Last updated: October 21, 2024
Article
Amphibian Monitoring at San Antonio Missions National Historical Park: Two-Year Summary through February 2024
GULN/NPS
Overview
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 US 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.
The Gulf Coast Network monitors amphibians at San Antonio Missions National Historical Park (NHP), Texas, as well as in park units in Louisiana and Florida. San Antonio Missions NHP consists of several properties within the city limits of San Antonio, which is the 7th most populous city in the United States. The city's namesake river is channelized through the city center, along the San Antonio River Walk, and later reaches the park in a more natural, but still highly-controlled waterway. In spite of the pressures of an urban setting, the park's forests and waters are home to several amphibians species, and these are the targets of our amphibian monitoring in the park.
The Network’s long-term monitoring approach for amphibians uses artificial refuges placed in forested areas near waterbodies, which at San Antonio Missions NHP includes channels and irrigation ditches connected to the San Antonio River. The refuges are untreated plywood coverboards placed flat on the ground, providing favorable microclimates for key amphibian species and allowing them to come and go unharmed. In three sites within San Antonio Missions NHP, field crews record the numbers and types of amphibian species under coverboards. Sampling is completed over one morning, once every other month. Ancillary environmental data are collected during each sampling event, including air temperature, relative humidity, and the ground temperature under each board. There are also continuous data loggers installed in each site that take hourly measurements of air temperature and relative humidity. Data on rainfall and barometric pressure are obtained from the nearby airport weather station.
The Network’s two monitoring objectives for this project are to:
- determine species composition of the amphibian communities that use the monitored coverboards at each site, including native and non-native species, and
- determine the relative abundance (counts per 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.
Across all three monitoring sites in San Antonio, three species of amphibians are found regularly: Gulf Coast Toad (Incilius nebulifer; Figure 1A), Western Narrow-mouthed Toad (Gastrophryne olivacea; Figure 1B), and the non-native Rio Grande Chirping Frog (Eleutherodactylus cystignathoides; Figure 1C).
GULN/NPS
Monitoring Design and Effort
In January 2018, 60 coverboards were installed in three forested sites: 20 boards near San Juan Mission (San Juan site), 20 west of the San Antonio River near Rillings Rd (West Side site), and 20 near the now-closed state hospital (Hospital site). In each study site, boards were arranged in clusters of four (Figure 2), with each cluster separated by 20-30 meters. After two months of settling-in for the boards, the first monitoring event took place in April 2018, after which sampling occurred on a single day, once every other month, with new monitoring years (MY) beginning in April. Sampling methods and effort have remained consistent from MY2018 to present with the exception of the Hospital site, which was dropped at the end of MY2022 due to repeated vandalism to coverboards over a three-year period. Rather than reduce the overall effort at the park, the 20 boards at Hospital site were moved to an area directly adjacent to West Side, called West Side extension. From April 2023 to present, crews then sampled 20 boards at San Juan, 20 boards at West Side, and 20 boards at West Side extension. Fieldwork was mainly led by Marvin Lutnesky and his students from Texas A&M University San Antonio. Datasheets were reviewed by Gulf Coast Network amphibian field lead Billy Finney, and network staff also occasionally visited the park for site maintenance and to assist with sampling. See the Supplementary Materials section for more information on project participants and project timeline since 2018.
The current report summarizes findings from MY2022 (April 2022-February 2023) and MY2023 (April 2023-February 2024). These are the 5th and 6th MY under the current sampling design. Results from MY2018 - MY2021 are covered in the first and second 2-year reports for the park, with a few of the major findings repeated here for additional context. Prior to the installation of the current design in MY2018, network staff had checked similarly-placed pilot coverboards at all three sites every month since November 2011. More information on project design and the full project history are available in the protocol narrative “Monitoring Amphibians in Gulf Coast Network Parks” and its 8 standard operating procedure documents.
GULN/NPS
Key Findings
Results for the Past Two Monitoring Years
Total Captures
There were 171 amphibian captures in San Antonio Missions NHP over the 12 sampling events in MY2022 and MY2023 (April 2022 to February 2024; Table 1). Almost two-thirds of these captures (106) were of the Gulf Coast Toad, a native species restricted to Mississippi, Louisiana, Texas, and south into Mexico (Figure 1A). The second most common species was the non-native Rio Grande Chirping Frog, with 58 captures (Figure 1C). This species is indigenous to Mexico and southern-most Texas and has been introduced elsewhere through the plant trade, facilitated by its ability to reproduce without standing water. Together, these two species comprised 96% of captures across MY2022 and MY2023. The third and final amphibian species for this two-year period was the Western Narrow-mouthed Toad, with 7 captures total from West Side and West Side Extension only (Table 1; Figure 1B). This species is native to Texas, Oklahoma, Kansas, and portions of a few neighboring states, as well as Mexico. A low detection rate for this species is unsurprising, given that its presence under coverboards is episodic and dependent on heavy rainfall prior to sampling.
Table 1. Number of individuals of each amphibian species recorded under coverboards by monitoring year (MY) at Hospital, San Juan, West Side, and West Side Extension sites. The total effort over this time period was 702 board-checks. At San Juan and West Side, all 120 board-checks were completed during each of the two years. At Hospital, 102 board-checks were completed in MY2022 and none in MY2023. At West Side Extension, 120 board-checks were completed in MY2023.
| Species Name | MY2022 San Juan | MY2022 West Side | MY2022 Hospital | MY2023 San Juan | MY2023 West Side | MY2023 West Side Extension |
Total |
| Gulf Coast Toad | 10 | 26 | 0 | 5 | 45 | 20 | 106 |
| Western Narrow-mouthed Toad | 0 | 6 | 0 | 0 | 0 | 1 | 7 |
| Rio Grande Chirping Frog | 8 | 9 | 0 | 6 | 15 | 20 | 58 |
| Total amphibians captured | 18 | 41 | 0 | 11 | 60 | 41 | 171 |
Differences Among Sites and Between Years
Of the two sites with monitoring events in both MY2022 and MY2023, West Side had higher yields over the 2-year monitoring period, in terms of all species combined as well as each species individually (Table 1). There were less than a third as many amphibians at San Juan (29) as there were at West Side (101). When West Side Extension was added for MY2023, this site had yields that were only slightly less high than the 20 original West Side boards (60 versus 41; Table 1). At Hospital site, in contrast, no amphibians were detected in MY2022, and no monitoring events took place there in MY2023. Half of the events at Hospital site in MY2022 had at least five coverboards that were unsamplable during monitoring visits due to recent vandalism, such that data were not representative of normal sampling events. Additionally, the site regularly served as a campsite for a local unsheltered population that MY, which may have impacted amphibian presence as well.
Between the two years, yields in MY2023 were higher than in MY2022, but this increase was strongly influenced by the change in sampling effort, i.e., moving boards from the low-yielding Hospital site to the higher-yielding West Side Extension site. For the two sites where data were collected consistently, West Side and San Juan, the year with higher yields differed between sites. At West Side, yields of both species were higher in MY2023 than in MY2022, and at San Juan, this pattern was reversed (Table 1). Western Narrow-mouthed Toads were only captured at West Side in MY2022 and at West Side Extension in MY2023, but overall, representation was low with 7 individuals total.
Long-Term Patterns in Amphibian Detections
Patterns Over Time Within Species
When comparing the current 2-year period to previous monitoring years back to MY2018, MY2022 and MY2023 had relatively low yields of Rio Grande Chirping Frogs across the three sites (Figure 3A). Peak yields for this non-native species were recorded just after the current project began, between October 2018 and February 2019, and they have not been as high since. Detections have remained low in all three sites since February 2022.
For Gulf Coast Toads, yields in MY2022 and MY2023 were on par or higher than yields of previous years, with the exception of MY2021 (Figure 3B). Monitoring year 2021 had record-high yields of Gulf Coast Toads in all three sites, particularly in October 2021-February 2022. The winter of MY2021 was preceded by an unusually wet May-August, which may have favored toad reproduction to increase abundance the following winter. In an earlier study of Gulf Coast Toads in central Texas, toad reproductive activity was indeed higher during years with more rainfall in the spring and summer, and the greatest number of toads were detected in the wettest year of that 6-year study (Blair 1960; see Supplementary Materials for literature cited and more information). Also notable in the time-series at San Antonio Missions NHP was the pattern of higher yields of Gulf Coast Toads during colder months, presumably because coverboards served as a thermal refuge.
Patterns Over Time Among Sites
The park's the three original monitoring sites often differed from each other in total yields within each of the six monitoring years (Figure 3A and B), and for Gulf Coast Toads in particular, the highest yielding site changed over time (Figure 3B). In the first two MY, West Side was the lowest-yielding site, but in the most recent four MY, it was consistently the leader. Lower yields at the Hospital site may have been driven by its frequent use as a campsite in recent years, but there were no apparent, persistent changes at San Juan or West Side near the end of MY2019 that would be readily associated with their changes in rank. Further interpretation will be possible with the upcoming trend report, which will be based on a statistical analysis of the full six years of amphibian detections and a broad suite of ancillary data.
In spite of fluctuating yields among sites and over time, Gulf Coast Toads and Rio Grande Chirping Frogs were recorded on nearly every event over the 6-year period. The non-native Rio Grande Chirping Frog was first documented in the park during the Inventory of Amphibians and Reptiles in 2002, and its ecological impacts remain unclear. The Gulf Coast Toad, in contrast, is a native member of the park fauna, but it is also a well-known habitat generalist and is common in many suburban and urban areas throughout its range. Although its presence at San Antonio Missions NHP is not particularly surprising, sustained losses of this native species --in the context of the long-term dataset-- would raise significant concern.
* no sampling due to COVID-19 travel restrictions. **Data from Hospital site excluded from this event because 5 or more boards were not sampled due to vandalism
Notable Events from the Past Two Monitoring Years and Supplemental Observations During Monitoring
Weather Events
During MY2022 and MY2023 (April 2022 to February 2024), San Antonio experienced one severe drought and two abnormally hot summers. Conditions were extremely dry throughout the county for nearly a year, from May 2022 to March 2023. Several rainfall events in April and May 2023 improved conditions partially, yet the county remained relatively dry through to the end of the reporting period. The lack of rainfall was coupled with excessively high summertime temperatures in both monitoring years, in terms of the number of days above 100 degrees F (37.8 degrees C) and the average daily temperatures in the summer (June 1 and August 31). The National Weather Service reported that the summer mean temperature for 2022 was the hottest ever recorded for San Antonio (88.3 degrees F [31.3 C]; Stinson Airport weather station data), but this record was surpassed by nearly a full degree the very next summer (89.0 F [31.7 C]).
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 coverboards as shelter. In MY2022 and MY2023, there were no amphibian species recorded other than the three found under coverboards. Since the project began, four additional amphibian species have been recorded, all of which are native to the San Antonio area: Blanchard's Cricket Frog (Acris blanchardi), American Bullfrog (Lithobates catesbeianus), Green Treefrog (Dryophytes cinereus), and Rio Grande Leopard Frog (Lithobates berlandieri; 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 or elsewhere in the site. In MY2022 and MY2023, five reptile species were recorded. Most common were Little Brown Skinks (Scincella lateralis), Green Anoles (Anolis carolinensis), and Checkered Gartersnakes (Thamnophis marcianus), all of which had at least six detections over the two-year period. Two additional species were each seen only once. A Texas Coralsnake (Micrurus tener) was seen in August 2022 at the Hospital site, and a Texas Threadsnake (Rena dulcis) was seen in June 2023 at the San Juan site. The Texas Threadsnake was the first record by I&M in San Antonio since this monitoring project began, but the species was detected in the Inventory of Reptiles and Amphibians for the park in 2002-2003. Since pilot monitoring began in 2012, a total of 17 reptile species have been recorded during network monitoring events in San Antonio, with the only non-native species being the Mediterranean Gecko (Hemidactylus turcicus; see Supplementary Materials for more information). This species was last seen in a monitoring site in 2019, although it is regularly observed on nearby buildings in the evening. In recent years, another non-native gecko has been recorded in the San Antonio area, often in the same habitats as the Mediterranean Gecko. This is the Moorish Gecko (Tarentola mauritanica), and although potentially already present in the park, it has not yet been detected in the Gulf Coast Network's monitoring sites. Learn more about this new non-native species in this recent article by Drew Davis and others.
Additional Resources and Online Supplementary Materials
A supplementary materials package was prepared as part of this 2-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 on the park during monitoring,
- a list of reptiles detected during the 2-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 links to the amphibian and environmental covariates datasets for the time-period of interest.
Article created by Jane E. Carlson, Ph.D., Ecologist for the Gulf Coast I&M Network.All Reports, Briefs, and Summaries in this Series
Carlson, J.E., 2024. Amphibian Monitoring at San Antonio Missions National Historical Park: Two-Year Summary through February 2024. Gulf Coast I&M Network, Lafayette, LA.
Carlson, J.E., 2024. Amphibian Monitoring at Gulf Islands National Seashore: Two-Year Summary through March 2024. Gulf Coast I&M Network, Lafayette, LA.
Carlson, J.E., 2024. Amphibian Monitoring at Barataria Preserve,Jean Lafitte National Historical Park and Preserve: Two-Year Summary through March 2024. Gulf Coast I&M Network, Lafayette, LA
Carlson, C. and J.E. Carlson. 2022. Scientists Spy on Treefrogs Using Plastic Pipes in Trees. Frontiers for Young Minds 10:718524. https://doi.org/10.3389/frym.2022.718524
Carlson, J.E. and M. Toussel. 2022. Two-Year Report for Amphibian Monitoring at Barataria Preserve 2020-2022. Gulf Coast I&M Network, Lafayette, LA
Carlson, J.E. and M. Toussel. 2022. Two-Year Report for Amphibian Monitoring at Gulf Islands National Seashore 2020-2022. Gulf Coast I&M Network, Lafayette, LA
Carlson, J.E. and M. Toussel. 2022. Two-Year Report for Amphibian Monitoring at San Antonio Missions NHP 2020-2022. Gulf Coast I&M Network, Lafayette, LA
Carlson, J.E. 2022. Resource Brief: Amphibian Monitoring Program Summary. Gulf Coast I&M Network
Carlson, J.E. and Others. 2020. Two-Year Report for Amphibian Monitoring at Barataria Preserve 2018-2020. Gulf Coast I&M Network, Lafayette, LA
Carlson, J.E. and Others. 2020. Two-Year Report for Amphibian Monitoring at Gulf Islands National Seashore 2018-2020. Gulf Coast I&M Network, Lafayette, LA
Carlson, J.E. and Others. 2020. Two-Year Report for Amphibian Monitoring at San Antonio Missions NHP 2018-2020. Gulf Coast I&M Network, Lafayette, LA
Supplementary Materials
This link leads to this summary's supplementary materials document, as well as data exports and the full, online reference for the web article, including a pdf printout