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Indirect signs of species presence (e .g . , tracks , scats , hairs ) are frequently used to
detect target species in occupancy , presence /absence , and other wildlife studies . Indirect
signs are often more efficient than direct observation of elusive animals , making such
signs well suited for long -term and broad -scale monitoring programs . However , error
associated with misidentification of indirect signs can be high , and should be measured
if meaningful inferences about population parameters are to be made . This study
addressed the need for systematic approaches to estimate and minimize variation due to
observer error in identifying indirect signs . I reanalyzed data from 4 replicates of a
presence /absence survey of northern river otters (Lontra canadensis ) that had been
conducted by Texas Parks and Wildlife Department (1996 -2003 ) . Sixteen observers had
recorded tracks at sample points under bridges (n = 250 ) distributed throughout 27
counties in the Piney -Woods ecoregion of east Texas . My objectives were to 1 )
determine if observers were a source of bias in the survey , 2 ) estimate the proportion of
error associated with track identification skill , and 3 ) evaluate the use of an international
certification procedure that measured observer tracking skill . The null hypothesis that observers had no effect on the variation in reported sign was rejected . Indeed , binary
logistic regression tests indicated that observers were significantly associated with
variation in reported track presence . Observers were not randomly distributed among
bridge sites , and therefore were significantly correlated with 4 habitat variables that may
have influenced heterogeneity in otter occupancy and probability of detection
(watershed , vegetation -type , water -type , bridge -area ) . On average , experienced
observers (n = 7 ) misidentified 44 % of otter tracks , with a range of 0 % to 100 % correct
detection . Also , 13 % of the tracks of species determined to be 'otter -like' were
misidentified as belonging to an otter . During the certification procedure , participants
misidentified the tracks of 12 species as otter . Inaccurate identification of indirect signs
is a likely source of error in wildlife studies . I recommend that observer skill in
identification of indirect signs be measured in order to detect and control for observer
bias in wildlife monitoring . |
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