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Cave Ecology

Mexican free-tailed bat (Tadarida brasiliensis) maternity colony ranging well over ten thousand individuals, El Malpais National Monument, New Mexico, USA. A colony this size produces a substantial guano deposit each year. In this setting, bats are a keystone species in the cave ecosystem. Their guano supports an entire community of animals. Without the bats, the community would be dramatically changed. From Wynne 2013.

Beyond the darkness, the development of cave habitats are driven by environmental conditions and the availability of energy (nutrients). As there are numerous books devoted to cave ecology (including one of which Jut is editor), this page provides some broad brush strokes concerning how the interplay of these three variables dictate both evolution unfolds and communities are assembled underground.

The four principal environmental zones. Nutrient and stability gradate from low (-) to high (+). Adaptation to the cave environment occurs within the transition and deep zones.

While most caves would be fairly barren environments without energy from the surface, nutrients are deposited randomly along the length of the cave — within the four environment zones. Typically, a cave may be divided into four principal zones. The entrance zone is characterized as a combination of surface and cave environmental conditions. The cave’s twilight zone has both diminished light conditions and climatic influence of surface environment, while the next zone, the transition zone, is an aphotic region where barometric and diurnal shifts are observed at a significantly diminished rate approaching near stable climatic conditions. Finally, the deep zone is completely dark, and largely environmentally stable with a near constant temperature, near water-saturated atmosphere, and low to no airflow. 

Entrance to an impressive diversity of cave life — including several troglomorphic arthropod species and a bat maternity roost, Runaway Creek Nature Reserve, Belize.

Photosynthesis fuels most of life on our planet. However, as one traverses beyond the cave’s twilight zone, photosynthesis ceases. Nutrients that reach the deep cave interior are transported from the surface. This has resounding implications on how ecological communities are formed and evolve.

Nutrient types within caves are quite varied. They include bat guano piles, plant roots penetrating the cave from the surface, and flood-transported surface vegetation at one end of the food spectrum with more nebulous sources such as dissolved organic material percolating into the cave with surface water at the other. Regardless of the nutrient type, these energy sources are typically patchily distributed across the four environmental zones of a given cave. The combination of zone and nutrient source can result in the establishment of relatively simple, yet fascinating ecological communities.

As with surface ecosystems, cave habitats undergo a continuum of colonizations and extinction events over time. Organisms with certain pre-adaptive traits are those who successfully colonize the underground realm. Over evolutionary time, an established population may become increasingly better equipped to the cave environment and ultimately the deep zone due to genetic drift, mutations, and selective pressures. This process can result in the development of a troglomorphic (or subterranean-adapted) species.

However, not all species that live in caves are troglomorphic. Cave-dwelling organisms have varying degrees of dependency on the subterranean environment. The most reliant taxa are troglobionts — obligate cave dwellers that can only complete their life cycle within underground, and exhibit morphological and physiological traits indicative of subterranean adaptation (i.e., troglomorphies) and stygobionts, the aquatic counterpart to troglobionts. Obligate troglophiles are taxa seemingly restricted to the subterranean environment, but lack clear traits indicative of troglomorphy. These animals can be tricky to identify. In some cases, they may represent climate relicts, while in others they could be confused with edaphic (soil dwelling) taxa. Troglophiles are species that occur facultatively within caves, can complete their life cycles there, but also occur in similar surface microhabitats. Trogloxenes are organisms that frequently use caves for shelter, but forage on the surface. Organisms that stray into caves, but are evolutionarily ill-equipped to establish viable populations are considered accidental species.

Exemplars of the five evolutionary groups of subterranean-dwelling organisms. [A] Troglobiont: the highly specialized dragon millipede, Hylomus yuani from Guangxi, China (Liu & Wynne 2019). [B] Stygobiont: a troglomorphic crayfish, Cambarus cryptodytes Hobbs, 1941, from Georgia, USA, courtesy Dante Fenolio. [C] Troglophile: mating pair of cave crickets, Rhaphidophoridae n. gen. n. sp. from Grand Canyon Parashant National Monument, Arizona (Wynne & Voyles 2014), courtesy Kyle Voyles. [D] Obligate troglophile: Pratherodesmus voylesi from north rim Grand Canyon, Arizona (Shear et al. 2009). [E] Trogloxene: Peridroma sp. from Big Island, Hawaii, courtesy Fred D. Stone. Prior the arrival of the introduced rat to the Hawaiian Islands, these moths would reportedly darken the sky as they emerged from their cave roosts in the evenings. From Howarth & Wynne, 2022.

Over the past 50 years, the discipline of cave ecology has flourished. In particular, the past 20 years has witnessed a proliferation of knowledge and understanding of these oft difficult-to-study environments. This leap is due to the advent of geographic information systems for examining biogeographical patterns; a myriad of accessible statistical programs (many of which are freeware) granting robust inference into ecological patterns; and, the warp speed advancement of molecular and genomic techniques that have broadened our understanding related to the evolutionary mechanisms of troglomorphic taxa. Armed with these now contemporary techniques and approaches, our apperception of how the surface environment and nutrients sculpt cave ecosystems will continue to grow with many exciting discoveries evinced along the way.

Going deeper on cave ecosystems and biodiversity…

Presentation delivered as part of the 2020 virtual Global Biodiversity Festival organized by Paul Rose and Exploring by the Seat of Your Pants. This talk expounds upon some of the concepts driving cave arthropod diversity.

Further Reading

Huntsman spider (family Sparassidae) consuming a cricket nymph (family Rhaphidophoridae), Guangxi, China.

Wynne, J.J. 2022. Cave Biodiversity: Speciation and Diversity of Subterranean Fauna. John Hopkins University Press (JHUP), MD, Pps. 352.

Howarth, F.G. & J.J. Wynne. 2022. Influence of the physical environment on terrestrial cave arthropod diversity. In Cave Biodiversity: Speciation and Diversity of Subterranean Fauna (J.J. Wynne, editor), 1–56, JHUP.

Wynne, J.J., M.L. Niemiller, & K.J. Chapin. 2022. Evolutionary models influencing subterranean speciation. In Cave Biodiversity: Speciation and Diversity of Subterranean Fauna (J.J. Wynne, editor), 57–94, JHUP, MD.

Mammola, S., […], J.J. Wynne et al. 2020. Fundamental research questions in subterranean biology. Biological Reviews 1-18.

Wynne, J.J., et al. 2019. Fifty years of cave arthropod sampling: techniques and best practices. International Journal of Speleology 48: 33–48.

Wynne, J.J., et al. 2019. La biota de las cuevas del Parque Natural de la Sierra de las Nieves, Andalucía, con recomendaciones para futuras investigaciones y gestión. Andalucía Subterránea 31: 30–50.

Wynne, J.J., et al. 2018. Capturing arthropod diversity in complex cave systems. Diversity and Distributions 24: 1478–1491.

Wynne, J.J. 2014. Reign of the Red Queen: The future of bats hangs in the balance. The Explorers Journal 92: 40–45.

Wynne, J.J., & K.D. Voyles. 2014. Cave-dwelling arthropods and vertebrates of North Rim Grand Canyon, with notes on ecology and management. Western North American Naturalist 74: 1–17.

Wynne, J.J. 2013. Inventory, conservation and management of lava tube caves at El Malpais National Monument, New Mexico. Park Science 30: 45–55, +appendix.

Wynne, J.J., & W. Pleytez. 2005. Sensitive ecological areas and species inventory of Actun Chapat Cave, Vaca Plateau, Belize. Journal of Cave and Karst Studies 67: 148–157.

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