Responses of plant communities to mammalian herbivores vary widely, due to variation in plant species composition, herbivore densities, forage preferences, soils, and climate. In this study, we evaluated simultaneous changes in 11 plant assemblages on the 100,000 ha Sevilleta National Wildlife Refuge (SNWR) in central New Mexico, USA, over a 20-yr period following removal of the major mammalian herbivores (livestock and prairie dogs) in 1972-1975. Thirty study sites were established in 1976 within and outside of the SNWR, and these sites were resampled in 1986 and 1996 using line transect methods. At the landscape scale, repeated measures ANOVA of percentage cover measurements showed no significant overall net changes in total perennial plant basal cover, either with or without herbivores present; however, there was an overall increase in annual forbs and plant litter from 1976 to 1996. At the site scale, significant changes in species composition and dominance were observed both through time and across the SNWR boundary; each plant assemblage exhibited varying degrees of change, with sites dominated by Bouteloua eriopoda (black grama grass) being the most dynamic and sites dominated by Scleropogon brevifolius (burro grass) being the most persistent. Species-specific changes also were observed across multiple sites: B. eriopoda cover increased while Gutierrezia sarothrae (a small, short-lived shrub) greatly decreased. The non-uniform, multi-directional changes of the different plant assemblages acted to prevent detection of overall changes in perennial vegetation at the landscape level. Some plant assemblages displayed significant changes after removal of herbivores, while others appeared to respond primarily to climate dynamics. Certain species (e.g., G. sarothrae) that were not preferred by livestock or prairie dogs showed overall declines during drought periods, while other preferred species (e.g., B. eriopoda) exhibited widespread increases during wetter periods regardless of herbivore presence. Therefore, the vegetation dynamics cannot be attributed solely to removal of mammalian herbivores, and in some cases can be explained by short- and long-term fluctuations in climate. These results emphasize the variety of responses of different plant assemblages to mammalian herbivores under otherwise similar climatic conditions, and illustrate the value of site- and landscape-scale approaches to understanding the impacts of plant-herbivore interactions.
This data set contains observations of vegetation change in the Sevilleta National Wildlife Refuge of central New Mexico, USA, from 1976 to 1996 following the removal of all livestock and prairie dogs in 1972-75. Specifically, we addressed the following: (1) Would the removal of the two "keystone" mammal herbivores (livestock and prairie dogs) have significant effects on plant species composition or cover at the landscape level? (2) Would the various site-specific plant assemblages within the region exhibit similar changes through time and/or to herbivore removal? (3) What are the influences of species-specific dynamics in changing the landscape and site plant assemblages? (4) What are the relative roles of herbivory and climate dynamics in influencing vegetation change at both landscape and site-level scales?
Study Design This study takes advantage of a series of events during the mid-1970's that coincided to create a unique opportunity for evaluating multi-scale responses of plant communities to the removal of keystone mammalian herbivores. The Sevilleta NWR, a 100,000 ha former Spanish land grant and cattle ranch containing a wide range of vegetation communities, was created in December, 1973. The Sevilleta land grant had been heavily grazed by cattle since the late 1800's, and prairie dogs had been present even earlier. As part of its new "wildlife reserve" status, the entire refuge was fenced and all livestock were removed during 1974-75. In summer 1972, coincidentally just prior to the creation of the SNWR, rangeland pest control agents eradicated virtually all prairie dogs through an intensive poisoning campaign. Livestock and prairie dogs remained in areas outside the SNWR. Since 1973, the SNWR has been without livestock and prairie dogs, although prairie dogs have just recently begun to return to portions of the refuge. These events created a unique situation in which to evaluate the roles of keystone mammalian herbivores on different vegetation communities that (1) were subjected to similar climatic dynamics (i.e., all sites experienced the same high and low rainfall periods), (2) were formerly grazed at high densities by the same species of mammalian herbivores, and (3) were released and protected from grazing during the same time periods. In 1976, range scientists of the United States Bureau of Land Management (BLM) established and sampled 30 "range trend plots" (described below) in and around the SNWR to monitor the influences of cattle grazing on the vegetation; however, the plots were never resampled by the BLM. In 1986, ecologists from the University of New Mexico's Technical Applications Center resampled the sites. In 1988, the SNWR became the main study site of the Sevilleta Long-Term Ecological Research program (LTER); as part of this LTER program, in 1996 we returned to the sites for a 20-yr sample. These samples over two decades allowed us to evaluate the changes in plant cover and species composition at both a landscape scale (SNWR and bordering regions) and at individual sites representing multiple plant assemblages.
Study sites (n=30) are widely distributed and encompass a variety of plant assemblages. Biomes represented included desert, grassland, shrub-steppe, and woodlands. A line transect was established at each site by BLM range scientists in 1976; transects are 30.48 m (100 ft) in length and marked with permanent steel stakes at each end of the transect. The transects are sampled with a "line loop" technique (Parker 1951), a modification of the line point method. Instead of a point, a steel loop with an inner diameter of 1.91 cm (0.75 inch) is used. At every 0.305 m (1 foot) interval of the measuring tape, the loop is placed on the ground and the contents inside the loop recorded. Perennial plants in the loop are identified, and if two species were present, both are recorded. Overhanging shrub canopies are counted, but perennial grasses and forbs are recorded only if the base or stem fell inside the loop. It is important to note that this method does not include measurements of plant height and canopy cover for grasses and forbs, and thus prevents an assessment of plant assemblage architectural changes through time or across the SNWR fence lines.
Annual forbs are classified as "litter" in the August 1976 data set (as per BLM field methods). The 1986 measurements were conducted in January-March when annual forbs were dead, and hence were also categorized as "litter"(perennial plants were still present and easily identifiable). The 1996 samples were collected in August, near the peak of the growing season. Although we identified every plant to species, for consistency of comparisons we also pooled annual forbs and litter in our analyses. If no plants were present, the point was recorded as soil, rock, or litter. Rock or litter need to occupy at least the opening of the loop to be counted. Soil is recorded if no plants are present and the loop is not occupied by rock or litter.
We have located 29 of the original 30 transects, and resampled 28 of the still-existing sites. Photographs and GPS locations were taken of the transects for documentation and comparison with photographs taken in 1976.
Plant codes in the following data file correspond to the old Sevilleta plant code list.
File created 15 August 1996 by Dan Ryerso.n2/18/00 Metadata added to data files by Robert Parmenter.
When the Samples/Data were Collected
August, 1976; Winter of 1986, August 1996, August 2006.
Additional Information on the personnel associated with the Data Collection / Data Processing
Employee History for Sevilleta Field Crew: Mike Friggens, 1999-September 2001;Karen Wetherill, February 7, 2000-Present; Terri Koontz, February 2000-August 2003, August 2006-Present; Shana Penington, February 2000-August 2000;Heather Simpson, August 2000-August 2002; Chris Roberts, September 2001-August 2002; Caleb Hickman, September 9, 2002-November 15, 2004; Seth Munson, September 9, 2002-June 2004; Maya Kapoor, August 9, 2003- January 21, 2005;Tessa Edelen, August 15, 2004-August 15, 2005; Charity Hall, January 31, 2005-January 3, 2006; Yang Xia, January 31, 2005-Present; Michell Thomey, September 3, 2005-August 2008; Jay McLeod, January 2006-August 2006;Amaris Swann, August 25, 2008-Present
Additional Study Area Information
The study was conducted on rangelands within and adjacent to the SNWR, Socorro County, New Mexico. Elevations of the study sites ranged from 1400 to 1870 m. Annual precipitation records of Socorro, New Mexico (30 km south of SNWR) from 1973 to 1996 ranged from a minimum of 150 mm to a maximum of 367 mm. The average annual precipitation during the first decade of this study (1976--1986) was 266 mm, and in the second decade (1986--1996) was 245 mm. During 1976, precipitation was slightly below average and had been preceded by a year of high precipitation. Precipitation in 1985, prior to the 1986 sample, also had been above average. Measurements taken in 1996 followed a 2-yr period of low precipitation, but the summer monsoons of 1996 produced above-average moisture. Mean monthly temperatures ranged from 2.7C during January to 24.3C in July.Site Location DescriptionTable 1. Locations, soils, and commuity classification of study sites Community type is defined by species composition in 1996. Of the original 30 transects established in 1976, only 28 were resampled in 1996. Site #3 was not found, and Site #24 had been destroyed by road construction. Location PredominantTransect Latitude Longitude Soil Type Species_____________________________________________________________________________________________ 01 34 24' 37" 106 55' 49" Turney loam Burrograss 02 34 24' 47" 106 55' 43" Turney loam Burrograss 04 34 25' 45" 106 55' 27" Turney loam Burrograss 05 34 26' 02" 106 59' 47" Nickel-Caliza very gravelly sandy Galleta grass loam 06 34 25' 10" 107 01' 07" Nickel-Caliza very gravelly sandy Blue grama grassloam 07 34 15' 46" 106 56' 55" Armijo-Glendale-Bluepoint Galleta grass association 08 34 15' 05" 106 56' 01" Arizo-Riverwash complex Black grama grass 09 34 13' 46" 106 55' 40" Arizo-Riverwash complex Broom dalea 10 34 13' 29" 106 55' 37" Arizo-Riverwash complex Broom dalea 11 34 13' 27" 106 56' 44" Nickel-Caliza very gravelly sandy Broom dalea loam 12 34 13' 17" 106 47' 08" Bucklebar sandy clay loam Burrograss 13 34 11' 49" 106 48' 28" Barana loam Burrograss 14 34 13' 09" 106 47' 20" Bucklebar sandy clay loam Galleta grass 15 34 11' 48" 106 48' 44" Barana loam Black grama grass 16 34 24' 35" 106 32' 16" Sedillo-Clovis association Blue grama grass 17 34 23' 16" 106 31' 05" Ponciano very bouldery clay loam Black grama grass 18 34 24' 11" 106 40' 07" Turney loamy sand Black grama grass 19 34 24' 08" 106 40' 08" Turney loamy sand Galleta grass 20 34 24' 14" 106 40' 21" Turney loamy sand Black grama grass 21 34 24' 12" 106 40' 22" Turney loamy sand Black grama grass 22 34 23' 42" 106 40' 40" Turney loamy sand Black grama grass 23 34 21' 00" 106 37' 22" Sedillo-Clovis association Blue grama grass 25 34 20' 15" 107 00' 59" Armijo-Glendale-Bluepoint Saltbush/dropseed association grass 26 34 20' 17" 107 01' 09" Armijo-Glendale-Bluepoint Saltbush/dropseed association grass 27 34 20' 20" 107 01' 31" Armijo-Glendale-Bluepoint Saltbush/dropseed association grass 28 34 15' 55" 106 43' 52" Campana Yesum association Burrograss 29 34 16' 18" 106 44' 56" Elbutte-Courthouse Variant-Rock Galleta grass outcrop complex 30 34 16' 07" 106 44' 05" Campana Yesum association Black grama grass