Do you see what I see? Variation in detection, identification and enumeration of mammals during transect surveys

Effective monitoring, management and conservation of wildlife axiomatically depend on accurate data but causes of variation, including inter-observer variation, are rarely explicitly quantified. Here, under controlled conditions, we demonstrate considerable variation in detection, identification and enumeration of (theoretically) readily identifiable African mammals at a reserve with a known species assemblage. Detection: 97.8% of sightings were missed by & GE;1 observer; frequency of detection was affected by observer ID, detection distance, visibility and animal group size. Identification: just 3/14 species were identified consistently at all sightings. Enumeration: lack of consensus for 60.5% of sightings; consensus likelihood was affected by visibility and group size. La surveillance, la gestion et la conservation efficaces de la faune depend de facon axiomatique de donnees precises, mais les causes de variation, y compris la variation entre observateurs, sont rarement quantifiees de maniere explicite. Ici, dans des conditions controlees, nous demontrons une variation considerable dans la detection, l'identification et le denombrement de mammiferes africains (theoriquement) facilement identifiables dans une reserve dont l'assemblage d'especes est connu. Detection : 97,8 % des observations ont ete manquees par & GE;1 observateur ; la frequence de detection a ete affectee par l'identite de l'observateur, la distance de detection, la visibilite et la taille du groupe d'animaux. Identification : seuls 3/14 des especes ont ete identifies de maniere coherente lors de toutes les observations. Denombrement : absence de consensus pour 60,5 % des observations ; la probabilite de consensus etait affectee par la visibilite et la taille du groupe.