Modelling the effects of predation on the population dynamics and survivorship of the New Zealand pigeon or 'kereru' (Hemiphaga novaeseelandiae)

Abstract

The arrival of Maori and then Europeans in New Zealand, dramatically changed the islands’ biota with the extinction and ongoing decline of many species. Three factors are considered to be important consequences of human arrivals: (i) introduction of exotic animals that become invasive species, (ii) habitat loss and fragmentation due to deforestation and land clearing for human settlement and agriculture, and (iii) hunting for (traditional) harvest. Scientist believe that the most significant and current threat to New Zealand’s avifauna is the ongoing adverse ecological impacts of introduced animals, especially exotic mammals that are now considered important pests. An increasingly used tool for biodiversity conservation management is establishing a dynamic model to forecast prey-predator relationships and to assist the decision-making process in developing conservation strategies. A four-species systems dynamics model, representing kereru (Hemiphaga novaeseelandiae), ship rat (Rattus rattus), brushtail possum (Trichosurus vulpecula), and stoat (Mustela erminea) populations, was built using the Vensim® PLE Plus modelling package. The behaviour of the kereru population was explored under two predation scenarios: (i) impacts of different predators (single predation) and (ii) combined effect or predators (multiple predation). The risk of functional extinction of the total kereru population under two predator-driven settings: (i) ship rat + stoat predation and (ii) all predators, was also examined. The system dynamics model forecast an exponentially increasing population for the total kereru after 40 years, translated into a population growth rate of 1.061, which is comparable from a previous model by Lyver et al. (2009), in the absence of predation. However, the presence of each predator (single predation) can trigger a marked decline in the population and growth rate of the total kereru population. The magnitude of the impact on the total kereru population caused by the combination of ship rat and stoat predation is comparable to the effect caused by all predators (multiple predation). Furthermore, the presence of ship rat + stoat predation and all predators can drive the total kereru population to extinction in 198 and 127 years, respectively. These findings provide substantial information for the management of predators in sites and areas inhabited by the New Zealand pigeon.