Island Biogeography of Bats in Baja California, Mexico

Journal of Biogeography (J. Biogeogr.) (2008) 35, 353–364

ORIGINAL Island biogeography of bats in Baja ARTICLE California, Mexico: patterns of bat species richness in a near-shore archipelago Winifred F. Frick1*, John P. Hayes1 and Paul A. Heady III2

1Department of Forest Science, 321 Richardson ABSTRACT Hall, Oregon State University, Corvallis, OR Aim We studied the relationship between the size and isolation of islands and 97331, USA and 2Central Coast Bat Research Group, PO Box 1352, Aptos, CA 95001-1352, bat species richness in a near-shore archipelago to determine whether USA communities of vagile mammals conform to predictions of island biogeography theory. We compared patterns of species richness in two subarchipelagos to determine whether area per se or differences in habitat diversity explain variations in bat species richness. Location Islands in the Gulf of California and adjacent coastal habitats on the Baja California peninsula in northwest Mexico. Methods Presence–absence surveys for bats were conducted on 32 islands in the Gulf of California using acoustic and mist-net surveys. We sampled for bats in coastal habitats of four regions of the Baja peninsula to characterize the source pool of potential colonizing species. We ﬁtted a semi-log model of species richness and multiple linear regression and used Akaike information criterion

model selection to assess the possible inﬂuence of log10 area, isolation, and island group (two subarchipelagos) on the species richness of bats. We compared the species richness of bats on islands with greater vegetation densities in the southern gulf (n = 20) with that on drier islands with less vegetation in the northern gulf (n = 12) to investigate the relationship between habitat diversity and the species richness of bats. Results Twelve species of bats were detected on islands in the Gulf of California, and 15 species were detected in coastal habitats on the Baja peninsula. Bat species richness was related to both area and isolation of islands, and was higher in the

southern subarchipelago, which has denser vegetation. Log10 area was positively related to bat species richness, which increased by one species for every 5.4-fold increase in island area. On average, richness declined by one species per 6.25 km increase in isolation from the Baja peninsula. Main conclusions Our results demonstrate that patterns of bat species richness in a near-shore archipelago are consistent with patterns predicted by the equilibrium theory of island biogeography. Despite their vagility, bats may be more sensitive to moderate levels of isolation than previously expected in near- shore archipelagos. Differences in vegetation and habitat xericity appear to be *Correspondence and present address: Winifred associated with richness of bat communities in this desert ecosystem. Although F. Frick, Department of Ecology and observed patterns of species richness were consistent with those predicted by the Evolutionary Biology, Earth and Marine Science equilibrium theory, similar relationships between species richness and size and A308, UC Santa Cruz, Santa Cruz, CA 95064, USA. isolation of islands may arise from patch-use decision making by individuals E-mail: [email protected] (optimal foraging strategies). Present address: John P. Hayes, Department of Wildlife Ecology and Conservation, University Keywords of Florida, Gainesville, 110 Newins-Ziegler Hall, Baja California, bats, equilibrium theory, island biogeography, isolation, species PO Box 110430, Gainesville, FL 32611, USA. richness, species–area relationship.

ª 2007 The Authors www.blackwellpublishing.com/jbi 353 Journal compilation ª 2007 Blackwell Publishing Ltd doi:10.1111/j.1365-2699.2007.01798.x W. F. Frick, J. P. Hayes and P. A. Heady III

isolated habitats (Lomolino, 1984, 1986; Lawlor, 1986; Carvajal INTRODUCTION & Adler, 2005). A few biogeographical studies have analysed Insular populations have long been of interest in ecology, in insular bat faunas in oceanic archipelagos and report species– part because islands provide simplified model systems for area relationships similar to those for birds, the other volant understanding how ecological communities are structured. homeotherms (Wright, 1981; Lomolino, 1984; Lawlor, 1986; The equilibrium theory of island biogeography (MacArthur & Ricklefs & Lovette, 1999; Carvajal & Adler, 2005). Wilson, 1963, 1967) provided a theoretical foundation for Our first research objective was to determine whether bat understanding patterns of species richness in island systems by communities on a near-shore archipelago conform to predic- proposing that richness is a dynamic product between the tions of the equilibrium model of island biogeography. By opposing forces of extinction and colonization. Four decades investigating the influence of area and isolation of islands on of research have been inspired by MacArthur and Wilson’s the species richness of bats, we aimed to describe species–area influential theory, and numerous studies have field-tested the and species–isolation relationships for vagile mammals (Brown predictions of the theory by estimating the influence of the size & Kodric-Brown, 1977; Lomolino, 1984). Little research on and isolation of islands on the richness of various taxa in island biogeography of bats has been conducted on near-shore various archipelagos (Brown, 1986; Rosenzweig, 1995; Lomo- archipelagoes, perhaps because of assumptions that the lino, 2000; Whittaker & Fernańdez-Palacios, 2007). mobility of bats would preclude the detection of a relationship The species–area relationship is now one of the most widely between isolation and species richness except at very large documented patterns in ecology (Arrhenius, 1921; Preston, distances (Lomolino, 1984; Lawlor, 1986). 1962; MacArthur & Wilson, 1967; Rosenzweig, 1995). There Our second goal was to explore how area per se and habitat are several competing hypotheses about mechanisms driving diversity may independently influence bat richness. Earlier species–area relationships, including: (1) direct effects of area studies have suggested that the richness of bats may be less per se resulting from a fundamental link between area and affected by habitat diversity than the richness of some other population size that affects extinction probability (Preston, taxa (e.g. birds, herptiles, and butterflies) because bats tend to 1962; MacArthur & Wilson, 1967); (2) indirect effects of area be habitat generalists (Ricklefs & Lovette, 1999). Other studies resulting from a strong correlation between area and habitat suggest that habitat characteristics may influence the occur- diversity (Williams, 1943; MacArthur & Wilson, 1967; Ricklefs rence of bats in some isolated habitats (Carvajal & Adler, & Lovette, 1999); and (3) the passive sampling hypothesis, 2005). Islands in the southern subarchipelago in the Gulf of which results from larger areas being larger targets for California tend to have denser vegetation than islands in the colonizing individuals (Connor & McCoy, 1979; Coleman northern Midriff and Bahıá de Los A´ ngeles groups (Cody et al., 1982). Discriminating among these hypotheses has et al., 2002). Vegetation differences relate to bird richness, with proved difficult, as measures of area and habitat diversity are higher numbers of bird species on southern islands (Cody & often confounded, making their contributions to richness Velarde, 2002). The links to vegetation complexity are less well difficult to disentangle (Ricklefs & Lovette, 1999; Whittaker & established for bat species than they are for birds (MacArthur Fernańdez-Palacios, 2007). & MacArthur, 1961; Tomoff, 1977; Cody, 1989; Gill, 1995), but The species–isolation relationship has historically received we hypothesized that patterns of bat richness would mirror less attention than the species–area relationship (Lomolino, those observed for birds and be higher on southern islands as a 1986), but has equally important implications for the influ- result of differences in habitat between the two subarchi- ences of dispersal and vagility of species on distribution pelagos. patterns. Ecologically, isolation is a combination of both physical distance from a source population and the relative METHODS vagility, or dispersal capability, of the organism under study (Moilanen & Nieminen, 2002; Taylor et al., 2006). Negative Study region relationships between species richness and isolation may occur when colonization rates decline as isolation from a source The Gulf of California (Fig. 1) in northwest Mexico contains population increases (MacArthur & Wilson, 1967). Isolation more than 100 islands and islets that range in size from a few can also influence population persistence by means of a ‘rescue hectares to 1123 km2 (Carrenõ & Helenes, 2002). There are two effect’, if populations are ‘rescued’ from extinction by high subarchipelagos off the gulf coast of Baja California (Cody & immigration rates, i.e. by low effective values of isolation Velarde, 2002; Cody et al., 2002): the northern subarchipelago, (Brown & Kodric-Brown, 1977). which includes the Bahıá de Los A´ ngeles and Midriff island This study focuses on the influences of area and isolation of groups; and the southern subarchipelago, which extends from islands on bats. Although bats comprise a quarter of all Loreto to La Paz, Baja California Sur. Islands and adjacent gulf mammal species, our understanding of factors influencing coast habitats of the Baja California peninsula conform to a their community structure in isolated or fragmented habitats is Sonoran Desert ‘sarcocaulescent’ vegetation type (Shreve, 1951; minimal (Racey & Entwistle, 2003). Bats, the only volant Wiggins, 1980), dominated by columnar cacti (Pachycereus mammals, are highly mobile and are therefore generally pringlei (S. Watson) and Stenocereus thurberias Britton & Rose assumed to have high immigration and dispersal rates among Engelm.) and desert trees (Cercidium, Bursera, and Jatropha).

354 Journal of Biogeography 35, 353–364 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd Island biogeography of bats

0 20 40 80 km

Figure 1 Map of study islands (in black) in the southern and northern subarchipelagos in the Gulf of California, Baja California, Mex- ico. Dots on the Baja peninsula represent acoustic sites used to sample regional source faunas, grouped into four regions. Sampled regions on the peninsula from north to south 02040 80 0 135 270 540 are here referred to as: Bahı´a de Los A´ ngeles, km km Agua Verde, San Evaristo, and Tecolote.

The climate in the region is hot and dry, with unpredictable pelagos in the Gulf of California from 1 April to 1 June 2004– rainfall averaging between 100 and 150 mm year–1 (Cody 2006. We sampled for the presence of bat species on each et al., 2002). The northern islands receive most of their rainfall island for a 5-day period using passive Anabat acoustic stations in winter and are considerably more arid and barren than (Titley Electronics, Ballina, Australia). On 10 islands, active islands in the southern part of the Gulf, where approximately acoustic surveys and mist-net surveys were conducted to verify 40% of the rainfall occurs in the summer, resulting in denser species detected with passive acoustic stations. vegetation (Cody et al., 2002). A species was considered present if it was detected at least Five of the islands included in this study are oceanic (Partida once, without determination of breeding or residency status. Norte, Rasa, Salsipuedes, San Lorenzo, San Ildefonso, and Generally one island was sampled at a time, but occasionally Santa Catalina); the remainder are land-bridge islands that multiple small islands were sampled simultaneously. Historical were once connected to the Baja peninsula (Carrenõ & Helenes, records of bat observations exist for some islands in the study 2002). Most islands are composed of granitic or volcanic rocks region (for a review see Lawlor et al., 2002). In the few cases and have steep terrain (Carrenõ & Helenes, 2002). Apart from for which a historical record existed for a species that we did temporary fishing camps on beaches, almost all islands are not detect during our sampling, we did not include that species uninhabited by humans (Bahre & Bourilloń, 2002). in the analysis.

Data collection Acoustic sampling We conducted presence–absence surveys for bats on 32 islands We recorded bat echolocations using broadband ultrasonic bat across the northern (n = 12) and southern (n = 20) subarchi- detectors (Anabat II; Titley Electronics) to determine the

Journal of Biogeography 35, 353–364 355 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd W. F. Frick, J. P. Hayes and P. A. Heady III

Table 1 Sampling effort and characteristics of islands (n = 32) in the Gulf of California, Mexico, sampled for the presence of bat species. Number of detectors equals the number of passive acoustic detectors that ran for ‡ 3 nights. Espiritu Santo is combined with Partida Sur as a single island.

No. of Area Isolation No. of Mist-net Island Archipelago species (ha) (km) detectors surveys Year

San Jose´ South 12 18,494.5 4.75 11 yes 2005 Carmen South 9 14,801.4 5.50 13 yes 2004, 2006 Danzante South 8 423.7 2.67 6 yes 2004 Espiritu Santo South 8 10,367.1 6.21 10 yes 2005 San Francisco South 8 419.0 7.16 2 2005, 2006 Coronados South 7 715.8 2.60 7 yes 2004 Gallina South 6 2.0 7.18 1 2005 Monserrat South 6 1,902.8 13.66 8 yes 2004–2006 San Lorenzo North 6 3,632.3 16.31 9 yes 2006 Cabezo Caballo North 5 71.0 1.89 1 2004, 2006 Gemelitos East North 5 3.9 0.82 1 2004 Pardo South 5 4.3 0.36 1 2004 Santa Catalina South 5 3,995.6 25.06 13 yes 2005 Santa Cruz South 5 1,315.1 19.81 3 yes 2005 Ventana North 5 128.2 3.09 2 2004, 2006 Blanco South 4 1.3 0.84 1 2004 Cayo South 4 6.7 6.22 1 2005 Coronados Smith North 4 852.1 2.22 2 2006 Galeras East South 4 5.4 16.40 1 2004–2006 Islitas South 4 3.3 0.41 1 2004 San Ildefonso South 4 104.2 10.01 7 yes 2004, 2006 Tijeras South 4 4.0 1.90 1 2004 Galeras West South 3 3.2 16.77 1 2004–2006 Las Animas Sur South 3 9.1 16.49 1 2005 San Diego South 3 62.9 19.06 2 2005 Bota North 2 9.6 2.64 1 2004 Gemelitos West North 2 2.4 0.86 1 2004, 2006 Pata North 2 14.5 2.57 1 2004, 2006 Piojo North 2 67.6 4.57 1 2006 Salsipuedes North 2 102.6 17.70 1 2006 Partida Norte North 1 94.0 17.84 1 2006 Rasa North 1 59.2 20.75 1 2006 presence of species (Hayes, 1997; O’Farrell et al., 1999; Gehrt & Table 2 Sampling effort and number of species detected at four Chelsvig, 2004). Passive monitoring stations contained an areas sampled on the Baja peninsula, Mexico. Anabat II detector attached to a high-frequency microphone No. of No. of Mist-net housed in a waterproof shroud with a 45 reflector mounted Name Region species detectors surveys Year on a 1-m pole. The detector was connected to an Anabat Compact Flash Zero-Crossings Interface Module (Titley Elec- San Evaristo South 13 11 yes 2005 tonics) recording device. Agua Verde South 12 10 yes 2005 The number of passive acoustic stations placed on an island Tecolote South 9 13 yes 2005 ´ increased with island size (range: 1–13 detectors per island). Bahıá de Los Angeles North 7 6 yes 2006 We placed detectors at randomly determined distances between 100 and 1000 m from safe boat landings. The number fied with visual confirmation in the spotlight were used to of landings ranged from 1 to 8 per island, and 18 islands (all verify the presence of species detected with passive acoustic < 105 ha) were sampled with only one detector. We sampled detectors and to build a reference call library of echolocation 113 sites with passive acoustic stations on 32 islands (Table 1), signatures of free-flying bats. and 40 sites on four areas of the Baja peninsula (Table 2). On 10 islands, active monitoring of bat activity was Mist-net sampling conducted at mist-net survey locations using a spotlight, an Anabat II detector, and a zero-crossings analysis recorder Mist-net surveys were conducted to verify the identification of (anapocket v.2.4) on a hand-held computer. Species identi- species detected with acoustic sampling, to build an echoloca-

356 Journal of Biogeography 35, 353–364 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd Island biogeography of bats tion call reference library from hand-release recordings, and to and, although the approach taken may have negatively biased train observers to recognize flight patterns and body shapes in overall species richness, our identification of recorded species the spotlight for identifying free-flying bats with active moni- should be a reasonable index of community richness. toring. Hand-release recordings were conducted using ana- Repeated sampling across years on 10 islands (Table 1) pocket and a bright spotlight. Bats were released and recorded determined that patterns of species detection were typically as long as they remained in constant view in the spotlight. consistent across years, allowing islands sampled in different Mist-net sites on islands were selected so as to maximize years to be pooled in a single analysis. However, two species captures and were typically placed in dry arroyos (flyways) and (Nyctinomops femorosaccus and Tadarida brasiliensis I. Geof- desert scrub habitats, or over freshwater pools when available. frey) were not re-detected on islands re-sampled in 2006 in Typically, five locations were sampled on each island, except in either the northern or southern subarchipelago. Our sampling two cases where access was limited. We opened mist-nets at was limited to spring, and we make no inferences regarding the sunset and monitored them at least every 15 min for 4 h. seasonality of the presence of bats on islands. Captured bats were identified to species, age, sex, and As the distribution of bats in Baja California is poorly reproductive status (Anthony, 1988; Racey, 1988). known, we determined the potential pool of colonizing species by sampling for bats in coastal habitats on the peninsula. Four regions of the Baja gulf coast (Fig. 1) were sampled to Echolocation analysis for species identification determine the regional source pool for islands; the number of We developed a graphical and descriptive key for identifying sample sites ranged from six to 13 per region (Table 2). anabat echolocation calls based on recordings from active Regions were selected for sampling based on accessibility and monitoring of free-flying bats and recordings of hand-released proximity to islands. Sampling on the peninsula was con- individuals in Baja California, Mexico (Frick, 2007). anabat ducted in a similar manner to that on islands, with 5-day uses a zero-crossings analysis (ZCA) (Parsons et al., 2000), sampling periods. which produces files displaying echolocation calls on time– frequency graphs. Sequences were identified to species if they Data analysis had greater than two diagnostic pulses that met defined criteria based on reference calls. Call parameters such as characteristic Species-richness estimation frequency (flattest part of the call), minimum and maximum frequency, characteristic slope (slope of the flattest part of the The count of observed species was used to estimate species call), call duration, interpulse interval, and shape of the body richness: this is a reliable estimate of species richness when a of the call were measured from known reference calls and were species accumulation asymptote has been reached (Gotelli & used to characterize call sequences of species (O’Farrell et al., Colwell, 2001). Inspection of the species accumulation curves 1999; Gannon et al., 2004). indicated that species richness was asymptotic by the fourth The identification of echolocation calls can produce false- night (n = 28) of sampling, and on nine islands no new species negative and false-positive errors. False-negative errors were detected after the initial night of sampling. Asymptotic (a species is present and not detected) occur when a species species accumulation curves indicate that any potential is present and is recorded by the detector, but the call is detection biases resulting from the use of acoustic and mist- insufficiently diagnostic to be labelled as being produced by net sampling were uniform across islands and that the number that species. False-positive errors (a species is absent and is of species detected during sampling provides a consistent index falsely detected) occur when an echolocation call is misclas- of species richness. sified as a species that was in fact not present. Our approach to To determine whether five nights of sampling resulted in an classifying echolocation calls was designed to minimize both exhaustive sample, we sampled 10 islands for up to eight false-negative and false-positive errors, but greater emphasis nights. In only one case was there an additional species was placed on avoiding false-positive errors. In general, the detected after night five (Leptonycteris curasoae Miller on Isla species in the assemblage were easily identifiable to the species San Francisco on the sixth night). level using the key. Because of our efforts to minimize false-positive errors, our Island characteristics approach was not sensitive to detecting rare taxa with echolocation-call morphologies similar to those of common Island characteristics were measured using a ‘heads-up’ taxa. For example, some calls from the regionally rare species digitized GIS layer created in arcview 3.2 (ESRI, Inc.) from Lasiurus cinereus Beauvois may be confused with those from Landsat 7 satellite images (Table 1). We measured the shortest the common Nyctinomops femorosaccus Merriam, resulting in over-water path (km) to the Baja peninsula (the source an underestimate of species richness. In addition, species with population) as an index of isolation. This metric allowed for weak echolocation calls can be difficult to detect using stepping-stone-type movements by summing over-water legs if echolocation sampling methods, which may have led to a stepping-stone paths were the shortest route to the peninsula. further underestimation of species richness. We consider that This approach accounts for the presence of neighbouring these biases were consistent across islands and peninsular sites, islands by assuming that they function as stepping stones, but

Journal of Biogeography 35, 353–364 357 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd W. F. Frick, J. P. Hayes and P. A. Heady III emphasizes the role of the Baja peninsula as the source pool. Model selection More complex measures of isolation, including area-based metrics, are often advocated (Moilanen & Nieminen, 2002; We used Akaike’s information criterion (AIC) model selection Bender et al., 2003; Matter et al., 2005), but these approaches methods to compare the set of 13 a priori candidate models are more applicable to fragmented systems for which there is (Table 3), using the small sample size correction form: no clear source population (Moilanen & Nieminen, 2002). AIC ¼2Lðdatajmodel Þþ2K þ 2KðK þ 1Þ=n K 1; These metrics may also be less appropriate for community c i questions as they are highly sensitive to variation in scales of where L is the log likelihood of the observed data, given model movement among species (Bender et al., 2003; Be´lisle, 2005; i, K is the number of parameters in a model, and n is the Taylor et al., 2006). sample size (Burnham & Anderson, 2002). All models had the same sample size (n = 32 islands). Models were ranked by Species-richness regression analysis AICc value (the lowest value of AICc has the most support from the data) and were compared using DAICc and AICc

We used the semi-log model of the species–area relationship model weights (wi). The DAICc values represent the relative

(number of species vs. log10 area) because it better met the support between the best approximating model (AICc, min) and assumptions of constant variance and fit than did the log–log each competing model (AICc,i). We considered models with form of the linearized Arrhenius (1921) power function DAICc £ 2 to be strongly competing models (Burnham & (Rosenzweig, 1995) based on inspection of residual plots of Anderson, 2002). univariate regression models. Explanatory variables included AICc model weights were calculated for R models as log10 area (ha), isolation (km), and a dummy variable for 1 subarchipelago for the comparison of regression lines among expð DAICiÞ w ¼ P 2 i R 1 the northern and southern island groups. We developed 13 i¼1 expð 2 DAICiÞ a priori candidate models (Table 3) corresponding to biological hypotheses that incorporate combinations of main effects, to measure the relative support for a given model from the set additive effects, and selected interactions among area and of a priori candidate models (Burnham & Anderson, 2002). isolation and subarchipelago. We did not model an interaction between isolation and subarchipelago because we saw no RESULTS a priori reason to suspect that the species–isolation relationship would differ between the two subarchipelagos. All models Regional source fauna presented met the assumptions of linear regression models, and all analyses were conducted using the package r v.2.4.1. Fifteen species of bats were detected in coastal habitats across four regions of the Baja peninsula (Table 4). Four species Table 3 Model selection results for 13 candidate models of detected on the peninsula were never detected on islands insular richness of bat species. (Eptesicus fuscus Beauvois, Lasiurus blossevilli Lesson and No. of AIC Garnot, Myotis volans Allen, and Myotis yumanensis Allen). Model name parameters DAIC weights Community composition was similar among three areas sampled in the southern and northern regions, with a few S = logArea + Isol + Arch 5 0.00 0.76 exceptions. Mormoops megalophylla Peters and Nyctinomops S = logArea + Isol + Arch + 6 2.85 0.18 macrotis Gray were detected in the southern region but not in logArea*Isol the northern region. Eptesicus fuscus, M. volans, and Lasiurus S = logArea + Isol + Arch + 7 5.46 0.05 xanthinus Thomas were detected in at least one of the three logArea*Isol + logArea*Isol*Arch southern areas but were not detected at northern sampling S = logArea + Isol + Arch + 8 8.73 0.01 sites. However, three of these species detected in the southern logArea*Arch + logArea*Isol + logArea*Isol*Arch region, but not the northern were captured at Misioń San ´ S = logArea + Arch 4 19.62 0.00 Borjas in the mountains above Bahıá de Los Angeles, S = logArea + Isol 4 21.54 0.00 indicating that they occur in the north, but may not be S = logArea + Arch + logArea*Arch 5 21.90 0.00 common in coastal habitats. The fish-eating bat, Myotis vivesi S = logArea + Isol + logArea*Isol 5 24.00 0.00 Menegaux, was not detected at sites in the north, but was S = logArea 3 29.64 0.00 detected on southern peninsular sites and both northern and S = Isol + Arch 4 32.99 0.00 southern islands. S = Arch (group means) 3 34.83 0.00 S = null (overall mean) 2 41.76 0.00 S = Isol 3 41.98 0.00 Species-richness patterns

S, species richness; Isol, isolation variable representing the shortest The model best explaining variation in bat species richness on route to the Baja peninsula; Arch, grouping variable for subarchipel- islands (w = 0.76) incorporated parallel slopes for the explan- ago. +, additive effects; *, interactive effects. atory variables of log10 area and isolation in the two

358 Journal of Biogeography 35, 353–364 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd Island biogeography of bats

Table 4 Bat species detected in northern and southern regions of the Baja peninsula Detected on peninsula Detected on islands and subarchipelagos with acoustic sampling Northern Southern Northern Southern methods. Species region region archipelago archipelago

Family: Mormoopidae Mormoops megalophylla Family: Phyllostomidae Macrotus californicus Leptonycteris curasoae Family: Vespertilionidae Antrozous pallidus Eptesicus fuscus * Lasiurus blossevilli Lasiurus xanthinus * Myotis californicus Myotis volans * Myotis yumanensis Myotis vivesi Pipistrellus hesperus Family: Molossidae Tadarida brasiliensis Nyctinomops femorosaccus Nyctinomops macrotis Eumops sp.

*Species captured in mist-nets, but not detected with acoustic sampling. subarchipelagos (Table 3). This suggests that area and isolation 12 are associated with species richness of bats in both subarchipelagos, but overall richness differs between the island groups 10 after accounting for area and isolation. The penultimate model 8 (DAICc = 2.85, w=0.18) included the same terms plus an interaction term for log area and isolation. These two models 10 6 combined accounted for 94% of model weights. The null model had no support (DAICc = 41.76, w=0.00). 4 The intercept for bat richness based on the best-ﬁt model is Species richness 1.79 species (SE = 0.49; 95% CL: 0.78, 2.81) for the northern 2 subarchipelago and 4.35 species (SE = 0.48; 95% CL: 3.36, 0 5.34) for the southern subarchipelago. This difference indicates 1 10 100 1000 10 000 that northern islands had 2.55 fewer species than southern Area (ha) islands after accounting for the size and isolation of islands

(Fig. 2). The regression coefficient for log10 area was 1.38 Figure 2 Species–area relationship of bat richness for southern (SE = 0.18; 95% CL: 1.01, 1.75), indicating that richness islands (solid line) and northern islands (dashed line) in the Gulf increases by one species for every 5.37-fold increase in island of California, Mexico. Regression lines were estimated using the area. The coefficient estimate for isolation was )0.16 average isolation for the archipelago to account for the negative (SE = 0.03; 95% CL: )0.22, )0.10), indicating that richness influence of isolation on species richness. The regression equations used were: S = 2.98 + 1.38(log area) for the southern archipelago; decreases by one species for every 6.25 km farther away from and S = 0.42 + 1.38(log area) for the northern archipelago. the Baja peninsula. Across the range of isolation values in the region, this corresponds to a decrease of about two bat species at the most isolated islands (c. 25 km from the peninsula) for shape distributions (Haila, 1990; Ricklefs & Schluter, 1993; islands of similar size. Whittaker & Fernańdez-Palacios, 2007). The importance of scale, both spatial and temporal, is at the centre of modern island biogeography and is key to evaluating the continuing DISCUSSION relevance of the equilibrium theory (Haila, 1990; Whittaker & Patterns of species diversity can be understood from three Fernańdez-Palacios, 2007). As Haila (1990) suggests, the distinct perspectives: the influence of the ecological factors on applicability of the equilibrium model should be restricted to (i) individuals, (ii) populations, and (iii) historical factors that the ‘population dynamics’ scale, as colonization and extinction

Journal of Biogeography 35, 353–364 359 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd W. F. Frick, J. P. Hayes and P. A. Heady III dynamics may not be relevant forces at the scale of the 1977) could explain patterns of reduced species richness on individual. distant islands if travelling costs for individuals are higher than Whether a system should be considered insular at the foraging rewards on distant islands but are not prohibitive on individual or population scale is a function of the spatial scale near islands. If few or no individuals of a species commute to at which the organism perceives its environment relative to the distant islands for foraging, overall species richness would be isolation of the ‘island’ (Haila, 1990; Whittaker & Fernańdez- reduced on remote islands. Further research needs to be Palacios, 2007). However, Haila (1990) further acknowledges conducted to tease apart these competing hypotheses, as both that these scales often form a blurred continuum rather than theories predict similar patterns in this context. The high discrete levels. The mechanisms responsible for driving vagility of bats and relatively moderate distances of isolation of patterns of species richness of bats in near-shore archipelagos islands in this near-shore archipelago offer a good opportunity probably operate in that transition zone between the individual for understanding how the scale of movement of individuals and population dynamics scales, as many bats have high could potentially influence community patterns. vagility and some species may be capable of moving among Analysis of ecomorphological characteristics relating to cost islands on a daily or seasonal basis (Sahley et al., 1993; of flight, such as wing aspect ratios, could provide further Whittaker & Jones, 1994; Horner et al., 1998). insight into the role of mobility in shaping community patterns. In a separate analysis, we investigated whether wing aspect ratios of species in this desert bat community were Influence of isolation on species richness related to patterns of community nestedness (Frick, 2007). The Because of their vagility, the species richness of bats has been results from that analysis did not indicate a strong relationship hypothesized to be influenced by isolation only in very distant between community nestedness and wing aspect ratios, but archipelagos (Lomolino, 1984; Lawlor, 1986; Carvajal & Adler, this may have been a result of the small sample sizes (n =12 2005). However, our results and two studies of bat commu- species). This type of analysis could be a fruitful area of future nities on near-shore archipelagos in Scandinavia, (Ahleń, 1983; research in regions that support richer bat communities. Johansson & de Jong, 1996) demonstrate that isolation Regardless of whether the presence of species represents correlates with reduced numbers of insular species and that foraging activity related to patch-selection decisions or bat faunas are richer on larger islands. The similarity of equilibrium of immigration and extinction rates, isolated patterns in Baja and Scandinavia suggests that bat communi- habitats in this archipelago appear to support fewer species ties in insular habitats may be negatively influenced by of bats even at distances that do not affect richness of other relatively modest distances (< 10 km) of isolation in different vertebrates, such as birds and reptiles (Case, 2002; Cody & habitat settings. Velarde, 2002). Reduced bat richness in isolated habitats Although many bat species have large home ranges and the may have important consequences for the conservation of ability to travel large distances in a single foraging bout (Kunz bats in both naturally and anthropogenically fragmented & Lumsden, 2003; Tidemann & Nelson, 2004), the ability or landscapes. willingness to cross open water is unknown for most species (Ahleń, 1983). Experimental research on orientation and Influences of area and habitat on species richness navigation suggests that acoustic cues and landmarks are important for some bat species (Jensen et al., 2005), but these Increased richness of bats with island size is consistent with the studies are focused on close-range orientation rather than on equilibrium theory of island biogeography, but does not large-scale navigation. At least one species (E. fuscus) may use distinguish between competing hypotheses about causal factors magnetic fields to navigate over large distances (c. 20 km) responsible for observed species–area relationships (Gotelli & (Holland et al., 2006). Some species appear to have no Graves, 1996). Species–area relationships may result from problems travelling over water. Studies on the recolonization direct influences of area per se on population persistence of the Krakatau islands have shown that both mega- and or from indirect influences from the correlation of habitat micro-chiropterans have colonized these islands, which are diversity with area (Rosenzweig, 1995). Ricklefs & Lovette some 30–40 km from potential source pools (Tidemann et al., (1999) reported that bats in the Lesser Antilles displayed strong 1990; Whittaker & Jones, 1994). Previous studies in the Gulf species–area relationships, but that habitat diversity did not of California suggest that L. curasoae regularly commutes significantly contribute to richness. Many species of bats occur c. 30 km from a maternity cave on Isla Tiburon on the in a wide variety of habitats and may be less susceptible to Sonoran side of the Gulf to forage near Bahia Kino, Sonora direct effects of habitat diversity on species richness (Ricklefs & (Sahley et al., 1993; Horner et al., 1998). Lovette, 1999). Two possible mechanisms could underlie the observed Both area and habitat diversity may be related to bat negative relationship between isolation and bat richness: richness in our study. Larger islands support greater topo- reduced rates of immigration as isolation increases (equilib- graphical diversity, which creates greater habitat diversity and rium theory of island biogeography hypothesis) or patch-use produces formations, such as rocky canyons, that may contain decisions by individuals (optimal foraging use hypothesis) semi-permanent pools of freshwater (tinajas) (Cody & Velarde, (Russell et al., 2006). Optimal foraging theory (Pyke et al., 2002; Cody et al., 2002). In Sonoran desert ecosystems,

360 Journal of Biogeography 35, 353–364 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd Island biogeography of bats topographical diversity is strongly related to habitat diversity subarchipelagos were subject to a similar pool of potential (Bu´ rquez et al., 1999). colonizing species. Although habitat diversity was not measured directly on islands, and surrogates such as elevation were too strongly CONCLUSIONS correlated with area to be included in our models, there are substantial differences in vegetation structure between islands Our results present an important step in understanding how in the southern subarchipelago, which are more exposed to bat communities are structured in isolated habitats. The summer monsoons, and islands in the northern subarchipel- patterns we observed in this near-shore archipelago suggest ago, which tend to be drier (Cody et al., 2002). Southern that bat communities may be more sensitive to isolation islands supported higher numbers of bat species than the more than previously expected (Lomolino, 1984; Lawlor, 1986; xeric northern islands. The reduced richness in the northern Carvajal & Adler, 2005). In addition, our results demon- subarchipelago in our study is consistent with patterns of bat strate that the larger islands that have greater topographical richness in tropical Pacific islands, where frugivorous bats are and vegetative diversity tend to support greater numbers of absent from islands with low plant and topographical diversity bat species, as is predicted by island biogeography theory (Carvajal & Adler, 2005). and the species–area relationship (Rosenzweig, 1995; Whit- Vegetation structure and xericity could affect the persistence taker & Fernańdez-Palacios, 2007). The relationships of bat populations by affecting food resources (i.e. insect between species richness of bats, island area and isolation, availability) and access to semi-permanent freshwater in tinajas and habitat diversity have potentially important conse- that persist for long periods after infrequent rainstorms. quences for our understanding of the population persistence Although some desert bat species may be completely water- and community dynamics of bats. independent or only need infrequent access to freshwater (Geluso, 1978; Bell et al., 1986), many species need to drink ACKNOWLEDGEMENTS regularly to maintain a positive water balance (Carpenter, 1969; Geluso, 1978; Kurta et al., 1990; Webb et al., 1995). We thank Robert J. Whittaker, S¸erban Proches¸, Stuart During our surveys, we found freshwater only on Isla Carmen Parsons and one anonymous referee for helping to improve and Isla Espiritu Santo-Partida Sur. The influence of xericity our manuscript. We would also like to thank Doug Robin- on community structure is also evident for bird populations, son, Dave Lytle, and William Rainey for comments on earlier in that species that prefer brushier, denser vegetation are more drafts. Many thanks to Araceli Samaniego at Conservaciońde commonly distributed in the southern islands, and species that Islas and Roberto Martıńez Gallardo for providing local prefer more open, drier habitats frequent the northern islands support to conduct our research in Mexico. We thank Chris (Cody & Velarde, 2002). Corben, William Rainey and Tony Messina for assistance with anabat and anapocket. John Gilreath, G. Emma Flores Rojas and Willa Veber assisted in fieldwork. Amy Historical influences on species richness Whitesides, Willa Veber and Eric Abelson assisted in bat call Historical factors could also influence present-day patterns of identification. The Central Coast Bat Research Group species richness on islands (Ricklefs & Schluter, 1993). provided funding support. W.F.F. was supported by a Defining the potential pool of colonizing species is critical National Science Foundation Graduate Research Fellowship for accurately comparing diversity patterns among archipel- during this project. agos (Gotelli & Graves, 1996). If source faunas of different archipelagos were substantially dissimilar, differences among REFERENCES archipelagos could in part be attributable to historical legacies rather than to current ecological conditions on islands. We Ahleń, I. (1983) The bat fauna of some isolated islands in characterized the source pools of species for both subarchi- Scandinavia. Oikos, 41, 352–358. pelagos to provide a context for understanding the relation- Anthony, E.L.P. (1988) Age determination in bats. Ecological and ship between island characteristics and community structure behavioral methods for the study of bats (ed. by T.H. 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(1999) The roles of island area per Wright, S.J. (1981) Intra-archipelago vertebrate distributions: se and habitat diversity in the species–area relationships of the slope of the species-area relation. The American Natu- four Lesser Antillean faunal groups. Journal of Animal ralist, 118, 726–748. Ecology, 68, 1142–1160. Ricklefs, R.E. & Schluter, D. (1993) Species diversity in ecological communities: historical and geographic perspectives. The University of Chicago Press, Chicago. BIOSKETCHES Riddle, B.R., Hafner, D.J., Alexander, L.F. & Jaeger, J.R. (2000) Cryptic vicariance in the historical assembly of a Baja Cal- Winifred F. Frick is currently a research associate in ecology ifornia Peninsular Desert biota. Proceedings of the National and evolutionary biology at the University of California, Santa Academy of Sciences USA, 97, 14438–14443. Cruz. This paper represents part of her doctoral studies on Rosenzweig, M.L. (1995) Species diversity in space and time. island biogeography and community ecology of bats in Baja Cambridge University Press, Cambridge. California, Mexico. Her research interests include ecology and Russell, G.J., Diamond, J.M., Reed, T.M. & Pimm, S.L. conservation of bats as well as demography and population (2006) Breeding birds on small islands: island biogeogra- modelling.

Journal of Biogeography 35, 353–364 363 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd W. F. Frick, J. P. Hayes and P. A. Heady III

John P. Hayes is currently Professor and Department Chair Paul A. Heady III is a researcher with the Central Coast Bat of the Department of Wildlife Ecology and Conservation at the Research Group in Aptos, California. His research interests University of Florida. His research interests include the include habitat ecology and conservation of bats. ecology and conservation of bats, the inﬂuences of land management practices on vertebrates, and the inﬂuences of ¸ ¸ spatial scale on habitat selection. Editor: Serban Proches

364 Journal of Biogeography 35, 353–364 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd