Distribution of road surveys for monitoring guanaco populations. Shading shows the expected relative abundance of guanacos assuming it is a positive function of the probability of guanaco occurrence. Probabilities of occurrence calculated by Pedrana et al. (2010) were divided in three classes: <0.4, 0.4 to 0.6, and >0.6. Lines denote one realization of the possible location of 30-km sampling units needed for monitoring guanaco numbers under a hypothetical objective of detecting a 50% population change in the next 10 years, with α = 0.3 and a minimum power of 1-β = 0.8. a Example of 40 randomly placed sampling units required for detecting a population increase. b Example of 80 randomly placed sampling units needed to detect a population decrease.