FACTORS INFLUENCING POPULATION-DYNAMICS OF THE PLAINS VISCACHA (LAGOSTOMUS-MAXIMUS, MAMMALIA, CHINCHILLIDAE) IN SCRUB HABITAT OF CENTRAL ARGENTINA

Population dynamics of the plains viscacha (Lagostomus maximus), a highly social burrowing rodent, were studied in desert scrub habitat of central Argentina from 1984 to 1991. The viscacha population declined between 1984 and 1987 from 96 to three individuals, and all but one of eight social groups went extinct. Our data provide support for both food shortage and predation as important factors in the population crash. During this study, rainfall was above average for the first three years and then fell below average. Vegetative cover, especially plant species consumed by viscachas, declined concurrently with the drop in rainfall. A decrease in body weights of adult viscachas and growth rates of young indicated that food was limited. Predation by mountain lions (Felis concolor) was high throughout the study. However, observations of mountain lions foraging in viscacha colonies increased during the decline of the viscacha population. Predation rates may have increased because viscachas spent more time searching for food, and they were in poor body condition. Following the decline in the viscacha population and after a slight increase in rainfall, vegetation cover increased. However, the viscacha population did not recover except in areas that were burned subsequent to the population crash. Fire, a natural part of the ecosystem, may be necessary to reverse changes in vegetation that occur when viscachas are removed from the landscape, and to allow viscacha populations to recover.