Unrestricted guided transect sampling for surveying sparse species

We present a modification of an earlier presented method using prior auxiliary information in the layout of survey strips. The idea is to imitate a skilled surveyor who purposively seeks the species of interest. Yet, the method "unrestricted guided transect sampling" (UGTS) is a probability sampling method. In comparison with a strip survey using no auxiliary information, UGTS gave 11%-64% lower standard errors for estimates of species population size in three simulated forest types. In a test in six stands where European aspen (Populus tremula L.) and an epiphytic moss (Orthotrichum speciosum Nees) had been mapped, UGTS gave a small improvement in some stands but considerably higher standard errors in other stands with kNN estimates of volume of deciduous trees derived from satellite images as covariate values. With covariates values simulated from aspen basal area, UGTS gave 8%-75% lower standard error than a strip survey using no auxiliary information. The study shows a gain in precision by using auxiliary information both in the design and in estimation when surveying sparse species but also that the correlation between the covariate and the variable of interest has to be relatively strong to make the method worthwhile.