Niche Axes Research Articles

Complementarity in spatial subsidies of carbon associated with resource partitioning along multiple niche axes

The co-occurrence of several species, all of which share similar resource requirements, remains a paradox in ecology. Here, I evaluated resource use along multiple environmental axes to understand the potential for niche partitioning and complementarity in a guild of suspension-feeding rocky shore invertebrates. I focused on the mussels Perna canaliculus, Mytilus galloprovincialis, Aulacomya maoriana, and Xenostrobus pulex, all of which coexist along the coastline of the South Island of New Zealand. I quantified the mussels' distributions, both vertically on the shore and within the three-dimensional mussel bed matrix, and used carbon (δ13C) and nitrogen (δ15N) stable isotope ratios to compare species' diets. Mussels exhibited niche partitioning along all resource axes. Given the mussels' use of different spatial and food resources, I evaluated the potential for complementarity with respect to mussels' roles as mediators of an important spatial subsidy, carbon inputs from the nearshore ocean into rocky-shore ecosystems. In these systems, mussels are basal species, capturing and consuming particulate organic matter in the ocean and making it available for local consumption within the benthic community. I found that mussel diversity matters; even the most productive species-Perna canaliculus-only contributed about half of the mussel-mediated carbon that accumulated over a year. Multiple co-occurring species are, therefore, likely to provide more carbon at the base of the intertidal food web than any one species living independently, and complementarity in resource use along multiple environmental axes is likely an important mechanism underlying this relationship.

Horizontal distribution affects the vertical distribution of native and invasive container-inhabiting Aedes mosquitoes within an urban landscape.

The vertical dimension constitutes an important niche axis along which mosquitoes may adjust their distribution. Here, we evaluated whether the vertical distribution of container-inhabiting Aedes mosquitoes differs along a gradient of anthropogenic land-use intensity within an urban landscape. Using a pulley system, we hung oviposition cups at three heights (ground level, 4.5, and 9 m) and in three habitats: forest, park, and a built environment. We hypothesized that mosquito abundance and diversity would be highest in the least disturbed forest habitat, decrease in the park, and be lowest at the UNC-Greensboro campus. We also expected Aedes albopictus (Skuse) and Ae. triseriatus (Say) to mainly oviposit at ground level and Ae. hendersoni (Cockerell) at canopy height. Aedes albopictus was the most common species (68.8%) collected in all three habitat types and was the only species found in the built environment. In that habitat, Ae. albopictus exhibited a bimodal distribution with the lowest activity at the intermediate height (4.5 m). Aedes triseriatus (28.9%) did not differ in egg abundance between the forest and park habitats but did exhibit diverse vertical habitat use while avoiding the canopy in the park habitat. Aedes hendersoni (2.3%) was the most sylvatic species and oviposited only at ground level. Our results indicate that the vertical distribution of mosquitoes is affected by the type of habitat in which they occur, and that this variation could be driven via local-scale modification of microclimatic factors.

Phylogenetic signal in the topographic niche of trees: Current and historical significance of habitat structure on the species arrangement pattern within East Asian rugged forests

AbstractTopography often promotes habitat heterogeneity and is a major factor in fine‐grained changes in vegetation. Especially in temperate mountainous regions of East Asia, the distribution of tree species is largely explained by topographic niche differentiation. Because species niche is at least partially a historical product through the evolution of functional traits, phylogenetic signals are expected in the topographic arrangement pattern of species, although this has not been fully investigated. Thus, we examined common temperate trees in a 306 ha watershed on Mt Tanzawa, central Japan. The topographic niche position of each species was explained with two principal component analysis (PCA) axes, which aligned with the topographic structure of the watershed. High scores on PC1 reflected lower elevations, steeper slopes and nearby valleys. Higher scores on PC2 indicated thicker soil, more south‐facing slopes, slighter slope inclinations and nearby valleys. The former indicated the species were aligned on a habitat gradient of land‐surface instability, for which the gradient is typical under the current geological conditions. The latter indicated that a niche axis for resource availability relating to water, nutrients and light or heat existed. A phylogenetic signal, identified by Pagel's λ and the Mantel test, was detected for PC2 scores. However, the species arrangement along PC1 was independent of phylogeny. Our results suggested that the topographic niche of tree species in this area is caused by both relatively recently derived traits regarding adaptation to unstable land surfaces and conservative traits derived through plant evolution.

Life in the canopy: community trait assessments reveal substantial functional diversity among fern epiphytes.

The expansion of angiosperm-dominated forests in the Cretaceous and early Cenozoic had a profound effect on terrestrial biota by creating novel ecological niches. The majority of modern fern lineages are hypothesized to have arisen in response to this expansion, particularly fern epiphytes that radiated into the canopy. Recent evidence, however, suggests that epiphytism does not correlate with increased diversification rates in ferns, calling into question the role of the canopy habitat in fern evolution. To understand the role of the canopy in structuring fern community diversity, we investigated functional traits of fern sporophytes and gametophytes across a broad phylogenetic sampling on the island of Moorea, French Polynesia, including >120 species and representatives of multiple epiphytic radiations. While epiphytes showed convergence in small size and a higher frequency of noncordate gametophytes, they showed greater functional diversity at the community level relative to terrestrial ferns. These results suggest previously overlooked functional diversity among fern epiphytes, and raise the hypothesis that while the angiosperm canopy acted as a complex filter that restricted plant size, it also facilitated diversification into finely partitioned niches. Characterizing these niche axes and adaptations of epiphytic ferns occupying them should be a priority for future pteridological research.

A Small Parasitoid of Fire Ants, Pseudacteon obtusitus (Diptera: Phoridae): Native Range Ecology and Laboratory Rearing

The high diversity within a guild of Pseudacteon Coquillet (Diptera: Phoridae) parasitoid flies hosted by the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), may be based on several niche axes, including host size specialization, host location cues, climatic responses, and tolerances. The recently described species, Pseudacteon obtusitus Plowes, Folgarait, and Gilbert (Diptera: Phoridae), is one of the smallest species in this group (with a thorax width of only 0.3 mm), and appears to locate ant hosts on foraging trails as well as at nest disturbances. Here we examine the ecology, phenology, and development of this parasitoid at sites in Corrientes, Argentina. Pseudacteon obtusitus has 2 seasons of peak abundance, and exhibits low tolerance for cold temperatures or extended cold periods. It preys on the smallest ant workers (typically 0.6 mm head width) independent of the ant size offered, and has a male biased sex ratio (mean 2.78:1). Developmental times ranged from 35 d at 27 °C to 44 d at 22 °C. This species holds interest for inclusion among the multi-species assemblage of parasitoid flies considered for biological control of S. invicta in its invasive range in North America.

Evolution of ecological structure of anole communities in tropical rain forests from north-western South America

AbstractThe coexistence of several anoles in the same place is attributed to differential partitioning of resources. Although several mainland and island communities show a similar structure, differences in life-history traits, absence of niche complementarity, higher food supply and higher numbers of predators in mainland environments support the idea that predation, rather than competition, is a more important structuring force in mainland than in island anole communities. To analyse the pattern of ecological structure in mainland anole communities, we studied communities in three tropical rain forests of north-western South America to obtain data about the use of resources on three niche axes [spatial, thermal and morphological (as a proxy of diet)] for 17 species of anoles. We analysed the patterns of niche overlap for each axis and found that overlap on the dietary axis was less than the overlap on the other axes, indicating that species using similar spatial or thermal resources diverge strongly in their diet. In addition, we identified a niche complementarity among niche axes, suggesting that intraspecific competition is also an important process in those communities. Finally, this study revealed a similar ecological structure in different communities of mainland rain-forest anoles, which share seven ecomorphs, suggesting ecological adaptation and convergence in mainland anoles.

Soil-borne pathogens as determinants of regeneration patterns at community level in Mediterranean forests.

Emergent diseases are an increasing problem in forests worldwide. Exotic pathogens are now threatening forests where pathogens have not traditionally been considered to be major ecological drivers of tree demography, such as water-limited Mediterranean forests. However, how pathogens might limit regeneration in invaded forests is largely unknown. Here we used fungicide to analyse the impact of soil-borne oomycete pathogens on seedling establishment at community level in Mediterranean forests invaded by the exotic oomycete Phytophthora cinnamomi. Fungicide effects were modelled as a function of the tree neighbourhood composition, the seed mass of the target species, and the abiotic environment. Fungicide application had positive effects on seedling performance that varied in magnitude and spatial structure among coexisting species. Seed mass predicted fungicide effects on seedling emergence, but not on survival or growth. Positive fungicide effects were modulated by levels of abiotic resources, mainly water, increasing with soil moisture. Our results support a novel role for soil-borne oomycete pathogens as one more axis of the regeneration niche of woody species in water-limited forests. Given the increasing numbers of exotic oomycete pathogens worldwide, more research is needed to understand the role of this relevant microbial group as a factor shaping seedling establishment.

Otolith morphological divergences of successful Lessepsian fishes on the Mediterranean coastal waters

Here we used a method of morphological niche analysis, previously shown to be an effective predictor of invasion success, to investigate morphological relationships of sagittae otoliths of Lessepsian (Red sea species entering the Mediterranean through the Suez Canal) fishes within the morphospace of the corresponding receiving taxonomic groups. Overall, sagittae of 13 Lessepsian species and 49 closely related native fishes/taxa, distributed in 9 different families or subfamilies were considered, for a total of 305 analyzed otoliths. Based on wavelet functions of 512 Cartesian coordinates, we quantified the degree of morphological disparity produced by the introduction of these species in the hosting community. Lessepsian otoliths tended to be added outside or at the margins of the receiving morphospace, being morphologically divergent from the ones of native species. Considering that many ecological traits of fishes (especially feeding) are mirrored in the shape of otoliths, our findings agree with the idea that these successful invaders may count on novel sensory strategies to compete for available resources and to thrive in newly colonized habitats. Nevertheless, different measurements of morphological disparity did not show any variation before and after invasion and with the specific richness and further investigation is needed to understand to what extent these differentiated sensorial adaptations are linked to novel opportunities along the niche axes, such as in relation to feeding, predatory and anti-predatory behaviour and exploitation of the temporal niche.

Macroevolutionary convergence connects morphological form to ecological function in birds.

Animals have diversified into a bewildering variety of morphological forms exploiting a complex configuration of trophic niches. Their morphological diversity is widely used as an index of ecosystem function, but the extent to which animal traits predict trophic niches and associated ecological processes is unclear. Here we use the measurements of nine key morphological traits for >99% bird species to show that avian trophic diversity is described by a trait space with four dimensions. The position of species within this space maps with 70-85% accuracy onto major niche axes, including trophic level, dietary resource type and finer-scale variation in foraging behaviour. Phylogenetic analyses reveal that these form-function associations reflect convergence towards predictable trait combinations, indicating that morphological variation is organized into a limited set of dimensions by evolutionary adaptation. Our results establish the minimum dimensionality required for avian functional traits to predict subtle variation in trophic niches and provide a global framework for exploring the origin, function and conservation of bird diversity.

Integrating univariate niche dynamics in species distribution models: A step forward for marine research on biological invasions

AbstractAimThe development of approaches to predict the distribution and potential expansion of invasive species is still an open challenge. Here our goal is to improve the modelling procedure for marine invaders by coupling Species Distribution Models (SDMs) with an analysis of their univariate niche dynamics. In particular, we tested for the first time whether choosing model predictors among the stable niche dimensions was effective in improving predictions of invasive species expansion.LocationMediterranean Sea.TaxonDusky spinefoot, Siganus luridus.MethodsWe analysed the univariate niche dynamics for S. luridus across its native and invaded ranges, by applying a standardized framework that allowed the identification of cases of niche stability or shift. We compared inter‐range transferability of SDMs fitted with different combinations of labile or stable predictors. Finally, we evaluated interactions in SDM settings (calibration area, model technique and predictors set) on models’ predictive ability, using independent data from the most recent phase of invasion.ResultsWe detected a pattern of niche stability for several variables, especially salinity and bathymetry, which positively influenced model inter‐ranges transferability: when the models calibrated in the native range include only stable niche axes, predictive ability is improved. We also identified a shift towards lower surface temperatures in the introduced range, which were almost never experienced by the species before invasion. The model calibrated within the combined ranges was the most ecologically congruent. Also, models calibrated in the invaded range allowed a correct prediction of range expansion, with the predicted suitable areas only slightly underestimated.Main conclusionsWe provide the first evidence that using conserved predictors in SDMs improves inter‐range projections of expanding invasive species. Variable selection, calibration area and modelling technique all matter when modelling invasive species, with important interaction effects. We provide guidelines on how to improve SDMs applications in biological invasion research.

Mechanisms of trophic niche compression: Evidence from landscape disturbance.

Natural and anthropogenic disturbances commonly alter patterns of biodiversity and ecosystem functioning. However, how networks of interacting species respond to these changes remains poorly understood. We described aquatic food webs using invertebrate and fish community composition, functional traits and stable isotopes from twelve agricultural streams along a landscape disturbance gradient. We predicted that excessive deposition of fine inorganic sediment (sedimentation) associated with agricultural activities would negatively influence aquatic trophic diversity (e.g. reduced vertical and horizontal trophic niche breadths). We hypothesized that multiple mechanisms might cause trophic niche 'compression', as indicated by changes in realized trophic roles. Food-web properties based on consumer stable isotope data (δ13 C and δ15 N) showed that increasing sediment disturbance was associated with reduced trophic diversity. In particular, the aquatic invertebrate community occupied a smaller area in isotopic niche space along the sedimentation gradient that was best explained by a narrowing of the invertebrate community δ13 C range. Decreased niche partitioning, driven by increasing habitat homogeneity, environmental filtering and resource scarcity all seemingly lead to greater trophic equivalency caused by the collapse of the autochthonous food-web channel. Bayesian mixing-model analyses supported this contention with invertebrate consumers increasingly reliant on detritus along the sedimentation gradient, and predatory invertebrates relying more on the prey using these basal resources. The narrowing of the fish community δ13 C range along the sedimentation gradient contributed to an apparent 'trophic shift' towards terrestrial carbon, further indicating the loss of the autochthonous food-web channel. On the vertical trophic niche axis, fish became increasingly separated from aquatic invertebrates with an increase in their estimated trophic position. In combination, these responses were most likely mediated through reduced fish densities and a diminished reliance on aquatic prey. Although species losses remain a major threat to ecosystem integrity, the functional roles of biota that persist dictate how food webs and ecosystem functioning respond to environmental change. Sedimentation was associated with nonlinear reductions in trophic diversity which could affect the functioning and stability of aquatic ecosystems. Our study helps explain how multiple mechanisms may radically reshape food-web properties in response to this type of disturbance.

Assessment of a large number of empirical plant species niche models by elicitation of knowledge from two national experts.

Quantitative models play an increasing role in exploring the impact of global change on biodiversity. To win credibility and trust, they need validating. We show how expert knowledge can be used to assess a large number of empirical species niche models constructed for the British vascular plant and bryophyte flora. Key outcomes were (a) scored assessments of each modeled species and niche axis combination, (b) guidance on models needing further development, (c) exploration of the trade‐off between presenting more complex model summaries, which could lead to more thorough validation, versus the longer time these take to evaluate, (d) quantification of the internal consistency of expert opinion based on comparison of assessment scores made on a random subset of models evaluated by both experts. Overall, the experts assessed 39% of species and niche axis combinations to be “poor” and 61% to show a degree of reliability split between “moderate” (30%), “good” (25%), and “excellent” (6%). The two experts agreed in only 43% of cases, reaching greater consensus about poorer models and disagreeing most about models rated as better by either expert. This low agreement rate suggests that a greater number of experts is required to produce reliable assessments and to more fully understand the reasons underlying lack of consensus. While area under curve (AUC) statistics showed generally very good ability of the models to predict random hold‐out samples of the data, there was no correspondence between these and the scores given by the experts and no apparent correlation between AUC and species prevalence. Crowd‐sourcing further assessments by allowing web‐based access to model fits is an obvious next step. To this end, we developed an online application for inspecting and evaluating the fit of each niche surface to its training data.

Novel insights into how the mean and heterogeneity of abiotic conditions together shape forb species richness patterns in the Allegheny plateau ecoregion.

While plant community theory tends to emphasize the importance of abiotic heterogeneity along niche axes, much empirical work seeks to characterize the influence of the absolute magnitude of key abiotic variables on diversity. Both magnitude (as reflected, e.g., by a mean) and heterogeneity (variance) in abiotic conditions likely contribute to biodiversity patterns in plant communities, but given the large number of putative abiotic drivers and the fact that each may vary at different spatiotemporal scales, the challenge of linking observed biotic patterns with the underlying environment remains acute. Using monitoring data from a natural resource agency, we compared how well statistical models of the mean, heterogeneity, and both the mean and heterogeneity combined of 17 abiotic factor variables explained patterns of forb species richness in Northeast Ohio, USA. We performed our analyses at two spatial scales, repeated in spring and summer across four forest types. Although all models explained a great deal of the variance in species richness, models including both the mean and heterogeneity of different abiotic factors together outperformed models including either the mean or the heterogeneity of abiotic factors alone. Variability in forb species richness was mostly due to changes in mean calcium levels regardless of forest type. After accounting for forest type, we were able to attribute variation in forb species richness to changes in the heterogeneity of different abiotic factors as well. Our results suggest that multiple mechanisms act simultaneously according to different aspects of the abiotic environment to structure forb communities, and this underscores the importance of considering both the magnitude of and heterogeneity in multiple abiotic factors when looking for links between the abiotic environment and plant community patterns. Finally, we identify novel patterns across spatial scales, forest types, and seasons that can guide future research in this vein.OPEN RESEARCH BADGES This article has earned an Open Data Badge for making publicly available the digitally‐shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.kp3cb17.

Evidence of limiting similarity revealed using a conservative assessment of coexistence

AbstractThe concept of limiting similarity is important in ecology because it encapsulates the expectation under niche theory that differences among species are fundamental to coexistence. A growing body of research has tested for evidence of limiting similarity, but only a small number of studies have produced support. Here, using relevant field data, we highlight one possible explanation for the paucity of support for limiting similarity. We test whether coexisting plant species that share bees as pollinators flower asynchronously, a form of temporal niche separation consistent with limiting similarity. Our results provide evidence of limiting similarity, adding to the small collection of null modeling studies that have thus far done so. Our work also provides evidence that temporal niche variation may be an important niche axis that broadly contributes to species coexistence. Finally, we demonstrate that a more conservative assessment of coexistence that includes only individual plants that have achieved reproduction, is consequential in whether we find evidence of significant flowering asynchrony in this study. We conclude that the conservative approach to assessing coexistence that we present here can reduce noise in coexistence data, improving our power to test for evidence consistent with limiting similarity. Using this approach may or may not result in an increase in evidence supporting limiting similarity; however, it will certainly give researchers more confidence that they have not missed existing evidence of limiting similarity.

Resource spectrum engineering by specialist species can shift the specialist-generalist balance

The mechanisms which influence coexistence of specialist and generalist species in the same environment are a key focus of ecological theory. We use an agent-based model of community assembly to show that the available resource spectrum (distribution of resources along a niche axis) can play an important role in determining the specialist-generalist balance, even in the absence of spatial structure. Our results reveal a phenomenon that we term ‘resource spectrum engineering’, in which opportunistic specialists occupying small niches in a mostly generalist community can change the resource spectrum that is experienced by other species, in a way that disfavours generalists and causes a community-wide shift towards specialist strategies. More generally, this suggests a mechanism by which apparently minor changes in the specialist composition of an ecological community could have knock-on effects across the entire community.

Spatial variation in the feeding strategies of Mediterranean fish: flatfish and mullet in the Gulf of Gaeta (Italy)

Marine coastal areas are highly productive due to the presence of various inputs of organic matter, including terrestrial material, which fuels food webs. However, the ecological mechanisms underlying the productivity of benthic and demersal fish species in estuarine areas are poorly understood. By means of C and N stable isotope analysis and Bayesian mixing models, we investigated the trophic niches of three common fish species: Citharus linguatula, Pegusa lascaris (flatfish) and Liza ramada (mullet) in the Gulf of Gaeta (Italy). Fish were collected from the north-western area and the south-eastern area of the Gulf of Gaeta, the latter affected by organic inputs from the Garigliano River. The results highlighted the riverine terrestrial origin of the organic matter at the base of the food web in the south-eastern area and marine autochthonous input in the north-western area. All fish species increased their trophic specialisation in proximity to the river mouth. L. ramada specialised on seston of terrestrial origin, reducing its niche overlap with C. linguatula and P. lascaris. Away from the river mouth, all species were characterised by longer individuals, increased intraspecific diet variability and higher interspecific similarity in resource use. Organic input from the river represented a complementary trophic niche axis that enabled lower interspecific niche overlap in the south-eastern area, where fish populations were found at higher densities. In conclusion, this study provided information about the effects of the flow of material from the basal compartment up to abundant fish species in areas enriched by organic matter of varying origin.

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 0.1–173.6kg) 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.

Regeneration niches in Nothofagus-dominated old-growth forests after partial disturbance: Insights to overcome arrested succession

Arrested succession may lead to losses of productivity and biodiversity in forests after disturbance. In this condition, recalcitrant understory vegetation often dominates over long periods of time and delays forest recovery and succession. Arrested succession has been defined as a type of forest degradation because ecosystem processes and functions that underlie successional dynamics are dramatically reduced. We used the Hutchinson’ niche concept to understand the relative importance of environmental variables that define niche axes of species leading to arrested succession (recalcitrant understory vegetation) and to identify a shift along niche axes that promote the successional progression of long-lived early- and late-seral tree regeneration after partial disturbance due to restoration treatments. We studied Nothofagus old-growth forests in the Chilean Andes, where after selective harvests (i.e., a partial overstory disturbance) the understory is commonly dominated by dense thickets of native bamboo (Chusquea spp.), which has been shown to arrest succession by preventing tree regeneration. To overcome this, sites are typically partially scarified (24 to 45% of the area) following harvest to control bamboo and to encourage early-seral tree regeneration. The partial overstory and ground disturbance leaves a wide array of conditions (environmental gradient) that allowed us to investigate the factors under which a given plant group may regenerate, grow, and dominate the forest. Ordination and niche modeling were used to investigate the niche breadths for bamboo and regenerating tree species, as well as the environmental niche axes at this early successional stage, i.e., eight years after disturbance. The results showed different trends in the shift of centroid and extent of realized niches among three plant groups. For instance, performance of the early-seral group was related to litter cover (–), exposed mineral soil (+) and soil water content (+), which resulted in a niche expansion. In contrast, soil water content affected bamboo (–) and may be responsible for the reduction in realized niche space. Light had less influence on defining niches for tree regeneration during early successional stages. Linking the niche concept to succession provided a simple display of multifaceted issues and thus a better understanding of underlying processes and mechanisms. It provides an example of how application of this concept can help to determine situations in which such regeneration can (or cannot) be established successfully and promote the progression of succession following disturbance. This knowledge provides important insights for designing management and restoration practices based on strong ecological foundations.

Three-dimensional partitioning of resources by congeneric forest predators with recent sympatry

Coexistence of ecologically similar species can be maintained by partitioning along one or more niche axes. Three-dimensional structural complexity is central to facilitating resource partitioning between many forest species, but is underrepresented in field-based studies. We examined resource selection by sympatric northern spotted owls (Strix occidentalis caurina), a threatened species under the US Endangered Species Act, and nonnative barred owls (S. varia) in western Oregon, USA to explore the relative importance of canopy heterogeneity, vertical complexity of forest, and abiotic features to resource selection and identify potential differences that may facilitate long-term coexistence. We predicted that within home range selection of understory densities, measured with airborne lidar, would differ between species based on proportional differences in arboreal and terrestrial prey taken by each owl species. We used discrete choice models and telemetry data from 41 spotted owls and 38 barred owls monitored during 2007–2009 and 2012–2015. Our results suggested that while both species used tall canopy areas more often than low canopy areas, spotted owls were more commonly found in areas with lower tree cover, more developed understory, and steeper slopes. This is the first evidence of fine-scale partitioning based on structural forest properties by northern spotted owls and barred owls.

Is dietary or microhabitat specialization associated with environmental heterogeneity in horned lizards (Phrynosoma)?

Niche breadth is predicted to correlate with environmental heterogeneity, such that generalists will evolve in heterogeneous environments and specialists will evolve in environments that vary less over space and time. We tested the hypothesis that lizards in a heterogeneous environment were generalists compared to lizards in a homogeneous environment. We compared niche breadths of greater short‐horned lizards by quantifying resource selection in terms of two different niche axes, diet (prey items and trophic level), and microhabitat (ground cover and shade cover) between two populations occurring at different elevations. We assessed the heterogeneity of dietary and microhabitat resources within each population's environment by quantifying the availability of prey items, ground cover, and shade cover in each environment. Overall, our results demonstrate that despite differences in resource heterogeneity between elevations, resource selection did not consistently differ between populations. Moreover, environmental heterogeneity was not associated with generalization of resource use. The low‐elevation site had a broader range of available prey items, yet lizards at the high‐elevation site demonstrated more generalization in diet. In contrast, the high‐elevation site had a broader range of available microhabitats, but the lizard populations at both sites were similarly generalized for shade cover selection and were similarly specialized for ground cover selection. Our results demonstrate that environmental heterogeneity of a particular resource does not necessarily predict the degree to which organisms specialize on that resource.