Biology subjects

Tobias, J. A.

Publications and source records attributed to Tobias, J. A..

17 recordsLinked to original sources

bioRxiv2025

A functional Red List Index for monitoring trends in functional diversity

Functional diversity supports ecosystem function and resilience but is in decline due to ongoing extinctions. Despite this, no functional diversity metrics are included in the set of indicators adopted by governments to measure progress towards the Global Biodiversity Framework. Monitoring changes in functional diversity is essential to understand differences between taxonomic and functional diversity trends. The Red List Index (RLI) is a widely used metric in conservation policy, and tracks trends in aggregate extinction risk across sets of species over time. We present a functional Red List Index (fRLI), which weights species by their morphological uniqueness at either local or global scales, to indicate the risk that future extinctions could pose to functional diversity. We applied the fRLI on birds and found higher aggregate extinction risk for both local and global fRLI compared with the standard RLI. Species with higher local uniqueness were more likely to be at higher risk of extinction while we found no difference for globally unique species. Globally or locally unique species were not more likely to show escalation in extinction risk between 1988 and 2024 than less unique species; however, locally unique birds had a lower proportion of their resident range covered by site-based conservation measures, leaving them vulnerable to future decline. The fRLI provides a tool to monitor the impact of extinction risk on functional diversity and could be applied to other taxonomic groups with comprehensive or representative coverage of Red List assessments and trait data.

ecology↗

bioRxiv2025

Song complexity in suboscine birds: evolutionary drivers and ecological constraints

Acoustic signal complexity varies widely in the animal kingdom for reasons that remain unclear. In birds, it is widely proposed that vocal complexity evolves as an honest signal of individual quality driven by sexual selection. Other hypotheses related to social interactions include competition for ecological resources (social selection) and intra-group communication in group- living animals, both of which may favour signal complexity. However, these hypotheses are rarely explored at macroevolutionary scales, particularly in the context of constraints on sound production, transmission and detection, leading to ongoing uncertainty about the evolutionary origins of complex vocal signals. Using Bayesian phylogenetic models, we test whether different forms of social communication and ecological constraints predict the temporal and spectral complexity of songs in 1,288 species of suboscine passerine birds. We found that song complexity was reduced by sexual selection, along with other limiting factors including large body size and dense vegetation. Conversely, territoriality boosted the temporal complexity of songs. These findings challenge the common assumption that sexual selection is the main driver of increased signal complexity, and instead highlight the role of social selection as a key component of multiple inter-related drivers and constraints.

evolutionary biology↗

bioRxiv2025

Threat reduction must be coupled with targeted recovery programmes to conserve global bird diversity

Biodiversity is undergoing severe declines while threats to natural populations continue to escalate worldwide. Ambitious international commitments have been made to preserve biodiversity, with the goal of preventing extinctions and maintaining ecosystem resilience, yet the efficacy of large-scale protection remains unclear. Here we use a trait-based approach to show that global actions - such as the immediate abatement of all threats across at least half of species ranges for [~]10,000 bird species - will only prevent half of projected species extinctions and functional diversity loss attributable to current and future threats in the next 100 years. Nonetheless, targeted recovery programmes prioritizing protection of the 100 most functionally unique threatened birds could avoid 68% of projected functional diversity loss. Actions targeting habitat loss and degradation will prevent the greatest number of species extinctions and proportion of functional diversity loss relative to other drivers of extinction, whereas control of hunting and collection and disturbance and accidental mortality would save fewer species but disproportionately boost functional richness. These findings show that conservation of avian diversity requires action partitioned across all drivers of decline, and highlights the importance of understanding and mitigating the ecological impacts of species extinctions predicted to occur even under optimistic levels of conservation action.

ecology↗

bioRxiv2024

Elevational constraints on flight efficiency shape global gradients in avian wing morphology

Wings with elongated shape or larger surface area are associated with increased flight efficiency and dispersal ability in a wide range of animals from insects to birds 1-4. Inter- and intraspecific variation in these attributes of wing shape is determined by a range of factors - including foraging ecology, migration and climatic seasonality 5-8 - all of which may drive latitudinal gradients in wing morphology 9,10. A separate hypothesis predicts that wing shape should also follow an elevational gradient because air density and oxygen supply decline with altitude 11, altering the aerodynamics of flight, and driving the evolution of more efficient wings in high-elevation species to compensate for reduced lift 12,13. However, previous analyses have found only mixed support for the thin-air hypothesis 14-18, and we currently lack a global synthesis of elevational gradients in wing design for any taxonomic group. In this study, we use phylogenetic comparative models to explore elevational effects on wing morphology in 9986 bird species, while accounting for multiple climatic and ecological attributes, including latitude, temperature seasonality, body mass, aerial lifestyle and migration. We found that relative wing elongation (hand-wing index) and wing area increase with elevation, particularly in the upper montane zone (>4 km above sea level). These results confirm a pervasive elevational gradient in avian wing morphology, highlighting the role of aerodynamic constraints as key mechanisms shaping global patterns of trait evolution in flying animals.

evolutionary biology↗

bioRxiv2023

The evolutionary age-range size relationship is modulated by insularity and dispersal in plants and animals

Earth is home to millions of plant and animal species, with more than 40 thousand species facing extinction worldwide (Diaz et al. 2019). Species range size is particularly important in this context because it influences extinction risk (Purvis et al. 2000, Gaston & Fuller 2009), but the causes underlying the wide natural variation in range size remain poorly known. Here, we investigate how evolutionary age is related to present-day range size for over 25,000 species of mammals, birds, reptiles, amphibians, reef fishes, and plants. We show that, on average, older species have significantly larger ranges, but the effect of age on range size is modulated by clade, geographical context and dispersal ability. Specifically, age does not affect range size for island species, because islands limit dispersal and hence range size, regardless of species age. Furthermore, species from clades with high dispersal capabilities obtain large ranges faster, thereby further neutralizing the relationship between age and range size. Our results can help supporting global conservation priorities, by showing that species that are young, occupy islands, and/or are dispersal limited often have small ranges and therefore increased extinction risk.

ecology↗

bioRxiv2023

Resolving the climatic and ecological drivers of geographical gradients in avian sexual selection

Sexual selection, one of the central pillars of evolutionary theory, has powerful effects on organismal morphology, behaviour and population dynamics. However, current knowledge about geographical variation in this evolutionary mechanism and its underlying drivers remains highly incomplete, in part because standardized data on the strength of sexual selection is sparse even for well-studied organisms. Here we use information on mating systems - including the incidence of polygamy and extra-pair paternity - to quantify the intensity of sexual selection in 10671 (>99.9%) bird species distributed worldwide. We show that avian sexual selection varies latitudinally, peaking at higher latitudes, although the gradient is reversed in the worlds most sexually selected birds - specialist frugivores - which are strongly associated with tropical forests. Phylogenetic models further reveal that the strength of sexual selection is explained by temperature seasonality coupled with a suite of climate-associated factors, including migration, diet, and territoriality. Overall, these analyses suggest that climatic conditions leading to short, intense breeding seasons, or highly abundant and patchy food resources, increase the potential for polygamy in birds, driving latitudinal gradients in sexual selection. Our findings help to resolve longstanding debates about spatial variation in evolutionary mechanisms linked to reproductive biology, and provide a comprehensive species-level dataset for further studies of selection and phenotypic evolution in the context of global climatic change.

evolutionary biology↗

bioRxiv2023

Exploring the Macroevolutionary Signature of Asymmetric Inheritance at Speciation

AO_SCPLOWBSTRACTC_SCPLOWPopular comparative phylogenetic models such as Brownian Motion, Ornstein-Ulhenbeck, and their extensions, assume that, at speciation, a trait value is inherited identically by the two descendant species. This assumption contrasts with models of speciation at the micro-evolutionary scale where phenotypic distributions of the descendants are sub-samples of the ancestral distribution. Various described mechanisms of speciation can lead to a displacement of the ancestral phenotypic mean among descendants and an asymmetric inheritance of the ancestral phenotypic variance. In contrast, even macro-evolutionary models that account for intraspecific variance assume symmetrically conserved inheritance of the ancestral phenotypic distribution at speciation. Here we develop an Asymmetric Brownian Motion model (ABM) that relaxes the hypothesis of symmetric and conserved inheritance of the ancestral distribution at the time of speciation. The ABM jointly models the evolution of both intra- and inter-specific phenotypic variation. It also allows the mode of phenotypic inheritance at speciation to be inferred, ranging from a symmetric and conserved inheritance, where descendants inherit the ancestral distribution, to an asymmetric and displaced inheritance, where descendants inherit divergent phenotypic means and variances. To demonstrate this model, we analyze the evolution of beak morphology in Darwin finches, finding evidence of character displacement at speciation. The ABM model helps to bridge micro- and macro-evolutionary models of trait evolution by providing a more robust framework for testing the effects of ecological speciation, character displacement, and niche partitioning on trait evolution at the macro-evolutionary scale.

evolutionary biology↗

bioRxiv2023

Bird wings are shaped by thermoregulatory demand for heat dissipation

The tendency for animals in warmer climates to be longer-limbed (Allens Rule) is widely attributed to the demands of thermoregulation. However, the underlying mechanism remains unclear, because variation in limb-length can typically be driven by selection for both efficient heat retention and increased heat dissipation capacity. Using comparative phylogenetic models, we find that occurrence in warmer climates is associated with longer wing bones for 1,520 species of passerine birds. The highly vascularized musculature along these bones is only uncovered during flight, when the wings function as the primary site of heat exchange, cooling the organism by dissipating excess heat generated by muscular activity. Conversely, the musculature along the wing bones is insulated by feathering when at rest, playing a negligible role in heat retention, even in colder climates. Given this asymmetry in thermoregulatory roles, we can identify the positive relationship between temperature and wing bone length as a phenotypic gradient shaped by increased demand for heat dissipation in warmer climates. Our findings provide a clear illustration of the mechanism by which global warming can drive spatial and temporal trends in appendage length, and also highlight the role of heat dissipation in reshaping even the most critical features of vertebrate anatomy. Significance StatementAnimals tend to be longer-limbed in warmer climates, but it remains unclear whether this pattern is driven by selection for cold tolerance at low temperatures or efficient heat dissipation at high temperatures. We show that for 1,520 species of passerines, bird wing bones are relatively longer in warmer climates. The vascularized musculature along these bones primarily functions in heat exchange during flight, when the overwhelming thermoregulatory challenge is dissipating heat, suggesting longer wing-bone length is driven by heat dissipation demands. Our findings reveal the pervasive impacts of thermoregulatory demands on even the most important functional traits.

ecology↗

bioRxiv2023

Ecological and life history drivers of avian skull evolution

One of the most famous examples of adaptive radiation is that of the Galapagos finches, where skull morphology, particularly the beak, varies with feeding ecology. Yet increasingly studies are questioning the strength of this correlation between feeding ecology and morphology in relation to the entire neornithine radiation, suggesting that other factors also significantly affect skull evolution. Here, we broaden this debate to assess the influence of a range of ecological and life history factors, specifically habitat density, migration, and developmental mode, in shaping avian skull evolution. Using 3D geometric morphometric data to robustly quantify skull shape for 354 extant species spanning avian diversity, we fitted flexible phylogenetic regressions and estimated evolutionary rates for each of these factors across the full dataset. The results support a highly significant relationship between skull shape and both habitat density and migration, but not developmental mode. We further found heterogenous rates of evolution between different character states within habitat density, migration, and developmental mode, with rapid skull evolution in species which occupy dense habitats, are migratory, or are precocial. These patterns demonstrate that diverse factors impact the tempo and mode of avian phenotypic evolution, and that skull evolution in birds is not simply a reflection of feeding ecology. Impact summaryAlmost 200 years ago, Darwin found that the beaks of Galapagos finches were different shapes in birds with different diets. Nowadays, it is well established that phylogeny, allometry, and ecology can also be key factors in shaping skulls. Yet, the influence of specific aspects of ecology, as well as life history, on morphological evolution remain poorly constrained. Here, we examined whether three novel factors also influence the shape of bird skulls and rates of evolution: habitat density, migration, or developmental mode. To do so, we combine high resolution 3D quantification of skull shape with dense taxonomic sampling across living birds. Our analyses revealed that skull shape varies in birds based on the density of vegetation in their habitats and on the extent to which they migrate. However, how independent birds are when they are born does not appear to influence overall skull shape. Despite these differences in how much they influence the shape of the skull, habitat density, migration and life history all influence the rate at which bird skulls evolve. Birds evolved fastest if they live in densely vegetated habitats, migrate long distances, or are precocial. These results add to the growing body of evidence that skull evolution in birds is impacted by a diverse range of factors, and suggests that habitat density, migration and life history should be considered in future analyses on drivers of phenotypic evolution.

evolutionary biology↗

bioRxiv2022

Trait diversity metrics can perform well with highly incomplete datasets

O_LICharacterizing changes in trait diversity at large spatial scales provides insight into the impact of human activity on ecosystem structure and function. However, the approach is often based on trait datasets that are incomplete and unrepresentative, with uncertain impacts on trait diversity estimates. C_LIO_LITo address this knowledge gap, we simulated random and biased removal of data from a near complete avian trait dataset (9579 species) and assessed whether trait diversity metrics were robust to data incompleteness with and without using imputation to fill data gaps. Specifically, we compared two commonly used metrics each calculated with two methods: trait richness (calculated with convex hulls and trait probabilities densities) and trait divergence (calculated with distance-based Rao and trait probability densities). C_LIO_LIWithout imputation, estimates of global avian trait diversity (richness and divergence) were robust when 30-70% of species had missing data for four out of 11 continuous traits, depending on severity of bias and the method used. However, when missing traits were imputed based on present morphological trait data and phylogeny, trait diversity metrics consistently remained representative of the true value, even when 70% of species were missing data for four out of 11 traits and data were not missing at random (biased with respect to body mass). Trait probability densities and distance-based Rao were particularly robust to missingness and bias when combined with imputation, with convex hull-based trait richness being less reliable. C_LIO_LIExpanding global morphometric datasets to represent more taxa and traits, and to quantify intraspecific variation, remains a priority. In the meantime, our results show that widely used methods can successfully quantify large-scale trait diversity even when data are missing for two-thirds of species, so long as missing traits are estimated using imputation. C_LI

ecology↗

bioRxiv2021

Global plant-frugivore trait matching is shaped by climate and biogeographic history

Species interactions are influenced by the trait structure of local multi-trophic communities. However, it remains unclear whether mutualistic interactions in particular can drive trait patterns at the global scale, where climatic constraints and biogeographic processes gain importance. Here we evaluate global relationships between traits of frugivorous birds and palms (Arecaceae), and how these relationships are affected, directly or indirectly, by assemblage richness, climate and biogeographic history. We leverage a new and expanded gape size dataset for nearly all avian frugivores, and find a positive relationship between gape size and fruit size, i.e., trait matching, which is influenced indirectly by palm richness and climate. We also uncover a latitudinal gradient in trait matching strength, which increases towards the tropics and varies among zoogeographic realms. Taken together, our results suggest trophic interactions have consistent influences on trait structure, but that abiotic, biogeographic and richness effects also play important, though sometimes indirect, roles in shaping the functional biogeography of mutualisms.

ecology↗

bioRxiv2020

Global impacts of climate change on avian functional diversity

Climate change is predicted to drive geographical range shifts in many taxa, leading to the formation of novel species assemblages and fluctuations in species richness worldwide. However, the effect of these changes on functional diversity is not yet fully understood, in part because comprehensive species-level trait data are generally lacking at global scales. Here we use morphometric and ecological trait data for 8269 terrestrial bird species to compare functional diversity (FD) of current and future bird assemblages under a medium emissions scenario. We show that future assemblages are likely to undergo substantial shifts in trait structure, with the direction and magnitude of these shifts varying with geographical location and trophic guild. Specifically, invertivore FD is projected to increase at higher latitudes with concurrent losses at mid-latitudes, reflecting poleward shifts in range, whereas frugivore FD is projected to fluctuate in many tropical regions with major declines in much of South America and New Guinea. We show that these projected changes in FD are generally greater than expected from changing species richness alone, indicating that projected FD changes are primarily driven by the loss or gain of functionally distinct species. Our findings suggest that climate change will drive continental-scale shifts in avian functional diversity, with potentially far-reaching implications for ecosystem functions and resilience.

ecology↗

bioRxiv2020

The island rule explains consistent patterns of body size evolution across terrestrial vertebrates

Island faunas can be characterized by gigantism in small animals and dwarfism in large animals, but the extent to which this so-called island rule provides a general explanation for evolutionary trajectories on islands remains contentious. Here we use a phylogenetic meta-analysis to assess patterns and drivers of body size evolution across a global sample of paired island-mainland populations of terrestrial vertebrates. We show that island rule effects are widespread in mammals, birds and reptiles, but less evident in amphibians, which mostly tend towards gigantism. We also found that the magnitude of insular dwarfism and gigantism is mediated by climate as well as island size and isolation, with more pronounced effects in smaller, more remote islands for mammals and reptiles. We conclude that the island rule is pervasive across vertebrates, but that the implications for body size evolution are nuanced and depend on an array of context-dependent ecological pressures and environmental conditions.

ecology↗

bioRxiv2020

Comparative physiology of five tropical montane songbirds reveals differential seasonal acclimatisation and cold adaption

The physiology of tropical birds is poorly understood, particularly in how it relates to local climate and changes between seasons. This is particularly true of tropical montane species, which may have sensitive thermal tolerances to local microclimates. We studied metabolic rates (using open flow respirometry), body mass and haemoglobin concentrations of five sedentary Mesoamerican songbirds between the summer and winter at two elevations (1550 m and 1950 m, respectively). We asked whether there were uniform seasonal shifts in physiological traits across species, and whether higher elevation species displayed evidence for cold tolerance. Seasonal shifts in metabolic rates differed between the three species for which data were collected. Basal metabolic rates in one species - black-headed nightingalethrushes Catharus mexicanus - were up-regulated in summer (~19% increase of winter metabolism), however two other species displayed no seasonal regulation. No species exhibited shifts in haemoglobin concentrations across season or across elevation, whereas body mass in two species was significantly higher in the summer. One species restricted to higher elevations - ruddy-capped nightingale-thrushes C. frantzii - displayed physiological traits indicative of cold-tolerance. Although only summer data were available for this species (C. frantzii), metabolic rates were constant across temperatures tested (5-34{degrees}C) and haemoglobin concentrations were significantly higher compared to the other four species. Our results suggest that seasonal acclimatisation in physiological traits is variable between species and appear unrelated to changes in local climate. As such, the distinct physiological traits observed in ruddy-capped nightingale-thrushes likely relate to historic isolation and conserved physiological traits rather than contemporary climatic adaption.

ecology↗

bioRxiv2019

The latitudinal gradient in hand-wing-index: global patterns and predictors of wing morphology in birds

An organisms ability to disperse influences many fundamental processes in ecology. However, standardised estimates of dispersal ability are rarely available, and thus the patterns and drivers of broad-scale variation in dispersal ability remain unclear. Here we present a global dataset of avian hand-wing index (HWI), an estimate of wingtip pointedness widely adopted as a proxy for flight efficiency and dispersal in birds. We show that HWI is correlated with geography and ecology across 10,391 (>99 %) bird species, increasing at higher latitudes and in migratory and/or non-territorial species. After controlling for these effects, the strongest predictor of HWI is temperature variability (i.e. seasonality), with secondary effects of diet and habitat type. Our analyses (1) reveal a prominent latitudinal gradient in HWI shaped by ecological and environmental factors, and (2) provide a global index of avian dispersal ability for wider use in community ecology, macroecology, and macroevolution.\n\nStatement of authorshipThe study was conceived by CS and JAT. Data collection was led by JAT, SC, and CS, with contributions from CS, MNC, NA, SEIJ, CV, HEAM, TPB, and SC. CS performed the analyses. CS and JAT wrote the manuscript and all authors revised the text.

evolutionary biology↗

bioRxiv2019

Integrating behaviour and ecology into global biodiversity conservation strategies

Insights into animal behaviour play an increasingly central role in species-focused conservation practice. However, progress towards incorporating behaviour into regional or global conservation strategies has been far more limited, not least because standardised datasets of behavioural traits are generally lacking at wider taxonomic or spatial scales. Here we make use of the recent expansion of global datasets for birds to assess the prospects for including behavioural traits in systematic conservation priority-setting and monitoring programmes. Using IUCN Red List classification for >9500 bird species, we show that the incidence of threat can vary substantially across different behavioural syndromes, and that some types of behaviour--including particular foraging, mating and migration strategies--are significantly more threatened than others. When all factors are included in a combined model, behavioural traits have a weaker effect than well-established geographical and ecological factors, including range size, body mass and human population pressures. We also show that the association between behavior and extinction risk is partly driven by correlations with these underlying factors. Overall, these results suggest that a multi-species approach at the scale of communities, continents and ecosystems can be used to identify and monitor threatened behaviours, and to flag up cases of latent extinction risk, where threatened status may currently be underestimated. Our findings also highlight the importance of comprehensive standardized descriptive data for ecological and behavioural traits, and point the way forward to a deeper integration of behaviour into quantitative conservation assessments.

animal behavior and cognition↗

bioRxiv2016

Tempo and timing of ecological trait divergence associated with transitions to coexistence in birds

Summary paragraphOrganismal traits may evolve either gradually or in rapid pulses followed by periods of stasis, but the relative importance of these evolutionary models in generating biodiversity has proven difficult to resolve1,2. In addition, while it is often assumed that pulses of trait evolution are associated with speciation events, few studies have explicitly examined how the tempo of trait divergence varies with respect to different geographical phases of speciation. Thus, we still know little about the trajectories of trait divergence over timescales relevant to speciation, or the extent to which these trajectories are shaped by variation in geographical isolation and overlap (sympatry) among incipient species. Here, we combine divergence time estimates, trait measurements, and geographic range data for avian sister species pairs worldwide to examine the tempo and timing of trait divergence during allopatric speciation. We show that divergence in two important ecological traits--?body mass and beak morphology--is best explained by a model including pulses of divergence and periods of relative stasis. We also infer that trait divergence pulses often precede sympatry, and that pulses leading to greater trait disparity are associated with earlier transitions to sympatry. These findings suggest that early pulses of trait divergence promote subsequent transitions to sympatry, rather than such pulses occurring after sympatry has been established, for example via character displacement3. Incorporating pulsed divergence models into allopatric speciation theory helps to resolve some apparently contradictory observations, including widespread instances of both rapid sympatry and prolonged geographical exclusion4-6.

evolutionary biology↗