Re-examination of recent loss of indigenous cover in New Zealand and the relative contributions of different land uses

Loss of indigenous habitat is a key factor in the decline of New Zealand’s biodiversity. A recent contribution by Walker et al. (2006, New Zealand Journal of Ecology 30: 169–177) described losses of indigenous vegetation between 1996/97 and 2001/02 (some 17 000 ha) based on an analysis of changes in the Land Cover Database, LCDB1 and LCDB2, respectively. We agree that the general approach of using these and other spatial datasets appears to be appropriate to investigate changes in land cover and the types of land uses that are responsible, but we would like to offer some comments to aid with the interpretation of this and other studies that use LCDB comparisons and similar techniques. Using aerial photography, satellite imagery, site visits, and other methods, we evaluated a stratified sample of 67 of the 449 polygons that were indicated to have changed from the most affected indigenous classes (‘Tall Tussock Grassland’, ‘Manuka and/or Kanuka’, and ‘Broadleaved Indigenous Hardwoods’) to the exotic forest classes ‘Afforestation (not imaged)’ and ‘Afforestation (imaged, post LCDB1)’. Our assessment of the entire area of each of these polygons covered 56.6% of the total area that was identified to have changed, and this revealed an error rate of c. 70% for this particular comparison of LCDB1 and LCDB2 data. This indicates the accuracy of such analyses may be too low to be meaningful and requires verification of the data that are primarily based on remote sensing, even when the overall aggregate accuracy is very high. In addition, we comment on the relative merits of different land uses in relation to the conservation of indigenous biodiversity, particularly the contributions of low-producing exotic grassland and exotic plantation forests. This is important because much indigenous biodiversity remains in exotic forests and embedded indigenous remnants, and the current clearing of potentially over 100 000 ha of such land for exotic pasture will cause significant losses of indigenous biodiversity.