The influence of crocodile density on the prevalence of human attacks.
Abstract
Large predator attacks on humans often provoke calls for animal population reduction, assuming it will reduce such incidents. Whilst this seems logical, there is currently little evidence supporting a consistent link between large predator density and attacks on humans. Here, we assessed whether large predator density is linked to the frequency of attacks on humans using estuarine crocodile (Crocodylus porosus) data in the Northern Territory (NT), Australia. Over the past 50 years, the estuarine crocodile population in the NT has grown from a few thousand to over 100,000 non-hatchling individuals. Crocodile and human population densities have been closely monitored throughout this period, allowing the frequency of attacks on humans to be assessed over a wide range of densities for both populations. Our analysis showed an increase in the frequency of attacks on humans as the crocodile population recovered from very low levels in the 1970s. However, the attack rates stabilised around 2009, despite crocodile density and the human population continuing to increase. Based on the relationship between crocodile density and human-attack frequency, scenario modelling suggested that the crocodile population would need to be culled to a critically endangered level (e.g. 90% population reduction) to reduce attacks on humans from 2.16 to 1.16 attacks per year. We conclude that whilst crocodile density significantly influences crocodile attack rates at low crocodile population sizes, this relationship becomes weaker as the density increases. For estuarine crocodiles in the NT, a plateauing of attack risk occurred once crocodile density attained ~2 crocodiles per km of river, and we argue that this was because high crocodile densities instigated management (e.g. removal of bold animals, exclusion zones) and education initiates by the government (e.g. 'Be crocwise' campaign) that subsequently evoked a change in human behaviour around waterways and stabilised the attack rate. Read the free Plain Language Summary for this article on the Journal blog.