging is essential as it may influence the vital rates, which, in turn, can affect trophic relationships between species, and population dynamics. The aims of this study were to evaluate resource and trophic niche partitioning in summer/autumn between the endangered Atlantic‐Gaspésie caribou (Rangifer tarandus caribou) population, moose (Alces americanus) and their incidental predators, the black bear (Ursus americanus) and coyote (Canis latrans), and to quantify the extent to which these predators consumed caribou. Bayesian isotopic analysis showed a small overlap in trophic niche for the two sympatric ungulates suggesting a low potential for resource competition. Our results also revealed that caribou occupied a larger isotopic niche area than moose, suggesting a greater diversity of resources used by caribou. Not surprisingly, coyotes consumed mainly deer (Odocoileus virginianus), moose, snowshoe hare (Lepus americanus), and occasionally caribou, while bears consumed mainly vegetation and, to a lesser extent, moose and caribou. As coyotes and bears also feed on plant species, we documented trophic niche overlap between caribou and their predators, as searching for similar resources can force them to use the same habitats and thus increase the encounter rate and, ultimately, mortality risk for caribou. Although the decline in the Gaspésie caribou population is mostly driven by habitat‐mediated predation, we found evidence that the low level of resource competition with moose, added to the shared resources with incidental predators, mainly bears, may contribute to jeopardize the recovery of this endangered caribou population. Highlighting the trophic interaction between species is needed to establish efficient conservation and management strategies to insure the persistence of endangered populations. The comparison of trophic niches of species sharing the same habitat or resources is fundamental to evaluate the mechanisms of coexistence or competition and eventually predict the consequences of ecosystem changes in the community.     

Sexual niche partitioning in two species of New Guinean Pachycephala whistlers

Sex differences in foraging behavior have been widely reported in the ornithological literature, but few examples are available from tropical avifaunas. Differences between males and females in foraging behavior have been hypothesized to be a byproduct of sexual size dimorphism or a result of niche partitioning to reduce intersexual competition for food or different reproductive roles. From 2010 to 2013, I used foraging data and mist‐net capture rates from multiple study sites to examine possible sex differences in the foraging behavior of two New Guinean Pachycephala whistlers. I found that male Regent (Pachycephala schlegelii) and Sclater's (Pachycephala soror) whistlers consistently foraged in higher strata than females. It is unlikely that these differences are due to sexual dimorphism because these species exhibit little sexual dimorphism. Sex differences in foraging behavior were consistent across years and study sites and did not appear linked to breeding behavior, supporting the food‐competition hypothesis, but not the reproductive‐roles hypothesis. Male territorial defense often occurs in relatively high strata in Pachycephala whistlers, possibly influencing male foraging strata. However, male territorial behavior cannot explain why females predominately forage in lower strata. Instead, intersexual competition for food resources is likely the primary driver of differences in the foraging behavior of male and female Regent and Sclater's whistlers.     

Seasonal variation in prey preference,diet partitioning and niche breadth in a rich large carnivore guild

Large carnivore community structure is affected by direct and indirect interactions between intra-guild members. Co-existence between different species within a carnivore guild may occur through diet, habitat or temporal partitioning. Since carnivore species are highly dependent on availability and accessibility of prey, diet partitioning is potentially one of the most important mechanisms in allowing carnivores to co-exist. Intra-guild interactions may vary over time as carnivore prey preference and diet overlap can change due to seasonal changes in resource availability. We conducted scat analysis to compare the seasonal changes in prey preference, diet partitioning and niche breadth of four large carnivore species, namely leopard Panthera pardus, spotted hyena Crocuta crocuta, brown hyena Parahyaena brunnea and wild dog Lycaon pictus in central Tuli, Botswana. Large carnivores in central Tuli display a high dietary overlap, with spotted hyena and brown hyena displaying almost complete dietary overlap and the other carnivore species displaying slightly lower but still significant dietary overlap. Dietary niche breadth for both hyena species was high possibly due to their flexible foraging strategies, including scavenging, while leopard and wild dog showed a relatively low niche breadth, suggesting a more specialised diet. High dietary overlap in central Tuli is possibly explained by the high abundance of prey species in the area thereby reducing competition pressure between carnivore species. Our research highlights the need to assess the influence of diet partitioning in structuring large carnivore communities across multiple study sites, by demonstrating that in prey rich environments, the need for diet partitioning by carnivores to avoid competition may be limited.     

The roles of morphological traits,resource variation and resource partitioning associated with the dietary niche expansion in the fish‐eating bat Myotis pilosus

Niche expansion and shifts are involved in the response and adaptation to environmental changes. However, it is unclear how niche breadth evolves and changes toward higher‐quality resources. Myotis pilosus is both an insectivore and a piscivore. We examined the dietary composition and seasonality in M. pilosus and the closely related Myotis fimbriatus using next‐generation DNA sequencing. We tested whether resource variation or resource partitioning help explain the dietary expansion from insects to fish in M. pilosus. While diet composition and diversity varied significantly between summer and autumn, the proportion of fish‐eating individuals did not significantly change between seasons in M. pilosus. Dietary overlap between M. pilosus and M. fimbriatus during the same seasons was much higher than within individual species across seasons. We recorded a larger body size, hind foot length, and body mass in M. pilosus than in M. fimbriatus and other insectivorous trawling bats from China. Similar morphological differences were found between worldwide fishing bats and nonfishing trawling bats. Our results suggest that variation in insect availability or interspecific competition may not play important roles in the dietary expansion from insects to fish in M. pilosus. Myotis pilosus has morphological advantages that may help it use fish as a diet component. The morphological advantage promoting dietary niche evolution toward higher quality resources may be more important than variation in the original resource and the effects of interspecific competition.     

Stream microhabitat selectivity,resource partitioning,and niche shifts in grazing caddisfly larvae

Microhabitat selectivity, resource partitioning, and niche shifts in five species of grazing caddisfly larvae (Glossosoma califica, G. penitum, Dicosmoecus gilvipes, Neophylax rickeri, and N. splendens) were quantified by underwater measurement of microhabitat availability and utilization in three northern California streams. The microhabitat parameters water depth and velocity and rock size, roughness, and slope were measured. Comparisons of habitat available to habitat used revealed significant selection for at least two microhabitat parameters by each population, with depth and velocity being the most important. Comparisons of habitat used by different species showed significant partitioning of at least two microhabitat parameters at each site, with depth being partitioned at all sites. Non-parametric discriminant analysis revealed significant microhabitat partitioning on a multivariate level at two sites. Comparisons of habitat used at different sites quantified a major niche shift by D. gilvipes in its preference for riffles versus pools. Size-selective predation by dippers (Cinclus mexicanus) and steelhead (Salmo gairdneri gairdneri) is proposed as a hypothesis to explain the observed resource partitioning and niche shift.     

Nest site availability and niche differentiation between two cavity‐nesting birds in time and space

Niche differentiation is a key concept in the field of ecology and refers to the process by which competing species within an ecological community partition utilization of environmental resources to achieve coexistence. The existence of niche differentiation is uniquely difficult to prove on account of the fact that historical interaction among species, which plays a key role in elucidating the current state of coexistence among species, is not well known. We created continuous niche gradients in nest‐site resources between two sympatric secondary cavity‐nesting birds, the green‐backed tit (Parus monticolus) and the russet sparrow (Passer cinnamomeus), and investigated whether nesting site is a factor contributing to limiting breeding overlap by regular inspection and 388,160 min of film recording. Our results indicate that although we manipulated nest site availability to be uniformly high along the habitat gradient, the two bird species have little overlap in nest sites and rarely compete for them. Furthermore, the green‐backed tit possessed a wide range of fundamental niche that covered that of the russet sparrow, while their reproductive time was largely segregated. The sparrow was more aggressive and outcompeted the tit in their overlapped range. These results suggest that even though nesting sites are crucial to the reproduction of cavity‐nesting birds, some other factor plays a more important role in limiting niche overlap between sparrows and tits in space and time. Given that these two cavity‐nesting birds continued to use different habitats and breed in segregated time after our manipulation, their relationship is better explained by the ghost of competition past theory.     

Optimal foraging constrains macroecological patterns: body size and dietary niche breadth in lizards

Aim  To explore and identify probable mechanisms contributing to the relationships among body size, dietary niche breadth and mean, minimum, maximum and range of prey size in predaceous lizards.
Location  Our data set includes species from tropical rainforests, semi-arid regions of Brazil, and from deserts of the south-western United States, Australia and the Kalahari of Africa.
Methods  We calculated phylogenetic and non-phylogenetic regressions among predator body size, dietary breath and various prey size measures.
Results  We found a negative association between body size and dietary niche breadth in 159 lizard species sampled across most evolutionary lineages of squamate reptiles and across major continents and habitats. We also show that mean, minimum, maximum and range of prey size were positively associated with body size.
Main conclusions  Our results suggest not only that larger lizards tend to eat larger prey, but in doing so offset their use of smaller prey. Reduction of dietary niche breadth with increased body size in these lizards suggests that large predators target large and more profitable prey. Consequently, the negative association between body size and niche breadth in predators most likely results from optimal foraging. Though this result may appear paradoxical and runs counter to previous studies, resources for predators may be predictably more limited than resources for herbivores, thus driving selection for more profitable prey.     

Isotopic niche is not equal to trophic niche

The work of Sheppard et al. (Ecol. Lett., 21, 2018, 1395) relies on the strong assumption that isotopic niche is equal to trophic niche. Here I raise three main concerns showing that classic hypotheses built upon trophic niche cannot be directly interpreted in isotopic space. Future studies should always keep isotopic and trophic niches distinct.     

Small differences in root distributions allow resource niche partitioning

1. Deep roots have long been thought to allow trees to coexist with shallow‐rooted grasses. However, data demonstrating how root distributions affect water uptake and niche partitioning are uncommon.
2. We describe tree and grass root distributions using a depth‐specific tracer experiment six times over two years in a subtropical savanna, Kruger National Park, South Africa. These point‐in‐time measurements were then used in a soil water flow model to simulate continuous water uptake by depth and plant growth form (trees and grasses) across two growing seasons. This allowed estimates of the total amount of water a root distribution could absorb as well as the amount of water a root distribution could absorb in excess of the other rooting distribution (i.e., unique hydrological niche).
3. Most active tree and grass roots were in shallow soils: The mean depth of water uptake was 22 cm for trees and 17 cm for grasses. Slightly deeper rooting distributions provided trees with 5% more soil water than the grasses in a drier season, but 13% less water in a wetter season. Small differences also provided each rooting distribution (tree or grass) with unique hydrological niches of 4 to 13 mm water.
4. The effect of rooting distributions has long been inferred. By quantifying the depth and timing of water uptake, we demonstrated how even small differences in rooting distributions can provide plants with resource niches that can contribute to species coexistence. Differences in total water uptake and unique hydrological niche sizes were small in this system, but they indicated that tradeoffs in rooting strategies can be expected to contribute to tree and grass coexistence because 1) competitive advantages change over time and 2) plant growth forms always have access to a soil resource pool that is not available to the other plant growth form.

     

Age‐ and sex‐related dietary specialization facilitate seasonal resource partitioning in a migratory shorebird

1. Dietary specialization is common in animals and has important implications for individual fitness, inter‐ and intraspecific competition, and the adaptive potential of a species. Diet composition can be influenced by age‐ and sex‐related factors including an individual''s morphology, social status, and acquired skills; however, specialization may only be necessary when competition is intensified by high population densities or increased energetic demands.
2. To better understand the role of age‐ and sex‐related dietary specialization in facilitating seasonal resource partitioning, we inferred the contribution of biofilm, microphytobenthos, and benthic invertebrates to the diets of western sandpipers (Calidris mauri) from different demographic groups during mid‐winter (January/February) and at the onset of the breeding migration (April) using stable isotope mixing models. Western sandpipers are sexually dimorphic with females having significantly greater body mass and bill length than males.
3. Diet composition differed between seasons and among demographic groups. In winter, prey consumption was similar among demographic groups, but, in spring, diet composition differed with bill length and body mass explaining 31% of the total variation in diet composition. Epifaunal invertebrates made up a greater proportion of the diet in males which had lesser mass and shorter bills than females. Consumption of Polychaeta increased with increasing bill length and was greatest in adult females. In contrast, consumption of microphytobenthos, thought to be an important food source for migrating sandpipers, increased with decreasing bill length and was greatest in juvenile males.
4. Our results provide the first evidence that age‐ and sex‐related dietary specialization in western sandpipers facilitate seasonal resource partitioning that could reduce competition during spring at the onset of the breeding migration.
5. Our study underscores the importance of examining resource partitioning throughout the annual cycle to inform fitness and demographic models and facilitate conservation efforts.

     

Fitness consequences of plants growing with siblings: reconciling kin selection, niche partitioning and competitive ability

Plant studies that have investigated the fitness consequences of growing with siblings have found conflicting evidence that can support different theoretical frameworks. Depending on whether siblings or strangers have higher fitness in competition, kin selection, niche partitioning and competitive ability have been invoked. Here, we bring together these processes in a conceptual synthesis and argue that they can be co-occurring. We propose that these processes can be reconciled and argue for a trait-based approach of measuring natural selection instead of the fitness-based approach to the study of sibling competition. This review will improve the understanding of how plants interact socially under competitive situations, and provide a framework for future studies.     

Contrasting patterns of niche partitioning between two anaerobic terminal oxidizers of organic matter

Understanding the ecological principles underlying the structure and function of microbial communities remains an important goal for microbial ecology. We examined two biogeochemically important taxa, the sulfate-reducing bacterial genus, Desulfobulbus, and the methanogenic archaeal genus, Methanosaeta, to compare and contrast niche partitioning by these two taxa that are ecologically linked as anaerobic terminal oxidizers of organic material. An observational approach utilizing functional gene pyrosequencing was combined with a community-based reciprocal incubation experiment and characterization of a novel Desulfobulbus isolate. To analyze the pyrosequencing data, we constructed a data analysis pipeline, which we validated with several control data sets. For both taxa, particular genotypes were clearly associated with certain portions of an estuarine gradient, consistent with habitat or niche partitioning. Methanosaeta genotypes were generally divided between those found almost exclusively in the marine habitat (∼30% of operational taxonomic units (OTUs)), and those which were ubiquitously distributed across all or most of the estuary (∼70% of OTUs). In contrast to this relatively monotonic distribution, for Desulfobulbus, there were many more genotypes, and their distributions represented a wide range of inferred niche widths from specialist genotypes found only at a single site, to ubiquitous or generalist genotypes found in all 10 sites examined along the full estuarine gradient. Incubation experiments clearly showed that, for both taxa, communities from opposite ends of the estuary did not come to resemble one another, regardless of the chemical environment. Growth of a Desulfobulbus isolated into pure culture indicated that the potential niche of this organism is significantly larger than the realized niche. We concluded that niche partitioning can be an important force structuring microbial populations, with biotic and abiotic components having very different effects depending on the physiology and ecology of each taxon.     

Non-linear relationship between body size of terrestrial carnivores and their trophic niche breadth and overlap

Studying food partitioning of mammalian predators is important for understanding trophic structures and interactions between coexisting carnivore species. This is particularly pertinent in the light of expanding ranges of populations of generalist species whose habitat and diet overlap with more specialized species. Here, we tested the resource partitioning hypothesis in terrestrial carnivores, predicting that trophic niche breadth and overlap relate positively to body mass. We used dietary data from 18 terrestrial carnivore taxa in four families (Canidae, Mustelidae, Felidae and Ursidae; body mass 0.1–173.6 kg) in three regions in Central and Eastern Europe, i.e. deciduous forest and forest-steppe region (DFR), temperate deciduous and mixed forest region (MFR) and transitory mixed forest regions (TFR). We ranked carnivores along an axis of trophic niche (breadth and overlap), and analysed the relationship between trophic niche and body mass (or pair-wise difference in body mass). A hierarchical cluster analysis of diet composition divided carnivores into four ecological groups: wild ungulate predators; small-mammal predators; amphibians and small mammal predators and omnivores. The relationship between body mass of predators and both trophic niche breadth and trophic niche overlap were hump-shaped. The trophic niche breadth to body mass ratio was significantly lower in DFR than in TFR and trophic niche overlap was significantly higher in DFR than in MFR and TFR. The predominant food resource is small mammals whose abundance is related to local agricultural and forestry management practices. Modifications of management techniques can affect population dynamics and community composition of carnivore species, especially in the case of small-mammal predators.     

Interpopulation resource partitioning of Lesser Frigatebirds and the influence of environmental context

Conspecific individuals inhabiting nearby breeding colonies are expected to compete strongly for food resources owing to the constraints imposed by shared morphology, physiology, and behavior on foraging strategy. Consequently, colony‐specific foraging patterns that effectively partition the available resources may be displayed. This study aimed to determine whether intraspecific resource partitioning occurs in two nearby colonies of Lesser Frigatebirds (Fregata ariel). A combination of stable isotope analysis and GPS tracking was used to assess dietary and spatial partitioning of foraging resources during the 2013 and 2014 breeding seasons. These results were compared to vessel‐derived estimates of prey availability, local primary productivity, and estimates of reproductive output to suggest potential drivers and implications of any observed partitioning. Isotopic data indicated a more neritic source of provisioned resources for near‐fledged chicks at an inshore colony, whereas their offshore counterparts were provisioned with resources with a more pelagic signal. Deep pelagic waters (>200 m) had higher availability of a preferred prey type despite a trend for lower primary productivity. Differences in foraging ecology between the two populations may have contributed to markedly different reproductive outputs. These findings suggest environmental context influences dietary and spatial aspects of foraging ecology. Furthermore, the effect of colony‐specific foraging patterns on population demography warrants further research.     

Variable effects of wolves on niche breadth and density of intraguild competitors

1. 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.
2. 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).
3. 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.
4. 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.
5. 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.

     

Predation risk and resource abundance mediate foraging behaviour and intraspecific resource partitioning among consumers in dominance hierarchies

Dominance hierarchies and the resulting unequal resource partitioning among individuals are key mechanisms of population regulation. The strength of dominance hierarchies can be influenced by size‐dependent tradeoffs between foraging and predator avoidance whereby competitively inferior subdominants can access a larger proportion of limiting resources by accepting higher predation risk. Foraging‐predation risk tradeoffs also depend on resource abundance. Yet, few studies have manipulated predation risk and resource abundance simultaneously; consequently, their joint effect on resource partitioning within dominance hierarchies are not well understood. We addressed this gap by measuring behavioural responses of masu salmon Oncorhynchus masou ishikawae to experimental manipulations of predation risk and resource abundance in a natural temperate forest stream. Responses to predation risk depended on body size and social status such that larger fish (often social dominants) exhibited more risk‐averse behaviour (e.g. lower foraging and appearance rates) than smaller subdominants after exposure to a simulated predator. The magnitude of this effect was lower when resources were elevated, indicating that dominant fish accepted a higher predation risk to forage on abundant resources. However, the influence of resource abundance did not extend to the population level, where predation risk altered the distribution of foraging attempts (a proxy for energy intake) from being skewed towards large individuals to being skewed towards small individuals after predator exposure. Our results imply that size‐dependent foraging–predation risk tradeoffs can weaken the strength of dominance hierarchies by allowing competitively inferior subdominants to access resources that would otherwise be monopolized.