The Burrowing Cascade

 

Florida's Hidden Labyrinths

Florida's landscape is often overlooked for its apparent lack of topographical interest. This conception may cause visitors and residents alike to overlook geological features worth visiting; locations such as our Sacred Hills of Hillsborough County's Edward Medard Conservation Park, Polk County's Iron Mountain at Bok Tower Gardens, or Devil's Den Spring in Levy County are all worth the trip. But, broadly speaking, the state of Florida is flat. However, it is anything but static. Florida's recent emergence as a land mass has resulted in the development of truly unique habitats, the conditions of which put huge demands on pioneering species to adapt.

A gopher tortoise (Gopherus polyphemus) peering out from its burrow entrance. Photo by Geoff Gallice, April 27, 2011. CC BY 2.0.

Technology has only recently revealed the extent to which our keystone species, the gopher tortoise (Gopherus polyphemus), transforms some of our most extreme habitats. These remarkable creatures are the architects of an underground phenomenon known as the burrowing cascade, a process that is quietly reshaping Florida's landscape from below. But, before we can understand how they reshape that environment, it is important to appreciate what makes Florida... Florida.

Florida's Geological History: One Giant Sand Dune

North America 75 million years ago. Appalachia is the major landmass exposed on the Eastern half of the continent. (Blakey, 2012). CC BY 1.0.

The east coast of North America is largely characterized by the Appalachian Mountains. This ~1 billion year old mountain range has endured above water for several hundred million years. The resultant land mass of "Appalachia" has been home to animals since perhaps they first left the ocean. Eons of soil activity, the constant perturbance of erosion, animal activity, plant growth, and other factors, have led to the development of complex and nutrient rich soils, divided into several layers – or horizons – with distinct properties. These soils, in combination with a temperate climate, led to the development of large, deciduous forests.

Orange Island (green) marked the first emergence of Florida since perhaps it was part of the Pangea supercontinent. Eocene to early Miocene epochs. Orange island became a refuge for displaced flora and fauna during the repeated emergence and submersion of Florida in the coming 25 million years. (Petuch & Roberts, 2009). Public Domain.

Florida's geological history diverges significantly from that of Appalachia. Florida spent most of the past 150 million years submerged under the ocean. The limestone peninsula only began to emerge during the Miocene epoch, about 25 million years ago, with the process largely completing around 5 million years ago due to colliding continental plates. The result is a vast expanse of limestone covered in sand - essentially an enormous, ancient beach. Still, huge swaths of land have repeatedly plunged into the ocean as sea levels fluctuate through interglacial periods.

Cross-section comparison of Entisol (left) and Alfisol (right) soil profiles. Entisols show minimal horizon development with a uniform, light-colored appearance, while Alfisols display distinct, well-developed horizons with color variations from topsoil to reddish-brown subsoil. Alfisol soil is common in western Appalachia. (USDA, n.d.). Public Domain.

This relatively recent geological emergence means that Florida's upland soils, classified as entisols, are still in their early stages of development. These young soils are characterized by minimal horizon development, high sand content, and low nutrient retention. The confluence of these conditions during the early Pleistocene (approximately 2.5 million years ago) led to the establishment of extensive xeric (dry) environments in Florida. Sandy soils, intense heat, and frequent wildfires resulted in hostile environments, which while not true deserts, necessitated the accumulation of specialized flora and fauna to form stable ecosystems.

The Arrival of the Gopher Tortoise

A gopher tortoise (Gopherus polyphemus) on a hiking path. This keystone species creates extensive burrow systems that provide habitat for many other animals. Photo by Philipp Michel Reichold, September 24, 2020. CC BY 4.0.

It was into this harsh, newly formed landscape that the gopher tortoise (Gopherus polyphemus) emerged during the early Pleistocene. These land tortoises, descendants of a lineage long adapted to xeric environments in North America, found a niche in Florida's newly formed uplands. Their specialized ability to dig burrows in loose, sandy soil proved crucial in these harsh, nutrient-poor environments.

A View of Paynes Prairie Preserve State Park, Florida, showcasing the expansive dry prairie habitat. While presently rare due to agricultural development and the diversification of pine and oak habitats, sights like these may have been common when the gopher tortoise first arrived in Florida during the early Pleistocene, sharing open prairie with megafauna such as mammoths and primitive horses. Photo by Hauck Gaudio, December 31, 2023. CC BY 4.0.

The gopher tortoise quickly established itself as a keystone species in Florida's upland habitats, earning the title of "ecosystem engineer." This term refers to organisms that significantly modify their environment, creating or maintaining habitats for other species which persist stably even in the absence of the originating organism.

The Burrowing Cascade Explained

The term "burrowing cascade" refers to the fractal-like, potentially exponential expansion of the original gopher tortoise burrow by other animals. This process dramatically increases the complexity and surface area of the underground environment, creating a network of interconnected spaces.

Ground-penetrating radar (GPR) images of a simple gopher tortoise burrow, showing the main burrow spiraling down (turquoise), relic burrows (dark blue), Florida mouse burrows (pink), and tree roots (green) from different perspectives. Figure 7 from Kinlaw, A. and Grasmueck, M. (2012) in Geomorphology. Used with permission from Elsevier.

The cascade begins with the gopher tortoise's initial burrow, which can extend over 30 feet in length and 10 feet deep. From there, other animals such as the Florida mouse expand on this initial burrow, creating side chambers, extending existing tunnels, or digging new passages branching off from the main burrow. Smaller creatures like insects and arachnids further expand the network, creating minute tunnels and chambers within the walls of the larger burrows. This process continues over time, with each generation of inhabitants potentially expanding the network further.

Immediate Impacts of Gopher Tortoise Burrows

A gopher tortoise at the entrance of their burrow. Photo by Hillsborough County CELM Staff.

These additional species who utilize the tortoises’ burrow are called commensals. Unlike parasites, the commensal benefits from the host, without causing any immediate detriments. But, the commensal species which participate in this cascade are not solely other burrowing animals.

These underground structures maintain a stable temperature and humidity year-round, providing refuge from Florida's extreme heat, cold snaps, and wildfires. This microclimate regulation makes the burrows essential shelters for over 350 animal species, including endangered ones like the indigo snake and gopher frog. Large, commensals such as marsh rabbits and burrowing owls depend almost exclusively on the gopher tortoise to dig initial burrows, later expanding and modifying these habitats to suit their own purposes.

Florida mice (Podomys floridanus) captured on camera inside a gopher tortoise burrow. This image demonstrates the commensal relationship between gopher tortoises and other species, with over 350 animals known to use these burrows. Image from Florida Fish and Wildlife Conservation Commission, February 10, 2015. CC BY-ND 4.0. (Full video link here.)

Beyond providing shelter, the burrowing activity of gopher tortoises constantly mixes soil layers, improving nutrient distribution in Florida's young, underdeveloped entisols. The “aprons” of sand that gopher tortoises excavate are ideal nesting spots for a variety of reptile species (this is why you should avoid stepping to a burrows entrance).

For more examples of commensal species, and an introduction to the species, check out this previous trailblazer article.

Long Term Benefits

In a 2012 study, High-resolution ground-penetrating radar (GPR) revealed numerous stable air voids underground - remnants of old, abandoned tortoise burrows. (Kinlaw and Grasmueck, 2012) These persist long after the entrances have collapsed, and even when cut off from the surface, they still participate in the burrowing cascade.

Ground-penetrating radar (GPR) images of a second gopher tortoise burrow, revealing a more complex network. The image shows the active main burrow (turquoise), multiple relic burrows (dark blue), and tree roots (green) from various perspectives. This figure highlights the accumulation of underground structures created by gopher tortoises over time. Figure 9 from Kinlaw, A. and Grasmueck, M. (2012) in Geomorphology. Used with permission from Elsevier.

the GPR’s accuracy was such that researchers were able to verify that smaller animal burrows intersect with these voids. This builds upon a 1997 study which found that Florida mice utilized and maintained collapsed tortoise burrows over a period of 10 years through their own auxiliary tunnel system. In this way, commensals extend the life of the gopher tortoises burrows. (Kinlaw and Grasmueck, 2012; Newman, 1997)

Burrow camera view of a gopher tortoise (Gopherus polyphemus) inside its burrow, demonstrating the tortoise's ability to turn around within the confined space. Figure 1e from Kinlaw, A. and Grasmueck, M. (2012) in Geomorphology. Used with permission from Elsevier.

In areas with gopher tortoise populations, this action creates a complex network of tunnels and chambers underground. To quote the same 2012 paper:

Tortoises can and often do dig several burrows during each yearly activity period. Because they are long-lived species (up to 80 years), each individual can easily produce hundreds of burrows. They also re-excavate and re-use burrows, effectively increasing the number of “active” burrows within their home range.

This underground labyrinth becomes an integral part of the ecosystem, persisting for decades even with tortoises no longer present. Over centuries the burrowing cascade produce geometrically complex environments, enabling a much wider range of plant and animal species to thrive in these otherwise harsh environments.

Conservation Implications

Hillsborough County Environmental Lands Research Analysts Team (EL-RAT) surveying for active gopher tortoise burrows.

The gopher tortoise's role in shaping Florida's upland ecosystems demands attention. Regions which have seen local disappearances of gopher tortoises may not see the effects of that absence until the last of the burrows collapses. As a keystone species, their long-term presence is crucial for maintaining the biodiversity within their range.

Programs like Hillsborough County's Environmental Lands Acquisition and Protection Program (ELAPP) play a vital role in preserving these habitats and the complex web of life they support. Gopher tortoises on ELAPP properties are the subject of monitoring and survey. By protecting these animals and their habitats, we help ensure the continuation of this burrowing cascade and the biodiversity it fosters in Florida's distinctive landscape for posterity.

Photo by Hillsborough County CELM Staff.

References:

Kinlaw, A., and Grasmueck, M. (2012). Evidence for and geomorphologic consequences of a reptilian ecosystem engineer: The burrowing cascade initiated by the Gopher Tortoise. Geomorphology, 157-158, 108-121.

Newman, C.M. (1997). The Florida Mouse (Podomys floridanus): long-term dynamics of a population in the high pine sandhills of Putnam County, Florida. M.S. Thesis. University of Florida, Gainesville.