Phillip W. Bettoli U.S. Geological Survey, Tennessee Cooperative Fishery Research Unit, Tennessee Technological University, Cookeville, TN 38505. E-mail:[email protected] Tucked away on the Highland Rim physiographic province in middle Tennessee is the Barrens Plateau region, home to one of the state’s most floristically diverse natural areas. This region is fairly unique in that it harbors plants from the Atlantic and Gulf Coastal Plains as well as species common to Midwest tall grass prairies. Early Anglo settlers named these savanna-like areas the “barrens” because of the scarcity of trees relative to surrounding areas. The undisturbed landscape was a mosaic of open canopy woodlands with a grassy understory and areas of essentially treeless grasslands. Barrens are also found in southern Kentucky along the Highland Rim of the Cumberland Plateau. Wildfires or fires intentionally set by Native Americans were a frequent feature of the barrens in both states, which favored grasses over trees. In precolonial times, bison and elk were also a part of the landscape, and their grazing and trampling would have favored grasslands and maintained the prairie-like character of the barrens. Tennessee’s Highland Rim and Barrens Plateau regions, due to geological complexity and an abundance of drainage systems, are also home to the most diverse fish fauna of any region of comparable size in North America. The barrens region is dissected by many surface streams and is underlain by subterranean channels that carve through the limestone geology. Prominent features of this landscape are numerous caves and springs. The waters seeping from these karst springs average about 15°C year-round. If you were to fly low and slow over the barrens region, you would see[su_members message=”This content is for members only. Please login.”
login_url=”/membership/member-login/” class=””] innumerable springs bubbling up in open pastures, connected to adjacent streams by runs of a few meters or up to several hundred meters in length. Small rocky outcropping and copses of shrubs and small trees in the open landscape would be prime spots to locate springs. Depending on recent rainfall, spring discharges might be imperceptibly low or substantial (>0.5 cms). Natural spring pools can be small, perhaps a square meter or so, whereas inundated sinkholes and dammed spring pools might be several hundred or several thousand square meters in size. These springs and spring-influenced habitats in the barrens region support fishes such as the Spring Cavefish Forbes ichthys agassizii, Flame Chub Hemitremia flammea, and Barrens Topminnow Fundulus julisia. The last two species are granted Greatest Conservation Need status in Tennessee, but the Flame Chub is in no imminent threat of extinction. The same cannot be said of the Barrens Topminnow. Due to its limited distribution (the species is endemic to the barrens of Tennessee) and the scarcity of undisturbed habitats, the Barrens Topminnow has long been considered one of the most critically endangered fishes in eastern North America.
The features of the landscape where this species evolved into a prototypical spring-habitat species may have contributed to its imperiled status. The gently rolling terrain and soils where the Barrens Topminnow is making its stand are very conducive to settlement and landscape manipulation, especially agriculture in the form of pastureland and plant nurseries. Spring ponds and runs are natural watering holes for livestock and the destruction of riparian habitat and aquatic vegetation would be an obvious problem for any species inhabiting those habitats. Additionally, an important feature of the barrens region that would be obvious to even a casual observer is the large number of plant nurseries. In fact, this region of Tennessee is touted to be the Nursery Capital of the World, with over 300 wholesale nurseries in operation. Suitable soils and precipitation contributed to the influx of nursery operations to this region; perhaps more important, it is too warm for plant nurseries further south to grow northern species, and nurseries in the barrens region have a longer growing season to work with than northern nurseries. No direct links have been identified between nursery operations and loss of Barrens Topminnow habitats, though early authors concluded that such large-scale habitat changes were probably limiting already rare spring dwelling fishes to only a few sites compared to precolonial times. When the Barrens Topminnow was first described as a new species in the early 1980s, there were at least 14 populations. Those early surveys noted a common feature of the spring habitats supporting Barrens Topminnows: lush aquatic vegetation such as watercress Nasturtium officinale, water primrose Ludwigia palustris, and milfoil Myriophyllum spp. Vegetation, especially mats of filamentous algae, is used as spawning substrate for the handful of eggs the female deposits during each spawning act. The crystal-clear waters discharging at a near constant temperature, fertile soils, and little shade provide ideal conditions for aquatic vegetation to flourish in springs of the barrens region. Subsequent distribution surveys revealed a plummeting number of populations, and now only three wild populations are known. Those surveys documented a feature of the landscape that probably plays the most important role in the extirpation of Barrens Topminnows at many sites: swarms of invasive Western Mosquitofish Gambusia affinis. Western Mosquitofish have been implicated in the decline of native fishes in other locales including other topminnow species. Mosquitofish are aggressive; in aquaria, they have been observed harrying juveniles and consuming larval Barrens Topminnows. Being livebearers capable of producing several broods per year, Western Mosquitofish rapidly overwhelm low-density Barrens Topminnow populations, especially in small springs. Status surveys since the 1980s also revealed another threat to Barrens Topminnows: drought. In fact, the type locale for the species (a small pool created by a private landowner who dammed the outflow from a spring) has completely dried up on occasion, which prompts “rescuing” Barrens Topminnows and holding them in captivity until spring flows return. The type locale pool had once been stocked with Rainbow Trout Oncorhynchus mykiss (thankfully, that practice ceased years ago), and the small dam prevents the colonization of the site by Western Mosquitofish. The Barrens Topminnow does not currently enjoy federally protected status. However, it has benefited from the efforts of a longrunning partnership between nonprofit conservation groups, university researchers, private landowners, and state and federal agencies to keep it from becoming extinct. Those efforts include annual monitoring of the few remaining wild populations and propagating and stocking juvenile Barrens Topminnow into existing or newly created spring habitats to establish a metapopulation throughout the barrens region. Working closely with landowners, the partners committed to protecting the Barrens Topminnows have erected fencing to protect riparian zones, installed watering systems for cattle, and investigated the use of barriers to prevent colonization of stocked sites by Western Mosquitofish. Thousands of Barrens Topminnows have been reared in hatcheries, tagged, and stocked into dozens of spring pools and runs and monitoring efforts continue. Finally, the looming threat of droughts to Barrens Topminnows has prompted efforts to understand and model the relationships between aquifers, surface waters, and water use in the barrens region.
ACKNOWLEDGMENTS The Tennessee Cooperative Fishery Research Unit is jointly sponsored by the U.S. Geological Survey, the U.S. Fish and Wildlife Service, the Tennessee Wildlife Resources Agency, and the Tennessee Technological University. In preparing this essay I reviewed information from various sources, especially the theses and reports coauthored by two of my graduate students (Andrea Johnson and Cory Goldsworthy), two Tennessee state government websites (www.state.tn.us/environment/natural-areas/natural-areas/may/; www.tn.gov/environment/conservationist/archive/grass.htm), and the works of Rakes (1989. Life history and ecology of the Barrens Topminnow, Fundulus julisia Williams and Etnier [Pisces, Fundulidae]. Master’s Thesis. University of Tennessee, Knoxville), Williams and Etnier (1982. Description of a new species, Fundulus julisia, with a redescription of Fundulus albolineatus and a diagnosis of the subgenus Xenisma [Teleostei: Cyprinodontidae]. Occasional Papers of the Museum of Natural History. University of Kansas, Lawrence), and Etnier and Starnes (2001. The fishes of Tennessee. University of Tennessee Press, Knoxville).