Juniper Invasion Poses Profound Threat to Prairies, Researchers Say at AAAS SWARM Meeting
TULSA, Oklahoma—When the first Europeans made their way to the Great Plains in the center of the North American continent, they found vast prairies extending to the horizon, with the waves of grass hardly broken by trees. But a visitor to those same prairies today—from Texas up to South Dakota—is likely to find an army of shrubby junipers overtaking the grass.
Far more than an aesthetic change, the shifting ecology places both wildlife and humans at risk: The trees crowd into grazing land and consume water that might otherwise nourish the prairie or provide supplies for local towns and cities. They discharge clouds of highly allergenic pollen and harbor mosquitoes that carry West Nile Virus. And they usurp habitat crucial to species such as the lesser prairie chicken and the prairie mole cricket.
“Juniper invasion has emerged as a dominant threat to some of the most threatened ecosystems of North America,” says Samuel Fuhlendorf, a professor in the Natural Resource Ecology and Management program at Oklahoma State University (OSU).
At the recent meeting of the AAAS Southwestern and Rocky Mountain Division, Fuhlendorf and others detailed how human activity over the past 150 years has upset a natural balance that endured on the prairies for millennia. The question now— for farmers and ranchers, hunters, conservationists, consumers, policymakers, and scientists—is whether humans can help restore the balance and the prairie’s health.
The 86th Annual Meeting of the Southwestern and Rocky Mountain (SWARM) Division convened 31 March-4 April at the University of Tulsa (TU) to explore regional issues with global resonance—cybersecurity, new digital tools for archaeology, research ethics, and the relationship between science and religion, among others.
The meeting was held in conjunction with the 15th Annual TU Research Colloquium and the 10th Annual University of Oklahoma-Tulsa Research Day. The program was assembled under the direction of SWARM President Austin Cooney, a stem cell researcher at the Baylor College of Medicine in Houston, and SWARM Executive Director David T. Nash.
Stresses Felt Across Ecosystems
In Oklahoma, as in many places on earth, the growing human population has put a heavy footprint on local ecosystems. Flora and fauna long adapted to the intense heat, the frigid winters, the winds, and the droughts of the southern Plains have struggled to endure the stresses imposed by human development in the past 150 years.
An in some cases, those species appear to be failing.
“Endangered species have narrow habitat requirements that are being threatened by natural change or human activity,” said biologist Stanley Rice, a professor at Southeastern Oklahoma State University. “Even very slight changes can have an effect.”
Rice is the president-elect of the Oklahoma Academy of Science and the author of the “Encyclopedia of Biodiversity.“ He organized a 3 April symposium at the SWARM meeting on endangered species in Oklahoma where student and faculty researchers described a range of animals and plants that are at risk, frequently because of human encroachment or development.
The endangered interior least tern nests on flat, sandy riversides, but that habitat has sustained disruptions ranging from dam construction to dune buggies. Populations of the critically endangered American burying beetle (Nicrophorus americanus) and many species of freshwater mussels also have suffered from development, researchers reported.
The prairie mole cricket (Gryllotalpa major Saussure), at a length of about two inches, is the largest cricket in North America. But for 11 months of the year it lives underground, preferring undisturbed tallgrass prairie, and this reclusive character makes it almost impossible to count. Researchers in the past thought it was extinct; some now advocate its listing as an endangered species, but Kristopher Keane, a master’s degree student in biology at the University of Tulsa, told the SWARM audience that the process has stalled because too little is known about it.
The prairie mole cricket “generally likes more diverse prairies, and more burning promotes a higher diversity of species,” Keane said. “But we definitely know that they don’t like disturbed fields that have been grazed or tilled….Hopefully we can ultimately get it listed [as endangered] because we’ve found that a lot of habitat is no longer viable for them.”
Rice studies seaside alder (Alnus maritima) trees; they grow in wet and marshy areas in Oklahoma, Georgia, and Maryland and Delaware, but some research suggests that it was once far more widespread. Today, the seaside alder is listed as nearly threatened, and according to Rice, researchers are noticing that seedlings are very rare.
Heat and drought appear to be an important stress—and Oklahoma last year endured its worst drought since the 1930s. Climate change, too, will be a factor in the alders’ survival, he said.
Heartland Prairies: Fire and Renewal
The symposia on the juniper invasion, organized by University of Tulsa aerobiologist Estelle Levetin, offered a broad view onto the health of the Great Plains—and researchers suggested that the troubling conditions there often apply to grasslands worldwide.
A hundred million years ago, a shallow inland sea covered what is now the Great Plains region in North America. As the water receded, the Plains and a sea of prairie grass emerged. Today, the Plains extend from Texas north through Oklahoma, Nebraska and Kansas, Eastern Colorado and Wyoming, the Dakotas and Montana, and further into the Canadian prairie provinces of Alberta, Manitoba, and Saskatchewan.
Eastern redcedar (Juniperus virginiana) and mountain cedar (Juniperus ashei) are native to the region, with mountain cedar concentrated in the southern portion. But for millions of years they were pinned back into canyon areas where they were protected from the lightning-sparked wildfires that regularly burned across the grasslands.
Initially, humans were able to work in concert with those cycles. Indigenous people saw how buffalo and other animals were attracted to green shoots that emerged after a fire, so they set their own fires to attract game.
But then, in the 19th century, European pioneers brought intensive cattle-grazing. The sodbusters soon followed, turning up millions of acres of native grass to plant wheat and other crops. Initial harvest were bountiful, but an extended drought made farming all but impossible and created conditions that yielded the Dust Bowl in the 1930s.
Recognizing how well-intentioned government agricultural had precipitated an unprecedented environmental disaster, federal and local agencies encouraged new soil conservation efforts. Among them were policies that supported the planting of thousands of Eastern redcedar as windbreaks to hold the soil in place. Wildfires, meanwhile, were suppressed as a threat to human communities.
Not until it was too late did people realize that the disrupted soil and lack of fires create highly favorable conditions for the juniper.
The U.S. Department of Agriculture’s Natural Resources Conservation Service has reported that by 2004, 8 million acres in Oklahoma alone were “infested” with 50-plus junipers per acre. The conquest was growing by 762 acres a day, doubling every 18 years. New birds follow the trees into the prairie; they eat the juniper berries and expel the seeds, aiding the invasion with each meal. Rising levels of atmospheric carbon dioxide, associated with climate change, may also be fueling the advance, researchers told the AAAS division audience.
The Risk to Animal and Human Communities
Researchers see the lesser prairie chicken as symptomatic of the plains’ stressed condition. Agriculture, overgrazing, and energy development have fragmented and reduced its habitat; the bird’s population has plunged by 90% since 1900. Fuhlendorf says junipers and proliferating wind turbines pose similar threats: The bird “tolerates basically no vertical structures,” he said, because it perceives them as haven for predators.
Similarly, the endangered black-capped vireo has suffered habitat disruption caused by agriculture, grazing, and fire-suppression; those same conditions have encouraged the invasion of Eastern redcedar, which grow taller than the brush in its preferred habitat, and the nest-stealing brown-headed cowbird.
Other research says that birds such as the Eastern wood pewee, the grasshopper sparrow, the Rio Grande turkey, and bobwhite quail, along with white-tailed deer, could be adversely affected by the junipers’ advance.
The concern of researchers and conservationists goes beyond sentimentality for those creatures. Their struggles, like the advance of the Eastern redcedar, are strong signals of the prairie in distress. Already, the researchers said, the juniper proliferation is having a health and economic impact in human communities, even big cities.
Research shows that Eastern redcedar are favored by two species of mosquito that carry diseases dangerous to wildlife and humans: Culex tarsalis, a vector for West Nile Virus, and Aedes albopictus, which is a vector for dengue and other viruses.
“Eastern redcedar could definitely enhance West Nile Virus or these types of transmission if you have woodland birds that are involved,” said Valerie O’Brien, a post-doctoral researcher in the OSU Department of Entomology & Plant Pathology. “If you eliminate Eastern redcedar, you eliminate woodland birds [which are supported by the redcedar], you still have tarsalis, but you may not have the same kind of transmission.”
Levetin, the TU aerobiologist, detailed a health issue that’s not lethal, but that could affect thousands of people even hundreds of miles away: highly allergenic juniper pollen. Eastern redcedars around Tulsa appear to be driving “a significant increase” in airborne juniper pollen, Levetin said. Early results in a study covering 25 years suggest a 110% increase in the city’s springtime pollen.
Other research indicates the junipers disrupt the prairie water cycle.
Mature trees may consume up to 35 liters of water a day, or even more if the soil holds more moisture or the tree is isolated, said Giulia Caterina, a master’s degree student at OSU. That would appear to mean the loss of millions of liters of water every day.
Don Turton, an OSU forest hydrologist at OSU, described how the juniper canopy captures significant portions of rain, especially in a light rainfall, allowing it to evaporate. Preliminary research results attribute a 20% average annual net loss of water to the canopies, about double that intercepted by prairie grasses. The effect: drier soil and reduced flow of surface streams. Heavy juniper infestation in watershed areas along the Canadian River may even reduce water supplies for Oklahoma City, researchers say.
In an arid state like Oklahoma, water is a rallying point for diverse interests, from ranchers and farmers to hunters, anglers, and environmentalists. Over the past decade, campaigns to control the junipers have intensified in many parts of the Great Plains. Groups such as the Nature Conservancy and the American Prairie Foundation are working to conserve blocks of native grassland and reintroducing herds of buffalo. Others have established cooperatives to manage controlled burns, using new science to help restore the prairie’s ancient balance.
But how far will Oklahoma and other Great Plains states go to protect the prairie? That remains to be seen.
Burning or bulldozing every invasive juniper would be an exhaustive effort with a price tag in the hundreds of millions of dollars. And efforts to aggressively protect endangered species have run into intense opposition.
The U.S. Fish and Wildlife Service is now considering whether to list the Lesser Prairie Chicken as an endangered species, which could bring a range of actions to protect its habitat. But in February, Oklahoma’s entire congressional delegation joined to warn that such action could have a huge toll—in jobs and revenue—on ranching, farming, and wind-power development.
A decision on the bird’s status is expected later this year.
[An abridged version of this story appeared in AAAS News & Notes (Science, 27 April 2012, p. 432)]
See the full program for the 2012 AAAS Southwestern and Rocky Mountain (SWARM) Division annual meeting.