Wind turbines as landscape impediments to the migratory connectivity of bats.

AuthorCryan, Paul M.
  1. INTRODUCTION II. THE UNIQUE AND MYSTERIOUS MIGRATIONS OF BATS A. Winter Survival Strategies B. The Elusive Bat: Research Challenges C. Unique Migration Behaviors of Bats III. PERILS OF MIGRATION IN BATS A. Species Characteristics Indicate Higher Natural Mortality in Migratory Bats B. Migratory Bat Collisions with Human-Made Structures IV. WIND TURBINES: AN EMERGING THREAT TO BAT MIGRATIONS A. Documenting Bat Fatalities at Wind Energy Sites B. Emerging Patterns of Bat Mortality at Wind Energy Sites C. Impacts on Migratory Bat Populations V. THE IMPORTANCE OF SCIENTIFIC RESEARCH INTO BAT MORTALITY AT TURBINES A. Gaps in Legal Protection B. Gaps in Monitoring and Conservation at Wind Energy Sites .. C. Benefits of Research on Bat Mortality at Wind Turbines ... VI. CONCLUSION I. INTRODUCTION

    Several species of insectivorous bats migrate hundreds to thousands of kilometers each spring and autumn, crossing a wide variety of landscape features and ecosystems on their journey. (1) These long-distance nocturnal flights, combined with the cryptic diurnal habits of migratory bats, have made it extremely difficult to uncover the details of their seasonal whereabouts, movements, and migration behaviors. Beginning around the turn of the millennium, a scatter of reports came to light regarding the surprising numbers of migratory bats found dead beneath wind turbines during autumn across both North America and Europe. (2) Since the release of these studies, mortality of migratory bats at wind turbines during late-summer and autumn has become a major conservation issue. (3) Whereas there were no known energy-related imminent threats to populations of migratory bats prior to about the year 2000, observed fatality rates of certain species at turbines now indicate the distinct possibility of population declines. At some sites, the estimated number of bats killed range from hundreds to over one thousand in a single autumn migration season, with cumulative estimates for North America ranging into the hundreds of thousands per year, eclipsing any previously observed mortality of these mysterious migrants. (4) Over the past decade it has become apparent that wind turbines have the potential to seriously impede and disrupt the migration--and therefore long-term persistence--of several species of bats at a continental scale. Importantly, none of the migratory bats most affected by wind turbines are protected by national conservation laws or international treaties, (5) so legal mandates for researching and finding practical solutions to the problem are lacking.

    This Article describes the unprecedented bat mortality caused by wind turbines, and the potentially disastrous effects on certain bat populations. Current laws may be insufficient to protect bat migrations, and additional research is needed to identify effective conservation solutions. Part II includes an overview of migration in bats and emphasizes how little we know about this phenomenon. Part III discusses the inherent risks of migration in bats, including both natural and human-caused mortality. Part IV provides background on the emerging problem of bat fatalities at wind turbines and compares turbine-induced mortality to other known sources of mortality in migrating bats. Part V highlights the importance of scientific research in understanding the scope and magnitude of bat mortality at turbines, and in developing effective solutions to the problem. The Article concludes by identifying the challenges associated with developing research and conservation strategies aimed at poorly understood migratory species that are not protected by law.

    H. THE UNIQUE AND MYSTERIOUS MIGRATIONS OF BATS

    There are approximately 5400 species of mammals on Earth, of which about 1100 are bats. (6) Bats occur nearly everywhere but Antarctica and some remote islands, playing important functional roles in the majority of our planet's terrestrial ecosystems. (7) In tropical and subtropical regions where species diversity is highest, bats pollinate flowers, disperse seeds, prey on small terrestrial vertebrates, and consume nocturnal insects. (8) Temperate zones of the world are occupied mostly by insectivorous bats that consume vast quantities of nocturnal flying insects during the warmer months. (9) Forty-five species of bats occur in the continental United States, and all but three species eat nothing but insects. (10)

    A. Winter Survival Strategies

    Bats, as flying insect predators, have evolved a spectrum of strategies for surviving seasonally harsh conditions and winter absence of prey. One general winter survival strategy, exhibited by about half of the bats occurring in the United States, involves moving tens to hundreds of kilometers from summer habitats to underground shelters, such as caves and mines. (11) There, bats lower their metabolism and body temperatures for several months to save energy during the process of their hibernation, which allows them to live off of stored fat reserves throughout the winter. At a different end of the spectrum, other bat species occurring in the United States either reside all year in warm areas or migrate from colder summering grounds across hundreds to thousands of kilometers to lower latitudes and warmer climates during the winter, where they continue feeding or sporadically enter daily torpor (12) when prey becomes scarce. (13) Unlike migratory birds, few, if any, species of migratory bats regularly make intercontinental movements. (14) For example, no bats migrate between the Americas and other continents nor are any known to consistently make long-distance movements from North to South America. (15) The longest known bat migrations are about 2000 kilometers. (16) Regardless, these mysterious migrations have been difficult for researchers to study.

    B. The Elusive Bat: Research Challenges

    Although many species of temperate zone bats migrate long distances, the details of their migratory movements and seasonal whereabouts are mostly lacking. As with many species of migratory songbirds, bat migration occurs at night under the cover of darkness. (17) Birds and other migratory animals that are active during the day may be tracked by observing where they appear during the course of day-to-day migration. This approach is not possible with many bats because most migratory bats are inactive during the day and hide so well that they are rarely found in their natural roosts during migration periods without substantial effort (e.g., radio tracking). (18) Caves, abandoned mines, rock crevices, tree cavities, and tree foliage conceal bats so well that, for many species, we are just beginning to form a coarse understanding of the seasonal whereabouts and habitats of their populations. However, very little is known of the migration behaviors or long-distance movements of individual bats.

    Compounding the difficulty of predicting where migratory bats are likely to occur during certain times of year is the fact that few observations of actively migrating bats have ever been recorded. A substantial body of direct and circumstantial evidence for long-distance migration leaves little doubt about the existence of such movements, but the details of how bats migrate remain almost completely unknown. (19)

    C. Unique Migration Behaviors of Bats

    Unique migration behaviors likely evolved in bats in response to natural selection acting on their distinctive thermoregulatory (20) strategies and life histories. (21) For example, temperate zone bats differ from many other long-distance aerial migrants in that they mate during autumn and winter. (22) Females often migrate in spring while pregnant, moving to habitats and regions of the continent separate from males during the early summer. (23) Additionally, most species can drop their metabolism throughout the year and enter torpor to save energy when climate, weather, or food availability is unfavorable. (24)

    Bats have high rates of adult survival, which results in longevity: individuals of at least ten to tenty years old have been documented in many species (25) and there are recent records of some bats reaching at least forty years old. (26) Reproductive success is high in bats, and for mammals of their size, they produce a small number of young and put considerable time and energy into taking care of them. (27) These life-history traits result in bat populations growing slowly and an inability to quickly rebound after rapid declines in population size. (28) For these reasons, migration behaviors of bats may be unlike those of any other migratory animals.

  2. PERILS OF MIGRATION IN BATS

    Migration is an inherently risky business. Like other taxa that migrate, difficulties associated with the conservation of migratory bats include their dependence on widely dispersed and mostly uncharacterized habitats, their propensity to become concentrated in distribution during migration, and their regular crossing of political and administrative boundaries. (29)

    A. Species Characteristics Indicate Higher Natural Mortality in Migratory Bats

    Certain species of migratory bats show signs of experiencing higher natural mortality than species that do not migrate. (30) Most bat species in the United States have one offspring per year. (31) Exceptions to this pattern are found in the species that dwell in the foliage or trunks of trees year round and migrate across several degrees of latitude, the so-called migratory "tree bats." (32) In North America, this unique group includes the hoary bat (Lasiurus cinereus), eastern red bat (Lasiurus borealis), western red bat (Lasiurus blossevillii), and silver-haired bat (Lasionycteris noctivagans), among others. These species regularly give birth to twins, and species of Lasiurus may sometimes successfully birth triplets and quadruplets. (33) The characteristic of higher reproductive output in species of tree bats may reflect a need to compensate for higher natural mortality associated with roosting...

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