erosion

In the 1950s, increased erosion, flooding and sedimentation was widely observed in New Zealand. The ruling opinion then was that the forests prevented erosion and floods, and browsing mammals were primarily responsible for the increased erosion of mountain lands. It followed that effective control of browsing mammal populations was necessary to prevent erosion and alleviate lowland flooding and alluviation. In the 19608 evidence was found for much severe erosion on the Ruahines in the 1840s -long before browsing mammals were there.

During the Late Holocene (1800 years BP to the present day) there have been three depositional episodes in coastal sand dune areas: Tamatean (1800 to 450 years BP), Ohuan (450 to 150 years BP) and Hoatan (150 years BP to the present day). Each episode comprised two phases: an unstable phase with a high rate of deposition, followed by a stable phase with a low rate of deposition and soil formation.

Polynesian settlement of New Zealand (c. 1000 yr B.P.) led directly to the extinction or reduction of much of the vertebrate fauna, destruction of half of the lowland and montane forests, and widespread soil erosion. The climate and natural vegetation changed over the same time but had negligible effects on the fauna compared with the impact of settlement. The most severe modification occurred between 750 and 500 years ago, when a rapidly increasing human population, over-exploited animal populations and used fire to clear the land.