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ISLAND BATS

The University of Chicago Press

Chapter One

Introduction

One of us (THF) recently polled colleagues in his department about their immediate visual response to the phrase "tropical islands." In addition to the usual images of aquamarine seas, turquoise lagoons, white sandy beaches, and lush green vegetation, people mentioned coral reefs, volcanoes, basalt, palm trees, and hammocks. By and large most of these respondents pictured benign scenes of tranquility and beauty—scenes that you would typically see in tourist brochures. But, as will become abundantly clear in this volume, the biological reality of island life is far from benign and tranquil. Ask these same people to describe "tropical islands" in less visual and more biological terms, and their responses might include such terms as "limited species diversity," "limited space and resources," "fewer predators," "physiologically harsh environments," "frequent disturbances caused by tropical storms, earthquakes, and erupting volcanoes," and, with increasing frequency and intensity, "human disturbance." For the organisms that have successfully colonized them, isolated oceanic islands worldwide can be anything but benign and Eden-like places to live.

Despite, or perhaps because of, their isolation, and limited space, resources, and species richness, islands and their species have long fascinated biologists. With their reputation as being living laboratories, islands have provided ecologists and evolutionary biologists with a much greater number of fundamental concepts than their total area (about 3% of Earth's surface; Whittaker 1998) might suggest. Beginning with Darwin's and Wallace's seminal idea of organic evolution via natural selection (Darwin and Wallace 1858), these concepts include adaptive radiation, Sewall Wright's (1931) island and stepping stone models of population genetics, Ernst Mayr's (1942) concept of allopatric speciation and the importance of founder effects, Edward O. Wilson's (1961) taxon cycle, Robert MacArthur and E. O. Wilson's (1963, 1967) equilibrium theory of island biogeography and r and K selection, Jared Diamond's (1975) community assembly rules, Graeme Caughley's (1994) small-population paradigm in conservation biology, and Ilka Hanski and Michael Gilpin's (1997) recent versions of metapopulation theory (table 1.1). Collectively, these concepts represent many of the basic cornerstones of modern evolutionary, ecological, and conservation thinking.

In addition to inspiring many important biological concepts, islands and their faunas and floras have been endlessly fascinating to biologists because of their intrinsic physical and biological features, many of which are summarized in table .2. The important physical features associated with islands are generally well-known. Two of those features—geological substrates and degree of natural disturbance—are particularly important for bats, the major subject of this book. Like their continental relatives, many island-dwelling bats use caves for their day roosts, and basic island geology can determine the extent and physical nature of caves. Many islands lie on the boundaries of crustal plates or above crustal "hot spots" and hence occur in areas of intense seismic activity. This activity can have strong negative effects on island floras and faunas. Additionally, many tropical islands occur in hurricane or typhoon zones, whose storms can also have devastating effects on populations of plants and animals.

Biological features of islands include reduced species richness (impoverishment) and taxonomically and ecologically skewed (disharmonic) faunas and floras favoring organisms with excellent over-water dispersal abilities (table 1.2). Interesting ecological features of island endemics often include reduced fecundity and dispersal ability (e.g., loss of flight), higher population densities, broader ecological niches, reduced fear of predators, larger or smaller body sizes, and elevated rates of extinction compared with their continental relatives. Island faunas and floras are also notable for harboring high proportions of endemic species such that islands contribute (or contributed) a disproportionately high number of species to Earth's biodiversity. Levels of endemism are uniformly high in flowering plants, birds, and bats on islands. In terms of numbers of endemic families or subfamilies and their genera and species, island birds are more diverse than island bats (tables 1.3 and 1.4). Fourteen families or subfamilies of birds containing 47 genera and 86 species are island endemics, compared with only 5 families or subfamilies of bats containing 7 genera and 25 species. With 5 endemic families, Madagascar has the greatest number of endemic bird families. With 2 endemic subfamilies of phyllostomid bats, the West Indies is the site of greatest endemism at higher taxonomic levels in bats. Two other groups of West Indian bats—family Natalidae and the phyllostomid tribe Stenodermatina of subfamily Stenodermatinae—evolved in the Caribbean and then colonized the mainland of Mexico and Central and South America (Dávalos, chapter 4, this volume) and hence are not strictly endemic to those islands.

Finally, island faunas and floras are notable because of their high conservation concerns. About one-quarter of the 25 global biodiversity hot spots identified by Myers et al. (2000) because of their exceptional conservation concern, for example, are island systems. These areas include the Caribbean, Madagascar, Sundaland, Wallacea, the Philippines, Polynesia/Micronesia, and New Zealand. Each of these areas contains endemic species of island-dwelling bats, many of which are considered to be "threatened" by the IUCN. More generally, extinction rates of island plants and animals are considerably higher than those of their continental relatives. In birds, for example, extinction rates on islands are 40 times higher than they are elsewhere in the world. Similarly, in the West Indies, 83–00% of nonvolant mammals, depending on family, are extinct, although only 4% of West Indian bats are known to be extinct. Likewise, most known extinctions of reptiles have occurred on islands. Most, but not all (e.g., West Indian bats; Morgan 200 ), of these extinctions have an anthropogenic cause resulting from habitat destruction, overhunting, and the introduction of exotic species (including pathogens).

Overview of Bats on Islands

Because they can fly, bats often represent most or all of the extant mammals on isolated oceanic islands. They are the only native land mammals on Hawaii, New Zealand, and many Pacific islands, for example. Island bats also contribute significantly to the overall species richness of bats. Jones et al. (chapter 16, this volume) report that fully 60% of all bat species live on islands (n = 925) and that 25% of all bats are island endemics; 8% of all bats are single-island endemic species. Thus islands have played an especially important role in the overall evolution of bats. In addition to being of considerable evolutionary interest, plant-visiting bats are particularly important as pollinators and seed dispersers in tropical island ecosystems (e.g., Cox et al. 1991; Cox et al. 1992; Elmqvist et al. 1992; Rainey et al. 1995). Banack's work (1998) in American Samoa, for example, indicates that two species of Pteropus flying foxes feed on flower and fruit resources of 78 plant species throughout their ranges and on 69 plant species on Samoa alone. Many of their food resources are produced by canopy trees in primary forests, and bats are likely to be their sole dispersers. Although fruit-eating bats appear to play a more important role in the early stages of ecological succession in the Neotropics than in the Paleotropics (Muscarella and Fleming 2007), pteropodid bats have played an important role in the recolonization of Krakatau by plants (e.g., Whittaker and Jones 994). The ability to retain seeds in viable condition in their guts for up to 9 hours makes pteropodid bats especially important as long-distance dispersers of the seeds of island plants (Shilton et al. 999). Finally, island bats are the source of considerable conservation concern. Jones et al. (chapter 6, this volume) indicate that nearly 50% of threatened bats worldwide (i.e., species designated as VU, EN, or CR in IUCN 2006) are island endemics; an additional 22% of threatened bats are single-island endemics. Not only will the loss of these bats contribute to a decrease in global biodiversity, but it also represents the loss of important ecological services such as predation on insects as well as pollination and seed dispersal. McConkey and Drake (2006), for instance, reported that seed dispersal by flying foxes declined nonlinearly with a decline in relative bat abundance on Vava'u (Tonga, Polynesia). Below a threshold abundance value, bats moved <1% of the seeds they handled <5 m from the canopies of fruit trees, whereas above this threshold they removed up to 58% of the seeds >5 m. From these results, McConkey and Drake (2006) concluded that flying foxes can become functionally extinct well before their actual numbers decline to zero. In summary, island bats and their fate are of considerable interest for many evolutionary, ecological, and conservation reasons.

Scope of this Book

This book is the outgrowth of a symposium on island bats held at the 2004 annual meeting of the Association for Tropical Biology and Conservation in Miami, Florida. Twelve of the thirteen papers delivered at that symposium appear as chapters in this book. After the meeting, we invited three additional colleagues or groups of colleagues to contribute chapters to this book, which is divided into three major sections.

Part deals with the evolution of island bats and contains four chapters. Two chapters discuss bats in the Wallacean region of Indonesia and the Philippines and two discuss bats in the West Indies. One common theme that runs through these chapters is the effect that changes in sea level has had on connections between island and mainland populations and among island populations. In both the Philippines and Wallacea, islands that are currently separated by shallow water channels formed larger islands in the Pleistocene, and bats that live on the same Pleistocene islands tend to be more closely related than bats that lived on other Pleistocene islands. Deepwater channels that persisted during the Pleistocene have also had a strong effect on the genetic structure of bats in the Philippines, Wallacea, and the Greater Antilles. Dávalos's analysis of phylogenetic relationships within seven West Indian bat lineages in three families reveals the strong imprint of geological history on bat relationships. Three periods of low water during the Miocene (between 16 and 5 Ma) promoted colonization of northern Caribbean islands by several lineages of bats and helped shape patterns of divergence within these lineages. A second common theme is that, contrary to theoretical expectations, populations of many island bats do not contain lower amounts of genetic variation than mainland populations and that even on small, isolated islands, bat populations tend to contain substantial genetic variation. Historically, island bat populations have often been large and resistant to abiotic disturbances such as hurricanes, typhoons, and volcanic eruptions. Rather than being extinction-prone, as postulated by the MacArthur-Wilson equilibrium theory of island diversity, island bats have actually been extinction-resistant (prior to the arrival of humans).

Several notable findings emerge from these four chapters. First, Heaney and Roberts used both allozymes and mitochondrial DNA (mtDNA) to estimate rates of between-island gene flow in six species of Philippine pteropodid bats. While the two data sets generally give congruent results, these authors note that allozymes reflect gene flow over much longer periods of time than does mtDNA. This is because mutations in allozyme loci don't often create new alleles, whereas nucleotide mutations in DNA are immediately visible. As a result, estimates of genetic subdivision will be higher and rates of gene flow lower when they are measured using mtDNA than when they are measured using allozymes. Second, Schmitt et al. point out that, contrary to the common view that Wallacea is merely a transition zone between the floras and faunas of Asia and Australasia, it is an evolutionarily dynamic region on its own right. Their studies show that these islands have produced a number of endemic species of bats as well as other mammals and reptiles. Third, Dávalos's analysis of West Indian bats helps to dispel the common belief that colonization of islands from mainlands is a one-way street. Her analyses show that three to six lineages of bats that are currently distributed from northern Mexico to Paraguay may trace their ancestry to the West Indies. Island groups that have successfully colonized (and radiated in some cases) the Neotropical mainland include mormoopid, natalid, and phyllostomid bats representing a variety of trophic adaptations. Finally, Fleming et al.'s analysis of three West Indian lineages of phyllostomid bats indicates that island-mainland gene exchange is still occurring in Artibeus jamaicensis; that, contrary to general expectations, two "old island endemics" (Erophylla sezekorni and E. bombifrons) show no evidence of low genetic diversity or high levels of between-island subdivision; and that the species Macrotus waterhousii, whose geographic distribution includes Mexico and the Greater Antilles, actually consists of a series of endemic island species that have been isolated from the mainland and each other for substantial periods of time. Differences in the degree of genetic subdivision within the latter two taxa are striking and point to the importance of ecological lifestyle (E. sezekorni and E. bombifrons are feeding generalists; M. waterhousii is a specialist on large insects) and trophic position, rather than length of island residency per se, in determining the genetic structure of island bats.

Part 2 deals with the ecology of island bats and contains seven chapters. Topics included in this section range from the physiological challenges that island bats face to the effects of major abiotic disturbances (hurricanes and volcanoes) on bat populations. Included here is a fascinating discussion of a fourtrophic- level interaction among humans, fruit-eating bats, their food plants, and symbiotic cyanobacteria that has important health implications. In his review of the metabolic and life-history characteristics of island bats and birds, McNab points out that energy reduction and a lower cost of living are common themes. Smaller size, lower basal metabolic rates, reduced fecundity, and, in at least 11 families of birds, the evolution of flightlessness are the major ways by which island endotherms have reduced their costs of living in resource-limited environments. Unlike birds, island bats have not evolved flightlessness, but the New Zealand endemic short-tailed bats (Mystacina) spend considerable amounts of time foraging on and under forest litter for insects. McNab points out that many of the metabolic and life-history adaptations found in island bats and birds have made them highly vulnerable to humans and the exotic predators they have brought to islands.

Shilton and Whittaker provide the first detailed account of the role of pteropodid bats as seed dispersers in the Krakatau island system. Their data indicate that these bats have played an important role in the revegetation of these four volcanic islands for over a century. Although bats of the genus Cynopterus—the most common pteropodids in this system—are generally sedentary foragers, they sometimes fly between islands and move seeds from one island to another. Strong-flying Pteropus vampyrus bats occasionally visit the islands from Sumatra and Java and are also important long-distance seed dispersers. They conclude that the role of bats as long-distance seed dispersers and as agents of revegetation of Krakatau has been underestimated.

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