A Hydrologists Contribution to the Debate on Wild Animal Management

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. Furthermore, four severe periods of erosion had affected the Ruahines, Ureweras and elsewhere, since the 13th century and each had ended regardless of humans, and without their assistance. By 1985 I was able to show that there have been at least seven other periods of accelerated erosion since 180AD, similar to, but greater than, the current one. These were long before even Polynesians and their animals had arrived in New Zealand.
The present period of increased erosion and alluviation is primarily the consequence of atmospheric warming and the bigger rainstorms and floods it brings. Browsing mammals have not contributed significantly but they do reduce the density of the vegetation which minimises normal soil erosion and hence the concentrations of fine sediments in streams. The primary objectives in managing browsing mammals should be to: (a) protect the indigenous vegetation for itself and as habitats of native animals, and (b) enhance the beneficial effects of vegetation on the stability of surface soil and on water quality.