A simulation model for the management of vicuña populations

Abstract

The population dynamics of the vicuña was modelled based on field data from the Central Galeras Sector, Perú. Rains were simulated in order to obtain net primary productivity and grass availability which act upon density-dependent fecundity and mortality. The model produces a population growth curve that tends to stabilize at densities around 100 vicuñas per km 2. Harvest and shearing processes were simulated as part of the vicuña population management model. Harvest was based upon a fixed threshold density D e below which no harvest occurs; for densities above D e harvesting is applied at a rate proportional to the difference between D e and the current population density (fixed escapement or ‘bang-bang’ harvest rule). Management optimization was analyzed by determining the optimal escapement density D e, which maximized either net profits or the number of animals harvested. Analyzing the results as cumulative totals over 20 years of simulation, an optimum harvest production was obtained for a D e of about 40 vicuñas per km 2, and maximum profit was obtained for a D e around 70 vicuñas per km 2. Analyzing the results of 20 years of simulation as annual averages with an original population density of 40 vicuñas per km 2, the harvest production and profits were maximized for a D e of 40 and 60 vicuñas per km 2, respectively. The model was validated using data from the Reserva Nacional San Guillermo, Argentina, where vicuña and guanaco populations coexist (although their interaction was not modelled). The sensitivity analysis was performed with three different techniques: (a) stepwise multiple linear regression, (b) visual graphic analysis based on a polar coordinates system, and (c) direct evaluation of the effect on management decisions. The curve shape parameters of the fecundity and mortality functions proved to be the most important ones in determining the outcome of the model.