Vertebrate Populations Research Articles

The Effect of Temperature on the Distribution of Zoonotic Pathogens in Livestock and Wildlife Populations: A Systematic Review

Background. Evidence for the impact of climate change on the distribution of zoonoses has largely focussed on the burden in humans and is lacking information on the effect of temperature on nonvectorborne zoonoses that are transmitted indirectly through contaminated environments. We present a systematic literature review on the impact of temperature on the distribution of zoonotic pathogens in mammalian livestock and wildlife populations, with a focus on nonvectorborne zoonoses that can be spread through air, water, food, and soil. Methods. We systematically searched PubMed, Scopus, and Web of Science, as well as grey literature, and screened titles, abstracts, and full text. English, peer-reviewed, and full text studies were included if they: focused on temperature; considered incursion, distributional burden or risk; and focused on a zoonotic pathogen in livestock and/or wildlife populations of mammalian vertebrates that can be transmitted through indirect pathways without a nonmammalian and nonvertebrate intermediate host. Results. Temperature was an important determinant of zoonoses distribution across all 17 studies included in the final review, with 11 studies finding a positive association. The majority of studies focused on parasites (7) and bacteria (9) and were conducted in the northern hemisphere. Two studies provided future climate projections that identified areas of increasing prevalence and expanded risk for pathogens that were already established. However, no studies specifically investigated the risk of zoonotic incursion with increasing temperature. Few studies explored how local variations in temperature and urbanisation interact with distal changes like Arctic warming to affect the distribution and spread of nonvectorborne pathogens through food, water, and soil. Conclusions. The review’s findings point to the value of a One Health approach to biosecurity that builds on the interconnected relationship between human, animal, plant, and environmental health. Such research is urgently needed to inform the prioritisation and risk assessment of zoonoses more comprehensively in a rapidly changing climate.

Genetic variation at innate and adaptive immune genes - contrasting patterns of differentiation and local adaptation in a wild gull.

Immunogenetic variation in natural vertebrate populations is expected to respond to spatial and temporal fluctuations in pathogen assemblages. While spatial heterogeneity in pathogen-driven selection enhances local immunogenetic adaptations and population divergence, different immune genes may yield contrasting responses to the environment. Here, we investigated population differentiation at the key pathogen recognition genes of the innate and adaptive immune system in a colonial bird species, the black-headed gull Chroicocephalus ridibundus. We assessed genetic variation at three toll-like receptor (TLR) genes (innate immunity) and the major histocompatibility complex (MHC) class I and II genes (adaptive immunity) in gulls from seven colonies scattered across Poland. As expected, we found much greater polymorphism at the MHC than TLRs. Population differentiation at the MHC class II, but not MHC-I, was significantly stronger than at neutral microsatellite loci, suggesting local adaptation. This could reflect spatial variation in the composition of extracellular parasite communities (e.g., helminths), possibly driven by sharp differences in habitat structure between colonies. Despite contrasting patterns of population differentiation, both MHC classes showed similar regimes of diversifying selection. Some significant population differentiation was also observed at TLRs, suggesting that innate immune receptors may respond to fine-scale spatial variation in pathogen pressure, although this pattern could have been enhanced by drift. Our results suggested that local adaptation at the pathogen recognition immune genes can be maintained at relatively small or moderate spatial scales in species with high dispersal potential and they highlighted the complexity of immunogenetic responses of animals to heterogeneous environments.

Macrogenetics reveals multifaceted influences of environmental variation on vertebrate population genetic diversity across the Americas.

The broad scale distribution of population-specific genetic diversity (GDP ) across taxa remains understudied relative to species diversity gradients, despite its relevance for systematic conservation planning. We used nuclear DNA data collected from 3678 vertebrate populations across the Americas to assess the role of environmental and spatial variables in structuring the distribution of GDP , a key component of adaptive potential in the face of environmental change. We specifically assessed non-linear trends for a metric of GDP, expected heterozygosity (HE ), and found more evidence for spatial hotspots and cold spots in HE rather than a strict pattern with latitude. We also detected inconsistent relationships between HE and environmental variables, where only 11 of 30 environmental comparisons among taxa groups were statistically significant at the .05 level, and the shape of significant trends differed substantially across vertebrate groups. Only one of six taxonomic groups, freshwater fishes, consistently showed significant relationships between HE and most (four of five) environmental variables. The remaining groups had statistically significant relationships for either two (amphibians, reptiles), one (birds, mammals), or no variables (anadromous fishes). Our study highlights gaps in the theoretical foundation upon which macrogenetic predictions have been made thus far in the literature, as well as the nuances for assessing broad patterns in GDP among vertebrate groups. Overall, our results suggest a disconnect between patterns of species and genetic diversity, and underscores that large-scale factors affecting genetic diversity may not be the same factors as those shaping taxonomic diversity. Thus, careful spatial and taxonomic-specific considerations are needed for applying macrogenetics to conservation planning.

Geographic isolation reduces genetic diversity of a wide‐ranging terrestrial vertebrate, Canis lupus

AbstractGenetic diversity is theorized to decrease in populations closer to a species' range edge, where habitat may be suboptimal. Generalist species capable of long‐range dispersal may maintain sufficient gene flow to counteract this, though the presence of significant barriers to dispersal (e.g., large water bodies, human‐dominated landscapes) may still lead to, and exacerbate, the edge effect. We used microsatellite data for 2421 gray wolves (Canis lupus) from 24 subpopulations (groups) to model how allelic richness and expected heterozygosity varied with mainland–island position and two measures of range edge (latitude and distance from range center) across >7.3 million km2 of northern North America. We expected low genetic diversity both at high latitudes, due to harsh environmental conditions, and on islands, but no change in diversity with distance to the range center due to the species' exceptional dispersal ability and favorable conditions in far eastern and western habitats. We found that allelic richness and expected heterozygosity of island groups were measurably less than that of mainland groups, and that these differences increased with the island's distance to the species' range center in the study area. Our results demonstrate how multiple axes of geographic isolation (distance from range center and island habitation) can act synergistically to erode the genetic diversity of wide‐ranging terrestrial vertebrate populations despite the counteracting influence of long‐range dispersal ability. These findings emphasize how geographic isolation is a potential threat to the genetic diversity and viability of terrestrial vertebrate populations even among species capable of long‐range dispersal.

The African killifish: A short-lived vertebrate model to study the biology of sarcopenia and longevity.

Sarcopenia, the age-related decline in muscle function, places a considerable burden on health-care systems. While the stereotypic hallmarks of sarcopenia are well characterized, their contribution to muscle wasting remains elusive, which is partly due to the limited availability of animal models. Here, we have performed cellular and molecular characterization of skeletal muscle from the African killifish-an extremely short-lived vertebrate-revealing that while many characteristics deteriorate with increasing age, supporting the use of killifish as a model for sarcopenia research, some features surprisingly reverse to an "early-life" state in the extremely old stages. This suggests that in extremely old animals, there may be mechanisms that prevent further deterioration of skeletal muscle, contributing to an extension of life span. In line with this, we report a reduction in mortality rates in extremely old killifish. To identify mechanisms for this phenomenon, we used a systems metabolomics approach, which revealed that during aging there is a striking depletion of triglycerides, mimicking a state of calorie restriction. This results in the activation of mitohormesis, increasing Sirt1 levels, which improves lipid metabolism and maintains nutrient homeostasis in extremely old animals. Pharmacological induction of Sirt1 in aged animals was sufficient to induce a late life-like metabolic profile, supporting its role in life span extension in vertebrate populations that are naturally long-lived. Collectively, our results demonstrate that killifish are not only a novel model to study the biological processes that govern sarcopenia, but they also provide a unique vertebrate system to dissect the regulation of longevity.

Half of a forest bird community infected with haemosporidian parasites

IntroductionParasites play important roles in ecosystems. Through their interactions with host and vector species, they are capable of changing the behavior and population dynamics of their host species, and the shape of entire communities. Over the past years, many studies have acknowledged the role of parasitism for host populations and communities and discovered their important regulatory functions for many vertebrate populations. Although birds are a well-studied group of vertebrates, the infection patterns of endoparasites at the community level are not fully understood. Some bird species and families are known to have a higher susceptibility to certain endoparasites than others, which may be driven by their abundance in the community.MethodsOver the course of four consecutive breeding seasons (2019–2022), we monitored the patterns of endoparasite infections in a bird community of a temperate forest ecosystem. We sampled 483 birds belonging to 29 Palearctic species and investigated the prevalence of blood parasites (haemosporidian parasites) andTrichomonasspp. using molecular methods.ResultsWe found an overall prevalence of 48.1% of haemosporidians belonging to 53 genetic lineages of the three generaHaemoproteus,Leucocytozoon, andPlasmodiumspp. While the bird families Turdidae (94%) and Paridae (76%) showed a high prevalence of haemosporidians, Certhiidae, and Picidae were not infected (0%). Host–parasite network analysis detected high variability in interactions. Infections withTrichomonasspp. were not observed in the forest bird community.DiscussionWe found that the prevalence and lineage diversity of haemosporidian parasites differed between avian families and that the parasite prevalence of a family could not serve as a predictor of lineage diversity. To further assess the consequences of these host–parasite interactions for bird communities, future research should aim to disentangle the infection pathways in different ecosystems while also considering the vector community and environmental factors.

Pitfalls arising from site selection bias in population monitoring defy simple heuristics

Abstract Site selection bias can occur when researchers monitor animals at sites of great abundance, occupancy or quality. The prevailing heuristic is therefore that ecologists should never select sites using these criteria. There is thus concern that common monitoring schemes including surveys at colonies, migratory bottlenecks and artificial breeding sites intentionally monitor sites of great abundance. Whether such routine monitoring schemes likely succumb to site selection bias is unexamined. We simulate the dynamics of long‐lived vertebrate populations to test three potential pitfalls hypothesized to occur when sites are selected for monitoring based on abundance, occupancy or quality. (1) For regression to the mean, population levels might appear to decline at monitored sites because they were chosen during a peak of abundance and random fluctuations cause populations to decrease from that point. (2) Preferential sampling occurs when some sites are preferred by the focal organism and have an above‐average probability of being selected for monitoring. (3) The missing zero effect occurs when occupied sites are chosen more often for monitoring because of initial or historical occupancy. Our simulations demonstrate that regression to the mean should only occur when inter‐annual fluctuations in abundance are relatively large compared with the average difference between sites. If researchers monitor sites that are truly of great average abundance, regression to the mean is avoided. Preferential sampling can cause a delay in the detection of a decline because monitored sites are preferred by the focal species and thus should be the first to be occupied and the last to be abandoned. Finally, the missing zero effect can cause a perceived decline in a stable population because sites were chosen when they were initially occupied and thus can only be abandoned, whereas potential colonizations at initially unoccupied sites cannot be observed. Regression to the mean occurred during specific circumstances that do not seem biologically plausible for some study systems (e.g. colony surveys). It is therefore impractical to apply a single simple heuristic such as ‘never monitor animals at sites of great abundance’ across organisms of varying life histories.

Experimental reduction in blood oxygen-carrying capacity alters foraging behaviour in a colonial waterbird.

Oxidative metabolism is a key component of organismal physiology and it is primarily determined by aerobic capacity, which depends on the capacity of blood to carry oxygen. However, experimental manipulations of blood oxygen-carrying capacity are scarcely implemented to test ecophysiological hypotheses in vertebrate populations. Here we combined an experimental manipulation of blood oxygen-carrying capacity with GPS-tracking to test if suboptimal (reduced) haematological performance affects foraging behaviour in a colonial waterbird, the black-headed gull Chroicocephalus ridibundus. First, a validation of phenylhydrazine (PHZ) treatment in gulls revealed 9-18% reduction in haematocrit and blood haemoglobin concentration (via oxidative denaturation and haemolysis of erythrocytes). Then, GPS-tracking of experimental (PHZ-treated) and control (saline-treated) gulls during the incubation period provided no support for reduced or suspended engagement in energetically costly activities (long-distance foraging trips) by experimental birds. Instead, we found evidence for fine-scale alterations in foraging behaviour of PHZ-treated individuals, which resulted in fewer foraging trips per unit time, but trips that were longer in duration and in distance compared to control birds. This suggests reduced foraging performance of experimental birds (e.g. lower capacity to find and collect food during trips) or evasion of social competition, although no differences in the total investment in foraging may also suggest compensatory physiological responses to haemolytic anaemia. Our study contributes to better understanding of physio-ecological nexus in non-diving colonial avian species. At the same time, it should be subject to further investigation, if behavioural effects of reduced aerobic capacity may have any implications for gull condition and reproductive performance.

An evolutionary paradox on threadsnakes: Phenotypic and molecular evidence reveal a new and remarkably polymorphic species of Siagonodon (Serpentes: Leptotyphlopidae: Epictinae) from Amazonia

Threadsnakes are known for their conserved external morphology and historically controversial systematics, challenging taxonomic, biogeographic and evolutionary researches in these fields. Recent morphological studies—mostly based on µCT data of the skull and lower jaw—have resolved systematic issues within the group, for instance leading to the description of new taxa or re-positioning little-known scolecophidian taxa in the tree of life. Herein we describe a new polymorphic species of the genus Siagonodon from Amazonia based on morphological (external, osteology and hemipenis) and molecular data, and provide the first hemipenial description for the genus. We also reassign Siagonodon acutirostris to the genus Trilepida based on osteological data in combination with molecular evidence. The new species described represents an evolutionary paradox for scolecophidians because the species displays a remarkable variation in the shape of the snout region that is otherwise always highly conserved in this clade. Finally, this study reinforces the importance of protected areas as essential in maintaining vertebrate populations, including those that are not yet formally described.

Logistic-growth models measuring density feedback are sensitive to population declines, but not fluctuating carrying capacity.

Analysis of long-term trends in abundance of animal populations provides insights into population dynamics. Population growth rates are the emergent interplay of inter alia fertility, survival, and dispersal. However, the density feedbacks operating on some vital rates ("component feedback") can be decoupled from density feedbacks on population growth rates estimated using abundance time series ("ensemble feedback"). Many of the mechanisms responsible for this decoupling are poorly understood, thereby questioning the validity of using logistic-growth models versus vital rates to infer long-term population trends. To examine which conditions lead to decoupling, we simulated age-structured populations of long-lived vertebrates experiencing component density feedbacks on survival. We then quantified how imposed stochasticity in survival rates, density-independent mortality (catastrophes, harvest-like removal of individuals) and variation in carrying capacity modified the ensemble feedback in abundance time series simulated from age-structured populations. The statistical detection of ensemble density feedback from census data was largely unaffected by density-independent processes. Long-term population decline caused from density-independentmortalitywas the main mechanism decoupling the strength of component versus ensemble density feedbacks. Our study supports the use of simple logistic-growth models to capture long-term population trends, mediated by changes in population abundance, when survival rates are stochastic, carrying capacity fluctuates, and populations experience moderate catastrophic mortality over time.

Evidencing success: Data requirements to model the impacts of long-term management of invasive squirrels

Established widespread populations of invasive vertebrates can be challenging to eradicate and for small to medium-sized vertebrates, such as squirrels, is often not deemed feasible. However, long-term trapping campaigns are necessary to limit spread or reduce impacts. The scale of trapping programmes is often limited by budgets that are not commensurate to the scale of the problem. On-going costs and fundraising are key factors used by decision makers to assess the feasibility of complete removal or long-term management options of established populations. Predicting the time and effort required to successfully manage Invasive Alien Species (IAS) remains a challenge, particularly where different strategies need to be employed across different spatial scales and habitat types, and in response to changing population densities. Statistical methods such as removal models can be applied to quantify population abundance during management operations and could inform planning of resource requirements. However, in practice, the lack of awareness of data requirements results in unsuited or non-standardized data collection. We present data from an invasive grey squirrel (Sciurus carolinensis) management project in the United Kingdom as a case study to illustrate common issues that impact on quantifying project outcomes. We focus on human and operational aspects of implementation and suggest ways of prioritizing analytical requirements. Insights consider conservation targets, data collection and operational design and may be of relevance to stakeholders, analysts, field operators and managers.

Recovery of western black-legged tick and vertebrate populations after a destructive wildfire in an intensively-studied woodland in northern California.

Despite increasing severity and frequency of wildfires, knowledge about how fire impacts the ecology of tick-borne pathogens is limited. In 2018, the River Fire burned a forest in the far-western U.S.A. where the ecology of tick-borne pathogens had been studied for decades. Forest structure, avifauna, large and small mammals, lizards, ticks, and tick-borne pathogens (Anaplasma phagocytophilum, Borrelia burgdorferi, Borrelia miyamotoi) were assessed after the wildfire in 2019 and 2020. Burning reduced canopy cover and eliminated the layer of thick leaf litter that hosted free-living ticks, which over time was replaced by forbs and grasses. Tick abundance and the vertebrate host community changed dramatically. Avian species adapted to cavity nesting became most prevalent, while the number of foliage-foraging species increased by 83% as vegetation regenerated. Nine mammalian species were observed on camera traps, including sentinel (black-tailed jackrabbits) and reservoir hosts (western gray squirrels) of B. burgdorferi. One Peromyscus sp. mouse was captured in 2019 but by 2020, numbers were rebounding (n=37), although tick infestations on rodents remained sparse (0.2/rodent). However, western fence lizards (n=19) hosted 8.6 ticks on average in 2020. Assays for pathogens found no B. miyamotoi in either questing or host-feeding ticks, A. phagocytophilum DNA in 4% (1/23) in 2019, and 17% (29/173) in 2020 for questing and host-feeding ticks combined, and B. burgdorferi DNA in just 1% of all ticks collected in 2020 (2/173). We conclude that a moderately severe wildfire can have dramatic impacts on the ecology of tick-borne pathogens, with changes posited to continue for multiple years.

An infectious disease outbreak and increased mortality in wild alpine reindeer

AbstractClimate changes may lead to tipping points where the epidemiological characteristics of infectious wildlife diseases suddenly change, leading to outbreaks. However, empirically documented cases of the extent to which emerging infectious diseases (EIDs) affect populations of large vertebrates are few, partly because of the limited surveillance of remote wildlife populations. EIDs may potentially lead to higher mortality than mean mortality levels. Here, we quantified the likely impact of digital necrobacillosis on the survival and population dynamics of wild alpine reindeer (Rangifer tarandus) in Hardangervidda National Park (NP), Norway. The outbreak of digital necrobacillosis in 2019 contributed to a decrease in the summer survival rate of juveniles to 51% compared to an average of 88% in 2005–2018 and 77% in 2020. However, since the outbreak mainly reduced juvenile survival, the impact on short‐term population dynamics was moderate, lowering the population growth rate (λ) before harvest from an average value of 1.18 (2005–2018) to 1.08 (2019) before harvest. In 2019, anomalous climate conditions with a high daily mean temperature combined with the highest recorded daily precipitation in late summer were observed. Further studies are required to document the pathological development of digital necrobacillosis across demographic groups and proximate and ultimate causes of mortality.

Change in the Guild Structure of a Snake Community in Japan over 30 Years

Long-term field studies have provided evidence of worldwide declines in vertebrate populations, including amphibians and reptiles. However, studies investigating the decline in snake populations are scarce. As obligate predators, snakes play important roles in local ecosystems and are in turn affected by their food resources. In the present study we investigated the seasonal abundance of snakes and their food habits in a rice paddy field and compared the results of the present study with those of a previous study that was conducted at the same site approximately 30 years ago. Our results showed that Elaphe quadrivirgata, a dietary generalist snake that had been one of the dominant species at the site, has drastically declined and that Rhabdophis tigrinus and Gloydius blomhoffii have remained as dominant species. Furthermore, in the present study, most of the stomach contents of R. tigrinus consisted of the Japanese tree frog, Dryophytes japonicus, whereas three frog species, D. japonicus, Rana japonica, and Pelophylax porosus porosus, had been the main prey of R. tigrinus 30 years ago. In addition, fewer individuals of R. tigrinus and G. blomhoffii contained food in their stomachs than 30 years ago. Our findings imply that a change in prey availability may have affected the guild structure of snakes at this study site, although further investigation is necessary to verify the reasons for the decline of the dietary generalist, E. quadrivirgata.

The impact of an extreme climatic event on adder (Vipera berus) demography in southern Sweden

AbstractClimate change has been shown to have significant negative effects on wildlife populations owing to both gradual shifts in mean climatic conditions and an increased frequency of extreme climatic events. Based on a long-term study spanning 40 years, we explore the demographic effect of the exceptionally hot and dry summer of 2018 on the demography of an isolated adder (Vipera berus) population in southern Sweden. In spring 2019, we observed a dramatic decline in adder relative body mass and a concomitant 50% reduction in population size. Our results strongly suggest that the negative impacts on adder demography were the result of a combination of low prey abundance and adder dehydration caused by the extreme climatic conditions during the previous summer. The study provides an example of the significant negative effects of ongoing climate change on vertebrate populations and emphasizes the importance of long-term studies to document the demographic effects of extreme climatic events.

Evolution of viruses in immunized populations of vertebrates

Scientists have described thousands of species of viruses, many of which are pathogens of vertebrates. Given that vertebrates have their highly sophisticated adaptive immune systems capable of memorizing pathogens, interaction with such systems should theoretically be one of the most important factors influencing the evolution of viruses. The review focuses on how acquired immunity (infection-induced and vaccine-induced) affects the most important medical characteristics of viral pathogens – transmissibility, infectivity, and virulence. Both known real examples of the evolution of viruses in immunized populations, as well as theoretical articles and the results of mathematical modeling, are considered. Special attention is paid to the SARS-CoV-2 pandemic. Methodological recommendations are given for creating vaccines and conducting vaccination campaigns in the light of the raised evolutionary issues.

Daily roadkill monitoring and long-term population census reveal female-biased mortality for a small mammal along a wildland-urban interface

Expanding road networks in the wildland-urban interface may increase wildlife mortality from vehicle collisions. However, our knowledge of long-term roadkill rates and impacts on vertebrate populations is poor. We capitalized on a 31-year study to determine how roadkill rates varied with demography and traffic volume in a population of golden-mantled ground squirrels (Callospermophilus lateralis; GMGS) along 1 km of rural road in Colorado, USA. Despite low annual numbers of roadkill events (0–7 GMGS), adult female mortality was high (10.9 %) and increased with female cohort size. Adult roadkill rates increased with our traffic volume index. We estimate 71 % of roadkill events would have been undetected if we performed weekly instead of daily surveys. Hence, population-level impacts of road mortalities are likely underestimated in short-term studies that lack paired demographic data. Understanding true rates of roadkill in wildland and developing areas will help inform conservation strategies.

Wildlife in climate refugia: Mammalian diversity, occupancy, and tiger distribution in the Western Himalayas, Nepal.

Anthropogenic land-use change continues to be predicated as a major driver of terrestrial biodiversity loss for the rest of this century. It has been determined that the effect of climate change on wildlife population will accelerate the rate and process of decline of global vertebrate populations. We investigated wildlife composition, occupancy, and activity pattern along the larger climate resilient forests that serve as microrefugia for a wide range of species under the escalating climate change. We used camera trap survey covering 250 km2 of climate microrefugia in Dadeldhura hills in far western region of Nepal. We used 62 trapping locations accumulating 1800 trap nights taking 98,916 photographs in 62 days-survey period during the summer season of 2020. We photographed 23 mammalian species with estimated species richness of 30 species (95% CI: 25-34) based on multi-species occupancy model. We estimated overall species occupancy ψ(SE(ψ)) to be 0.87 (0.09) in climatic microrefugia. While human activity predominated throughout the day, the majority of animals was found to exhibit nocturnal temporal patterns. Tiger and hyaena, two of the top predators, were newly discovered in the western Himalayan range of Nepal, with their discovery at the 34 highest elevations of 2511 meters and 2000m, respectively. In Nepal, high-altitude tiger range is characterized by tiger distribution above a 2000 m cutoff representing habitats in the physiographic zone of high mountains and above. Our findings establish a baseline and show that the climatic microrefugia that have been identified have high levels of species richness and occupancy, which characterize the Dadeldhura hill forest ranges as biologically varied and ecologically significant habitat. These areas identified as climatic microrefugia habitats should be the focus of conservation efforts, particularly efforts to reduce human disturbance and adapt to climate change.

Defaunation changes leaf trait composition of recruit communities in tropical forests in French Guiana.

Hunting impacts tropical vertebrate populations, causing declines of species that function as seed dispersers and predators, or that browse seedlings and saplings. Whether and how the resulting reductions in seed dispersal, seed predation, and browsing translate to changes in the tree composition is poorly understood. Here, we assess the effect of defaunation on the functional composition of communities of tree recruits in tropical rainforests in French Guiana. We selected eight sites along a gradient of defaunation, caused by differences in hunting pressure, in otherwise intact old-growth forests in French Guiana. We measured shifts in functional composition by comparing leaf and fruit traits and wood density between tree recruits (up to 5 cm diameter at breast height) and adults, and tested whether and how these compositional shifts related to defaunation. We found a positive relationship with defaunation for shifts in specific leaf area, a negative relationship for shifts of leaf toughness and wood density, and a weak relationship for shifts in fruit traits. Our results suggest that the loss of vertebrates affects ecological processes such as seed dispersal and browsing, of which browsing remains understudied. Even though these changes sometimes seem minor, together they result in major shifts in forest composition. These changes have long-term ramifications that may alter forest dynamics for generations.

Great tits differ in glucocorticoid plasticity in response to spring temperature.

Fluctuations in environmental temperature affect energy metabolism and stimulate the expression of reversible phenotypic plasticity in vertebrate behavioural and physiological traits. Changes in circulating concentrations of glucocorticoid hormones often underpin environmentally induced phenotypic plasticity. Ongoing climate change is predicted to increase fluctuations in environmental temperature globally, making it imperative to determine the standing phenotypic variation in glucocorticoid responses of free-living populations to evaluate their potential for coping via plastic or evolutionary changes. Using a reaction norm approach, we repeatedly sampled wild great tit (<i>Parus major</i>) individuals for circulating glucocorticoid concentrations during reproduction across five years to quantify individual variation in glucocorticoid plasticity along an environmental temperature gradient. As expected, baseline and stress-induced glucocorticoid concentrations increased with lower environmental temperatures at the population and within-individual level. Moreover, we provide unique evidence that individuals differ significantly in their plastic responses to the temperature gradient for both glucocorticoid traits, with some displaying greater plasticity than others. Average concentrations and degree of plasticity covaried for baseline glucocorticoids, indicating that these two reaction norm components are linked. Hence, individual variation in glucocorticoid plasticity in response to a key environmental factor exists in a wild vertebrate population, representing a crucial step to assess their potential to endure temperature fluctuations.