Individual study: The effect of cattle removal on vegetation and breeding birds in mesquite grassland, San Pedro River, Arizona, USA
Krueper D., Bart J. & Rich T.D. (2003) Response of vegetation and breeding birds to the removal of cattle on the San Pedro River, Arizona, USA. Conservation Biology, 17, 607-615
Cattle and sheep grazing have been widespread and intensive across the semi-arid shrublands, grasslands and riparian habitats of western USA. The ecological consequences of such grazing are suggested to be severe, including reductions in both plant and avian biodiversity and aiding the spread of alien species. In this study, the effect of exclosures (no grazing) on the plant and avian communities in mesquite grassland is investigated.
Study site: The study was conducted within the 23,000 ha San Pedro Riparian National Conservancy Area (NCA) along the San Pedro River, which is
perennial for 35 km and ephemeral for 34 km within the NCA.
In 1987, the mesquite grassland community covered 5,000 ha and was dominated by mesquite Prosopis sp., whitethorn acacia Acacia neovernicosa and sacaton grass Sporobolus wrightii.
Cattle removal: On 1 January 1988, a 15 year grazing moratorium was introduced in the NCA, and cattle were removed. In the years previous, there was a season-long cow-calf regime of 6,500-13,000 head (no further details available).
Monitoring: Seven transects 0.8-1.6 km in length were established. Vegetation was sampled at the peak of summer growth (September or October) in 1986, 1989 and 1992. Avian surveys were conducted at regular intervals, three times per month from 1986-1990.
Vegetation censuses: Vegetation was measured every 50 m along transects at 12 heights between 0.15-21.3 m. At each height, the perpendicular distance to the nearest patch of live vegetation that covered >22.5 cm x 40 cm in area was recorded on each side of the transect. Using these data, foliage density indices were calculated for herbs, shrubs and short trees (a higher number corresponding with denser vegetation).
Avian censuses: A fix-width transect, with survey area extending to 124 m on each side of the transect, was walked at 30 m/minute.
Vegetation community: The density of herbaceous species increased substantially after removal of cattle. The density indices between 1986 and 1992 at 0.15 m height quadrupled from 0.25 to 0.94 and at 0.61 m height increased five fold from 0.16 to 0.89. Furthermore, there was also an increases in the density index between 1986 and 1992 for low trees (0.26 to 0.42) suggesting that there was an overall trend of habitat regeneration after cattle removal. Shrub density did not change.
Avian community: Of 38 species identified as mesquite grassland specialists, 13 species increased and four species decreased significantly. Furthermore, average detections (all species combined) per km increased over the study: 70 in 1986, 81 in 1987, 85 in 1988, 122 in 1989, to 130 in 1990. This equates to a significant annual increase of 18%.
Conclusions: Removal of cattle grazing caused significant regeneration of vegetation and increase in bird populations over seven years. Cattle removal is therefore a successful management strategy for regenerating mesquite grassland communities. (See also Cases 418 and 420 for effect of cattle removal in riparian and Chihuahuan desert-scrub communities).
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