Ecology of Mammoth Cave National Park
Today Mammoth Cave National Park (MACA) is designated as an UNESCO Biosphere Reserve, but humans have long been fascinated with the park’s other inhabitants. Formal study of Mammoth Cave’s biodiversity dates back to the 1820s, but native peoples had explored the caves centuries before Europeans ever landed in the New World.
Mammoth Cave (MACA) is the world’s longest explored cave system, which reflects not only the extensive size and developed karst landscape but also a history of extensive exploration compared to other caves. Mammoth Cave is home to 426 miles of cave passage not including the 500 smaller caves within park boundaries.
In a typical ecosystem, virtually all energy present originates from the Sun. Plants capture this energy in a usable form, are then consumed, and this energy is dispersed throughout the food web. A cave ecosystem, however, doesn’t receive sunlight. There are photosynthetic bacteria present in some areas of Mammoth Cave, but these are due to human activity. They have not historically been part of the ecosystem and are not known to be eaten by anything in Mammoth Cave.
NUTRIENTS IN CAVES
So where do organisms in caves get their food? The Mammoth Cave landscape is karst, meaning that it is made of rocks which are easily dissolved such as limestone or gypsum. It is formed and sculpted by water. Water filtering down from the surface contains dissolved organic matter which feeds microbes in the cave. Aquatic invertebrates feed on these microbes, which are then fed on by predators such as cave fish and crayfish.
However, animals in caves often directly move nutrients from the surface into the cave. For example, the cave cricket Hadenoceus subterraneus commutes regularly to the surface to feed but spends most of its time in caves. It lays both its eggs and its waste in the cave, which support consumer organisms such as carabid beetles. Many more species living at cave entrances such as spiders and salamanders also depend on the crickets themselves as prey items. Cave crickets in Mammoth Cave are an example of a keystone species, a species that supports many others and which the ecosystem could not function without.
Cave animals can be sorted into three categories based on habitat preferences:
Troglobites (terrestrial organism which is restricted to caves and cannot complete its life cycle outside of them. Troglo- is a Greek root meaning cave dwelling, -bite is a composite of bios (life) and –ite (inhabitant of a location)) are restricted to caves and cannot complete their life cycle outside of them. They do not participate in the import of nutrients from the surface. The most common troglobionts in MACA are arthropods, especially cave beetles, but include many other kinds of insects, snails, and even vertebrates such as cave fish; aquatic troglobites are commonly called stygobites (aquatic organism which is restricted to caves and cannot complete its life cycle outside of them. Stygo- refers to the River Styx of Greek myth means dark, damp, and subterranean. -bite is a composite of bios (life) and –ite (inhabitant of a location)).
Neaophaenops tellkampfi is the most abundant troglobitic beetle species in MACA. They feed on the eggs of cave crickets Beetle taxa are the most well studied troglobitic species in Mammoth Cave; eight species have been identified.
Amblyopsis spelaea is one of two cave fishes inhabiting the Mammoth Cave system; they are the only two cave-limited vertebrates known in MACA. The fish displays a loss of color and is completely blind, both of which are associated with adaptation to cave life.
Troglophiles are terrestrial organisms with a preference for cave habitat, but can survive and reproduce outside of them. (Troglo- is a Greek root meaning cave dwelling, while -phile comes from Greek for ’beloved’.) Examples include spiders, crayfish, and salamanders. Some can be commonly found living at cave entrances, taking advantage of specific resources found there such as cave crickets emerging to feed.
Cave-limited crayfish occur in MACA, but Cambarus tenebrosus pictured above is not one of them. This species prefers cave habitat, but is occasionally known from surface collections, making it a troglophile.
Trogloxenes are organisms which regularly commute between the surface and subsurface, typically to feed, mate, etc., and can complete their entire life cycle outside of caves. (Troglo- is a Greek root meaning cave dwelling, -xene is a Greek root which means foreign or outside.) These play the most significant role in moving nutrients from the surface. Examples in MACA include cave crickets and bats, both of which live in caves but leave to hunt in the more abundant surface ecosystem. Their droppings can then support many more species within the cave itself. White-nose syndrome (a disease caused by the fungus Pseudogymnoascus destructans, which infects bat colonies, has a high mortality rate, and has lead to a great decline to bat populations in the US and abroad. ‘White-nose’ refers to the white nose of infected individuals.) has reduced bat populations in MACA to the point that they no longer provide a significant source of nutrients to ecosystems there.
Woodrats are not limited to caves but often choose to nest there. They are trogloxenes as they must regularly return to the surface to find food.
Cave crickets are often considered trogloxenes as they leave the cave regularly at night to hunt. However, their particular situation is a bit more complicated; they must leave caves to feed and so they cannot complete their life cycle inside, except in rare cases where food is abundant. But they’ve also never been observed reproducing outside of a cave which is required to be considered a trogloxene or troglophile.
CONSERVATION IN MCNP
The Mammoth Cave system is home to 39 troglobitic (and stygobitic) species of conservation concern; many of these have small ranges or have only very rarely been observed. These species face many threats including climate change and habitat loss, but the most common are threats to water quality. Karst is marked by its solubility and connections between underground passages, which make water resources in karst regions very vulnerable to contamination. Fertilizers, sewage, and runoff from I-65 are all contaminants which have been known to impact MACA, and regular backflooding of the Green River has been known to carry contaminants and even non-native species into the cave system. Conservation is also difficult to carry out without a complete picture of the habitats that are affected; while Mammoth Cave sits at over 400 miles mapped, the park estimates there may be another 600 miles of unmapped passages. The interconnected nature of caves only makes this issue more severe; the source or destination of a contaminant may be entirely unknown. MACA is a hotspot of subterranean biodiversity, much of which is only found here; advancing conservation practices in MACA will require advancing understanding and mapping of karst landscapes.