Abstract:
Tiritiri Matangi is an island located in the Hauraki Gulf of Auckland, New Zealand. After a long history of Maori and European occupancy the island was left with only 6% forest cover. Despite the reductions in habitat Tiritiri Matangi had the only remaining population of bellbird (Anthornis melanura) on the North Island. This led John Craig and Neil Mitchell to believe that a complete restoration of Tiritiri Matangi may be possible. A restoration plan was created and carried out that resulted in the planting of over 300,000 plants and the introduction of 16 species of fauna. Tiritiri Matangi thrived under the restoration efforts; however there were problems with the methods implemented. Regeneration was occurring in a patchy manner, with some areas demonstrating high regeneration density and others being almost barren. The aim of this study was to investigate the patterns of regeneration occurring within the restored forest on Tiritiri Matangi, and the potential influencing factors. As birds are one of the primary dispersal agents on the island, their roles were closely examined. A fine grained approach was used to better understand the influencing ecological processes. Ten restored plots were selected. They were of close age and evenly distributed around the island. Within each of the 10 plots, two subplots were determined: one objectively selected, based on high regeneration density, and a comparison subplot randomly selected 25 metres away. Two studies were carried out within the 20 subplots: fixed-radius point counts for birds, and evaluations of understory and canopy species. Separate specific canopy species understory evaluations were performed by randomly measuring the understory of seven selected canopy species within each plot. Results were compiled and examined using PAST, and SAS statistical software. This study identified a significant relationship between canopy species and bird abundances and compositions. Furthermore, the density and composition of understory regeneration varied significantly beneath different canopy species. There is a causal linkage between the two observations as all the birds observed are potential seed dispersers and the majority of the understory is composed of seed dispersed species. Regeneration and bird abundances were significantly higher underneath puriri (Vitex lucens) and five-finger (Pseudopanax arboreus) canopies. This indicates that the two canopies are important foraging opportunities within the restored forests and may even function as keystone species within the ecosystem. The different compositions and abundances of birds using canopy species are determining the successional pathways of restored areas. The mechanisms observed appear to be similar to those of Markovian chain succession. However, more research will be required to determine if this is true. Given the current understory compositions and understanding of successional pathways, the likely future canopy species of restored areas will be kohekohe (Dysoxylum spectabile) and pigeonwood (Hedycarya arborea). Trajectories established during the restoration of the island are helping to decide current spatial patterns. Coprosma spp. and kohekohe were among the most common species planted during restoration, providing early food sources for birdlife. As a result, current understories are dominated by the two species. Furthermore, the canopy species planted during restoration are affecting the way bird species are using the island. This results in a feedback loop occurring within the restored forests of Tiritiri Matangi - canopy species determine bird usage, bird usage determines understory composition and future successional pathways. This is an important finding as it might enable future restoration projects to better plan and manage sites to provide the optimal environment for species reintroductions. However, more work will be required to better understand the underlying ecological processes and make use of the theory. bird usage determines understory composition and future successional pathways. This is an important finding as it might enable future restoration projects to better plan and manage sites to provide the optimal environment for species reintroductions. However, more work will be required to better understand the underlying ecological processes and make use of the theory.