How many giraffes are there? A comparison of abundance estimators at Ongava Game Reserve, Namibia

Camera-traps are a versatile and widely adopted tool for collecting biological data for wildlife conservation and management. While estimating population abundance from camera-trap data is the primarily goal of many projects, the question of which population estimator is suitable for analysing these data needs to be investigated. We took advantage of a 21 day camera-trap monitoring period of giraffes (Giraffa camelopardalis angolensis) on the Ongava Game Reserve (Namibia) to compare capture-recapture (CR), rarefaction curves and N-mixture estimators of population abundance. A marked variation in detection probability of giraffes was observed both in time and between individuals. Giraffes were also less likely to be detected after they were seen at a waterhole (mean daily visit frequency of f = 0.25). We estimated the population size to be 119 giraffes (Cv = 0.10) using the most robust reference estimator (CR). All other estimators deviated from the CR population size by ca. -20 to > +80%. This was due the fact that these models did not account for the temporal and individual variations in detection probability. We found that modelling choice was much less forgiving for N-mixture models than CR estimators because it leads to very variable and inconsistent estimations of abundance. Double counts were problematic for N-mixture models, challenging the use of raw counts (i.e. when individuals are not identified) at waterholes, to monitor the abundance of giraffe or of other species without idiosyncratic coat patterns.