Article

Amphibian Monitoring at Gulf Islands National Seashore: Two-Year Summary through March 2024

a small, alert treefrog on the branch of an inkberry shrub — Squirrel treefrog in a daytime rain shower at Naval Live Oaks Area, Gulf Islands National Seashore, Florida.
GULN/NPS

The Gulf Coast Inventory and Monitoring (I&M) Network tracks amphibian presence and abundance over time in three National Park Service (NPS) units in the southeastern U.S. and Texas. This work is part of a nationwide I&M program that monitors ecological health on NPS lands. Most frogs, toads, and salamanders are good ecological indicators because they begin their lives underwater and then transition to land as adults. With their unique physiology and exposure to multiple habitats, they can be particularly sensitive when environmental quality degrades or hydrologic processes are altered. Amphibian populations also fluctuate naturally, however, in response to rainfall and other environmental cues. To gain the necessary context to distinguish between causes, the Gulf Coast Network collects data in a consistent manner over a long period of time and takes into account a wide range of environmental effects. The resulting long-term datasets can reveal persistent declines that may otherwise go unnoticed, as well as help identify the causes of change, which can sometimes be addressed through management actions.

Highlights

• The Gulf Coast Network completed its sixth year of amphibian monitoring in two sites in the Naval Live Oaks Area, Florida. This summary focuses on the most recent two years of monitoring, from May 2022 to March 2024, and it provides additional context from the first four years of data collection, beginning in May 2018.

• Over the past two years, crews recorded 358 individuals of 10 amphibian species in monitored PVC pipes and coverboards, with 337 of these belonging to just four species: Squirrel Treefrog, Pinewoods Treefrog, Green Treefrog, and the non-native Greenhouse Frog. Yields in the past two years were broadly similar to most previous reporting intervals, with the exception of monitoring year 2021, when capture rates of all four of the common species peaked.

• Across all six years of study, the four most common amphibian species fluctuated in numbers, but they also consistently occupied one or both sites throughout the project. Amphibian population sizes are known to fluctuate naturally, often in response to rainfall or other environmental conditions. Each amphibian species is affected differently by these changing conditions, and their preferred habitats differ as well. For these reasons, temporal changes are best contextualized for each species individually and in light of a wide range of ancillary, environmental data, as will occur in upcoming trend analyses.

The Gulf Coast Network monitors amphibians at the Naval Live Oaks Area of Gulf Islands National Seashore, Florida, as well as in park units in Louisiana and Texas. The Naval Live Oaks Area is a small parcel of oak scrub, forest, and pine flatwoods in the city of Gulf Breeze on the Florida Panhandle. It is bordered by Pensacola Bay and Santa Rosa Sound, both of which are connected to the Gulf of Mexico. There are several freshwater wetlands and ponds within the Naval Live Oaks Area that provide breeding habitat for amphibians. A total of 21 species of frogs, toads, and salamanders have been recorded in the Area, as documented in the 2008 Inventory of Reptiles and Amphibians and from subsequent observations. Several of these species are the targets of our amphibian monitoring in the park.

The Network’s long-term monitoring approach for amphibians uses artificial refuges placed in vegetated areas near two freshwater ponds within the Naval Live Oaks Area. The refuges are small PVC pipes hung from trees or shrubs and untreated plywood coverboards placed flat on the ground. Each refuge type provides a favorable microclimate for many amphibian species and allows them to come and go unharmed. Field crews check under coverboards and inside PVC pipes once every other month, recording the numbers and types of amphibian species present. Ancillary environmental data are collected during each sampling event, including air temperature, relative humidity, and the surface temperature inside PVC pipes or under coverboards. There are also continuous data loggers installed at each site that take hourly measurements of air temperature, relative humidity, barometric pressure, and water depth of the adjacent pond. Daily rainfall data are obtained from a nearby weather station.

The Network’s two monitoring objectives for this project are to:

determine species composition of the amphibian communities that use the monitored PVC pipes and coverboards at each site, including native and non-native species, and
determine the relative abundance (counts per pipe or board) of the more commonly encountered amphibian species at each site, particularly in the context of trends over time, while also accounting for the effects of ancillary environmental data.

The Gulf Coast Network initiated amphibian monitoring in the Naval Live Oaks Area with a pilot study from 2011 to 2017 and then transitioned to the current design in 2018. Since work began in 2011, crews have documented 19 of the 21 amphibian species known to occur in the Area, but only four are found frequently enough for comparisons of relative abundance over time. These are Squirrel Treefrogs (Dryophytes squirellus; Figure 1A), Pine Woods Treefrogs (Dryophytes femoralis; Figure 1B), Green Treefrogs (Dryophytes cinereus; Figure 1C), and non-native Greenhouse Frogs (Eleutherodactylus planirostris; Figure 1D). The remaining, less common species continue to be tracked by the Network, but mainly on the basis of presence or absence across years (see this article's photo gallery for pictures).

four different images depicting three treefrog species resting on plants and a greenhouse frog on the ground — Figure 1. The four amphibian species that are commonly detected during I&M monitoring at Naval Live Oaks. (A) Squirrel Treefrog; (B) Pine Woods Treefrog; (C) Green Treefrog; (D) Greenhouse Frog. Body color varies from green to brown or grey within all three treefrog species.
GULN/NPS

Monitoring Design and Effort

In March 2018, Gulf Coast Network crews installed 124 PVC pipes and 64 coverboards in the two monitoring sites at the Naval Live Oaks Area. There were 64 pipes and 32 boards installed at Brown's Pond, located along the Area's northern-most boundary (Brown's Pond site). The remaining pipes and boards were installed at Maintenance Pond, located northeast of the Area's maintenance facility (Maintenance Pond site). In each study site, pipes and boards were arranged into clusters of three pairs of PVC pipes and four coverboards (Figure 2A and B). Each cluster of pipes and boards was separated from other clusters by 20 to 30 meters. A final cluster per site had six or eight pairs of pipes hung in trees over water (Figure 2C and D). After two months of settling-in, the first event was in May 2018, after which sampling occurred once every other month, with new monitoring years (MY) beginning in May. From May 2018 to May 2023, fieldwork was led by Sarabeth Uriz and her assistants from the University of West Florida. After May 2023, fieldwork transitioned to Gulf Coast Network staff, led by Billy Finney with support from park staff and other network staff. See the Supplementary Materials section for more information on project participants and project timeline since 2018.

The current report summarizes findings from MY2022 (May 2022 to March 2023) and MY2023 (May 2023 to March 2024). These are the 5th and 6th MY under the current sampling design. Results from MY2018 - MY2021 are covered in the first and second two-year reports for the park, with a few of the major findings repeated here for additional context. More information on project design and sampling methods is available in the protocol narrative “Monitoring Amphibians in Gulf Coast Network Parks” and its eight standard operating procedure documents.

two different ponds where PVC pipes are hung from trees and two adjacent areas where coverboards and pipes are deployed — Figure 2. Amphibian monitoring sites at Naval Live Oaks Area. Clusters of coverboards and PVC pipes on land at (A) Brown's Pond and (B) Maintenance Pond (pipes not visible in photo). PVC pipes over water at (C) Brown's Pond and (D) Maintenance Pond.
In (C), there is a larger PVC pipe holding a data logger for water level, which is used as ancillary data. Plywood coverboards are 0.9 x 1.2 m (3 x 4 feet) in size. PVC pipes are 0.6 m long (2 feet) and 2.5 or 5 cm (1 or 2 inches) in diameter.

Key Findings

Results for the Past Two Monitoring Years

Total Captures

Over the 12 events from May 2022 to March 2024, there were 358 amphibian captures of 10 different species in the Naval Live Oaks Area. The majority of captures were in PVC pipes, consisting of 278 individuals of five different species (Table 1). Only 80 individuals were found under coverboards, but coverboard richness was higher at eight species (Table 2). The most common species in PVC pipes was the Squirrel Treefrog, comprising 51% of pipe captures (Figure 1A). Pine Woods Treefrogs were also fairly common, with 36% of pipe captures (Figure 1B), followed by Green Treefrogs, with 10% (Figure 1C). The remaining two treefrog species were captured only 1-3 times each, but among them was the park's first record of the non-native Cuban Treefrog, found as one individual in September 2022 at Maintenance Pond. The arrival of this species is a cause for concern, given that elsewhere, its presence has been associated with declines in native treefrog species.

The most common species under coverboards was the non-native Greenhouse Frog, with 66% of board captures or 53 records (Figure 1D; Table 2). This Caribbean species was first seen in the Naval Live Oaks Area in 1997. Because it can reproduce without standing water, it has successfully spread into many habitats throughout the southeast. The remaining species under boards were all native, but each was recorded as fewer than eight individuals (Table 2), making them too infrequent for estimating trends over time. The presence of treefrogs under coverboards was notable in that it occurred only during the coldest times of year. Outside of those few unusually cold events, PVC pipes were a more reliable method for monitoring treefrogs in the Area. Coverboard monitoring produced only one new amphibian species record for I&M crews in MY2022 and MY2023. This was the Eastern Newt, which was first documented under coverboards at Maintenance Pond in November 2022.

Table 1. Number of individuals of each amphibian species recorded inside PVC pipes by monitoring year (MY) at Brown's Pond and Maintenance Pond sites. Seven percent of pipe-samples could not be completed due to flooding or other sampling issues, out of 1488 possible pipe-samples across the two MY (384 at Brown's Pond per year and 360 at Maintenance Pond per year). The summary counts below exclude a total of five unidentified Dryophytes treefrog species. These counts also exclude a single Southern Cricket Frog found in a pipe at Maintenance Pond during a high-water sample event in MY2022, because this species would not normally be found in this refuge type.

Species Name	Scientific Name	MY2022 Brown's Pond	MY2022 Maintenance Pond	MY2023 Brown's Pond	MY2023 Maintenance Pond	Total
Green Treefrog	Dryophytes cinereus	17	6	7	0	30
Pine Woods Treefrog	Dryophytes femoralis	26	59	3	13	101
Barking Treefrog	Dryophytes gratiosus	0	3	0	0	3
Squirrel Treefrog	Dryophytes squirellus	77	16	31	19	143
Cuban Treefrog	Osteopilus septentrionalis	0	1	0	0	1
-	Total pipes sampled	360	321	374	332	1387
-	Amphibians captured in pipes	120	85	41	32	278

Table 2. Number of individuals of each amphibian species recorded under coverboards by monitoring year (MY) at Brown's Pond and Maintenance Pond sites. Out of the 768 possible board-samples over the two MY (192 per site per year), 12% could not be completed due to flooding or other sampling issues.

Species Name	Scientific Name	MY2022 Brown's Pond	MY2022 Maintenance Pond	MY2023 Brown's Pond	MY2023 Maintenance Pond	Total
Southern Cricket Frog	Acris gryllus	0	0	0	2	2
Oak Toad	Anaxyrus quercicus	0	0	0	2	2
Pine Woods treefrog	Dryophytes femoralis	0	5	0	1	6
Barking Treefrog	Dryophytes gratiosus	0	1	0	0	1
Squirrel Treefrog	Dryophytes squirellus	0	2	2	0	4
Greenhouse Frog	Eleutherodactylus planirostris	35	5	12	1	53
Eastern Narrowmouth Toad	Gastrophryne carolinensis	0	1	5	1	7
Eastern Newt	Notophthalmus viridescens	0	4	0	1	5
-	Total boards sampled	168	153	182	164	677
-	Amphibians captured under boards	35	18	19	8	80

Photo Gallery

Amphibian Monitoring Naval Liv...

19 Images

Select photos of amphibians and reptiles encountered during I&M monitoring in 2022-2024 at Naval Live Oaks in Gulf Islands National Seashore, Florida

Differences Between Sites and Years

The two monitoring sites differed markedly in both counts and composition, with Brown's Pond generally producing higher yields and Maintenance Pond generally having higher species richness. Even so, this pattern varied by refuge type and species. For PVC pipes, Brown's Pond had over three times as many Squirrel and Green Treefrogs as did Maintenance Pond, but the opposite was true for Pine Woods Treefrogs, albeit with fewer records overall (Table 1). Maintenance Pond was also the only source of Barking or Cuban Treefrogs during the current two MY.

For coverboards, Greenhouse Frogs made up 87% of the 54 total captures at Brown's Pond, but only 23% of the 24 total captures at Maintenance Pond. Five of the eight species recorded under coverboards were only documented at Maintenance Pond, although only in very small numbers (Table 2). The Eastern Narrow-mouthed Toad was the only native species that was more common at the Brown's Pond site than it was at the Maintenance Pond site.

The number and composition of captures not only differed between sites, but they also differed between the two monitoring years. Amphibian yields dropped between MY2022 and MY2023 for each of the four common species within each site, with the exception of Squirrel Treefrogs at Maintenance Pond (Table 1). Of the less common species, several were seen in one year but not the other, but this may be expected given their low detection rates overall. Even the common species are known to have fluctuating population sizes from year to year, often in response to natural variation in environmental conditions over time. For this reason, temporal changes in captures are best viewed in light of a well-documented baseline for variability, combining many years of long-term monitoring with a wide range of ancillary, environmental data.

Long-Term Patterns in Amphibian Detections

Patterns Over Time Within the Four Common Species

Monitoring data from the past six years showed a distinct peak in captures of all three common treefrog species during the winter of MY2021 (Figure 3). That year also had higher-than-normal water levels in the two ponds throughout the spring and summer, potentially favoring treefrog reproduction to increase abundance the following winter. The highest-yielding events for treefrogs were in November 2021 and March 2022, but the event between them, in late January 2022, was surprisingly low-yielding. This event was atypical in that both mornings of fieldwork followed nights with near freezing temperatures (<5 degrees C). Conditions this cold are known to occur on roughly 20% of winter nights in that area (25 to 35 nights between November and March). On such occasions, most treefrogs do not use PVC pipes as shelter, presumedly because they are insufficient protection when temperatures drop that low.

A figure showing changes in treefrog detections in PVC pipes over time — Figure 3. Detections of three treefrog species between May 2018 and March 2024 at two sites at Naval Live Oaks Area (A) Squirrel Treefrog, (B) Pine Woods Treefrog, and (C) Green Treefrog. Grey bars indicate the six cooler months of the year.
* 10 or more pipes were unsamplable at Brown's Pond. ** 10 or more pipes were unsamplable at Maintenance Pond.

The six-year dataset for Greenhouse Frogs showed a pattern similar to that of the treefrogs, in that MY2022 and MY2023 were on par with or higher than yields from MY2018 to MY2020, but MY2021 had unusually high yields (Figure 4). Also notable in the time-series at Naval Live Oaks was the pattern of higher yields during colder months. This was observed in Greenhouse Frogs and, to a lesser extent, the treefrog species. This seasonal variation in refuge use could be related to a preference for less-enclosed shelter types during hotter times of year.

Figure showing changes in Greenhouse Frog detections over time — Figure 4. Detections of Greenhouse Frogs between May 2018 and March 2024 at two sites at Naval Live Oaks Area. Grey bars indicate the six cooler months of the year. * six or more boards were unsamplable at Brown's Pond. ** six or more boards were unsamplable at Maintenance Pond.

Patterns Over Time, Compared Between Sites

The park's two monitoring sites had differing total and annual yields of amphibians across the six-year time series. In terms of overall detections, refuges at Brown's Pond yielded 666 amphibians, whereas those at Maintenance Pond yielded 473. For individual years, it varied by species which site had more individuals encountered. Even so, both sites featured a moderate to large peak in MY2021, as described in the last section (Figures 3 and 4). This peak was more pronounced at Brown's Pond for Squirrel Treefrogs and Greenhouse Frogs, and it was more pronounced at Maintenance Pond for Pine Woods Treefrog and Green Treefrogs. These site-associations in the peak year generally matched associations throughout the six-year period, in that Maintenance Pond generally had higher yields of Pine Woods Treefrogs, and Brown's Pond generally had higher yields of Squirrel Treefrogs and Greenhouse Frogs. Green Treefrogs were the least-often recorded species of the top four, and they did not show a strong tendency for higher yields in one site or the other, with the exception of the peak at Maintenance Pond in MY2021. This species is the least likely to be found away from standing waterbodies, and its higher capture rate in MY2021 may be linked to the extremely high water levels recorded at Maintenance Pond during that year, as compared to the moderately high water levels recorded at Brown's Pond.

The two long-term sites used for monitoring amphibians at Naval Live Oak Area each provided a distinct window into the Area's amphibian communities. Their differing yields and compositions were unsurprising, given that they also differed in surrounding plant communities, soils, and features of the waterbody. Brown's Pond is a spring-fed pond along the northern-most edge of the park, with dense tupelo trees along its margins (Figure 2C). The adjacent forest type is called live oak forest (Urbatsch et al. 2009), which features a mixed assemblage of mature trees, a relatively shady understory, and organic material in the soils (Figure 2A). The terrestrial amphibian species that use this site are generally those requiring moister soils, including Greenhouse Frogs, Eastern Narrow-mouth Toads, and Dwarf Salamanders. Among these three, Dwarf Salamanders were absent under coverboards in MY2022 and MY2023, but were recorded at Brown's Pond at least once in most previous MY. The relatively static water level of Brown's Pond may provide reliable amphibian breeding sites, but it also supports fish and other predators of amphibians during their aquatic life stages.

In contrast to Brown's Pond, the Maintenance Pond is on sandier soils and is surrounded by a ring of evergreen shrubs (primarily Cyrilla racemiflora) that quickly transitions upslope into a large expanse of shrubby oak scrub, saw palmetto, and intermittent sandy openings (Urbatsch et al. 2009). The pond is primarily rainwater fed and contains only a few live tupelo trees at its center (Figure 2D). The refuges at Maintenance Pond have been sampled by I&M crews when the pond appears dry, when the surrounding shrubs are partially submerged, and at many water depths in between. These fluctuating water levels may keep the pond relatively fish-free and favor tadpole survival when water levels are high. See the supplementary materials for more information on pond water levels and other environmental covariates recorded since 2018.

Notable Events from the Past Two Monitoring Years and Supplemental Observations During Monitoring

Weather Events during MY2022 and MY2023

At the start of MY2022, the Naval Live Oaks Area was at the tail end of a prolonged wet period, during which water levels in the Area's rainwater-fed ponds had been unusually high. The six-month period of continuous high water between April and December 2021 was the longest recorded so far by the I&M water level logger at the Maintenance Pond. By May 2022, water levels had dropped, but there were additional high water phases between July and September 2022 and between June and August 2023 (See Supplementary Materials for hydrographs). During most of these events, the lowest-elevation coverboards at the pond were below water and unsampleable, and the lower-hanging pipes were often dislodged. There was less rainfall overall in the final eight months of MY2023, and the current reporting period ended with a severe drought lasting from January to May 2024.

Supplemental Observations of Amphibians

As part of the fieldwork routine, crew members record any amphibian species present within the site to enrich knowledge of species that generally do not use PVC pipes or coverboards as shelter. In MY2022 and MY2023, there were 10 amphibian species recorded in PVC pipes and/or under coverboards. Three additional species were recorded outside of monitored refuges: Southern Toad (Anaxyrus terrestris), Pig Frog (Lithobates grylio), and Southern Leopard Frog (Lithobates sphenocephalus). Of these three, only Southern Leopard Frogs have been found under coverboards in previous years (two individuals in MY2021), but generally, these species do use monitored coverboards at Naval Live Oaks Area.

Since the project began, a total of 19 amphibian species have been recorded by I&M crews, either in monitored refuges, as supplemental observations, or in a few cases, using minnow traps, which were only deployed during the pilot study. Six of the 19 species were not recorded in the past two MY. These were Two-toed Amphiuma (Amphiuma means), American Bullfrog (Lithobates catesbeianus), Bronze Frog (Lithobates clamitans), Eastern Spadefoot (Scaphiopus holbrookii holbrookii), Southeastern Dwarf Salamander (Eurycea quadridigitata), and Southeastern Slimy Salamander (Plethodon grobmani). See Supplementary Materials for more information.

Supplemental Observations of Reptiles

Crews collecting amphibian data also document any reptiles seen in monitoring areas, either under coverboards, in PVC pipes, or elsewhere at the site. In MY2022 and MY2023, 15 reptile species were recorded, including seven lizard species and eight snake species. Most common were Little Brown Skinks (Scincella lateralis) and Green Anoles (Anolis carolinensis), both of which had at least 15 detections over the two-year period. The non-native Brown Anole (Anolis sagrei) was recorded twice at Maintenance Pond during the current reporting period, as a supplemental visual observation and under a coverboard. This species was first recorded by I&M under a coverboard at Brown's Pond in MY2021. Finally, there were three new reptile records for I&M crews in MY2022 and 2023: Scarletsnake (Cemophora coccinea), Eastern Diamondback Rattlesnake (Crotalus adamanteus), and Mole Skink (Plestiodon egregius; see the article's photo gallery for pictures). All three of these species were expected to occur in the Area and had been recorded on the Inventory species list, but they nevertheless went undetected by I&M crews during their first decade-plus of monitoring in the Area. The Supplementary Materials document lists all reptile species recorded during the current two MY, as well as the 23 reptile species recorded since the Gulf Coast Network began monitoring there.

Additional Resources and Online Supplementary Materials

A supplementary materials package was prepared as part of this two-year summary, allowing additional space for detailed project descriptions, supplemental tables, and data exports. The current Supplementary Materials package includes a document with:

a project timeline at the park,
an overview of ancillary weather and environmental data,
a list of all amphibian species detected in the park during monitoring,
a list of reptiles detected during the two-year monitoring period as well as during previous monitoring years, and
literature cited and other useful references for amphibians and amphibian monitoring at the park.

The Supplementary Materials package also includes the approved datasets of amphibian counts by monitored refuge for each sampling event as well as environmental covariate data for the time-period of interest.

Article created by Jane E. Carlson, Ph.D., Ecologist for the Gulf Coast I&M Network.

Data collection, data entry, data management ,and project management completed by Gulf Coast Network Staff members Billy Finney, Whitney Granger, Fabi Speyrer, Daniel Hiatt, Jeff Bracewell, Jane Carlson, and Martha Segura, with field assistance from park staff members, interns, and other cooperators.

All Reports, Briefs, and Summaries in this Series

Supplementary Materials

This link leads to this summary's supplementary materials document, as well as data exports and the online reference for the web article, including a pdf printout

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Monitoring Design and Effort

Key Findings