Degradation of the crocodile population in the Olifants River Gorge of Kruger National Park, South Africa

1. Top predators such as crocodiles often reflect ecosystem degradation. The recent spate of close to 200 Nile crocodile deaths may reflect the ecosystem state of the Olifants-Letaba River system in the Kruger National Park. This paper investigates whether the crocodile deaths were indeed the consequence of a rare and perhaps acute event given the variability in annual population growth rates of crocodilians. 2. Spotlight-and helicopter-based surveys designed to correct for availability and detectability bias were used to estimate population sizes of crocodiles from 2008 to 2010 in and around the Olifants River Gorge, the focal area of crocodile deaths. 3. Correction factors derived from individually observed crocodiles for both spotlight-and helicopter-based counts were lower than those typically used for Nile crocodiles as well as other crocodile species. Even so, corrected spotlight-and helicopter-based estimates were comparable and the number of crocodiles in the focal study area declined significantly from 780 (95% CI: 637-1222) to between 460 (spotlight estimate, 95% CI: 375-665) and 505 (aerial estimate, 95% CI: 559-1746) during the period of crocodile deaths. The average annual decline of 35% was at the lower end of the distribution of annual population growth rates across the rivers of Kruger National Park. 4. The crocodile deaths reflect a possible rare event that suggests a degraded crocodile population, possibly the consequences of broad-scale cascades of environmental deterioration of the Olifants-Letaba River system. 5. Even so, the potential risk of local demise of the population in the focal study area in the short term may be diminished through evolutionary, demographic and spatial resilience inherent within crocodiles that can accommodate the as yet unknown disease dynamics of pansteatitis. Copyright (C) 2011 John Wiley & Sons, Ltd.