Read an Excerpt

Bat Ecology

---

By M. Brock Fenton

University of Chicago Press

Copyright © 2003 M. Brock Fenton
All right reserved.
ISBN: 0226462064

---

Ecology of Cavity and Foliage Roosting Bats

Thomas H. Kunz and Linda F. Lumsden

Introduction

Bats occupy a wide variety of roosts in both natural and manmade structures. More than half of the approximately 1,100 species of living bats use plants exclusively or opportunistically as roosts. Others seek shelter in caves, rock crevices, mines, tombs, buildings, bridges, and other manmade structures. Some bats roost in cavities constructed by other animals, including bird nests and nests of ants and termites, whereas others roost in exposed places on branches and the trunks of trees. Thus, it is not surprising that the conditions and events associated with roosting have played a major role in the ecology and evolution of bats (Kunz 1982). Roosts are important sites for mating, hibernation, and rearing young. They often facilitate complex social interactions, offer protection from inclement weather, promote energy conservation, and minimize risks of predation.

Roosting habits of bats are influenced by the diversity and abundance of roosts, the distribution and abundance of food, and an energy economy influenced by body size and the physical environment. Roosting ecology is ultimately tempered by constraints of phylogenetic inertia and a compromise ofopposing selective pressures derived from both roost and nonroost sources (Kunz 1982). Morphological, physiological, and behavioral characteristics of bats commonly regarded as adaptations for roosting include flattened skulls, suction pads and disks on feet and wrists, cryptic markings and postures, clustering, torpor, and synchronous nightly departures. These traits reflect compromises imposed by manner of flight, body size, predator pressure, energy economy, and variations in the physical environment (Kunz 1982).

Bats seek shelter in a wide variety of roost types, ranging along a continuum from ephemeral to permanent. At one extreme, roosting sites in caves, mines, and some rock crevices offer the advantages of relative permanency, thermal stability, and protection from climatic extremes but may be patchy in distribution. At the other extreme, spaces beneath exfoliating bark and foliage generally are ephemeral and more subject to environmental extremes but are more abundant and ubiquitous (Kunz 1982). Associations between bats and roosts range from being obligatory to opportunistic, with selection of a particular type of roost dependent on its availability (Kunz 1996).

3

We focus this review primarily on bats that seek shelter in tree cavities, in foliage, on exposed branches and boles of trees, natural cavities, or structures modified by bats. This emphasis is timely because forests have become increasingly threatened by anthropogenic factors (e.g., timber management, deforestation, and associated habitat alteration and loss). Knowledge of roost requirements is a prerequisite to understanding the impact of disturbance on bat populations and to providing focus to conservation efforts (Fenton and Rautenbach 1998; Pierson 1998). Bats that occupy habitats that are highly susceptible to disturbance and loss are of special concern to conservation biologists. Only in recent history have manmade structures, such as mines, bridges, and buildings, provided alternative habitats comparable to caves and tree cavities.

Types of Roosts

Tree Cavities

Tree cavities are important roost resources in both temperate and tropical regions (Barclay and Brigham 1996; Boonman 2000; Kunz 1982, 1996; Pierson 1998; Rosevear 1965; Sedgeley and O'Donnell 1999b; Simmons and Voss 1998; Tuttle 1976; Verschuren 1957, 1966; figs. 1.1 and 1.2). In North America, Europe, Asia, and Australia, tree cavities are used mostly by members of the Vespertilionidae. In Africa, they are used mostly by members of the Vespertilionidae and Hipposideridae. In the Neotropics, tree cavities are used primarily by members of the Phyllostomidae and Emballonuridae. Although cavity-roosting habits are most common among microchiropterans, they may be more common in some small megachiropterans than previously recognized (Bonaccorso 1998; Flannery 1995; Rainey 1998).

Within temperate regions, the proportions of species assemblages that use tree cavities vary geographically (Humphrey 1975). In western North America and Australia, where extensive areas of native forests remain, a relative high proportion of bat species roost in tree cavities (Barclay and Brigham 1996; Churchill 1998; Pierson 1998; fig. 1.1A). By contrast, in western Europe and eastern North America, where natural tree cavities have been depleted by extensive clearing of forests and misguided forest management practices, many cavity-roosting species now rely considerably on manmade structures such as buildings (Kunz and Reynolds, in press), bridges (Kunz 1982), and bat houses (Mayle 1990; Tuttle and Hensley 1993). In some areas, tree cavities provide suitable roosting habitats for bats on a year-round basis. At higher latitudes, tree cavities may be too cold during winter months, and thus bats must seek alternative roosts, usually in caves or other subterranean structures (Mayle 1990). In lowland tropical regions, where caves are absent, tree cavities provide one of the primary roosting habitats for bats. On Barro Colorado Island, Panama, tree cavities are commonly occupied by small harem groups of Artibeus jamaicensis (Morrison 1979; fig. 1.1B).

Buttress cavities form semidarkened spaces on the exterior of lowland tropical trees (Kaufman 1988; Richards 1996; Whitmore 1998), and provide ideal roosting habitats for tropical bats (Simmons and Voss 1998; Tuttle 1976). In strangler figs, adjacent buttresses may fuse or anastomose to form deep, vertical cavities adjacent to the bole (fig. 1.1C). Such cavities and the spaces between adjacent buttresses are often used as day roosts by Saccopteryx bilineata (Bradbury and Emmons 1974; Bradbury and Vehrencamp 1976). Cavities that form in baobab trees (Adansonia digitata) are commonly used by Cardioderma cor (Vaughan 1976; fig. 1.1D) and several other micrchiropteran species in Africa (Verschuren 1957).

Cavities may form in the boles, trunks, or branches of live and dead trees. In general, large dead trees (snags) remain standing for longer periods than do small trees, with snags in old-growth Douglas fir (Pseudotsuga menziesii) thought to take 250 yr after death to completely decompose (Cline et al. 1980). In many areas, these snags provide important roosting sites for bats (e.g., Brigham et al. 1997; Crampton and Barclay 1998; Lumsden et al. 2002b; Matt-son et al. 1996; Ormsbee and McComb 1998; Zielinski and Gellman 1999).

In old-growth temperate and tropical forests, basal cavities sometimes form in the interior of living trees, when the heartwood is exposed to fire (Finney 1991). These basal cavities share some characteristics with caves, including stable temperatures and humidity, pronounced light gradients, protection from rain, relatively spacious internal flight space (Gellman and Zielinski 1996), and extended longevity, with some trees living up to 2,000 yr (Becking 1982). Because basal cavities may persist for a major portion of a tree's life, they are considered to be important resources for cavity-roosting bats (Gellman and Zielinski 1996). That hollow trees are prevalent in nutrient-poor soils, especially in tropical regions, led Janzen (1976) to suggest that rotted cavities may be selected as a mechanism to trap minerals and nitrogen from the accumulation of animal feces. If this hypothesis is correct, deposits of nitrogen-rich guano from bats may play an important role in forest dynamics (see Gellman and Zielinski 1996; Pierson 1998).

Cavity formation in trees results from a range of processes, including fungal infection, insect attack, excavation by termites and woodpeckers, lightning strikes, fire, and natural damage to branches (Bennett et al. 1994; Betts 1996; Gibbons 1994; Kalcounis and Brigham 1998; Mackowski 1984; Pierson 1998). The number and size of cavities vary with the diameter, age, and height of the tree, with larger and older trees having experienced longer periods of exposure to processes of cavity formation and development than smaller trees (Bennett et al. 1994; Lindenmayer et al. 1993; Sedgeley and O'Donnell 1999b). These relationships vary among species, with some trees forming cavities at earlier stages of growth than others (Bennett et al. 1994; Cline et al. 1980; Gibbons 1994; Raphael and Morrison 1987). In Australia and New Zealand, where no vertebrate species are known to excavate tree cavities, roosts used by bats are more often found in older trees. Tree cavities that form due to physical forces or actions of invertebrates generally do not develop until trees are large and old, usually after 100 or more years (Mackowski 1984; Mawson and Long 1994).

When trees with cavities and buttresses die and fall, they may continue to provide roosting spaces for bats (fig. 1.2). In the tropics, cavities in fallen tree trunks are commonly used as roosts by members of the Emballonuridae (e.g., Emballonura monticola, Cormura brevirostris, Peropteryx leucoptera, Saccopteryx bilineata [Bernard 1999; Lekagul and McNeely 1977; Reid 1997; Tuttle 1970]) and Phyllostomidae (e.g., Carollia perspicillata, Lonchophylla thomasi, Micronycteris hirsuta, M. megalotis, Mimon crenulatum, M. bennettii, Trachops cirrhosus [LaVal 1977; Reid 1997; Simmons and Voss 1998; Tuttle 1970]). In Central America, fallen trees appear to be the primary roosting habitat for Furipterus horrens (Reid 1997), with up to 59 individuals recorded from a single roost (LaVal 1977).

Selection of Tree-Cavity Roosts

Knowledge of how bats use tree cavities has increased in recent years, largely due to the use of radiotelemetry. Early research on cavity-roosting bats concentrated mostly on relatively large species (e.g., Barclay et al. 1988; Fenton 1983), but with radio transmitters currently weighing less than 0.5 g, knowledge of small cavity-roosting species (6 g), such as Myotis californicus (Brigham et al. 1997) and Vespadelus pumilus (Law and Anderson 2000) have markedly increased.

Several patterns have begun to emerge in the types of roosts used by cavity-roosting bats. Recent studies have focused on whether bats select particular roost attributes relative to their abundance (see references in table 1.1). To this end, roost and available habitat features have been measured and compared statistically. Because cavity-roosting bats generally do not modify their roost environment, they select roost sites from those that form from physical means, invertebrate activity, or facilitated by cavity excavators. In north temperate regions, several species of bats occupy abandoned woodpecker cavities (e.g., Betts 1996; Gaisler et al. 1979; Kalcounis and Brigham 1998; Pierson 1998; van Heerdt and Sluiter 1965), and in fact some species select these cavities in preference to those that form by physical or invertebrate action. For example, 85% of the 81 Nyctalus noctula roosts reported by Boonman (2000) in the Netherlands were observed in cavities excavated by woodpeckers. In these and similar situations, roost selection was strongly influenced by the preference of the original excavators for nesting sites and the decay characteristics of the tree (Kalcounis and Hecker 1996).

Selection pressures (in particular due to microclimate variables and predators) that govern the choice of tree and site characteristics should be similar regardless of the species of bat (Vonhof and Barclay 1996). Vonhof and Barclay (1996) and Brigham et al. (1997) predicted that cavity-roosting bats should require a number of large dead trees, in specific stages of decay and that project above the canopy in relatively open areas (fig. 1.3).

To assess the generality of these predictions, we summarize results from 26 studies (table 1.1) that have statistically assessed selection of various roost tree and site attributes. Some species in these studies roosted both in tree cavities and beneath exfoliating bark, and these were not separated in the assessment of roost selection. Thus, we include information on tree cavities and spaces beneath exfoliating bark. Roosts located beneath exfoliating bark are treated in more detail below.

Selection of roosts can occur at a number of different levels (cavity, tree, stand, and landscape), and various studies have addressed one or more of these criteria. Most studies have assessed selection at the level of the tree and/or the stand, with fewer studies providing information on cavity selection and landscape characteristics.

Cavity size and shape may directly influence the number of bats present, their social structure, and roost microclimate (Kunz 1982). Several investigators have demonstrated differences in the types of openings used by bats to gain access to roosts (Lumsden et al. 2002b; Vonhof and Barclay 1996), but few studies have measured "available" cavities to assess whether bats actually select particular types of cavities. A notable exception is the study by Sedge-ley and O'Donnell (1999a) in New Zealand, who compared characteristics of 84 tree cavities used by Chalinolobus tuberculatus with 57 other available but unoccupied cavities. Roosts were predominantly formed in knotholes with medium-sized openings and had thick cavity walls with dry, medium-sized internal spaces. Roosts typically were high above the ground, and the areas that surrounded the openings were uncluttered by adjacent vegetation. Sedge-ley and O'Donnell (1999a) suggested that these characteristics facilitated easy access to the roost and provided good insulation. The microclimate (temperature and humidity) in available cavities was also compared, with roost cavities being the most stable, as well as having higher humidities and temperatures that continued well into the night (Sedgeley 2001).

Cavity roosts may be selected by bats to increase their protection from predators and competitors. Predation on bats may occur within the roost or as they depart at dusk (Fenton et al. 1994; Speakman 1991). Bats that enter torpor on a daily (or extended) basis may not be sufficiently alert to escape from predators and, hence, need to select roosts that deny entry to predators. In temperate regions of Australia, predation by birds and arboreal animals, including goannas, pythons, and marsupial carnivores, may exert a strong selection pressure on bats to choose tree cavities with openings not much larger than their own body size (Tidemann and Flavel 1987). However, small openings to tree cavities may not be as important in areas that lack predators (e.g., in New Zealand [Sedgeley and O'Donnell 1999a]) or in tropical regions, where some species roost in cavities with large basal openings (e.g., Desmodus rotundus [Wilkinson 1985], Rhinolophus hildebrandti [Fenton and Rautenbach 1986], Noctilio albiventris [Fenton et al. 1993], and Nycteris thebaica [Aldridge et al. 1990]).

In the Neotropics, where Saccopteryx bilineata typically roosts in relatively accessible buttress cavities (Bradbury and Emmons 1974; Bradbury and Vehrencamp 1976) they may remain active to avoid predators (Genoud and Bonaccorso 1986). Alertness, however, does not guarantee protection from predators, as observed by Arendt (1986) when a St. Lucia boa (Boa constrictor) captured a Brachyphylla cavernarum that unsuccessfully retreated upward into a large tree cavity. That some tropical species select cavities with large openings may reflect the fact that thermal constraints generally are less than in temperate regions where well-insulated cavities should offer important thermoregulatory advantages.

A number of variables have been measured to assess selection at the level of the roost tree and in the immediate vicinity of the tree (roost stand; table 1.1). As predicted by Vonhof and Barclay (1996) and Brigham et al. (1997), most, but not all, bat species that have been studied select trees that are large in diameter, taller than surrounding trees, and relatively uncluttered by adjacent vegetation. Vonhof and Barclay (1996) and Betts (1998) suggest three benefits of such roosts: (1) increased conspicuousness and hence ease of bats finding the roost tree; (2) reduced predation risk; and (3) maintenance of an optimal microclimate. Tall trees with an open canopy generally experience elevated exposures to solar radiation that may increase the energetic benefits to bats.

In general, bats are not likely to discriminate between tree species per se but, rather, select trees based on the specific characteristics of the cavities associated with a particular species (Sedgeley and O'Donnell 1999b). Notwithstanding, the species of tree was an important variable in several studies (e.g., Boonman 2000; Sedgeley and O'Donnell 1999b; Vonhof 1996). This may reflect the fact that some species provide better insulation than others. Using infrared thermal imaging, Rieger (1996) showed that beech trees (Fagus sylvatica) used as roosts by Myotis daubentonii remained warmer during the day and night than other tree species. The size of the tree appears to influence the microclimate in the cavity roost, and thus large trees are often selected as roosts (table 1.1). Slender trunks offer less insulation against extreme temperatures than do large ones (Alder 1994; Gellman and Zielinski 1996; Sluiter et al. 1973).

The amount of bark present on a tree also appears to influence the internal microclimate of the roost, with thicker bark providing the greatest insulation (Nicolai 1986). Dead trees are generally less well insulated than live ones owing to a lack of bark and a lower water content (Maeda 1974). Dead trees often contain more cavities than do live ones; and many of the studies summarized in table 1.1 noted that most roost trees were dead, although this varied among tree species and areas. Some trees, such as conifers, generally do not form cavities until they begin to decay, whereas species such as eucalypts form cavities when they are alive and healthy. Thus, in the case of eucalypts, although dead trees are not categorically avoided, certain bat species roost primarily in live trees (Lumsden et al. 2002b).

Not only do bats select particular trees as roost sites, they may also select particular parts of the forest in which to roost. Several studies have compared variables within the roost stand to other areas of the forest, with the majority showing selection for one or more of these variables (table 1.1). For example, in North America, areas around roosts of Lasionycteris noctivagans had more roost-type trees, a lower canopy cover, shorter understory, and less vegetative cover than did random plots (Campbell et al. 1996). In southeastern Australia, Nyctophilus geoffroyi and Chalinolobus gouldii selected areas of forest that contained high densities of their respective preferred roost trees (Lumsden et al. 2002a).

Selection can also occur at the landscape scale with roosts of some species being closer to water (Boonman 2000; Ormsbee and McComb 1998), closer to the forest edge (Boonman 2000; Sedgeley and O'Donnell 1999b), or associated with other landscape elements (Lumsden et al. 2002a). In the Netherlands, Boonman (2000) found that Nyctalus noctula and Myotis daubentonii roosted closer to the edge of forested areas than was expected from randomly chosen cavities and suggested that these trees may experience greater exposure to solar radiation, resulting in warmer cavities. Moreover, bats that foraged outside the forested area were able to reduce the time and energy spent flying through the forest.

How dependent bats are on certain characteristics of roosts can be explored by determining ways that a single species reacts to the availability of roost resources. Dependence on certain characteristics would be indicated if some variables were consistently selected in different environments. If variables were used selectively it might indicate that the bats were more flexible in their use of these characteristics. Selection of roosts by Vespadelus pumilus was investigated at two sites in eastern Australia with different disturbance histories: old-growth and regenerating forest (Law and Anderson 2000). At sites where numerous large, old trees were available, bats selected those in preference to smaller trees. In the regenerating forest, the remaining dead trees and large trees in an adjacent area were preferentially used as roosts. In addition, understory trees, such as blackwood (Acacia melanoxylon), which forms cavities at a smaller tree diameter were used as roost sites. Although these understory trees were present in the old-growth forest, they were not used as roosts when more suitable cavities were available.

Trees selected by bats may not only vary regionally and by area but also intraspecifically by sex and season. Roosts selected by maternity colonies may be different from those used during the nonbreeding period. Adult males and nonreproductive females tend to select cooler roost sites at temperatures that allow them to enter torpor, thus minimizing energy expenditure (Hamilton and Barclay 1994; Kerth et al. 2000). During the nonbreeding season in southeastern Australia, both males and females of Nyctophilus geoffroyi occupy a wide range of structures, including buildings, under bark, and within cavities in relatively small trees. Within the same area, females selected large dead trees during the maternity period that were more than twice the diameter of those selected at other times (Lumsden et al. 2002b). The location of a roost within a given landscape may also vary between maternity and nonbreeding periods. Maternity roosts of Chalinolobus tuberculatus (Sedgeley and O'Donnell 1999b) and N. geoffroyi (Lumsden et al. 2002a) were located closer to the forest edge than were nonbreeding roosts.

Several studies have investigated roost use during the breeding season, but they have seldom separated data by reproductive status or age (e.g., Brigham et al. 1997; Callahan et al. 1997; Kalcounis and Brigham 1998; Ormsbee and McComb 1998). While it can be expected that pregnant and lactating females both would require a warm microclimate to enhance the rapid growth of the fetus and young, distinguishing use of roosts between these groups can reveal differences in their requirements at these times. For example, Kerth et al. (2000) found that pregnant females of Myotis bechsteinii preferred significantly cooler roosts than did lactating females.

Are Tree Cavities Limited Resources for Bats?

Long-term studies on roost selection and detailed information on the availability of roosts are needed to determine whether roosts are limiting to bats. Because many species of bats show strong selection for particular types of roosts, tree cavities may be limiting depending on their relative abundance (Crampton and Barclay 1998). From observations on cavity and tree selection by Chalinolobus tuberculatus, Sedgeley and O'Donnell (1999a) determined that only 1.3% of trees contained cavities that were suitable as roosts. Based on the density of suitable trees, they calculated that more than 3,000 potential roost trees were present in their study area. Although colonies of C. tuberculatus shifted roost sites almost every day and rarely reused roosts (O'Donnell 2000), they concluded that roosts were relatively abundant (Sedgeley and O'Donnell 1999a).

Little information is available on competition between different bat species and whether they partition available roost resources. Several studies have compared different species in areas where they may have access to the same tree cavities. In some situations, no differences in roost charactersitics were found between sympatric species (Crampton and Barclay 1998; Vonhof 1996), whereas significant differences were found in other situations, which had larger sample sizes (Boonman 2000; Lumsden et al. 2002b). Perkins (1996) suggested that local distribution, species composition, and population size of bats in managed forests in North America were related to interspecific competition for limited roost sites. There is also evidence that other vertebrate and invertebrate species compete for roosts and, at times, may evict bats from tree cavities (e.g., Maeda 1974; Mason et al. 1972; Sedgeley and O'Donnell 1999a; Start 1998; Tidemann and Flavel 1987).

Spaces beneath Exfoliating Bark

Spaces that form beneath exfoliating bark (fig. 1.4) also provide alternate roosting sites to cavities in branches and tree trunks for some species. In North America, crevices beneath exfoliating bark are used predominantly by Myotis spp. (e.g., M. californicus [Brigham et al. 1997], M. evotis [Vonhof and Barclay 1996, 1997], M. septentrionalis [Foster and Kurta 1999], M. sodalis [Humphrey et al. 1977; Kurta et al. 1993, 1996], and M. volans [Vonhof and Barclay 1996]; fig. 1.4A) and by Lasionycteris noctivagans (Mattson et al. 1996; Vonhof and Barclay 1996). In Australia, spaces beneath exfloliating bark are predominantly used as roosts by Nyctophilus spp. (e.g., N. arnhemensis [Churchill 1998], N. bifax [Lunney et al. 1995], N. geoffroyi [Hosken 1996; Lumsden et al. 2002b; Taylor and Savva 1988], N. gouldi [Lunney et al. 1988; Tidemann and Flavel 1987], and N. timoriensis [Churchill 1998]). In the Neotropics, spaces beneath



Continues...

---

Excerpted from Bat Ecology by M. Brock Fenton Copyright © 2003 by M. Brock Fenton. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

---