The European Commission Habitats Directive requires that changes in the conservation status of designated species are monitored. Nocturnal and elusive species are difficult to count directly and thus population trajectories are inferred by variation in the incidence of field signs.
The Eurasian otter (Lutra lutra), listed by the IUCN as ‘near threatened’, is monitored throughout Europe using the ‘Standard Otter Survey’ method. The report explores the reliability of this approach by analysing species incidence throughout Ireland.
Surveillance of wild animal populations is notoriously problematic due to the difficulty in detecting individuals directly and the associated costs of surveying remote areas or rough terrain. For nocturnal and elusive species, researchers frequently sacrifice quantifying abundance and concentrate on determining patch occupancy. Consequently, indirect survey methods that record species presence using tracks, faeces or scent markings have become standard protocol for many species. These have comparatively low costs and, therefore, are widely used not only for assessing distribution and abundance but also in studies of habitat selection, behaviour and diet. However, binary presence/absence data are vulnerable to both Type I (false positive), and more significantly, Type II (false negative) errors.
False positives occur when the target species is recorded erroneously, for example by the misidentification of scats or where transient individuals are detected but are not resident whilst false negatives occur when the target species goes undetected at a site at which it occurs due to the apparent absence of field signs. Such errors can result in highly biased estimates of site occupancy, population size and habitat use. False positives can be avoided by surveyor training and testing as provided by the CyberTracker Certification used in the USA to quantify the skills of field observers (see http://trackercertification.com) or by independent verification, for example, DNA testing faeces to confirm the target species identity.
Quantifying the skills of observers working on wildlife surveys to be used as an explanatory variable in data analysis would be helpful. Thus, it is essential to accurately record the identity of surveyors and estimate their reliability during pre-survey training. For otter surveys throughout Europe, it may be beneficial to develop a similar programme to that offered by the CyberTracker Certification in the USA to provide an objective test of an observer’s reliability (see http://trackercertification.com).