Understanding predator densities for successful co-existence of alien predators and threatened prey

The high failure rate of threatened species translocations has prompted many managers to fence areas to protect wildlife from introduced predators. However, conservation fencing is expensive, restrictive and exacerbates prey naivete reducing the chance of future co-existence between native prey and introduced predators. Here, we ask whether two globally threatened mammal species protected in fenced reserves, with a history of predation-driven decline and reintroduction failure, could co-exist with introduced predators. We defined co-existence as population persistence for at least 3years and successful recruitment. We manipulated the density of feral cats within a large fenced paddock and measured the impact on abundance and reproduction of 353 reintroduced burrowing bettongs and 47 greater bilbies over 3years. We increased cat densities from 0.038 to 0.46 per square km and both threatened species survived, reproduced and increased their population size. However, a previous reintroduction trial of 66 bettongs into the same paddock found one red fox (Vulpes vulpes), at a density of 0.027 per square km, drove the bettong population extinct within 12months. Our results show that different predator species vary in their impact and that despite a history of reintroduction failure, threatened mammal species can co-exist with low densities of feral cats. There may be a threshold density below which it is possible to maintain unfenced populations of reintroduced marsupials. Understanding the numerical relationships between population densities of introduced predators and threatened species is urgently needed if these species are to be re-established at landscape scales. Such knowledge will enable a priori assessment of the risk of reintroduction failure thereby increasing the likelihood of reintroduction success and reducing the financial and ethical cost of failed translocations.