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Evaluating the Effectiveness of the Standard Mist-netting Protocol for the Endangered Indiana Bat (Myotis sodalis)
Lynn W. Robbins, Kevin L. Murray, and Paul M. McKenzie

Northeastern Naturalist, Volume 15, Issue 2 (2008): 275–282

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2008 NORTHEASTERN NATURALIST 15(2):275–282 Evaluating the Effectiveness of the Standard Mist-netting Protocol for the Endangered Indiana Bat (Myotis sodalis) Lynn W. Robbins1,*, Kevin L. Murray1,2, and Paul M. McKenzie3 Abstract - Standardized mist-netting protocols set guidelines regarding the best way to sample a bat species or community. We evaluated the current mist-netting guidelines designed to determine presence or absence of endangered, Myotis sodalis (Indiana bat). This test was conducted in Deer Ridge Conservation Area (Lewis County, northeastern Missouri), an area known to have an abundance of Indiana bats, including several primary maternity colonies. We mist-netted according to recommended guidelines for Indiana bats for two consecutive nights at three different times during the reproductive season. Anabat II detectors were used in conjunction with mist nets to sample bat activity at the same locations. Captures and detections of Indiana bats and other species of bats varied substantially among the sampling periods. In addition, there was a significant decrease in number of Indiana bats captured with mist nets from night one to night two, although activity levels remained the same. Finally, our data show that augmenting mist nets with ultrasonic detectors can enhance the probability of determining the presence or absence of Indiana bats. Introduction A standard mist-netting protocol should be designed to provide an accurate sample of the chiropteran biodiversity in an area. However, the emphasis shifts to maximizing the accuracy of determining presence or absence when monitoring for an endangered species. Myotis sodalis (Miller and G.M. Allen) (Indiana bat) was listed as endangered by the US Fish and Wildlife Service in May 1967. Since then, populations in the southern states, including Missouri, have continued to decline or have become stable at substantially lower numbers, although overall the population is increasing (US Fish and Wildlife Service 2007). Observations suggest (Gardner and Cook 2002) that most females of this species migrate north in the spring, where they form summer maternity colonies under the exfoliating bark of dead trees. Because Indiana bats inhabit forests, any disturbance to this habitat within the summer range of the species could disrupt their reproductive success in an area. As a result, the US Fish and Wildlife Service (1999, 2007), in collaboration with the Indiana Bat Recovery Team, provided standard mist-netting guidelines to determine presence or absence of Indiana bats in a particular area. 1Department of Biology, Missouri State University, Springfield, MO 65897. 2Current address - Department of Biology, University of Miami, Coral Gables, fl33124. 3US Fish and Wildlife Service, 101 Park Deville Drive, Suite A, Columbia, MO 65203. *Corresponding author - lynnrobbins@missouristate.edu. 276 Northeastern Naturalist Vol. 15, No. 2 Two recent studies have evaluated the effectiveness of these mist-netting guidelines. MacCarthy et al. (2006) tested a subset of the mist-netting guidelines by video-recording bat activity at net sites. They showed that 23% of bats were able to avoid the mist net and concluded that the overall bat capture rate would be increased by decreasing or eliminating the time between net checks. This decreased time period (10 min.) has been included in the 2007 revision. Carroll et al. (2002) suggested that, in addition to potential flight corridors, nets should be placed in forest interiors to more effectively document the presence of Indiana bats. Both of these studies demonstrate how mist-netting protocols can be improved with empirical data. The purpose of this project was to test the effectiveness of the accepted mist-net protocol in determining presence or absence of Indiana bats. Specifically, we investigated three questions regarding the Indiana bat mist-net protocol: 1) Does the probability of capturing an Indiana bat vary throughout the maternity period (15 May to 15 August)? 2) Does the probability of capturing an Indiana bat at a net site vary from night 1 to night 2? and 3) Are mist nets alone able to reliably detect Indiana bats when present? We conducted our study at Deer Ridge Conservation Area in northeastern Missouri. Because it had abundant Indiana bats, this site was an ideal location to evaluate the mist-netting protocol. We followed the standard guidelines used to survey Indiana bats except that sampling sites were always within 1.6 km of known maternity colonies. Methods The study was conducted May–August 2002 in conjunction with a larger study on the breeding and foraging habitat of Indiana bats and other vespertilionid bats in managed and unmanaged forests (Miller 2003, Timpone 2004, Timpone et al. 2006). The study area was Deer Ridge Conservation Area (DRCA) and adjacent private lands (Lewis County, northeastern Missouri). Three primary maternity roosts (Callahan et al. 1997), each with >100 individuals, were located within DRCA. All mist netting was conducted according to the Indiana Bat Recovery Plan standardized mist-netting guidelines. The guidelines are as follows: 1) netting must be conducted between 15 May and 15 August; 2) for every 1 square km (100 ha), two net sites are required; 3) each net site must consist of two nets no less than 30 m apart, each of which must be monitored for a minimum of two nights; 4) although not required, nets are usually placed in potential flight corridors (e.g., streams, road ruts, logging roads), perpendicular to the corridor, covering as much of the corridor as possible; 5) the sampling period must begin at sunset and last a minimum of 5 hours; and 6) surveys must not be conducted during precipitation, if temperatures are <10 °C (50 °F), during strong winds, or in bright moonlight. We surveyed three net sites (2 net sets each) on service roads and a stream within 1.6 km of primary roosts. Site 1 consisted of an open-canopy service road (12–15 m wide), site 2 consisted of an open-canopy service road and 2008 L.W. Robbins, K.L. Murray, and P. McKenzie 277 stream (10–12 m wide), and site 3 consisted of a closed-canopy service road (7–10 m wide). For each net set, we used standard 50-dernier/2-ply nylon mist nets with 38-mm mesh size (Avinet, Inc.; www.avinet.com) and stacked three 2.6-m high nets. Mist-net width varied from 6 m to 12 m depending on the site. We monitored each net site during three different reproductive periods of Indiana bats: pregnancy (10–20 June), lactation (15–21 July), and post-lactation (6–12 August). Within each reproductive period, we monitored each net site for two consecutive nights whenever possible (always within two nights). One bat detector set-up (Anabat II detector, ZCAIM, and laptop) was placed at each net site to record echolocation calls throughout the sampling period (see Britzke et al. 2002, Murray et al. 1999). Detectors were placed >25 m from all mist nets. We identified all bats captured in mist nets to species. Echolocation calls were identified to species according to the methods of Britzke et al. (2002). We compared the difference in capture success and species richness from night one to night two for both nets and bat detectors using a Wilcoxon signed ranks test. We conducted these tests for all species combined and for Indiana bats. To determine if bats are sampled more effectively with nets or detectors, we compared differences in species richness between techniques using a Wilcoxon signed ranks test. Species richness was defined as number of species. All statistical tests were two-tailed with α = 0.05. Finally, we calculated capture/detection success for each technique and both techniques combined for four of the most common species encountered in our survey. Mist-net success is often expressed as capture rate (number of bats per unit effort) and reported as a percent. We were interested in estimating the probability that a particular technique was successful in determining presence or absence of the target species. Therefore, we defined the capture/ detection success as number of net sets (detectors) that captured (detected) the target species divided by the total number of nets sets (detectors) used. A net set was defined as a set of 2 nets used on a single night. Results We captured 139 individuals representing seven species. Bat captures for each reproductive period and night are shown in Table 1. Data from three species, Lasiurus cinereus Palisot de Beauvois (Hoary bat, n = 1), Myotis lucifugus LeConte (little brown bat, n = 3), and Perimyotis subflavus F. Cuvier (eastern pipistrelle, n = 3), were not included due to small sample size. Substantial variation in capture success occurred among reproductive periods (Table 1). Patterns were not consistent among species. Small sample size precluded statistical analysis of variation among reproductive periods. With the exception of Nycticeius humeralis Rafinesque (evening bat), captures were consistently lower on the second night of netting than on the first night (Table 1). Both Indiana bats (meannight1 = 1.33 ± 0.55; meannight2 = 0.11 ± 0.11; Z = -2.07; P = 0.038) and total captures (meannight1 = 10.00 ± 1.68; 278 Northeastern Naturalist Vol. 15, No. 2 meannight2 = 5.56 ± 0.11; Z = -2.16; P = 0.031) were significantly lower on the second night of netting. This difference did not reflect a change in bat activity at net sites because neither the number of Indiana bats (meannight1 = 20.67 ± 10.12; meannight2 = 12.89 ± 4.65; Z = -0.840; P = 0.401) nor total bats (meannight1 = 124.56 ± 54.15; meannight2 = 98.11 ± 31.49; Z = -0.652; P = 0.514) recorded with bat detectors were significantly different from night 1 to night 2. Finally, we found no significant difference in species richness from night 1 to night 2 for either nets (meannight1 = 3.78 ± 0.49; meannight2 = 2.78 ± 0.49; Z = -1.558; P = 0.119) or detectors (meannight1 = 3.56 ± 0.44; meannight2 = 4.11 ± 0.35; Z = -1.121; P = 0.262). Species richness was not significantly different between nets and detectors (meannets = 3.28 ± 0.36; meandetectors = 3.83 ± 0.28; Z = -0.991; P = 0.322). Capture/detection success varied for nets, detectors, and combined (Table 2). Detectors usually had a higher capture/detection success than mist nets. Combining both techniques generally increased capture/detection success of a particular species. Table 1. Total bat captures by reproductive period and night (see methods). MYSO = Myotis sodalis; MYSE = Myotis septentrionalis; NYHU = Nycticeius humeralis; EPFU = Eptesicus fuscus Palisot de Beauvois (big brown bat); LABO = Lasiurus borealis. We sampled for 12 net nights during each reproductive period, 18 net nights each on 1st and 2nd nights, and 36 net nights total. Detector nights are always one half of net nights. MYSO MYSE NYHU EPFU LABO TOTAL Reproductive period Pregnant 11 7 0 4 12 34 Lactating 1 9 14 21 15 60 Post-lactating 1 6 5 4 22 38 Night 1st 12 17 9 16 33 87 2nd 1 5 10 13 16 45 Table 2. Capture/detection success measured as percent of nets (N%), detectors (D%), or combined (C%) capturing or recording at least one of the target species. Number of net nights is the same as Table 1. Total capture/detection success is per net site (2 nets x 2 nights) rather than per net set (2 nets x 1 night). MYSO MYSE EPFU LABO N%, D%, C% N%, D%, C% N%, D%, C% N%, D%, C% Reproductive period Pregnant 67, 100, 100 33, 83, 83 50, 67, 83 83, 100, 100 Lactating 17, 83, 83 83, 83, 100 83, 100, 100 83, 100, 100 Post-lactating 0, 50, 50 83, 33, 100 50, 67, 67 100, 100, 100 Night 1st 44, 56, 56 89, 78, 100 44, 67, 78 100, 100, 100 2nd 11, 100, 100 44, 56, 89 78, 89, 89 78, 100, 100 Total 44, 100, 100 89, 78, 100 78, 89, 89 100, 100, 100 2008 L.W. Robbins, K.L. Murray, and P. McKenzie 279 Discussion Two previous studies (Carroll et al. 2002, MacCarthy et al. 2006) have suggested that the mist-netting guidelines of the US Fish and Wildlife Service (1999) Indiana Bat Protocol should be modified to more accurately assess the presence or absence of Indiana bats. Carroll et al. (2002) showed that nets set in non-traditional (forest-interior) locations accounted for Indiana bats at 50% of their netting locations, and MacCarthy et al. (2006) recommended that nets be monitored either at intervals of less than 10 minutes or continuously. The present study followed the published guidelines as closely as possible and addressed three questions: 1) Does the probability of capturing an Indiana bat vary throughout the maternity period? 2) Does the probability of capturing an Indiana bat vary from night 1 to night 2? and 3) Are mist nets alone able to reliably detect Indiana bats when present? We found that capture rates varied seasonally for all species. However, our sample size (n = 3) was small and we were unable to explain the observed patterns of variation. The variation may not necessarily be associated with reproductive season, but may reflect factors such as ambient temperature or the availability of food resources. Our data highlight how much we do not know regarding the activity and foraging patterns of Indiana bats and other vespertilionids. It is difficult to explain why we did not capture more net-naïve, newly volant, juvenile Indiana bats during the post-lactation netting sample. Both Miller (2003) and the present study showed that activity in forest corridors remained high during all three reproductive periods. However, Miller (2003) also showed that activity increased over open-water sources during the post-lactation period. Perhaps juvenile Indiana bats are preferentially using these more open areas as they perfect their foraging strategies and therefore were not present at our net sites. Our most striking result was the decrease in bat captures, but not activity, from night 1 to night 2. Indiana bats and all species combined were captured significantly less on night 2. Kunz and Brock (1975) were the first to document a similar reduction in captures from one night to the next. In our study, bat detectors did not have a detection bias between nights, as bat activity levels were not significantly different between night 1 and night 2. This result indicates that prior knowledge of the mist-net location on the part of the bats is the most likely explanation for decreased capture success. We assume that bats use spatial memory to remember the location of nets, and that experience with a net makes them more inclined to avoid it in the future. Kunz and Brock (1975) drew a similar conclusion. Regardless of the mechanism of avoidance, it seems that netting two nights in the exact same location is of limited value, particularly for Indiana bats. It is notoriously difficult to capture an Indiana bat with a mist net. In our study, the capture success for Indiana bats was often very low using nets alone (Table 2), but higher when using ultrasonic detectors. However, both 280 Northeastern Naturalist Vol. 15, No. 2 nets and detectors have their own limitations and biases when used alone. The best results, for both successfully documenting presence/absence and estimating biodiversity (i.e., species richness) were found using both mist nets and ultrasonic detectors. As other studies have shown, the most effective way to survey bat communities is to incorporate both of these techniques whenever possible (Flaquer et al. 2007, Kalko and Handley 2001, Murray et al. 1999, Sampaio et al. 2003, ). Other studies (Carroll et al. 2002, MacCarthy et al. 2006) showed that there was room for improvement in the original Indiana Bat Recovery Plan mist-netting protocol. The revised protocol was modified based upon these studies. We feel that the current guidelines should also be modified to enhance the probability of capturing or detecting Indiana bats at a site. Currently, there are simply too few data available to determine how variation in seasonal abundance and activity affect capture success. Future studies should attempt to address this issue. Our results provide good evidence that bats are not fooled twice by the same net. We see limited utility to netting multiple nights in the exact same location and advocate moving nets to a new location on the second night to enhance the probability of capturing Indiana bats. Finally, we recommend that ultrasonic detectors be used to augment survey work involving Indiana bats. Numerous studies in the past decade have made acoustic identification of bat echolocation calls much more commonplace (Biscardi et al. 2004, O’Farrell et al. 1999, Parsons and Jones 2000, Preatoni et al. 2005, Russo and Jones 2002, Rydell et al. 2002, Vaughan et al. 1997), and a substantial amount of work has been done to characterize and identify the echolocation calls of Indiana bats (Britzke et al. 2002; Ford et al. 2005; Murray et al. 1999, 2001; Yates and Muzika 2006). Overall, we recommend that comparable studies in other portions of the range of Indiana bats be conducted and that further work be done to characterize the echolocation calls of Indiana bats, particularly in different clutter conditions and with different types of ultrasonic detectors. Acoustic identification is only as good as the call library it relies upon (Broders et al. 2004, Tibbels 2000). In conclusion, we suggest that the protocol as written may be improved by moving mist nets between night one and night two and by using ultrasonic detectors to supplement mist-netting effort. Acknowledgments Funding for field work was provided by the US Fish and Wildlife Service, Missouri Department of Conservation, Dickerson Park Zoo, and Missouri State University. We thank the Missouri Department of Conservation for permission to work on Deer Ridge Conservation Area and in particular, Richard Clawson and Brian Root for their helpful interactions. For their considerable help in the field, we thank Matt Miller, John Timpone, Justin Boyles, and Scott Kelly. We thank T. Carter and one anonymous reviewer for providing comments that improved the quality of this manuscript. 2008 L.W. Robbins, K.L. Murray, and P. McKenzie 281 Literature Cited Biscardi, S., J. Orprecio, M.B. Fenton, A. Tsoar, and J.M. Ratcliffe. 2004. 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