Patterns of landscape occupancy of Hyla cinerea and Osteopilus septentrionalis in a mesic hammock North-Central

Abstract

Post breeding migration from wetlands into drier upland habitat is a behavior shared by many . Unlike that of breeding habitat, very few studies determine what factors are important in how amphibians select this non-breeding habitat. We explored how amphibians of the family occupied a mesic hardwood hammock in north-central Florida, considering differences using distance from breeding pond and amount of canopy cover. We are particularly interested in the habitat of the native green tree (Hyla cinerea)—GTF—and the invasive

Cuban tree frog (Osteopilus septentrionalis)—CTF. We used an array of PVC pipe refugia to sample the uplands outwards from the edge of water. We found that there was little movement of individuals between transects and there was no movement between the two ponds. There was no apparent pattern of where juvenile or adult H. cinerea were found in regard to distance from water. We collected presence data for each of the 54 pipe “sites” and ran an occupancy model and found that distance from the water is a good predictor of where H. cinerea can be found during the nonbreeding season, but not canopy cover. Unfortunately, we were unable to collect enough data for O. septentrionalis to draw any conclusions about their occupancy in the hammock.

Introduction

Amphibia as a Linnaean class and a unique phylogenetic clade faces worldwide decline and extinction (Maccallum 2007). Amphibians face many barriers to survival through the anthropogenic sixth mass extinction; habitat loss and fragmentation, virulent chytrid fungus epidemics, pollution, global climate change, and even the spread of new predators and competitors from around the world. There is a desperate and growing need for conservation and research that will guide effective management (Moore 2008). Observing and measuring spatial pattern is a vital part of landscape ecology and is the basis of protected area planning. Much of the focus on spatial pattern lies on the breeding habitat of amphibians and its relation to reproductive processes (Garton & Brandon 1975, Gerdhardt 1987, Salinas 2006). This focus on amphibian breeding habitat biases what we think is important to the life history of (Strofer 2003), which may have important implications to monitoring amphibians and protecting critical habitat.

The American green tree frog is a cosmopolitan anuran throughout the eastern United

States and is the only member of the genus Hyla in this region that breeds in permanent bodies of water with predatory fish. The species is very widespread and subject to climatic and spatial variability throughout its range (Gunzburger 2006). Although this species isn’t a species of conservation concern—listed as Least Concern (LC) by the IUCN—populations of this North

American hylid tend to decline or disappear when the large, invasive is present

(Meshaka 2001). Meshaka (2001) details the invasion success of this large Caribbean hylid, owing its success to: 1) large body size, 2) year-round opportunistic breeding, 3) large clutch sizes, 4) and generalist diet. Cuban tree frog tadpoles and adults have been found with native anurans in their stomach contents (Meshaka 2001, Hoffman 2007, and Granatosky 2011), cause breeding disruptions through amplexing other species (Salinas 2006). Despite overlap in the range and habitat of GTF and CTF, there appear to be important differences: GTFs occupy permanent water bodies and open canopy vegetation (Meshaka 2001, Horn et al 2005, and

Gunsburger 2006), whereas CTF prefer to breed in temporary aquatic habitats and closed canopy forests (Meshaka 2001 and Salinas 2006). Canopy cover appears to be an important, phenomenological factor in the distribution of hylids (Demaynadier & Hunter 1999, Horn et al

2005, and Binckley & Resetarits 2007), perhaps for food availability, water conservation, or cover from predators. When breeding season ends for green tree , adults and recent juveniles migrate into upland habitat away from breeding sites (Garton & Brandon 1975,

Demaynadier & Hunter 1999, and Gunsburger 2006), and thus distance away from the breeding habitat may be an interesting parameter of seasonal amphibian habitat.

We will attempt characterize the occupancy of wetlands by green tree frog (Hyla cinerea), GTF, and Cuban tree frog (Osteopilus septentrionalis), CTF, in Alachua county using measures of canopy cover and distance from the breeding habitat where we find individuals.

With this information we hope to find patterns of occupancy that can help us define habitat requirements of both species and develop methods for characterizing non-breeding habitat in anurans. We will measure canopy cover and record at what distance we find frogs in PVC refugia on private property in Gainesville, Florida. We hypothesize that we will find that green tree frogs occupy pipes further from the water than CTF and that there will be spatial differences between adult and juvenile GTFs.

Methods

Data were collected through nonrandom sampling during the non-breeding season. We selected a site on private property—with the owner’s permission—that during the breeding season had three permanent to semi-permanent bodies of water where breeding choruses of various species could be heard, including our target species—Hyla cinerea and Osteopilus septentrionalis. The site is located in north central Florida in Gainesville, Alachua county just north and adjacent to Payne’s Prairie State Park. The predominant habitat type according to the vegetation present and in reference to the Florida Natural Areas Inventory (FNAI 2010) appears to be a mesic hardwood hammock disturbed with coral ardisia (Ardisia crenata), which surrounds each of the breeding ponds and has a patch of pine flatwoods separating some of the ponds.

Boughton (2000) and Hoffman (2007) have shown that PVC pipe act as refugia and are good sampling tools when you are aware of the sampling bias between species. We set up 54

PVC pipes—1.25 diameter approximately 40 inches in length, cut at a diagonal at the base—in a total of 9 transects around Pond B and C (Figure 1), with 3 transects surrounding Pond B and 6 transects surrounding Pond C. Pond A was not used for the study because some regions were inaccessible and it was near a busy road, which could interfere with dispersal. Each transect consisted of 6 PVC pipes each. The pipes were placed in arrays at 10 meters intervals, with the first pipe placed at the water’s edge. Each transect was placed a minimum of 10 meters away from the next transect over in pond B, and a minimum of 20 meters away from each other in pond C. We collected data on 5 different trap days. On each sampling day, we used pipe plungers made with a ½ inch thick wooden dowel approximately 4 feet in length with spongy material cut in a cylindrical shape 2 inches tall about the same diameter as the PVC pipe (or slightly smaller)—stuck onto one end of the pipe with the dowel through the middle and hot glued in place—to remove the frogs from the pipe and put them into plastic Ziploc bags.

We then identified the species of frog, and clipped the toes of Hyla cinerea and

Osteopilus septentrionalis according to the diagram in Figure 2 in order to identify what transect they were located in. We also took note of whether the individuals were juvenile or adult by relative size (body length and mass). After identification and clipping, frogs were released back into the same pipe that they were originally found. We took note of frogs that were found to have moved across transects. Throughout the study we also found squirrel tree frogs (Hyla squirella)—STF— and pine woods tree frogs (Hyla femoralis). When species of frogs that were not targeted for the study were captured, we documented their presence but did not mark them and they were re-released in the same general location.

We tested the relationship between life stages (juvenile or adult) and between the two species and distance from water by setting up 54 ground-placed PVC pipes at 20-meter intervals beginning from water’s edge. We separated the PVC pipes into 9 different transects and marked

H. cinerea and O. septentrionalis individuals by transect group in order to identify in which transect they were found.

We modeled occupancy as single season simple models in the PRESENCE software using canopy cover and distance as covariates of occupancy(훙) and the different trap days as a covariate of detection(p), treating each of the 54 pipes and a 5-meter radius as an individual site.

Figure 1. Study site in Gainesville, Florida detailing the locations of the ponds on the property. Only ponds B and C were used in this study due to flooding during the rainy season at pond A that prevented setting PVC pipe transects.

Figure 2. Toe clip mark codes where each number represents the corresponding transect. All frogs found within a single transect will receive a group mark to note movement between transects and ponds. (Artist: Keara Clancey, University of Florida)

Figure 3. We plotted the distribution of canopy cover values across each of the 54 pipe refugia and described 3 classes of canopy cover to use in analysis. We opted not to use a continuous canopy cover measurement due to gaps amount of cover and we did not use a binary model of “open” or “closed canopy” because we did not find a strong value that demarcated these two classes.

Figure 4. We plotted the distribution of canopy cover values and instead described 5 classes of canopy cover to use as a comparison in the analysis. We found that this classification system had less significance than that of the 3- class description.

Results

We looked at relative abundance of the hylids found at the property on a per trap day basis and found that green tree frogs were the most common hylids on the property, followed by squirrel tree frogs, and then Cuban tree frogs. Juveniles were by far the most common life stage capture across all days and pipes, except for the last trap day, wehere very few individuals were caught the whole day. We also counted the sum of adults and juveniles found at each pipe on a per trap day basis to determine if there was any pattern in the distribution or differences between adults and juveniles and found that there is no significant difference between adults and juveniles found in the pipes. We found that refugia 30 meters from the water had the most individuals and refugia 40 meters from the water had the second highest amount of captures. There were very few and sometimes no catches at the edge of water and 50 meters away, although there were more captures at 50 meters than at 0 meters. We ran several occupancy models in PRESENCE

(Single season simple sample) to determine if distance and canopy cover are good predictors of occupancy, which can be found in Table 6 and 7.

Table 1. Trap Day 1 counts of tree frogs, “Distance” representing the distance from water corresponding to the PVC pipe. “GTF”, “CTF”, and “STF” referring to green tree frog, Cuban tree frog, and respectively. Distance No. GTF No. CTF No. STF 0 22 0 7 10 28 0 2 20 23 1 1 30 46 0 4 40 28 1 0 50 14 1 4 Grand Total 161 3 18

Table 2. Trap Day 2 counts of tree frogs. Distance No. GTF No. CTF No. STF 0 18 0 1 10 34 0 2 20 34 0 6 30 60 0 4 40 56 1 7 50 41 1 6 Grand Total 243 2 26

Table 3. Trap Day 3 counts of tree frogs. Distance No. GTF No. CTF No. STF 0 12 0 3 10 19 0 7 20 26 0 6 30 38 0 9 40 35 0 13 50 21 0 3 Grand Total 151 0 41

Table 4. Trap Day 4 counts of tree frogs. Distance No. GTF No. CTF No. STF 0 9 0 8 10 27 0 5 20 32 0 13 30 33 0 16 40 35 0 11 50 11 1 10 Grand Total 147 1 63

Table 5. Trap Day 5 counts of tree frogs. Distance No. GTF No. CTF No. STF 0 3 0 24 10 2 0 17 20 4 0 26 30 3 0 31 40 4 1 20 50 3 0 19 Grand Total 19 1 137

45 40 35 30 25

20 Sum of Adult Counts 15 Sum of Juv 10 5 0 0 10 20 30 40 50 Distance

Figure 5. Day 1 counts of adult and juvenile green tree frogs.

50 45 40 35 30 25 Sum of Adult Counts 20 15 Sum of Juv 10 5 0 0 10 20 30 40 50 Distacne

Figure 6. Day 2 counts of adult and juvenile green tree frogs.

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0 0 10 20 30 40 50 Distance Figure 7. Day 3 counts of adult and juvenile green tree frogs.

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Figure 8. Day 4 counts of adult and juvenile green tree frogs. 4.5

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Figure 9. Day 5 counts of adult and juvenile green tree frogs.

Table 6. PRESENCE Model of Green Tree Frog (GTF) Occupancy w/AIC Model # of Parameters AIC dAIC (distance),p(day) 7 240.89 0.00 (dist+canopy),p(day) 8 241.98 1.09 (.),p(day) 6 242.37 1.48 (canopy),p(day) 7 244.19 3.30 (distance),p(canopy) 4 298.04 57.15 (dist+canopy),p(.) 4 304.27 63.38 (.),p(.) 2 304.62 63.73 (canopy)p(.) 3 306.44 65.55 (distance),p(.) 3 308.66 67.77

Table 7. PRESENCE Model of Cuban Tree Frog (CTF) Occupancy w/AIC Model # of Parameters AIC dAIC (.),p(.) 2 81.13 0.00 (distance),p(.) 3 82.49 1.36 (canopy2),p(.) 3 82.57 1.44 (dist+canopy2),p(.) 4 83.97 2.84 (.),p(day) 6 85.64 4.51 (distance),p(day) 7 87.00 5.87 (canopy2),p(day) 7 87.09 5.96 (dist+canopy2),p(day) 8 88.48 7.35

Discussion

Green tree frogs were the most common species found in the PVC refugia, followed closely by squirrel tree frogs. Cuban tree frogs and pine woods tree frogs were uncommon, and we only found a few individuals of each. Based on AIC scores in the Presence program, using distance as a covariate of occupancy and trap day as a covariate of detection [휓(distance), p(day)] seemed to be the best model to describe H. cinerea occupancy. This may be related to atmospheric humidity and temperature variation on the different trap days and the need for frogs to maintain internal body temperature and water/electrolyte balances, though we did not include these nuances in our analyses. We found that the null occupancy model [휓(.), p(.)] was the best fit to describe O. septentrionalis occupancy, however this is not a significant result because there were very few captures of CTFs.

The results of our study are dependent on the assumptions that there were no misidentifications of frog species. For the occupancy model our assumptions include a closed population and sites are independent. Although weather and temperature data were collected, they were not used as variables in the occupancy model. Limitations of our study include misidentification between GTF and STF. Often times identifying H. squirella from H. cinerea was difficult, especially in earlier life stages. Additionally, identifying GTF and CTF as either juveniles or adults was relatively arbitrary. To better distinguish life stage, future studies should use more concrete methods to categorize individuals as juveniles and adults. Green tree frogs seem to have high site fidelity (Gerhardt et al 1987 and personal obs.) and often use PVC pipes more than natural tree cavities (Hoffman 2007), and thus we initially thought to treat each PVC pipe and the area within a 5 meter radius of the pipe as individual site, however sites were too close together to be considered independent as easily recognizable individuals were consistently found in the same pipe as well as adjacent pipes. On some occasions, individuals that were located in a pipe escaped when being transferred into the Ziploc bags and did not contribute to the counts for that trap day, though we did make note of these escaped individuals.

Limited data on O. septentrionalis was available, likely because they either did not utilize the PVC pipes as refuge, they were not present in the habitat, or are not as abundant as the native hylids. We know from hand captures, refugia captures, and known history of frogs that CTF are on site, however they we may have had few observations because 1) the property owner’s home was located within 100 meters of pond B and 200 from pond C and frogs may be preferentially selecting to live in the urban habitat (Meshaka 2001), 2) Cuban tree frogs are less likely to be found in PVC refugia than native hylids (Hoffman 2007), and 3) there may be individuals too large to fit in the 1.25” diameter pipes—the largest individual found (__mm) barely fit inside the pipe. Canopy cover did not appear to have an effect on dispersal in this study, however we did not have clearly demarcated “open” and “closed” canopy cover or a normally distributed range of canopy cover. Forest floor cover data was not collected, which may be biologically important for movement for tree frogs than tall overstory vegetation, and thus more studies would help us elucidate the importance of this cover.

The study period was limited as we were only able to obtain data for 5 trap days. This may have affected our results if these 5 days were not truly representative of the entire non- breeding season. However, we varied the times in which we would collect data in order to ensure that we accounted for variation in times in which individuals are active.

Despite its limitations, this study has provided insight on the various factors that influence H. cinerea and O. septentrionalis dispersal. This preliminary study on non-breeding habitat of anurans may show more interesting findings with stratified sampling of independent sites and broader spatial scales. Future studies related to these species should also focus on how other factors, such as presence or absence of other species, ground cover, or temperature/humidity levels in PVC pipes affect dispersal. The aim of this study was to begin to inquire on the importance and characteristics of non-breeding habitat moving forward looking into landscape level dynamics of hylid ecology and serve as a basis for further growth and development.

Literature Cited

1. Mccallum, M.L. (2007) Amphibian Decline or Extinction? Current Declines Dwarf Background Extinction Rate. Journal of Herpetology, 41, 483–491.

2. Moore, R.D. & Church, D.R. (2008) Implementing the Amphibian Conservation Action Plan. International Zoo Yearbook, 42, 15–23.

3. Brooks, T.M., Mittermeier, R.A., Mittermeier, C.G., Fonseca, G.A.B.D., Rylands, A.B., Konstant, W.R., Flick, P., Pilgrim, J., Oldfield, S., Magin, G. & Hilton-Taylor, C. (2002) Habitat Loss and Extinction in the Hotspots of Biodiversity. Conservation Biology, 16, 909–923.

4. Garton, J., & Brandon, R. (1975). Reproductive Ecology of the Green Treefrog, Hyla cinerea, in Southern Illinois (Anura: Hylidae). Herpetologica, 31(2), 150-161. R

5. Storfer, A. (2003) Amphibian declines: future directions. Diversity and Distributions, 9, 151–163.

6. Gunzburger, M.S. (2006) Reproductive Ecology of the Green Treefrog (Hyla cinerea) in Northwestern Florida. The American Midland Naturalist, 155, 321–328.

7. Salinas, F.V. (2006) Breeding Behavior And Colonization Success Of The Cuban Treefrog Osteopilus Septentrionalis. Herpetologica, 62, 398–408.

8. Boughton, R.G., Staiger, J. & Franz, R. (2000) Use of PVC Pipe Refugia as a Sampling Technique for Hylid Treefrogs. The American Midland Naturalist, 144, 168–177.

9. Meshaka, W.E. (2001) The Cuban Treefrog in Florida: Life History of a Successful Colonizing Species. University Press of Florida, Gainesville.

10. Granatosky, M.C. (2011) Modeling the role of stage-structured agonistic interactions in ontogenetic habitat shifts Florida Museum of Natural History, Division of Herpetology, P. O. Box 117800, University of Florida, Gainesville, Florida 32611, USA

11. Demaynadier, P.G. & Hunter, M.L. (1999) Forest Canopy Closure and Juvenile Emigration by Pool-Breeding Amphibians in Maine. The Journal of Wildlife Management, 63, 441.

12. Horn, S., Hanula, J.L., Ulyshen, M.D. & Kilgo, J.C. (2005) Abundance of Green Tree Frogs and in Artificial Canopy Gaps in a Bottomland Hardwood Forest. The American Midland Naturalist, 153, 321–326.

13. Florida Natural Areas Inventory (FNAI). 2010. Guide to the natural communities of Florida: 2010 edition. Florida Natural Areas Inventory, Tallahassee, FL.

14. Binckley, C.A. & Resetarits, W.J. (2007) Effects of forest canopy on habitat selection in treefrogs and aquatic insects: implications for communities and metacommunities. Oecologia, 153, 951–958.

15. Hoffmann, K.E. (2007) Testing the Influence of Cuban Treefrogs (Osteopilus Septentrionalis) on Native Treefrog Detection and Abundance. Thesis.

16. Gerhardt, H.C., Daniel, R.E., Perrill, S.A. & Schramm, S. (1987) Mating behaviour and male mating success in the green treefrog. Behaviour, 35, 1490–1503.

17. Mackenzie, D.I., Bailey, L.L. & Nichols, J.D. (2004) Investigating species co-occurrence patterns when species are detected imperfectly. Journal of Animal Ecology, 73, 546–555.

18. Richmond, O.M.W., Hines, J.E. & Beissinger, S.R. (2010) Two-species occupancy models: a new parameterization applied to co-occurrence of secretive rails. Ecological Applications, 20, 2036–2046.