A global synthesis of survival estimates for microbats

A species-level trait dataset of bats in Europe and beyond

Knowledge of species' functional traits is essential for understanding biodiversity patterns, predicting the impacts of global environmental changes, and assessing the efficiency of conservation measures. Bats are major components of mammalian diversity and occupy a variety of ecological niches and geographic distributions. However, an extensive compilation of their functional traits and ecological attributes is still missing. Here we present EuroBaTrait 1.0, the most comprehensive and up-to-date trait dataset covering 47 European bat species. The dataset includes data on 118 traits including genetic composition, physiology, morphology, acoustic signature, climatic associations, foraging habitat, roost type, diet, spatial behaviour, life history, pathogens, phenology, and distribution. We compiled the bat trait data obtained from three main sources: (i) a systematic literature and dataset search, (ii) unpublished data from European bat experts, and (iii) observations from large-scale monitoring programs. EuroBaTrait is designed to provide an important data source for comparative and trait-based analyses at the species or community level. The dataset also exposes knowledge gaps in species, geographic and trait coverage, highlighting priorities for future data collection.

Bat responses to climate change: a systematic review

Understanding how species respond to climate change is key to informing vulnerability assessments and designing effective conservation strategies, yet research efforts on wildlife responses to climate change fail to deliver a representative overview due to inherent biases. Bats are a species-rich, globally distributed group of organisms that are thought to be particularly sensitive to the effects of climate change because of their high surface-to-volume ratios and low reproductive rates. We systematically reviewed the literature on bat responses to climate change to provide an overview of the current state of knowledge, identify research gaps and biases and highlight future research needs. We found that studies are geographically biased towards Europe, North America and Australia, and temperate and Mediterranean biomes, thus missing a substantial proportion of bat diversity and thermal responses. Less than half of the published studies provide concrete evidence for bat responses to climate change. For over a third of studied bat species, response evidence is only based on predictive species distribution models. Consequently, the most frequently reported responses involve range shifts (57% of species) and changes in patterns of species diversity (26%). Bats showed a variety of responses, including both positive (e.g. range expansion and population increase) and negative responses (range contraction and population decrease), although responses to extreme events were always negative or neutral. Spatial responses varied in their outcome and across families, with almost all taxonomic groups featuring both range expansions and contractions, while demographic responses were strongly biased towards negative outcomes, particularly among Pteropodidae and Molossidae. The commonly used correlative modelling approaches can be applied to many species, but do not provide mechanistic insight into behavioural, physiological, phenological or genetic responses. There was a paucity of experimental studies (26%), and only a small proportion of the 396 bat species covered in the examined studies were studied using long-term and/or experimental approaches (11%), even though they are more informative about the effects of climate change. We emphasise the need for more empirical studies to unravel the multifaceted nature of bats' responses to climate change and the need for standardised study designs that will enable synthesis and meta-analysis of the literature. Finally, we stress the importance of overcoming geographic and taxonomic disparities through strengthening research capacity in the Global South to provide a more comprehensive view of terrestrial biodiversity responses to climate change.

Size at Birth, Postnatal Growth, and Reproductive Timing in an Australian Microbat

Reproductive phenology, size at birth, and postnatal growth are important life history traits that reflect parental investment. The ability to document detailed changes in these traits can be a valuable tool in the identification and management of at-risk wildlife populations. We examined reproductive traits in a common, widespread Australian microbat, Chalinolobus gouldii, at two sites over two years and derived growth curves and age estimation equations which will be useful in the study of how intrinsic and extrinsic factors alter parental investment strategies. We found that male and female offspring did not differ significantly in their size at birth or their postnatal growth rates. Bats born in 2018 were smaller at birth but grew at a faster rate than those born in 2017. When date of birth was compared across sites and years, we found bats born in 2018 had a later median birthdate (by 18 days) and births were more widespread than those born in 2017. Cooler and wetter weather during late gestation (Nov) in 2018 may have prolonged gestation and delayed births. With many bats facing threatening processes it is important to study reproductive plasticity in common and widespread “model” species, which may assist in the conservation and management of threatened microbats with similar reproductive traits.

Novel passive detection approach reveals low breeding season survival and apparent lactation cost in a critically endangered cave bat

Capture-mark-recapture/resight (CMR) methods are used for survival-rate studies, yet are often limited by small sample sizes. Advances in passive integrated transponder (PIT) technology have enabled passive detection or ‘resight’ of marked individuals using large antennas with greater read-ranges than previously possible. We used passively-detected resight data and CMR models to study survival rates of the southern bent-winged bat Miniopterus orianae bassanii, a critically endangered, cave-dwelling bat. Over three years, we used PIT-tagging to monitor 2966 individuals at the species’ largest breeding aggregation, using daily detection data (> 1.6 million detections) to estimate seasonal survival probabilities, structured by age, sex and reproductive status, and parameterise population projection matrices. This has hitherto been impossible using traditional CMR methods due to disturbance risk and low recapture rates. Bats exhibited lowest apparent seasonal survival over summer and autumn, particularly for reproductive females in summer (when lactating) and juveniles in autumn (after weaning), and high survival in winter. Lowest survival rates coincided with severe drought in summer–autumn 2016, suggesting that dry conditions affect population viability. Under all likely demographic assumptions, population projection matrices suggested the population is in deterministic decline, requiring urgent action to reduce extinction risk. Passively-collected resight data can now be used in combination with CMR models to provide extensive, robust information for targeted wildlife population management.

Similar hibernation physiology in bats across broad geographic ranges

Species with broad geographic ranges may experience varied environmental conditions throughout their range leading to local adaptation. Variation among populations reflects potential adaptability or plasticity, with implications for populations impacted by disease, climate change, and other anthropogenic influences. However, behavior may counteract divergent selection among populations. We studied intraspecific variation in hibernation physiology of Myotis lucifugus (little brown myotis) and Corynorhinus townsendii (Townsend's big-eared bat), two species of bats with large geographic ranges. We studied M. lucifugus at three hibernacula which spanned a latitudinal gradient of 1500 km, and C. townsendii from 6 hibernacula spread across 1200 km latitude and 1200 km longitude. We found no difference in torpid metabolic rate among populations of either species, nor was there a difference in the effect of ambient temperature among sites. Evaporative water loss was similar among populations of both species, with the exception of one C. townsendii pairwise site difference and one M. lucifugus site that differed from the others. We suggest the general lack of geographic variation is a consequence of behavioral microhabitat selection. As volant animals, bats can travel relatively long distances in search of preferred microclimates for hibernation. Despite dramatic macroclimate differences among populations, hibernating bats are able to find preferred microclimate conditions within their range, resulting in similar selection pressures among populations spread across wide geographic ranges.

Local trends in abundance of migratory bats across 20 years

Hoary bats (Lasiurus cinereus) and silver-haired bats (Lasionycteris noctivagans) are species of conservation concern because of the documented annual mortality that occurs at wind energy facilities. Several recent studies have predicted continental-scale declines of hoary bat populations due to interactions with wind turbines. We predicted a decrease in captures at a summer site over 20 years where researchers have captured bats using generally consistent methods. We developed a hierarchical Bayesian model to estimate the relative change in the expected number of captures while controlling for time of year, temperature, and netting effort. We found no decrease in the number of captures for either species. We suggest that the lack of decrease observed at our study site may be a result of compensatory immigration, despite potential broader-scale population declines.

Nest boxes do not cause a shift in bat community composition in an urbanised landscape

Nest boxes are often used to provide supplementary roosts for cavity-dependent wildlife, but little is known about if they influence faunal community composition. Long-term monitoring of bat boxes in south-eastern Australia indicated that their use was dominated by one generalist species (Chalinolobus gouldii), causing concern that installing bat boxes could cause a shift toward less diverse bat communities. To test this, we conducted a large-scale before-after control-impact experiment at 18 sites, over five years. Sites were either: (1) those with existing bat boxes, (2) those where boxes were added during the study, or (3) controls without boxes. We used echolocation call data from 9035 bat detector nights to compare community composition, diversity, and species' relative activity between the sites. Chalinolobus gouldii continued to dominate the use of existing boxes, but we found little difference in community composition between sites based on the presence, absence, or addition of boxes. Our study is the first to explore the influence installing artificial hollows has on localized faunal assemblages over spatio-temporal scales relevant to management. We conclude that there is cause for optimism that bat boxes might not have perverse outcomes on local community composition in the short- to medium-term, as we had feared.

Survival and persistence of tricolored bats hibernating in Arkansas mines

White-nose syndrome (WNS) has caused large declines in bat populations across eastern North America, making information on demographics of affected species critical to determining their risk for extinction. We used Cormack–Jolly–Seber models to estimate apparent survival rates of hibernating tricolored bats (Perimyotis subflavus) for 5 years in four small abandoned mines in the Ouachita Mountains of Arkansas, located within the WNS endemic area of the United States. Populations in individual mines varied greatly in survival rates, with one mine displaying annual survival rates as high as 0.706 and another as low as 0.101. Differences in survival among bats in different mines could not definitively be attributed to WNS, but may have varied based on a combination of WNS, disturbance, mine climate, and other unknown factors. Further, some hibernacula may have served as temporary winter shelter for young transient males. Sites housing small colonies of hibernating bats may result in high survival rates despite WNS, and protecting these smaller sites may be important for overall species perseverance.

Multi-year population dynamics of a specialist trawling bat at streams with contrasting disturbance

Habitat degradation leads to homogenization of biological communities, often due to the dominance of generalist species over specialists. Yet data as to how life history attributes of specialists vary with such perturbations remain sparse. We compared long-term population dynamics of a specialist trawling bat, the large-footed myotis (Myotis macropus), between two forested catchments. One forest stream was nutrient-enriched from dairy farming in its headwaters and a portion of its surrounding catchment was harvested for timber during the study, while the other was located in primarily undisturbed forest. We caught and banded bats annually at their roosts over 14 years and banded 529 individuals with a 45% recapture rate. The maximum time to recapture was nine years and there was no evidence for transiency in our populations. Mark-recapture analyses allowed for investigation of the dependence of survival on time, sex, and age at marking. Our study spanned extreme El Niño and La Niña weather events, but we found little variation in survival, although recruitment was lower during drought. Mean minimum winter temperature (positive) and rainfall (positive) had weak influences on survival. Survival of adults (~0.70) and population size of adult females was similar between the two sites, suggesting that neither timber harvesting with retained riparian buffers nor eutrophication from farming influenced survival. Survival of adult males and females was similar, but survival of juveniles was less than half that of adults, probably due to a combination of mortality and dispersal. Survival was three times lower immediately after one of the timber bridges used as a roost fully collapsed. Specializing on aquatic habitats buffered M. macropus from most extreme weather, but there was also evidence for possible mortality and recovery after an intense rainfall and flooding event immediately prior to the study. More frequent intense rainfall predicted with global warming may reduce the species’ resilience over time.

Individual asymmetry as a predictor of fitness in the bat Carollia perspicillata

The measurement of fitness in wild populations is a challenging task, and a number of proxies have been proposed with different degrees of success. Developmental instability/stability (DI) is an organismal property associated with variance in bilateral asymmetry (fluctuating asymmetry-FA) and a correlated effect on fitness. This study provides evidence to corroborate the hypothesis that asymmetry partly reflects DI and is correlated with a reduction in fitness measured by survival and reproduction in bats. We studied two colonies of the bat Carollia perspicillata in southeastern Brazil over 5 years, marking and recapturing individuals. Gaussian mixture models for signed Forearm Asymmetry (ForA) distribution indicated that ~20% of asymmetry variation was due to DI heterogeneity among individuals. ForA, body condition (Scaled Mass Index-SMI) and Forearm Length (ForL) were used as predictors of survival probability in Cormack-Jolly-Seber models. Asymmetry was negatively associated with survival, whereas SMI and ForL were positively associated. The male C. perspicillata defend sites within the roost that are favoured by female harems, but there are mating opportunities for bachelor males, leading to both territorial disputes and sperm competition. As predicted by sexual selection, ForA was negatively associated with relative Testicle Length, a measure of reproductive potential. In females, ForA was negatively associated with the probability of two pregnancies (as opposed to one) in a given breeding season. The effect magnitudes and directions of associations suggest that asymmetry, even though not perfectly reflecting DI variation, is a useful predictor for fitness components in C. perspicillata.

Differences in seasonal survival suggest species-specific reactions to climate change in two sympatric bat species

Long-lived animals with a low annual reproductive output need a long time to recover from population crashes and are, thus, likely to face high extinction risk, if the current global environmental change will increase mortality rates. To aid conservation of those species, knowledge on the variability of mortality rates is essential. Unfortunately, however, individual-based multiyear data sets that are required for that have only rarely been collected for free-ranging long-lived mammals. Here, we used a five-year data set comprising activity data of 1,445 RFID-tagged individuals of two long-lived temperate zone bat species, Natterer's bats (Myotis nattereri) and Daubenton's bats (Myotis daubentonii), at their joint hibernaculum. Both species are listed as being of high conservation interest by the European Habitats Directive. Applying mixed-effects logistic regression, we explored seasonal survival differences in these two species which differ in foraging strategy and phenology. In both species, survival over the first winter of an individual's life was much lower than survival over subsequent winters. Focussing on adults only, seasonal survival patterns were largely consistent with higher winter and lower summer survival but varied in its level across years in both species. Our analyses, furthermore, highlight the importance of species-specific time periods for survival. Daubenton's bats showed a much stronger difference in survival between the two seasons than Natterer's bats. In one exceptional winter, the population of Natterer's bats crashed, while the survival of Daubenton's bats declined only moderately. While our results confirm the general seasonal survival pattern typical for hibernating mammals with higher winter than summer survival, they also show that this pattern can be reversed under particular conditions. Overall, our study points toward a high importance of specific time periods for population dynamics and suggests species-, population-, and age class-specific responses to global climate change.

Long-term monitoring suggests bat boxes may alter local bat community structure

Bat boxes are often used to provide supplementary roosting habitats; however, little is known of their impacts on community composition. Data collected from a 25-year box-monitoring and 31-year harp trapping case study provides preliminary evidence that the installation of boxes may have contributed to one species, Gould’s wattled bat (Chalinolobus gouldii), dominating the bat community of a periurban park in Melbourne. This highlights the need for systematic monitoring and empirical assessment of conservation-focused bat box programs.

Forest bat population dynamics over 14 years at a climate refuge: Effects of timber harvesting and weather extremes

Long-term data are needed to explore the interaction of weather extremes with habitat alteration; in particular, can 'refugia' buffer population dynamics against climate change and are they robust to disturbances such as timber harvesting. Because forest bats are good indicators of ecosystem health, we used 14 years (1999-2012) of mark-recapture data from a suite of small tree-hollow roosting bats to estimate survival, abundance and body condition in harvested and unharvested forest and over extreme El Niño and La Niña weather events in southeastern Australia. Trapping was replicated within an experimental forest, located in a climate refuge, with different timber harvesting treatments. We trapped foraging bats and banded 3043 with a 32% retrap rate. Mark-recapture analyses allowed for dependence of survival on time, species, sex, logging treatment and for transients. A large portion of the population remained resident, with a maximum time to recapture of nine years. The effect of logging history (unlogged vs 16-30 years post-logging regrowth) on apparent survival was minor and species specific, with no detectable effect for two species, a positive effect for one and negative for the other. There was no effect of logging history on abundance or body condition for any of these species. Apparent survival of residents was not strongly influenced by weather variation (except for the smallest species), unlike previous studies outside of refugia. Despite annual variation in abundance and body condition across the 14 years of the study, no relationship with extreme weather was evident. The location of our study area in a climate refuge potentially buffered bat population dynamics from extreme weather. These results support the value of climate refugia in mitigating climate change impacts, though the lack of an external control highlights the need for further studies on the functioning of climate refugia. Relatively stable population dynamics were not compromised by timber harvesting, suggesting ecologically sustainable harvesting may be compatible with climate refugia.

Spatial and temporal drivers of avian population dynamics across the annual cycle

Untangling the spatial and temporal processes that influence population dynamics of migratory species is challenging, because changes in abundance are shaped by variation in vital rates across heterogeneous habitats and throughout the annual cycle. We developed a full-annual-cycle, integrated, population model and used demographic data collected between 2011 and 2014 in southern Indiana and Belize to estimate stage-specific vital rates of a declining migratory songbird, the Wood Thrush (Hylocichla mustelina). Our primary objective was to understand how spatial and temporal variation in demography contributes to local and regional population growth. Our full-annual-cycle model allowed us to estimate (1) age-specific, seasonal survival probabilities, including latent survival during both spring and autumn migration, and (2) how the relative contribution of vital rates to population growth differed among habitats. Wood Thrushes in our study populations experienced the lowest apparent survival rates during migration and apparent survival was lower during spring migration than during fall migration. Both mortality and high dispersal likely contributed to low apparent survival during spring migration. Population growth in high-quality habitat was most sensitive to variation in fecundity and apparent survival of juveniles during spring migration, whereas population growth in low-quality sites was most sensitive to adult apparent breeding-season survival. These results elucidate how full-annual-cycle vital rates, particularly apparent survival during migration, interact with spatial variation in habitat quality to influence population dynamics in migratory species.

Rare catastrophic events drive population dynamics in a bat species with negligible senescence

Bats are remarkably long-lived with lifespans exceeding even those of same-sized birds. Despite a recent interest in the extraordinary longevity of bats very little is known about the shape of mortality over age, and how mortality rates are affected by the environment. Using a large set of individual-based data collected over 19 years in four free-ranging colonies of Bechstein's bats (Myotis bechsteinii), we found no increase in the rate of mortality and no decrease in fertility demonstrating no senescence until high ages. Our finding of negligible senescence is highly unusual for long-lived mammals, grouping Bechstein's bats with long-lived seabirds. The most important determinant of adult mortality was one particular winter season, which affected all ages and sizes equally. Apart from this winter, mortality risk did not differ between the winter and the summer season. Colony membership, a proxy for local environmental conditions, also had no effect. In addition to their implications for understanding the extra-ordinary longevity in bats, our results have strong implications for the conservation of bats, since rare catastrophic mortality events can only be detected in individual based long-term field studies. With many bat species globally threatened, such data are crucial for the successful implementation of conservation programs.

Surface reflectance drives nest box temperature profiles and thermal suitability for target wildlife

Thermal properties of tree hollows play a major role in survival and reproduction of hollow-dependent fauna. Artificial hollows (nest boxes) are increasingly being used to supplement the loss of natural hollows; however, the factors that drive nest box thermal profiles have received surprisingly little attention. We investigated how differences in surface reflectance influenced temperature profiles of nest boxes painted three different colors (dark-green, light-green, and white: total solar reflectance 5.9%, 64.4%, and 90.3% respectively) using boxes designed for three groups of mammals: insectivorous bats, marsupial gliders and brushtail possums. Across the three different box designs, dark-green (low reflectance) boxes experienced the highest average and maximum daytime temperatures, had the greatest magnitude of variation in daytime temperatures within the box, and were consistently substantially warmer than light-green boxes (medium reflectance), white boxes (high reflectance), and ambient air temperatures. Results from biophysical model simulations demonstrated that variation in diurnal temperature profiles generated by painting boxes either high or low reflectance colors could have significant ecophysiological consequences for animals occupying boxes, with animals in dark-green boxes at high risk of acute heat-stress and dehydration during extreme heat events. Conversely in cold weather, our modelling indicated that there are higher cumulative energy costs for mammals, particularly smaller animals, occupying light-green boxes. Given their widespread use as a conservation tool, we suggest that before boxes are installed, consideration should be given to the effect of color on nest box temperature profiles, and the resultant thermal suitability of boxes for wildlife, particularly during extremes in weather. Managers of nest box programs should consider using several different colors and installing boxes across a range of both orientations and shade profiles (i.e., levels of canopy cover), to ensure target animals have access to artificial hollows with a broad range of thermal profiles, and can therefore choose boxes with optimal thermal conditions across different seasons.

Host social organization and mating system shape parasite transmission opportunities in three European bat species

For non-mobile parasites living on social hosts, infection dynamics are strongly influenced by host life history and social system. We explore the impact of host social systems on parasite population dynamics by comparing the infection intensity and transmission opportunities of three mite species of the genus Spinturnix across their three European bat hosts (Myotis daubentonii, Myotis myotis, Myotis nattereri) during the bats’ autumn mating season. Mites mainly reproduce in host maternity colonies in summer, but as these colonies are closed, opportunities for inter-colony transmission are limited to host interactions during the autumn mating season. The three investigated hosts differ considerably in their social system, most notably in maternity colony size, mating system, and degree of male summer aggregation. We observed marked differences in parasite infection during the autumn mating period between the species, closely mirroring the predictions made based on the social systems of the hosts. Increased host aggregation sizes in summer yielded higher overall parasite prevalence and intensity, both in male and female hosts. Moreover, parasite levels in male hosts differentially increased throughout the autumn mating season in concordance with the degree of contact with female hosts afforded by the different mating systems of the hosts. Critically, the observed host-specific differences have important consequences for parasite population structure and will thus affect the coevolutionary dynamics between the interacting species. Therefore, in order to accurately characterize host-parasite dynamics in hosts with complex social systems, a holistic approach that investigates parasite infection and transmission across all periods is warranted.