Tentative Times for Treefrogs

December 13, 2021

By Melinda Schuman | Biologist

Humans have undoubtedly had an impact on the land in Florida. Occasionally the history of an area and how humans have interacted with it gets lost with time. Sometimes an area heals so well it may be easy to believe everything is completely back to how it was before. Scars on the land often have an almost magical ability to heal and grow green again from nearly any damage done to it. Not all species may be as resilient, however, and what has really been lost may not always be immediately obvious.

Through our research on treefrog communities in southwestern Florida, we found that treefrog populations can be negatively affected by damage to wetlands for much longer than the time it takes for the habitat to appear restored. Our study results indicated that a non-native species was dominating the treefrog aggregation during the past decade. Furthermore, more sensitive species may have been decimated during habitat alterations of the past century and numbers remain reduced despite the ensuing conservation efforts.

Green treefrogs

In our study, we looked at treefrog species compositions within three conservation areas. Florida Panther National Wildlife Refuge and Fakahatchee Strand Preserve State Park were reference areas (defined as an area with similar habitats typically used as restoration goals for evaluating the success of the area being restored).

The third conservation area, Picayune Strand State Forest, was undergoing habitat restoration (our sampling took place before, during, and after restoration). All conservation areas are located in the Big Cypress Basin bio-region, defined as the area west of the Florida Everglades, and this area is comprised of a mosaic of pine forest, cypress strands, hardwood hammocks and wet prairies.

The entire region experienced human disturbance and hydrologic changes at some point, primarily through logging, canal excavation for drainage, road building, cattle grazing, and farming. For the reference areas, however, this damage was inflicted decades prior to their designation as conservation lands and they are now considered to be in a much less impacted state.

Map of southwest Florida conservation areas and sampling sites. Source: https://www.biotaxa.org/hn/article/view/66904

Logging of pine timber began in the Big Cypress region in 1924, where 90% of the pine forests were cut. Highways and a railroad were constructed in the 1920s, which facilitated the logging operation.

Major cypress logging in Fakahatchee Strand began in 1944 and these operations continued northward through to Corkscrew Swamp. A network of tramways was created, including roads and drainage canals, and harvesting was completed by 1957.

In the 1960s and 70s a system of canals and roads was created in western Big Cypress laying the groundwork for what was supposed to become one of the biggest subdivisions in South Florida known as Golden Gate Estates. All of these activities contributed to not only severe loss of wetland habitat, but also altered the natural flow of water in the area.

As a result, land acquisition to create Fakahatchee Strand Preserve State Park began in 1974 and Florida Panther National Wildlife Refuge was established in 1989. Land acquisition began in the Southern Golden Gate Estates in 1985 and Picayune Strand State Forest was designated in 1995. The Picayune Strand Restoration Project, with a goal to restore habitat and watershed flow, officially began in 2007, although work filling the canals began in 2004.

Logging in southwest Florida the 1940s & 50s. Photo credit: https://www.news-press.com/

Treefrogs are a member of the amphibian order Anura, which contains all frog and toad species. Anurans are a fascinating group of amphibians that lead a “double life” — swimming as larvae at the beginning of life and then hopping around as adults on land. Their role in the food chain makes them vital contributors to the ecosystem. Furthermore, they have permeable skin that makes them extremely sensitive to changes in the quality or quantity of the water in their environment. As such, their life cycles are acutely tuned to the hydrology (water cycles) of their region.

Here in Southwest Florida, we have anuran species that range from the diminutive native little grass frog (that can fit its entire body on a dime) to the shockingly large non-native cane toad (that when fully grown can be comfortably held with two hands). If all this wasn’t extraordinary enough, the blend of unique characteristics allows scientists to use frogs and toads as indicators of ecological health, disturbance, or change.

Green treefrog tadpole

Regarding our research, the objectives were to compare spatial aspects of treefrog species compositions among conservation areas and to compare temporal patterns of the compositions in each area.

Treefrog sampling took place from 2005–2007 (prior to major restoration activity in Picayune Strand), 2009–2011 (during restoration) & 2016–2017 (first-year post-restoration for the eastern region). Sampling sites were located in cypress, prairie, pine, and hardwood habitats within each conservation area. Treefrogs were passively trapped using Polyvinyl chloride (PVC) pipes, which they could enter or exit freely. Any treefrogs found in the pipe at the time of sampling were gently pushed into a mesh bag, weight and length measurements were taken, and then treefrogs were released back at the site.

Treefrog sampling
Treefrog sampling

Our results indicated that the abundance of invasive Cuban treefrogs was relatively high in the restoration area, while native treefrog abundances were comparatively low.

The Cuban treefrog is a large species that is known to prey on native treefrogs and compete for resources. More surprising, however, was that the non-native species was also gaining a foothold at reference sites, despite the presumed restored state of both areas and a hearty presence of native treefrogs. Over time, the native green and squirrel treefrog populations in the reference areas declined and were seemingly replaced by the non-native species.

Cuban treefrog

The dominance of the Cuban treefrog, in addition to the low abundance of native species in the restoration area, may have been the result of the altered hydrology in this conservation area.

The life history requirements of the Cuban treefrog potentially make the species more able to adapt to the disturbances in the area. This non-native species dominance may also preclude any meaningful results for the restoration from this research. The decline of native treefrog abundances that occurred in the reference areas may be at least in part due to the increase in Cuban treefrogs, and the threat this species poses to the native species.

However, it also brings into question the suitability of these areas as reference locations. Despite outward appearances, they may also be in a state of recovery. All natural populations fluctuate and some change over time is expected, but reference site populations should remain somewhat stable, especially over the long term.

Finally, the complete absence of two sensitive indicator species, pinewoods and barking treefrogs, during our surveys was noteworthy.

Given their greater dependence on wetland integrity, these treefrogs were probably among the first species excluded from disturbed or degraded habitats. Other studies have observed both species within the conservation areas but in very low numbers, suggesting the habitats themselves may still be in a state of recovery and missing key components needed to sustain historical populations.

Barking treefrog

More research is needed to understand the shifts in treefrog communities and whether native species can persist despite the pressure from the exotic Cuban treefrog. This information will be vital to answer questions regarding the long-term health of these anuran populations. Habitat degradation and invasive species are currently affecting native treefrog species, but the threat of climate change may also result in difficulties for treefrogs in the years to come.

Decreases in precipitation will likely result in failed breeding attempts for many aquatic species and a higher risk for desiccation; saltwater intrusion from sea level rise has the potential to impact freshwater dependent species who are unable to tolerate salinity exposure, and changes in temperatures can lower disease resistance.

Additionally, we will likely see exotic species spread farther north and into other natural areas more easily owing to rising temperatures. However, restoring and conserving natural areas in the least disturbed state possible may give native treefrogs and many other native species a better chance to survive and thrive despite the challenges that lay ahead.

For more detailed information, the results of our study were recently published in the scientific journal Herpetology Notes.