Assessing the role of revegetation in achieving restoration goals on Tiritiri Matangi Island

The ecological restoration of Tiritiri Matangi Island is a community-driven initiative that has captured the interest of the international conservation movement. Ecological restoration commonly focuses on the establishment and maintenance of functioning indigenous ecosystems through the control or eradication of invasive weeds and animal pests, indigenous species translocations, and habitat enhancement, including revegetation. Revegetation of indigenous plant communities provides an opportunity to kick-start natural processes and facilitate succession towards a diverse ecosystem. However, revegetation initiatives are often conducted in an ad hoc manner, without clear objectives or monitoring to assess the effectiveness of the chosen approach. The objective of this study was to determine whether aspects of the revegetation of Tiritiri Matangi are meeting the restoration goals by providing habitat for indigenous diversity, particularly birds. To this end, we investigated forest structure, plant regeneration and bird numbers and species richness in three different densities of pohutukawa planting, specifically a) densely planted pohutukawa, b) thinned pohutukawa stands, and c) mixed species plantings. The Point Centered Quarter and Presence of Seedlings along a Transect methods were used to collect data on forest structure and regeneration. Five-minute bird counts were used to gather data on bird conspicuousness and species richness. Vegetation analysis showed there were low levels of regeneration in dense pohutukawa. Similarly, results showed low bird numbers and species richness in dense pohutukawa compared with the two other vegetation types. This suggests that dense pohutukawa plantings are inhibiting vegetation diversity and regeneration, and richness and relative abundance of indigenous birds, contrary to the objectives of the Tiritiri Matangi restoration. It appears that thinning dense pohutukawa stands on Tiritiri Matangi will encourage species diversity and better meet the restoration objectives. However, this may lower landscape heterogeneity overall and have negative effects on specialist species that rely on pohutukawa habitat, including invertebrates and insectivorous, ground-dwelling birds such as the kiwi.