In recent years much progress has been made in non-invasive genetic methods for various purposes including population estimation. Previous research focused on optimizing laboratory protocols and assessing genotyping errors. However, an important source of bias in population estimates still remains in the field sampling methods. The probability of animals being sampled can vary due to sex, age, social status or home range location. In this paper, relevant literature is reviewed to provide an overview of the occurrence of individual heterogeneity (IH) in the field and how it can be minimised, e.g. by adaptation of sampling design. Thirty-eight articles describing non-invasive population estimation for 12 mammal and two bird species were surveyed. The majority of studies discussed IH as a potential problem. The detectability of IH via goodness-of-fit testing depended on the average capture probability reported in the studies. Field tests for assessing variation in sampling probabilities or validating estimations were carried out in only 11 out of the 38 studies. Results of these tests point out that IH is a widespread problem in non-invasive population estimation which deserves closer attention not only in the development of laboratory protocols, but also concerning the sampled species’ characteristics and the field methods. IH can be reduced in the field by carefully adapting the sampling design to the characteristics of the studied population. If this is not reasonable, it may be better to switch to a different sampling strategy.
Key words: capture-recapture; genotyping; hair sampling; individual heterogeneity; population estimate; faeces; wildlife management