Monitoring vertebrate populations using observational data

Methods for monitoring temporal changes in population size vary from intens ive and potentially expensive to less intensive and more easily implemented techniques. In this paper we evaluate the utility of a monitoring techniqu e that can be used to follow many vertebrate species simultaneously at low cost and requires little training of personnel. Observers record the number of individuals seen per hour in the field and these rates of encounter are used as an index of population size. We examine whether encounter rates re flect population size by comparing them with independent censuses of three species over a 7-year period in the boreal forest near Kluane Lake in the s outhern Yukon Territory. Encounter rates were generally an accurate reflect ion of variation in population size. In our study system, inter-observer va riability did not influence our ability to detect fluctuations in populatio n size: the underlying fluctuations were detected whether data from all or only a group of "high-quality" observers were used. In our study, the benef it of using all available data outweighed the cost of variation among obser vers because sample sizes were large (averaging over 1200 data points from 33 observers per year). Variation in the length of observation periods did not affect the chance of detecting animals in our study. Encounter rates pr ovide a reasonable index of variation in population size, although caution should be used with species that are uncommon or difficult to detect.