Wednesday, February 09, 2011

Pronghorn Declines in Texas

The Pronghorn is one of the iconic mammals of the West in North America. Often referred to as 'antelope,' Antilocapra americana is the sole survivor of twelve species that roamed the North American continent before one of the Great Extinctions (Pleistocene) 10,000-12,000 years ago. (Unlike the pronghorn, true antelopes are a member of the cattle family and are not native to North America. Pronghorns have forked horns; antelopes have unforked horns.) Early observations recounted pronghorns covering the grassy plains. Their traditional range was documented reaching from south-central Canada down into Texas and northern Mexico, and west to Baja, California.

Observations of declines in pronghorn populations were noted in the early 1900's; hunting pressure reduced their population to 13,000 in the 1920's. Disease, especially Blue Tongue, also contributed to reductions. Luckily, conservation efforts helped to restore populations: 500,000 to 1,000,000 across the continent. However, local populations that rebounded have recently declined again. West Texas is one of those areas.

Similar to nationwide trends, overall declines in Texas have paralleled loss of habitat. Only two regions now support pronghorns: the Panhandle and Trans-Pecos areas. The latter has declined faster over the last 150 years, and has recently accelerated. Population fluctuations usually follow changes in water and forage availability, so decreases during years of drought are not surprising. Herds typically rebound by improved reproduction and survival with more favorable weather and food availability, as they did after the drought of the 1990's. However, population estimates of pronghorns in the Trans-Pecos region have continued to decline despite more favorable forage availability during the last three years. In fact, these populations are the lowest since the 1970's. The question is: why?

In 2007, state parks and wildlife officials, hunters, and landowners that profit from hunting licenses began to sit up and take notice of pronghorn populations. Based on preliminary studies and data collection, two hypotheses predominate: internal parasites and predation. The Borderlands Research Institute (BRI) at Sul Ross State University collected a few hundred samples since 2009. They discovered a high load of a common internal stomach parasite, the roundworm, in both hunter-harvested and regional habitat-dwelling animals. A high parasite burden can cause anemia (low red blood cell count), weakening an animal and increasing susceptibility to disease, predation and other stresses. Reproduction can also be affected contributing to decreases on populations. 

Predation is always the favored target of declining populations involving any mammalian species valued by human economics, especially livestock and hunting-target species. Although predators occupy a practical and valuable niche in mammalian ecosystems, large predators have historically served as an easy scapegoat for unexplained deaths. Regardless, considering the overall increasing urban and rural populations of coyotes, investigation into the role of predation on the Texas pronghorn is valid. Although the adult pronghorn can outrun a coyote and cougar, even a cheetah, fawns are more susceptible to predation.

The recruitment into a herd of pronghorn by growing fawns, called 'fawn recruitment,' is commonly a preferred determinant of populations than mortality of adults. This measure more realistically reflects the reproductive capacity of does as well as predation pressure, since they are the typical target by predators: mostly coyotes, cougars, eagles. The major predator of adult pronghorns is Man. 

A 1948-1977 summary of fawn recruitment rates in Canada and the U.S. varied from 43 (fawns per 100 does) in Texas to highs of 105 in South Dakota. More recent pooled data - 1978-2008 - revealed that despite fluctuations in numbers, fawn recruitment continued to decline in most of the same areas as the previous study. The data also revealed differences in fawn recruitment associated with vegetation communities. An independent study of fawn recruitment and forage availability in Arizona showed an increase in fawn recruitment parallel with forage conditions. Their findings strongly suggested that a diversity of plant species is optimal pronghorn habitat and maintenance of habitat diversity is important. This, along with supporting evidence from other independent studies, demonstrates differences in fawn survival may depend on the diets of adults as well as offspring. 

Despite increases in rainfall over the past three years and subsequent increases in forage availability, an important component may be missing: accessibility. The pronghorn display an unusual behavior apart all other wild ungulates: they are not good jumpers. They are disinclined to jump even a low brush fence over three feet high. Small bands of pronghorn have been known to almost starve within a fenced enclosure while plenty of food is available on the outside. When hardpressed, they can jump over moderately high obstructions, but fences will alter their ranges and access to forage. They have been observed to crawl under or between wires of barbed-wire fences, prompting many conservation cooperatives to remove the bottom wire or replace it with a barbless wire. 

The major factor in determining animal populations across the continent during modern times is habitat changes. Land use patterns, including development, resource extraction, and traffic, have changed not only the topographical features but also water and food availability and accessibility. Entire ecosystems are changing. As habitats shrink, many species are competing for the same food and water resources. Migration routes are cut off or severely restricted across the country. Along with habitat changes are subsequent down-stream pressures: increased density of animals and parasite loads, disease exposure, local vegetation exhaustion, decreased vegetation to hide from predators, traffic mortality increases, reductions in reproduction and subsequent offspring survival. Whether these factors have been taken into account in Texas pronghorn performance and survivability is unknown.

Last September the Texas Parks and Wildlife Department (TPWD) awarded a 3 year $111,210 grant to the BRI at Sul Ross State University in Alpine, TX. Their research aims to evaluate the two competing hypotheses described above, regarding pronghorn survival and productivity in the Trans-Pecos area. Meanwhile, the Trans-Pecos Pronghorn Working Group, a group of West Texas landowners, outfitters, TPWD biologists, BRI personnel and wildlife veterinarians, decided to transfer several hundred pronghorn adults from the Texas Panhandle area to West Texas. My objection to this move is that moving healthy animals to locations where populations are inexplicably and rapidly declining is premature and possibly deleterious to the health of the transplanted animals.

Because the decline of the Panhandle pronghorn population appears to be less than that in the Trans-Pecos area, I hope that those involved in this move have considered using the Panhandle population, and others living in similar habitats as those in the Trans-Pecos region, as controls. I also hope that those involved in this research expand their consideration of factors that govern population performance, including population displacement and decline. Rather than address the Trans-Pecos problem from a 'Band-aid' approach, and jeopardizing the lives of transplanted animals at the expense of another population, a thoroughly planned investigation based on scientific principles should precede premature and possibly detrimental short-term fixes.

Other regions have and continue to study and address pronghorn population declines. Several organizations -local, state and federal- are cooperatively working to facilitate habitat preservation. Even migration corridors are being preserved and maintained in several states, such as Montana and southern Canada. Many ranches in Oregon and Arizona are removing unnecessary fence lines and installing barbless wire, especially on bottom lines. Natural gas companies in Wyoming are cooperatively working with organizations to alter gas pads and traffic in order to preserve migration corridors and habitat. Underpasses are being constructed across major highways to divert migration routes away from fences and traffic. Ranches in the upper Great Plains are cooperatively restoring overgrazed ranges for pronghorn forage.

Maybe it is time for Texas to rejoin the Union to help explore and solve its problems. At the very least, they can expand their considerations from a few trees and to the entire forest. Animal populations exist in an interconnected community and system, and problems should be considering holistically. Let's all focus on finding answers to the questions and then examining long-term solutions. 

Pronghorn benefit a huge success; release set for late February

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