Responses of birds, rodents, and vegetation to livestock exclosure in a semidesert grassland site.

Livestock have been excluded from a 3,160-ha range in southeastern Arizona since 1968. Compared to an adjacent continuously grazed area, in 1981-82 a protected upland site supported 45% more grass cover, a comparatively heterogeneous grass community, and 4 times as many shrubs. Grama grasses (Bouteloua spp.) were equally common in and outside the exclosure, while a variety of other species, especially plains lovegrass (Eragrostis intermedia) and Arizona cottontop (Trichachne cal~ornicum) were much more abundant on the protected site. The grazed area supported significantly higher numbers of birds In summer, while densities did not differ in winter. Rodents were significantly more abundant inside the protected area. Species of birds and rodents more common in the grazed area included those typical of more xeric lowland habitats and those preferring open ground for feeding. Species more common on the protected site were those which characterize semidesert or plains grasslands, and which prefer substantial grass or shrub cover. Grazing appeared to favor birds as a class over rodents. Livestock exclosures have potential value in assessing the effects of grazing upon vegetation and wildlife (e.g., Turner et al. 1980). However, most protected sites are too small for meaningful evalution of grazing impacts on birds and mammals. The AppletonWhittel Research Ranch is a large (3,160 ha) National Audubon Society preserve in southeastern Arizona, protected from livestock grazing since 1968. In 1981-82, we compared vegetation, bird, and rodent populations between the exclosure and a moderately grazed but otherwise similar and adjacent grama grass-shrubland site. Study Area and Methods The study area lies at about 1,500 m elevation in Santa Cruz County, Ariz. Temperatures range from a mean January minimum of -I . 7O C to a mean June maximum of 32.4’ C. Average annual precipitation in this semidesert grassland is 43 cm, with about half occurring in July and August. The site sampled was a 300-ha mesa extending from the exclosure across a boundary fence onto an adjacent cattle ranch. This was the only topographically uniform area large enough to permit meaningful comparison of bird and rodent populations, where we could be confident that the differences across the fenceline were due entirely to effects of livestock grazing. Slopes were <5% throughout, and soils a uniform gravelly loam of the White House series (Richardson et al. 1979). Livestock density on the grazed pasture just prior to and during data collection was approximately I cow per 10 ha. Grazing levels doubtless were much higher historically, both on and off the exclosure (Bahre 1977), but stocking data are unavailable. Two grazed and two ungrazed bird census plots (50 X 500-m) Authors are at the Appleton-Whittell Research Ranch, P.O. Box 44, Elgi? Ariz. 8561 I, and Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder 80309. This study was supported by Earthwatch and the Center for Field Research. by the University of Colorado Council on Researchand Cnative Work, and by the Naclo? al AudubonSociety. We thank D. Armstrong, M. Holmgren, B. Johnston, P. San+nl, B. Wcbb,,and 23 Earthwatch volunteers for advice and field assistance. Mr. W’dham Brophy Lmdly allowed access to the Babocomari Ranch. Manuscript received February 16, 1983. JOURNAL OF RANGE MANAGEMENT 37(3), May 1984 were established perpendicular to the boundary fence. These were located to avoid mesa edges and ravines, but to be at the maximum possible lateral distance from one another (about 400 m). Teams of 6-8 observers walked abreast through the plots, counting and identifying individual birds flushed from them. It is unlikely that many birds escaped detection, since inter-observer distance was only 7-8 m, and the habitat was open and low in profile. Plots were sampled no more than once per day, in early mornings of clear and relatively calm days. Each count required about 30 min. Counts were made from late July to early August of 1981 and 1982 (I 1 counts/plot = I1 hours total census time per treatment); winter counts were made in January of 1982 and 1983 (I 6 counts/ plot q I6 hours total census time per treatment). Total numbers of sightings were compared within-season between the grazed and ungrazed areas using the chi-square statistic. Two 600-m lines of Sherman live-traps were located centrally on the mesa and perpendicular to the fenceline, one in grazed and one Table 1. Ground cover, expressed as percent of points on line transects, in grazed vs. ungrazed semidesert grassland in southeastern Arizona (XI q 800 points per treatment). Chi-squere values were calculated using 8ctual numbers of points. Species shown are those comprising 21% cover on at least one site. Category Percent ground cover Grazed Ungrazed Chi-square 55.6 80.4** 36.03 Grasses total Andropogon barbinodis Aristida gkwca A. purpurea A. temipes Bouteloua chondrosioides B. curtipendula B. hirsutum B. gracilis Eragrostis intermedia Hilaria betkngeri Lycurus phleoides Panicum obtusum Trichachne califomicum 1.3 0.3 2.1 2.5 3.1+ 15.6 4.5 20.1 0 2.0* 2.4 0.6 0.1 1.3 0 1.0 3.60 7.42’ 23.21 4.6* 5.07 1.5 4.56 14.0 0.71 3.5 1.00 22.0 0.67 6.1** 49.00 0.4 8.89