ACTIVITY PATTERNS OF THE POCKET GOPHER PAPPOGEOMYS MERRIAMI MERRIAMI IN A MEXICAN RANGELAND

Pocket gophers (Pappogeomys meiriami merriami) are a problem in both crops and rangelands of Mexico. In rangelands, damage results from the animals burrowing habits which cause much soil disturbance, and their feeding on rangeland vegetation. Although considered a pest, to date there have been no quantitative studies of the activity and damage caused this species. This study was initiated to document fluctuations in activity of P. m. merriami throughout the year in a Mexican rangeland. Proc. 16th Vertebr. Pest Conf. (W.S. Halverson& A.C. Crabb, Eds.) Published at Univ. of Calif., Davis. 1994. INTRODUCTION The pocket gopher (Pappogeomys merriami merriami) is found in agricultural areas throughout the Valley of Mexico. It is the largest species of this family having an average weight of 800 grams (Villa-C 1989). Reproduction occurs throughout the year with a peak in reproductive effort occurring from October through March (Villa-C and Engeman 1993). Information pertaining to activity and damage caused by this species is limited despite it being considered a major pest in agricultural areas. Pocket gopher activity may result in erosion (Ellison 1946), a reduction in forage production (Downhower and Hall 1966; Foster and Stubbendieck 1980; Grant et al. 1980) and a change in plant species composition (Tillman 1983; Grant et al. 1980; Hobbs and Mooney 1985). However, pocket gophers may also have a beneficial effect by loosening the soil and improving soil drainage, which is especially important in heavy clay soils subject to compaction under livestock grazing (Ellison 1946; Ellis and Aldous 1952; Downhower and Hall 1966; Laycock and Richardson 1975). To date there have been no quantitative studies of the activity and damage caused by P. m. merriami. This study was initiated to provide some basic data on seasonal activity and impact of this species in rangeland typical of that found in areas around Mexico City. METHODS The study was undertaken at Ranch Lorenzo, Tres Marias (3000 m elevation), 53 km south of Mexico City. The study site was a 1.3 ha field bordered on two sides by open forest (Figure 1). The field was occasionally grazed by sheep during the study period. The field was dominated by a variety of monocot and dicot species including Erigeron longipes, Melampodium repens, Sonchus oleraceus, Raphanus raphanistrum, Cyperus hermaphroditus, Lolicum multiflorum, Trifolium repens, Oxalis corniculata, Oxalis decaphylla. The amount of pocket gopher activity in the site was assessed each month. To begin sampling, all sign of pocket gopher was erased by levelling mounds and scraping soil over plugs. The site was revisited each day over the following six day period and the number of fresh mounds and plugs recorded. The width of each mound was also noted. Percentage ground cover of vegetation and species composition was recorded at monthly intervals. Figure 1. Study Site.