Collapse of a desert bird community over the past century driven by climate change

Flexible breeding performance under unstable climatic conditions in a tropical passerine in Southwest China

Parents may adjust their breeding time to optimize reproductive output and reduce reproductive costs associated with unpredictable climatic conditions, especially in the context of global warming. The breeding performance of tropical bird species in response to local climate change is relatively understudied compared with that of temperate bird species. Here, based on data from 361 white-rumped munia (Lonchura striata) nests, we determined that breeding season onset, which varied from 15 February to 22 June, was delayed by drought and high temperatures. Clutch size (4.52±0.75) and daily survival rate but not egg mass (0.95±0.10 g) were negatively affected by frequent rainfall. Daily nest survival during the rainy breeding season in 2018 (0.95±0.04) was lower than that in 2017 (0.98±0.01) and 2019 (0.97±0.00). The overall nesting cycle was 40.37±2.69 days, including an incubation period of 13.10±1.18 days and nestling period of 23.22±2.40 days. The nestling period in 2018 (25.11±1.97 days) was longer than that in 2017 (22.90±2.22 days) and 2019 (22.00±2.48 days), possibly due to the cooler temperatures. Climate also affected the total number of successful fledglings, which was highest under moderate rainfall in 2017 (115 fledglings) and lowest during prolonged drought in 2019 (51 fledglings). Together, our results suggest that drought and frequent rainfall during the breeding season can decrease reproductive success. Thus, this study provides important insights into bird ecology and conservation in the context of global climate change.

Toward a Monte Carlo approach to selecting climate variables in MaxEnt

MaxEnt is an important aid in understanding the influence of climate change on species distributions. There is growing interest in using IPCC-class global climate model outputs as environmental predictors in this work. These models provide realistic, global representations of the climate system, projections for hundreds of variables (including Essential Climate Variables), and combine observations from an array of satellite, airborne, and in-situ sensors. Unfortunately, direct use of this important class of data in MaxEnt modeling has been limited by the large size of climate model output collections and the fact that MaxEnt can only operate on a relatively small set of predictors stored in a computer’s main memory. In this study, we demonstrate the feasibility of a Monte Carlo method that overcomes this limitation by finding a useful subset of predictors in a larger, externally-stored collection of environmental variables in a reasonable amount of time. Our proposed solution takes an ensemble approach wherein many MaxEnt runs, each drawing on a small random subset of variables, converges on a global estimate of the top contributing subset of variables in the larger collection. In preliminary tests, the Monte Carlo approach selected a consistent set of top six variables within 540 runs, with the four most contributory variables of the top six accounting for approximately 93% of overall permutation importance in a final model. These results suggest that a Monte Carlo approach could offer a viable means of screening environmental predictors prior to final model construction that is amenable to parallelization and scalable to very large data sets. This points to the possibility of near-real-time multiprocessor implementations that could enable broader and more exploratory use of global climate model outputs in environmental niche modeling and aid in the discovery of viable predictors.

Hydrothermal physiology and climate vulnerability in amphibians

Concerns over the consequences of global climate change for biodiversity have spurred a renewed interest in organismal thermal physiology. However, temperature is only one of many environmental axes poised to change in the future. In particular, hydrologic regimes are also expected to shift concurrently with temperature in many regions, yet our understanding of how thermal and hydration physiology jointly affect performance and fitness is still limited for most taxonomic groups. Here, we investigated the relationship between functional performance, hydration state and temperature in three ecologically distinct amphibians, and compare how temperature and water loss can concurrently limit activity under current climate conditions. We found that performance was maintained across a broad range of hydration states in all three species, but then declines abruptly after a threshold of 20-30% mass loss. This rapid performance decline was accelerated when individuals were exposed to warmer temperatures. Combining our empirical hydrothermal performance curves with species-specific biophysical models, we estimated that dehydration can increase restrictions on species' activity by up to 60% compared to restriction by temperature alone. These results illustrate the importance of integrating species' hydration physiology into forecasts of climate vulnerability, as omitting this axis may significantly underestimate the effects of future climate change on Earth's biological diversity.

Desiccation limits recruitment in the pleometrotic desert seed-harvester ant Veromessor pergandei

The desert harvester ant Veromessor pergandei displays geographic variation in colony founding with queens initiating nests singly (haplometrosis) or in groups (pleometrosis). The transition from haplo- to pleometrotic founding is associated with lower rainfall. Numerous hypotheses have been proposed to explain the evolution of cooperative founding in this species, but the ultimate explanation remains unanswered. In laboratory experiments, water level was positively associated with survival, condition, and brood production by single queens. Queen survival also was positively influenced by water level and queen number in a two-factor experiment. Water level also was a significant effect for three measures of queen condition, but queen number was not significant for any measure. Foundress queens excavated after two weeks of desiccating conditions were dehydrated compared to alate queens captured from their natal colony, indicating that desiccation can be a source of queen mortality. Long-term monitoring in central Arizona, USA, documented that recruitment only occurred in four of 20 years. A discriminant analysis using rainfall as a predictor of recruitment correctly predicted recruitment in 17 of 20 years for total rainfall from January to June (the period for mating flights and establishment) and in 19 of 20 years for early plus late rainfall (January-March and April-June, respectively), often with a posterior probability > 0.90. Moreover, recruitment occurred only in years in which both early and late rainfall exceeded the long-term mean. This result also was supported by the discriminant analysis predicting no recruitment when long-term mean early and late rainfall were included as ungrouped periods. These data suggest that pleometrosis in V. pergandei evolved to enhance colony survival in areas with harsh abiotic (desiccating) conditions, facilitating colonization of habitats in which solitary queens could not establish even in wet years. This favorable-year hypothesis supports enhanced worker production as the primary advantage of pleometrosis.

Dehydration risk is associated with reduced nest attendance and hatching success in a cooperatively breeding bird, the southern pied babbler Turdoides bicolor

High air temperatures have measurable negative impacts on reproduction in wild animal populations, including during incubation in birds. Understanding the mechanisms driving these impacts requires comprehensive knowledge of animal physiology and behaviour under natural conditions. We used a novel combination of a non-invasive doubly labelled water (DLW) technique, nest temperature data and field-based behaviour observations to test effects of temperature, rainfall and group size on physiology and behaviour during incubation in southern pied babblers Turdoides bicolor, a cooperatively breeding passerine endemic to the arid savanna regions of southern Africa. The proportion of time that clutches were incubated declined as air temperatures increased, a behavioural pattern traditionally interpreted as a benefit of ambient incubation. However, we show that (i) clutches had a <50% chance of hatching when exposed to daily maximum air temperatures of >35.3°C; (ii) pied babbler groups incubated their nests almost constantly (99% of daylight hours) except on hot days; (iii) operative temperatures in unattended nests frequently exceeded 40.5°C, above which bird embryos are at risk of death; (iv) pied babblers incubating for long periods of time failed to maintain water balance on hot days; and (v) pied babblers from incubating groups lost mass on hot days. These results suggest that pied babblers might leave their nests during hot periods to lower the risk of dehydration associated with prolonged incubation at high operative temperatures. As mean air temperatures increase and extreme heat events become more frequent under climate change, birds will likely incur ever greater thermoregulatory costs of incubation, leading to compromised nest attendance and increased potential for eggs to overheat, with implications for nest success and, ultimately, population persistence.

Cellular and whole-organism effects of prolonged versus acute heat stress in a montane, desert lizard

Global climate change involves both prolonged periods of higher-than-normal temperatures and short but extreme heat waves. Both types of temperature increases are likely to be detrimental to ectotherms, and even if such temperature increases do not cause mortality directly, compensating for such temperature increases will likely entail costs to organisms. We tested the effects of prolonged periods of higher-than-average temperatures and short-term, acute heat stress in wild populations of greater short-horned lizards (Phrynosoma hernandesi), a temperate, montane lizard of the Colorado Plateau, UT, USA. We transplanted one group of lizards from a high- to a low-elevation site, exposing them to a prolonged period of warmer temperatures. These lizards, exposed to prolonged periods of higher-than-average temperatures, experienced no change in sprint speed, endurance, or heat shock protein (HSP) production after treatment compared to baseline levels; however, they had lower water content after the transplant to a warmer climate compared to before the transplant. We exposed a second group of lizards to acute heat stress by exposing them to thermally stressful temperatures for 4 h. These lizards, exposed to a short period of acute heat stress, had no change in endurance, water content, or HSP production following acute heat stress; however, lizards exposed to acute heat stress had slower sprint speeds than control lizards. Our results demonstrate that both prolonged temperature increases and acute heat stress, each of which are predicted to occur with climate change, had different cellular and/or whole organismal-level effects on lizards.

Ecological Niche Modeling: An Introduction for Veterinarians and Epidemiologists

Most infectious diseases in animals are not distributed randomly. Instead, diseases in livestock and wildlife are predictable in terms of the geography, time, and species affected. Ecological niche modeling approaches have been crucial to the advancement of our understanding of diversity and diseases distributions. This contribution is an introductory overview to the field of distributional ecology, with emphasis on its application for spatial epidemiology. A new, revised modeling framework is proposed for more detailed and replicable models that account for both the biology of the disease to be modeled and the uncertainty of the data available. Considering that most disease systems need at least two organisms interacting (i.e., host and pathogen), biotic interactions lie at the core of the pathogen's ecological niche. As a result, neglecting interacting organisms in pathogen dynamics (e.g., maintenance, reproduction, and transmission) may limit efforts to forecast disease distributions in veterinary epidemiology. Although limitations of ecological niche modeling are noted, it is clear that the application and value of ecological niche modeling to epidemiology will increase in the future. Potential research lines include the examination of the effects of biotic variables on model performance, assessments of protocols for model calibration in disease systems, and new tools and metrics for robust model evaluation. Epidemiologists aiming to employ ecological niche modeling theory and methods to reconstruct and forecast epidemics should familiarize themselves with ecological literature and must consider multidisciplinary collaborations including veterinarians to develop biologically sound, statistically robust analyses. This review attempts to increase the use of tools from ecology in disease mapping.

Hot droughts compromise interannual survival across all group sizes in a cooperatively breeding bird

Climate change is affecting animal populations around the world and one relatively unexplored aspect of species vulnerability is whether and to what extent responses to environmental stressors might be mitigated by variation in group size in social species. We used a 15-year data set for a cooperatively breeding bird, the southern pied babbler Turdoides bicolor, to determine the impact of temperature, rainfall and group size on body mass change and interannual survival in both juveniles and adults. Hot and dry conditions were associated with reduced juvenile growth, mass loss in adults and compromised survival between years in both juveniles (86% reduction in interannual survival) and adults (60% reduction in interannual survival). Individuals across all group sizes experienced similar effects of climatic conditions. Larger group sizes may not buffer individual group members against the impacts of hot and dry conditions, which are expected to increase in frequency and severity in future.

Evolution is a double-edged sword, not a silver bullet, to confront global change

Although there is considerable optimism surrounding adaptive evolutionary responses to global change, relatively little attention has been paid to maladaptation in this context. In this review, we consider how global change might lead populations to become maladapted. We further consider how populations can evolve to new optima, fail to evolve and therefore remain maladapted, or become further maladapted through trait-driven or eco-evo-driven mechanisms after being displaced from their fitness optima. Our goal is to stimulate thinking about evolution as a "double-edged sword" that comprises both adaptive and maladaptive responses, rather than as a "silver bullet" or a purely adaptive mechanism to combat global change. We conclude by discussing how a better appreciation of environmentally driven maladaptation and maladaptive responses might improve our current understanding of population responses to global change and our ability to forecast future responses.

In transition: Avian biogeographic responses to a century of climate change across desert biomes

Transition zones between biomes, also known as ecotones, are areas of pronounced ecological change. They are primarily maintained by abiotic factors and disturbance regimes that could hinder or promote species range shifts in response to climate change. We evaluated how climate change has affected metacommunity dynamics in two adjacent biomes and across their ecotone by resurveying 106 sites that were originally surveyed for avian diversity in the early 20th century by Joseph Grinnell and colleagues. The Mojave, a warm desert, and the Great Basin, a cold desert, have distinct assemblages and meet along a contiguous, east-west boundary. Both deserts substantially warmed over the past century, but the Mojave dried while the Great Basin became wetter. We examined whether the distinctiveness and composition of desert avifaunas have changed, if species distributions shifted, and how the transition zone impacted turnover patterns. Avifauna change was characterized by (a) reduced occupancy, range contractions, and idiosyncratic species redistributions; (b) degradation of historic community structure, and increased taxonomic and climatic differentiation of the species inhabiting the two deserts; and (c) high levels of turnover at the transition zone but little range expansion of species from the warm, dry Mojave into the cooler, wetter Great Basin. Although both deserts now support more drier and warmer tolerant species, their bird communities still occupy distinct climatological space and differ significantly in climatic composition. Our results suggest a persistent transition zone between biomes contributes to limiting the redistribution of birds, and highlight the importance of understanding how transition zone dynamics impact responses to climate change.

Migratory behavior and winter geography drive differential range shifts of eastern birds in response to recent climate change

Over the past half century, populations of neotropical migratory birds in North America have plummeted while populations of resident species have largely remained stable. We show that resident and migratory birds in eastern North America have responded differently to climate change over this period, with the ranges of resident species expanding along their northern margin while the ranges of migratory species have contracted at their southern margin. These results suggest that the ability to colonize newly suitable areas may make resident species resilient to future climate change but that climate-induced range contractions may make neotropical migrants vulnerable to these changes.

Over the past half century, migratory birds in North America have shown divergent population trends relative to resident species, with the former declining rapidly and the latter increasing. The role that climate change has played in these observed trends is not well understood, despite significant warming over this period. We used 43 y of monitoring data to fit dynamic species distribution models and quantify the rate of latitudinal range shifts in 32 species of birds native to eastern North America. Since the early 1970s, species that remain in North America throughout the year, including both resident and migratory species, appear to have responded to climate change through both colonization of suitable area at the northern leading edge of their breeding distributions and adaption in place at the southern trailing edges. Neotropical migrants, in contrast, have shown the opposite pattern: contraction at their southern trailing edges and no measurable shifts in their northern leading edges. As a result, the latitudinal distributions of temperate-wintering species have increased while the latitudinal distributions of neotropical migrants have decreased. These results raise important questions about the mechanisms that determine range boundaries of neotropical migrants and suggest that these species may be particularly vulnerable to future climate change. Our results highlight the potential importance of climate change during the nonbreeding season in constraining the response of migratory species to temperature changes at both the trailing and leading edges of their breeding distributions. Future research on the interactions between breeding and nonbreeding climate change is urgently needed.

Associations of breeding-bird abundance with climate vary among species and trait-based groups in southern California

The responses of individuals and populations to climate change vary as functions of physiology, ecology, and plasticity. We investigated whether annual variation in seasonal temperature and precipitation was associated with relative abundances of breeding bird species at local and regional levels in southern California, USA, from 1968-2013. We tested our hypotheses that abundances were correlated positively with precipitation and negatively with temperature in this semiarid to arid region. We also examined whether responses to climate varied among groups of species with similar land-cover associations, nesting locations, and migratory patterns. We investigated relations between seasonal climate variables and the relative abundances of 41 species as estimated by the North American Breeding Bird Survey. Associations with climate variables varied among species. Results of models of species associated with arid scrublands or that nest on the ground strongly supported our hypotheses, whereas those of species associated with coniferous forests or that nest in cavities did not. Associations between climate variables and the abundances of other trait-based groups were diverse. Our results suggest that species in arid areas may be negatively affected by increased temperature and aridity, but species in nearby areas that are cooler and less arid may respond positively to those fluctuations in climate. Relations with climate variables can differ among similar species, and such knowledge may inform projections of future abundance trajectories and geographic ranges.

Subpopulation augmentation among habitat patches as a tool to manage an endangered Mojave Desert wetlands-dependent rodent during anthropogenic restricted water climate regimes

Intensive management may be necessary to protect some highly vulnerable endangered species, particularly those dependent on water availability regimes that might be disrupted by ongoing climate change. The Amargosa vole (Microtus californicus scirpensis) is an increasingly imperiled rodent constrained to rare wetland habitat in the Mojave Desert. In 2014 and 2016, we trapped and radio-collared 30 voles, 24 were translocated and six remained at donor and recipient marshes as resident control voles. Soft-release was performed followed by remote camera and radio-telemetry monitoring. Although comparative metrics were not statistically significant, the mean maximum known distance moved (MDM) was longer for translocated (82.3 ± 14.6 m) vs. resident-control voles (74.9 ± 17.5 m) and for female (98.4 ± 19.9 m) vs. male (57.8 ± 9.1 m) voles. The mean area occupied (AO) tended to be greater in female (0.15 ± 0.04 ha) vs. male (0.12 ± 0.03 ha) voles, and control voles (0.15 ± 0.05 ha) compared with translocated voles (0.13 ± 0.03 ha). The mean minimum known time alive (MTA) was 38.2 ± 19.4 days for resident-control voles and 47.0 ± 10.6 days for translocated voles. Female survival (55.7 ± 14.3 days) exceeded that of males (31.5 ± 9.4 days) regardless of study group. Activity in bulrush/rushes mix and bulrush vegetation types was strongly and significantly overrepresented compared with salt grass and rushes alone, and habitat selection did not differ between resident and translocated voles. Our results provide ecological and methodological insights for future translocations as part of a strategy of promoting long-term survival of an extremely endangered small mammal in a wild desert environment.

Cooling requirements fueled the collapse of a desert bird community from climate change

Climate change—especially accelerated warming and drying—threatens to increase extinction risk, yet there is little evidence that physiological limitations have contributed to species declines. This study links species-specific water requirements for cooling body temperature to the collapse of a Mojave Desert bird community over the past century from climate change. Species occupying the hottest, driest sites were less likely to persist. Birds with the greatest water requirements for cooling their body temperature experienced the largest declines. Large-bodied carnivores and insectivores were especially vulnerable to cooling costs because they obtain water primarily from their food. Climate warming increases the evaporative cooling demand for birds, which will affect geographic patterns in body size and future extinction risk.

Climate change threatens global biodiversity by increasing extinction risk, yet few studies have uncovered a physiological basis of climate-driven species declines. Maintaining a stable body temperature is a fundamental requirement for homeothermic animals, and water is a vital resource that facilitates thermoregulation through evaporative cooling, especially in hot environments. Here, we explore the potential for thermoregulatory costs to underlie the community collapse of birds in the Mojave Desert over the past century in response to climate change. The probability of persistence was lowest for species occupying the warmest and driest sites, which imposed the greatest cooling costs. We developed a general model of heat flux to evaluate whether water requirements for evaporative cooling contributed to species’ declines by simulating thermoregulatory costs in the Mojave Desert for 50 bird species representing the range of observed declines. Bird species’ declines were positively associated with climate-driven increases in water requirements for evaporative cooling and exacerbated by large body size, especially for species with animal-based diets. Species exhibiting reductions in body size across their range saved up to 14% in cooling costs and experienced less decline than species without size reductions, suggesting total cooling costs as a mechanism underlying Bergmann’s rule. Reductions in body size, however, are unlikely to offset the 50 to 78% increase in cooling costs threatening desert birds from future climate change. As climate change spreads warm, dry conditions across the planet, water requirements are increasingly likely to drive population declines, providing a physiological basis for climate-driven extinctions.

Life history traits predict colonization and extinction lags of desert plant species since the Last Glacial Maximum

Variation in life-history strategies can affect metapopulation dynamics and consequently the composition and diversity of communities. However, data sets that allow for the full range of species turnover from colonization to extinction over relevant time periods are limited. The late Quaternary record provides unique opportunities to explore the traits that may have influenced interspecific variation in responses to past climate warming, in particular the rate at which species colonized newly suitable habitat or went locally extinct from degrading habitat. We controlled for differences in species climate niches in order to predict expected colonization and extinction sequences recorded in packrat middens from 15 localities in the Mohave, Sonoran, and Chihuahuan deserts of North America. After accounting for temperature niche differences, we tested the hypotheses that dispersal syndrome (none, wind, vertebrate), growth form (herb, shrub, tree) and seed mass mediated variation in postglacial colonization lags among species, whereas clonality (clonal, non-clonal), growth form, and seed mass affected extinction lags. Growth form and dispersal syndrome interactively affected colonization lags, where herbaceous species lacking long-distance dispersal mechanisms exhibited lags that exceeded those of woody, wind or vertebrate-dispersed species by an average of 2,000-5,000 yr. Growth form and seed mass interactively affected extinction lags, with very small-seeded shrubs persisting for 4,000-8,000 yr longer than other functional groups. Taller, vertebrate-dispersed plants have been shown in other studies to disperse farther than shorter plants without specialized dispersal mechanisms. We found that variation along this axis of dispersal syndromes resulted in dramatic differences in colonization rates in response to past climate change. Very small seeded shrubs may have a unique combination of long vegetative and seed bank lifetimes that may allow them to persist for long periods despite declines in habitat condition. This study indicates that readily measurable traits may help predict which species will be more or less sensitive to future climate change, and inform interventions that can stabilize and promote at-risk populations.

Chronic, sublethal effects of high temperatures will cause severe declines in southern African arid-zone birds during the 21st century

Birds inhabiting hot, arid regions are among the terrestrial organisms most vulnerable to climate change. The potential for increasingly frequent and intense heat waves to cause lethal dehydration and hyperthermia is well documented, but the consequences of sublethal fitness costs associated with chronic exposure to sustained hot weather remain unclear. Using data for species occurring in southern Africa's Kalahari Desert, we mapped exposure to acute lethal risks and chronic sublethal fitness costs under past, present, and future climates. For inactive birds in shaded microsites, the risks of lethal dehydration and hyperthermia will remain low during the 21st century. In contrast, exposure to conditions associated with chronic, sublethal costs related to progressive body mass loss, reduced nestling growth rates, or increased breeding failure will expand dramatically. For example, by the 2080s the region will experience 10-20 consecutive days per year on which Southern Pied Babblers (Turdoides bicolor) will lose ∼4% of body mass per day, conditions under which this species' persistence will be extremely unlikely. Similarly, exposure to air temperature maxima associated with delayed fledging, reduced fledgling size, and breeding failure will increase several-fold in Southern Yellow-billed Hornbills (Tockus leucomelas) and Southern Fiscals (Lanius collaris). Our analysis reveals that sublethal costs of chronic heat exposure are likely to drive large declines in avian diversity in the southern African arid zone by the end of the century.

Climate-Altered Wetlands Challenge Waterbird Use and Migratory Connectivity in Arid Landscapes

Wetlands in arid landscapes provide critical habitat for millions of migratory waterbirds across the world and throughout their annual cycle. The scope and scale of understanding avian use of these wetlands in conjunction with changes in climate are daunting yet critical to address lest we lose continent-wide migratory pathways. Here, we assess changes in waterbird use of North America's Pacific Flyway in the Great Basin by examining water availability and climate trends over the past 100 years. We found recent (1980-2015) climate warming has significantly reduced the amount and shifted seasonality of water flowing into wetlands. Further, we found remarkable changes in waterbird species composition over time. We propose that a reduced hydroperiod and lower water quality from reduction in water level and flow limits sites used by waterbirds. These factors reduce chick survivorship as they cannot metabolize saline water, which makes suitable freshwater conditions a limiting resource. Collectively, climate-induced changes in Great Basin wetlands suggest a major shift in freshwater ecosystems, resulting in degradation of a continental migratory route. This work illustrates the importance of examining multi-scale changes in critical regional resources to understand their impact across a hemispheric flyway and provides a model to examine other flyways.

Recent drought and tree mortality effects on the avian community in southern Sierra Nevada: a glimpse of the future?

Birds respond rapidly to changes in both habitat and climate conditions and thus are good indicators of the ecological effects of a changing climate, which may include warmer temperatures, changing habitat conditions, and increased frequency and magnitude of extreme events like drought. We investigated how a widespread tree mortality event concurrent with a severe drought influenced the avian community of the Sierra Nevada mountain range in California. We assessed and compared the separate effects of climate stresses and altered habitat conditions on the avian community and used this information to evaluate the changes that are likely to occur in the near future. We built tree mortality maps from freely available Landsat imagery with Google Earth Engine. We analyzed avian point counts from 2010 to 2016 in the southern Sierra Nevada, to model temperature, water deficit, and tree mortality effects on the abundances of 45 bird species, and then used these models to project abundances into the future based on three climate projections. A large portion of the avian community, 47%, had a positive relationship with temperature increase, compared to 20% that responded negatively. More species (36%) declined with drier conditions than increased (29%). More species declined in response to high tree mortality (36%) than increased (9%). A preponderance of species adapted to colder temperatures (higher elevation) had negative responses to high tree mortality and water deficit, but positive responses to increasing temperature. We projected the highest total bird abundances in the future under the warmest climate scenario that we considered, but habitat modification (e.g., tree mortality) and water deficit could offset the positive influence of temperature for many species. As other studies have shown, climate warming may lead to substantial but idiosyncratic effects on wildlife species that could result in community composition shifts. We conclude that future climate conditions may not have a universally negative effect on biodiversity in the Sierra Nevada, but probable vegetation changes and increased likelihood of extreme events such as drought should be incorporated into climate-smart forest and wildlife management decisions.

A century of climate and land-use change cause species turnover without loss of beta diversity in California's Central Valley

Climate and land-use changes are thought to be the greatest threats to biodiversity, but few studies have directly measured their simultaneous impacts on species distributions. We used a unique historic resource-early 20th-century bird surveys conducted by Joseph Grinnell and colleagues-paired with contemporary resurveys a century later to examine changes in bird distributions in California's Central Valley, one of the most intensively modified agricultural zones in the world and a region of heterogeneous climate change. We analyzed species- and community-level occupancy using multispecies occupancy models that explicitly accounted for imperfect detection probability, and developed a novel, simulation-based method to compare the relative influences of climate and land-use covariates on site-level species richness and beta diversity (measured by Jaccard similarity). Surprisingly, we show that mean occupancy, species richness and between-site similarity have remained remarkably stable over the past century. Stability in community-level metrics masked substantial changes in species composition; occupancy declines of some species were equally matched by increases in others, predominantly species with generalist or human-associated habitat preferences. Bird occupancy, richness and diversity within each era were driven most strongly by water availability (precipitation and percent water cover), indicating that both climate and land-use are important drivers of species distributions. Water availability had much stronger effects than temperature, urbanization and agricultural cover, which are typically thought to drive biodiversity decline.