Detecting population declines over large areas with presence-absence, time-to-encounter, and count survey methods
Pollock, JF 2006, Conservation Biology, vol. 20, no. 3, pp. 882-892.
This study compared three types of fauna surveying methods; presence-absence, count, and time-to-encounter, to determine which would be most cost-effective for monitoring regional population declines. The results found that presence-absence surveys were often more powerful than count data surveys. Presence-absence surveys worked better than count surveys when; animal abundance did not vary hugely between sites, the organism was rare, and/or the species was difficult to detect so that the time spent at the site was greater than the time taken to travel there. Count surveys worked best in all other scenarios.
Results from this study found that presence-absence surveys were the best method when variability and visitation cost were low but count surveys were better when visitation cost or variability was relatively large. Time-to-encounter surveys were usually the best choice when there were more than 150 sites and four or more observations could be made per site. Count data was always best when there was less than 150 sites and improved with fewer observations per site. Presence-absence surveys were most efficient when there were over 200 sites and fewer than four observations per site. The author created guidelines that can be followed in order to choose the best survey method under different scenarios. Each survey method also had a different threshold at which it had sufficient power to detect declines in populations. For both count surveys and time-to-encounter surveys, power increased with number of sites and number of observations. Presence-absence surveys were more limited, having power to detect declines only when there was a high number of sites and low observations per site.
This study will help other researchers choose the most cost-effective survey method to detect regional fauna population declines. Furthermore, the author has outlined when particular surveys methods have enough power to be able to achieve statistically sound results. These findings could be used to help choose the best survey method for monitoring declines in koala populations in different areas. The author acknowledges that many complexities exist when deciding which survey method to use, and not all of these could be captured in this study, but that the guidelines provided will at least be applicable in wide range of circumstances.
Summarised by Miranda Rew-Duffy
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