Desert Grassland Dynamics: Are Herbivorous Rodents Ecosystem Engineers?
Investigators (most current known information)
Black-tailed prairie dogs (Cynomys ludovicianus) have declined across their historic range by an estimated 98% as a result of habitat loss, disease and human persecution. Prairie dogs represent a unique conservation dilemma because of their importance to other species (e.g., as prey of the critically endangered black-footed ferret, Mustela nigripes), coupled with their very real perception as an agricultural pest. Conservation of black-tailed prairie dogs largely involves documenting their occurrence by estimating occupied habitat through some from of remote sensing and/or attempting to expand prairie dogs into formerly occupied areas via translocation. Both of these aspects of prairie dog conservation lack information about vital rates, which are important for assessing their status and evaluating translocation success. We explored the demography and estimated field metabolic rates in reintroduced colonies of black-tailed prairie dogs in south central New Mexico to: 1) more rigorously assess the data needed to evaluate their current status, 2) gain a better understanding of the factors influencing their vital rates (survival and reproduction) and 3) estimate levels of energetic expenditure and water turnover with respect to resource availability.
Black-tailed prairie dogs were originally considered for listing under the U.S. Endangered Species Act, but were removed from the candidate species list in 2004 after the U.S. Fish and Wildlife Service concluded that the area they occupied was greater than previously estimated and that threats to the species were exaggerated. This decision was based largely on remotely sensed estimates of occupied habitat — an index of abundance. Indices often are biased rendering them unreliable for monitoring populations. We used mark-recapture/resight techniques to estimate prairie dog densities. Estimates of prairie dog density from 3 colonies over a 3-year period varied considerably (1.3 to 28.3 prairie dogs/ha) and, when applied to the area potentially occupied by prairie dogs in the United States (745,400 ha), yielded a range-wide abundance of < 1 million to > 21 million prairie dogs. Exclusive use of indices and/or estimates of prairie dog density from a limited portion of their range may result in inaccurate estimates of range-wide abundance, and may adversely affect conservation decisions. Resource agencies should consider a multifaceted approach to prairie dog conservation that includes: 1) an assessment of range-wide occupancy coupled with, 2) a determination of colony extinction dynamics at the regional level that is, 3) augmented with demographic data at the colony level. Until such data are available, the status of the black-tailed prairie dog is really unknown.
Prairie dog conservation plans estimate that the greatest potential habitat for black-tailed prairie dogs occurs within desert grassland. The extent of this habitat is considerable (> 40,000,000 ha), but conservation plans have failed to explore differences in prairie dog vital rates across their range and in desert grasslands in particular. Desert grasslands are the driest habitat that black-tailed prairie dogs inhabit and are prone to large spatial and temporal fluctuations in precipitation that can lead to prolonged drought; this creates a circumstance that may hinder prairie dog population growth and establishment. We studied the demography of six reintroduced colonies of black-tailed prairie dogs for three years using mark-recapture/resight techniques to elucidate patterns of survival and reproduction. Rapid declines in monthly survival coincided with a significant loss in average adult body mass during a year of extreme below-average precipitation; both body mass and vital rates rebounded when precipitation neared the 100-yr average, but population size lagged. Vital rates were linked to body mass and resource limitation.
Field metabolic rates measured in the spring and fall in 2 years (2003 & 2004) did not vary significantly between the sexes, seasons or years. Total body water (TBW) and water intake (WI), however, varied with season and year, but not between the sexes. Both TBW, the inverse of body condition, and WI, in general, were higher in 2004 than in 2003 and correlated with plant water content. The year 2003 was the last year of a prolonged drought in which prairie dog colonies were characterized by low adult and juvenile body mass, low survival, and population crashes. Total body water increased and hence body condition decreased in 2004, when colony reproduction declined; one colony failed to produce any young. Our results suggest that the climate can influence resource availability and that understanding demographic processes of black-tailed prairie dogs in desert grasslands, and likely throughout their range, is important not only to conserve prairie dog populations but to further understand their ecological role.
Article in Journal
Facka, A.N., P. Ford, and G. W. Roemer. 2007. “A novel approach for assessing density and range-wide abundance of prairie dogs.” Journal of Mammalogy (in review).
Roemer, G.W. 2004. The demography and ecophysiology of the black-tailed prairie dog. USDA Jornada LTER 14th Annual Symposium. Las Cruces NM.
Facka, A.N., E. Geffen, M. Kam, P. Ford, and G. W. Roemer. 2007. Variable precipitation regime leads to vital rate fluctuations and population collapse in black-tailed prairie dogs. 87th Annual Meeting of the American Society of Mammalogists. Albuquerque NM.
Facka, A.N. 2006. Vitally important: The role of demography in assessing the current and future status of black-tailed prairie dogs . Master's Thesis, Department of Fishery & Wildlife Sciences, New Mexico State University. Las Cruces NM. 84 pp.