Abstract
Deforestation is a massive cause of global biodiversity loss. However, although natural and semi-natural forests are declining in global land-area, plantation forests are increasing. Conifer plantation forest cover in New Zealand has increased significantly since the 1950s but the influence of plantation forestry on native avian biodiversity is poorly known. In the Dunedin area, native forests and plantation forests intergrade seamlessly making this region ideal for investigating the life history traits of a native bird species that occupies both forest types, the South Island robin Petroica australis australis. I studied robin nest survival in plantation conifer and native forests in relation to ecologically relevant variables, following breeding of robins at 58 nests over two breeding seasons in conifer plantation forest and one in native forest. In addition relative measures of abundance for key mammalian predators were obtained from 120 tracking tunnels spread over 12 lines and lastly, suggestions for the conservation management of robins are provided.
I found no evidence to support the hypothesis that nest survival is higher in conifer plantation forest than in native forest. For the two years of the study, the opposite pattern was observed; daily nest survival was lower in the conifer site than in the native site. However, there was much better support for nest stage influencing nest survival than site (∆AICc 2.88 vs. 11.07), although the interaction between these two factors had the highest overall support (∆AICc 0). Importantly, this work suggests that native kanuka forests may not be poor robin nesting habitat compared to conifer, as previous survey work has inferred, but further work would be required to confirm this pattern. These results are valuable considering there are no previous estimates of nest survival for the Dunedin robin population, or for robins in conifer plantation forests in New Zealand.
Rats appeared to be more abundant, and mustelids less abundant, in the native forest than in the conifer. Although it is difficult to determine population-level interactions from one year of data, the seasonal changes in rat and mustelid tracking in this study suggest some degree of mustelid control of rats. This might explain why historically, fewer robins were recorded in native forest where there are fewer stoats to control the rats and thus more rats depredating forest birds such as robins; however, nest survival was higher in the native forest than the conifer plantation forest during the two seasons of study.
These results are important for understanding the distribution of South Island robins in the Dunedin area and more importantly, for the future conservation management of this robin population. Ongoing collaborative management and research of the Dunedin robin population, facilitated by the Department of Conservation, is recommended. Suggested management actions include continuing predator control to increase robin nest survival in the Silverstream native forest; drafting an agreement for City Forests to communicate to interested parties when they are going to harvest robin-occupied areas (allowing sufficient time for pre-harvest surveys or other management actions such as translocations ‘out of harm’s way’); harvesting plantation areas containing robins outside of the robin breeding season; and continuing population monitoring to allow management action should the population decrease to an unacceptably low level.