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Population Demographics of the Florida Bog Frog (Lithobates okaloosae)
Jeronimo Gomes Da Silva Neto, Thomas A. Gorman, David C. Bishop, and Carola A. Haas

Southeastern Naturalist, Volume 13, Issue 1 (2014): 128–137

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Southeastern Naturalist J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 128 2014 SOUTHEASTERN NATURALIST 13(1):128–137 Population Demographics of the Florida Bog Frog (Lithobates okaloosae) Jeronimo Gomes Da Silva Neto1, Thomas A. Gorman1,*, David C. Bishop1,2, and Carola A. Haas1 Abstract - Conservation of a target species is often hindered by the absence of basic demographic information. Lithobates [Rana] okaloosae (Florida Bog Frog) was discovered in 1982 and occupies only three counties in northwest Florida, with its geographic range occurring almost exclusively on Eglin Air Force Base. This limited distribution has led to the listing of the Florida Bog Frog as a species of special concern in Florida. We used a mark-recapture approach to 1) estimate population sizes and densities of male Florida Bog Frogs at four sites; 2) estimate recapture rates of males among sites, within years, and between years; 3) examine the relationship between male body condition and residency time in breeding sites; and 4) examine the variation in male body condition across sites. Florida Bog Frog population sizes and densities differed among sites, but not across years. Recapture rates were high within a breeding season but low between years, suggesting that males have high site fidelity and survival rates during the breeding season, but that annual mortality or dispersal rates are high. Lastly, body condition of males did not differ across sites or affect residency time during the breeding season. Understanding basic demographics of this species will aid future conservation efforts and management decisions. Introduction Lithobates [Rana] okaloosae (Moler) (Florida Bog Frog) is a rare North American ranid endemic to Walton, Okaloosa, and Santa Rosa counties in northwest Florida. The species occurs in the Yellow and East Bay river drainages, and all but three tributaries are within the boundaries of Eglin Air Force Base (Bishop 2005, Gorman 2009, Moler 1993). This species is associated with shallow, acidic, flowing seeps and bog overflows that have higher amounts of emergent and submergent vegetation, and woody debris, and lower levels of canopy cover than unused sites (Gorman 2009, Gorman and Haas 2011). Factors that impact the structural characteristics of the streams, such as altered fire regimes or changes in hydrology, have the potential to impact Florida Bog Frog populations (Jackson 2004, Gorman 2009). Since the discovery of the Florida Bog Frog in 1982 (Moler 1985), research on this species has focused on its distribution, movements, habitat selection, and conservation genetics (e.g., Austin et al. 2011a, b; Bishop 2005; Gorman 2009; Gorman and Haas 2011; Gorman et al. 2009); however, there is still a lack information on basic population demographics for this species, which is essential for future conservation efforts (Bielby et al. 2008). 1Department of Fish and Wildlife Conservation, Virginia Tech, Blacksburg, VA 24061. 2Current address - The Nature Conservancy, 1417 Stuart Engals Blvd, Mt. Pleasant, SC 29464. *Corresponding author - gormant@vt.edu. Manuscript Editor: John Placyk Southeastern Naturalist 129 J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 Common population metrics include population size, recapture rates, and density (Govindarajulu et al. 2005). Estimates of population size provide baseline data that can be used to monitor changes through time. By understanding recapture rates, we can make predictions on survivorship or dispersal within and between breeding seasons. Density estimates can be used in combination with measures of habitat condition to make assumptions on how habitat quality and patch sizes influence breeding populations. In addition, factors influencing survival during the breeding season can elucidate the trade-offs between chorus attendance and individual survival. Even though mate selection may favor males that breed longer, chorus attendance is an activity that requires an increased energy demand and may negatively affect survival (Bevier 1997, Greer and Wells 1980, Jacobson 1985, Murphy 1994). The relationship between body condition and how many nights each male spends calling (i.e., residency) may help explain this trade-off. Further, if we assume that male body condition can serve as an index of site quality, we can compare the quality of each site across years. We present a mark-recapture study of Florida Bog Frogs at four sites. We estimated population sizes, densities, and recapture rates at multiple sites across multiple years. Further, we investigated if body condition of males affected residency time, and whether male body condition differed among sites. Methods Study area Eglin Air Force Base is a large military installation (>187,000 ha) in northwestern Florida. The topography of the study area is level to rolling, with the highest elevation at ≈75 m and slopes that generally range from 0–30% (Eglin Air Force Base 2002). The upland habitat is dominated by Pinus palustris Mill. (Longleaf Pine) and Quercus laevis Walter (Turkey Oak) sandhills with interspersed streams. Our four study sites occurred in the riparian area of three perennial streams: Live Oak Creek, East Bay River, and Weaver Creek. The study sites were all associated with either a steephead (i.e., a small ravine associated with perennially wet firstorder streams that originate from springs; Means 1975, Schumm et al. 1995), such as Live Oak Tributary, or seepage springs that originated upslope of the “boggy” riparian areas that constituted the site, such as Live Oak Creek, East Bay River, and Weaver Creek. All four sites were characterized by predominantly shallow water with high amounts of emergent vegetation, submergent vegetation, and woody debris, and moderate levels of canopy cover (Gorman and Haas 2011). The overstory vegetation associated with these sites was predominantly Cliftonia monophylla (Lam.) Britton ex Sarg. (Black Titi or Buckwheat Tree), Magnolia virginiana L. (Sweetbay Magnolia), Chamaecyparis thyoides (L.) Britton, Sterns, & Poggenb. (Atlantic White Cedar), Cyrilla racemiflora L. (Swamp Titi), and Nyssa sylvatica Marsh. (Black Gum). All sites were separated from each other by at least 1.3 km, and the two most distant sites were 20.9 km apart (Fig. 1). Southeastern Naturalist J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 130 Sampling methods 2002–2003 sampling. We sampled the Live Oak Tributary study site during the 2002 and 2003 breeding season of Florida Bog Frogs. Each night of sampling usually lasted 2–3 person h and was terminated when all calling males were captured and the study site was completely traversed by the observers. We searched the study site for Florida Bog Frogs on 30 nights in 2002 (27 May–10 September) and 84 nights in 2003 (27 March–3 August). However, we reduced the dataset to no more than 2 consecutive sampling nights per week to make the distribution of sampling effort more comparable across years. Unless otherwise noted, we included 16 sampling nights or 8 two-night sampling periods from 27 May–01 August 2002 and 20 sampling nights or 10 two-night sampling periods from 29 May–29 July 2003. We used all 84 days from the 2003 sampling season to examine the relationship between body condition and residency time (see below). We permanently marked individual Florida Bog Frogs by toe-clipping between 1–3 toes, with never more than one toe clipped on one foot. Each toe was numbered from 1–18, and the resulting code uniquely identified each individual (e.g., 6 or 4-10 or 3-7-9). We georeferenced capture locations with an Asset Surveyor TDC2 GPS unit (Trimble Navigation Limited, Sunnyvale, CA) with sub-meter resolution. When each individual was captured for the first time in a season, we measured snout–vent length Figure 1. Study sites where male Lithobates okaloosae (Florida Bog Frog) where captured from 2002–2003 and 2006–2008 on Eglin Air Force Base, FL. Southeastern Naturalist 131 J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 (SVL) to the nearest 0.1 mm using dial calipers and weighed each individual to the nearest 0.5 g using a 10- or 30-g spring scale (Pesola AG, Baar, Switzerland). 2006–2008 sampling.We resampled Live Oak Tributary and sampled three additional sites (Live Oak Creek, Weaver Creek, and East Bay River) from May to August in 2006–2008 (Gorman and Haas 2011). We selected two of the additional sites to represent different drainages (Weaver Creek and East Bay River), and selected Live Oak Creek to represent a different stream position on the same drainage as Live Oak Tributary. All the study sites were sites at which Gorman (2009) had documented multiple Florida Bog Frogs calling in May 2006. We searched the study sites for Florida Bog Frogs during two periods (each period consisted of four consecutive nights) in 2006 and three periods in 2007 and 2008. During each sampling event, we extensively searched each site, usually for 2–3 person hours, to capture all calling males. We implanted visible implant alphanumeric (VIA) tags (Northwest Marine Technologies, Inc., Olympia, WA) into the hindlimb of captured frogs. We used a blood lancet to make a small incision in the skin and then, using a tag implanter, inserted the tag between the skin and the adductor muscles (Buchan et al. 2005, Gorman and Haas 2011). When each individual was captured for the first time in a season, we measured its SVL to the nearest 0.1 mm using dial calipers and its weight to the nearest 0.5 g using a 10- or 30-g Pesola™ spring. We georeferenced capture locations with an Asset Surveyor TDC2 GPS unit (Trimble Navigation Limited, Sunnyvale, CA) with sub-meter resolution. Statistical analyses We used minimum number alive (MNA; i.e., the minimum number of unique individuals captured within a year) and Lincoln-Peterson (LP) estimates to estimate population sizes of adult males at individual sites. We calculated LP estimates by dividing the sampling season in half, so that LP period 1 was the first half of the sampling season and LP period 2 was the second half. To estimate densities at each site, we calculated a minimum convex polygon (MCP) in ArcGIS 10.0 (Esri, Redland, CA) for each site using all capture data from all years in which the site was sampled. This approach resulted in a single area estimate for each of the four study sites (Table 1). We calculated male Florida Bog Frog densities by dividing the population size (MNA) at each site during each year by the area of the corresponding MCP. We presented the data as the number of male Florida Bog Frogs per 1000 m² because all of the study sites were larger than this conversion and it provided interpretable density estimates. We used an analysis of variance (ANOVA) to compare MNA and densities across three sites (Live Oak Tributary, Live Oak Creek, and Weaver Creek) over three years (2006–2008). We excluded one site (East Bay River) from the population, density, and recapture analyses, because it was not sampled in 2006. If the overall ANOVA was significant, then we used a Student’s t-test on least squares means to evaluate pair-wise comparisons. Lastly, we calculated recapture rates within years, between years, and at each site, and compared them among three sites (Live Oak Tributary, Live Oak Creek, and Weaver Creek) over three years (2006–2008) using an ANOVA. We used an alpha level of 0.05 to evaluate differences. Southeastern Naturalist J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 132 To examine the relationship between body condition and residency time, we used data that were collected during the entire sampling period 27 March–3 August 2003 (84 days) from one site (Live Oak Tributary). We developed a linear regression model that related log-transformed snout–vent length (SVL) and logtransformed body weight of captured male Florida Bog Frogs and used the residuals as the body condition index (BCI) (Băncilă et al. 2010). We then developed a linear regression to relate BCI to the number of nights individual frogs spent in the chorus during the 2003 breeding season. Additionally, we used an ANOVA to compare BCI of males from all sites over two years (2007–2008). Results We captured a total of 161 adult male Florida Bog Frogs at four sites. Out of the 161 males captured, 94 were captured at Live Oak Tributary from 2002–2003 and 2006–2008, 24 at East Bay River from 2007–2008, 22 at Weaver Creek from 2006–2008, and 21 at Live Oak Creek from 2006–2008 (Table 1). MNA was higher for 2002 compared to 2003 (28 vs. 16). During 2006–2008, we captured the most males in 2006 (43), and the least in 2007 (32) (Table 1). Male Florida Bog Frog densities were different among sites (F2,4 = 24.90, P = 0.006), but population sizes (all values for population size given in the text are for MNA unless otherwise indicated) were not quite statistically different at the α = 0.05 level among sites (F2,4 = 5.42, P = 0.07), and across years there was no difference (population size: F2,4 = 2.24, P = 0.22; density: F2,4 = 2.80, P = 0.17) during the 2006–2008 sampling period. Live Oak Tributary had the largest population size and highest density, and both measures exceeded those at Weaver Creek (population size: P = 0.05, density: P = 0.003) and Live Oak Creek (population size: P = 0.04, density: P = 0.005), neither of which was different from the other (population size: P = 0.93, density: P = 0.41) (Table 1). Recapture rates were the lowest in 2002 ( average = 0.54) during the 2002–2003 (Table 2) sampling period. During 2006–2008, recapture rates were highest in 2007 (0.64) and lowest in 2006 (0.58) (Table 2). The highest recapture rates between Table 1. Minimum number alive (MNA), Lincoln–Peterson estimates (LP) (standard deviation in parentheses), and density estimates (D; number of individuals/1000 m²) of adult male Lithobates okaloosae (Florida Bog Frog) captured during five years at four different sites (area of each site is listed after each site name) on Eglin Air Force Base, FL. Live Oak Live Oak East Bay Weaver Tributary (1415 m²) Creek (2609 m²) River (1638 m²) Creek (7518 m²) Year MNA LP D MNA LP D MNA LP D MNA LP D 2002* 28 29.0 (1.4) 19.8 - - - - - - - - - 2003* 16 17.2 (2.0) 11.3 - - - - - - - - - 2006 23 26.8 (8.9) 17.9 14 18.2 (13.4) 5.4 - - - 6 6.0 (0.0) 0.8 2007 13 12.3 (2.5) 10.1 2 2.0 (0.0) 0.8 9 9.0 (0.0) 5.5 8 8.0 (0.0) 1.1 2008 14 12.7 (1.2) 10.9 5 5.0 (0.0) 1.9 15 15.0 (0.0) 9.2 8 9.5 (3.9) 1.1 *Sampling methods used in 2002–2003 differ from those used in 2006–2008. Southeastern Naturalist 133 J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 years (0.14) occurred 2002–2003; the lowest (0.07) occurred 2006–2007, but 2007–2008 was just slightly higher (0.09). From 2006–2008, Live Oak Creek had the highest average within-year recapture rate (0.63), and Weaver Creek had the lowest (0.54) (Table 2). Within-year recapture rates (F8,84 = 0.74, P = 0.65) were not different among sites or across years from 2006–2008. Overall, recapture rates were high within a season (range = 0.47–0.78, mean = 0.61, SE = 0.02), but low between years (range = 0.07–0.14, mean = 0.10, SE = 0.02). As expected, there was a strong relationship between weight and SVL (R2 = 0.91). We used the residuals of this relationship as our estimate of BCI. We found no relationship between BCI on initial capture and the number of nights a male attended the chorus (i.e., number of capture nights; R2 = 0.035, P = 0.26; Fig. 2). Also, when examining data at all four sites from 2007–2008, we detected no differences in male body condition among sites or among years (F7,66 = 1.53, P = 0.17). Discussion This study provides the first quantitative assessment of population metrics of Florida Bog Frogs. From 2006–2008, one site (Live Oak Tributary) had both higher population size and density than the other two sites (Table 1). This result suggests that Table 2. Within-year recapture rates at four different sites where adult male Lithobates okaloosae (Florida Bog Frog) were captured on Eglin Air Force Base, FL. Site 2002 2003 2006 2007 2008 Mean East Bay River - - - 0.73 0.58 0.65 Live Oak Tributary 0.54 0.62 0.62 0.67 0.64 0.62 Live Oak Creek - - 0.62 0.50 0.78 0.63 Weaver Creek - - 0.49 0.66 0.47 0.54 Mean - - 0.58 0.64 0.62 - Figure 2. Relationship between initial body condition and number of nights in the chorus for male Lithobates okaloosae (Florida Bog Frog) captured in 2003 at Live Oak Creek Tributary, on Eglin Air Force Base, FL. Body conditions were derived from the residuals resulting from a linear regression model that related log-transformed snout–vent length (SVL) and log-transformed body weight of captured male Florida Bog Frogs. Southeastern Naturalist J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 134 some sites (e.g., Live Oak Tributary) may be capable of supporting larger breeding populations and thus have greater conservation value for this rare anuran. However, we did not find a difference in BCI among sites, and this result may indicate that despite the higher population size supported at some sites, males even in smaller populations are finding adequate resources. Additionally, the MCP method to calculate the area for each study site is likely an over-estimate, because it may include habitat that is not suitable for Florida Bog Frogs. Florida Bog Frogs are capable of using small patches of habitat as noted by their limited movements (Gorman et al. 2009), and it is likely that some sites were more heterogeneous than others. The site with the highest densities, Live Oak Tributary, appeared to be more homogenous and provide a greater amount of suitable contiguous habitat despite being the smallest study site overall. The difference in densities may reflect the spatial distribution of microhabitats (Gorman and Haas 2011) or the habitat quality of the sites. While we did not explicitly estimate survival rates because of small sample sizes, the relatively high recapture rates within a year suggest that survival is high within a breeding season. During the breeding season, males usually have limited movement, spending several nights at a calling location before moving to a new nearby location (Gorman et al. 2009). Conversely, recapture rates were very low between years and may indicate a low overwinter survival rate. Relatively little information on ranid frog longevity or annual survival is available, and several previous studies used recapture rates as uncorrected estimates of minimum annual survivorship (Wells 2007). Recapture rates of Florida Bog Frogs of 7–14% between years in this study fell within two uncorrected estimates of minimum annual survivorship (i.e., recapture rates) reported for Ranidae (Hylarana eythraea = 5% and Rana temporaria = 31%; Wells 2007). Uncorrected estimates of between-year recapture rates for seven species of Hylidae, several of which are similar in body size to L. okaloosae, averaged 32% (Wells 2007), more than twice as high as our observed recapture rates for Florida Bog Frogs. In addition, estimates of annual survivorship of ranid frogs based on recapture data corrected for capture probability or based on life-table data were substantially higher in eight studies reviewed by Wells (2007), with an average of 54% (range = 0–84%). However, the rates we observed may be biased low because we calculated recapture rates using data for males only, and male anurans are known to have higher annual mortality (Wood et al. 1998). Our annual survival data were calculated across three periods, separated by four years, and we saw consistently low survival. By contrast, survival rates may vary greatly within and across years in frog species from more strongly seasonal climates, as has been reported in a six-year study of European water frogs (Pelophylax [Rana] lessonae (Camerano) [Pool Frog] and P. kl. Esculenta (L.) [Edible Frog]; Anholt et al. 2003). Florida Bog Frog tadpoles overwinter at breeding sites (Bishop 2005, Moler 1992), hence adults likely are already more than two years old when they first breed. Metamorphs are often encountered in late spring and early summer and are unlikely to attain breeding status within the same season (based on their body size compared to breeding adults). It is not known whether some tadpoles metamorphose Southeastern Naturalist 135 J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 within the same year as they hatch, which is a strategy used by the closely related Lithobates clamitans Latreille (Green Frog; Martof 1956) and may lead to breeding at an earlier age. Martof (1956) found that Green Frogs were capable of both overwintering as a tadpole or as a metamorph, depending on the date that an egg mass was deposited. The low recapture rates between successive years may also be related to dispersal after the breeding season. During rain events, frogs are able to disperse farther, possibly moving outside of the sampling area between years (Bishop 2005, Gorman et al. 2009). We currently have very limited knowledge on the dispersal patterns of this species, which limits our ability to make predictions about the effects of dispersal and mortality on the population dynamics at a location. There was no relationship between body condition and the number of nights individual males were detected (i.e., residency time) within a season. Some anurans show a relationship between energy expenditure and number of days active in a chorus (Murphy 1994). The condition of an individual at the beginning of the chorus tenure and how fast an individual loses such condition may influence residency in the chorus. Because we did not record the weight of each individual at each time they were captured, we are unable to calculate rate of change in body condition. Body condition likely affects species of frogs that have short and intense breeding seasons (e.g., toads) or form large, dense choruses (e.g., treefrogs) more than prolonged breeders like Florida Bog Frogs. Species with prolonged breeding seasons may space themselves throughout the breeding habitat, which may influence foraging behavior, mating success, and chorus tenure (Gorman et al. 2009, Wells 1977). In addition, aggressive interactions between prolonged breeders have been reported (Ritchie et al. 2008). Finally, rainfall patterns, permanence of breeding sites, and predator pressure may influence chorus tenure (Bateson 1993). Body condition alone was not a good indicator of how long a male Florida Bog Frog remains active in the chorus. Our intensive study at four locations confirms that Florida Bog Frog breed in very small aggregations (2–29 males) that persist for several years. Although our study sites likely were located too far apart to detect dispersal of marked individuals, we assume that these small breeding aggregations must be connected through dispersal either of tadpoles, metamorphs, or adults, a pattern confirmed by genetics work (Austin et al. 2011a). We recaptured very few marked individuals between years. Further study will be necessary to explore whether this low recapture rate is the result of high mortality rates or non-breeding season dispersal events. In conclusion, it appears that maintaining a network of small breeding sites is necessary to ensure persistence of species that occur in naturally patchy wetland habitats (Shoemaker et al. 2013), including Florida Bog Frogs. Acknowledgments This research was supported by the Natural Resources Branch (Jackson Guard) of Eglin Air Force Base and Fralin Life Science Institute’s Summer Undergraduate Research Fellowship (SURF) Program. We thank K. Gault and B. Hagedorn of Jackson Guard for their Southeastern Naturalist J. Gomes Da Silva Neto, T.A. Gorman, D.C. Bishop, and C.A. Haas 2014 Vol. 13, No. 1 136 long-term support of this project. We thank S. Eddie, J. Homyack, J. Kern, K. Landry, L. Oztoloza, L. Phillips, B. Rincon, and S. Ritchie for help collecting this data. Also, we thank C. Proctor for guidance on the GIS portion of this paper. Literature Cited Anholt, B.R., H. Hotz, G.-D. Guex, and R.D. Semlitsch. 2003. 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