Demography and viability of a guanaco population (Lama guanicoe) in Río Negro province, Argentina. A preliminary analysis.

OVEJERO A. RAMIRO; SCHROEDER, NATALIA; MORRIS, WILLIAM; GREGORIO, PABLO AND CARMANCHAHI, PABLO.

Mendoza

Congreso; International Mammalian Congress; 2009

The guanaco (Lama guanicoe) is the dominant herbivore and undoubtedly one of the most important species for the South American arid ecosystems. Despite its key role in these ecosystems, there are very few studies of demographic behavior of populations over time. We analyzed demographic parameters (proportion of sexes and ages, birth and survival rates) and developed a preliminary demographic PVA (population viability analysis) for a guanaco population that inhabits a 40,000 ha ranch, in Río Negro province (Argentina). We used data from shearing in 2005, 2006, 2007 and 2008, as well as field surveys during the breeding season maximum (January-February) along 50 km of transects in 2007 and 2008. We identified three age classes: chulengos (individuals between birth and 1 year old), juveniles (1-2 years) and breeding adults (2 years or more). We used a binomial statistical test to analyze proportion of sexes, and deterministic and stochastic projection matrix models for structured populations, by calculating the sensitivity and elasticity of population growth rate. Environmental stochasticity was incorporated through variability in birth rate (probability of breeding). Proportion of sexes was 50:50 for chulengos, but 61:39 in favor of adult females, which would indicate a differential mortality for adult males. The proportion of age was 76:8,7:15,3 for adults, juveniles and chulengos, respectively. Both arithmetic (non stochastic) and geometric (stochastic) growth rates resulted in values less than unity, indicating that the population decreases, even under the assumption of no environmental variability. To ensure the population avoids extinction, it will be not enough to achieve maximum fertility. This could be because the vital rate that most contribute to population growth is not the birth rate but the survival of adult females. However, more years of continuous population sampling will be necessary to improve the estimation of such models.