Assessing the impact of owl predation on the growth rate of a rodent prey population

Data from a comparative survey of rodent population dynamics and predation by owls were used to estimate the impact of owl predation on the demography of the sigmodontine rodent Akodon azarae. A stage-structured matrix model based on mass classes was used to estimate rodent demographic parameters through the analysis of field estimated transition matrices. Owl pellet analysis allowed estimation of predation rate upon different mass classes of A. azarae and computation of estimated predation-free transition matrices. Owl predation affected exclusively small individuals of initial body mass stages. Although population growth rate in A. azarae was most sensitive to transitions involving initial stages, exclusive consumption of small individuals by owls did not have a significant impact on population growth rate. Owls preyed upon individuals of A. azarae only slightly, possibly on account of specific features of its habitat use and escape response, which may result in predators not affecting prey abundance or demography. The proposed methods can be of special convenience for endangered or rare vertebrate species in which large-scale experimental manipulations might be questionable.