Experimental Investigations of the Effects of Takahe and Deer Grazing on Chionochloa pallens Grassland, Fiordland, New Zealand

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Abstract
(1) The takahe (Notornis mantelli Owen), an endangered flightless rail, restricted to the Murchison and Stuart Mountains of Fiordland National Park, New Zealand, shares the alpine snow tussock grasslands with introduced red deer (Cervus elaphus L.) in summer. The herbivores eat the same tussock species but, whereas takahe pull out tussock tillers and eat only the succulent basal butt portion of the tiller, deer eat the leaf blades. (2) Experimental manipulations of individual Chionochloa pallens Zotov tussocks simulating the feeding mode of deer and takahe were undertaken to evaluate the effects of each herbivore, and the combined effect of both, on the productivity and nutrient status of the plants. Tussocks cut to simulate deer grazing were monitored to assess whether and when takahe would eat deer-grazed tussocks. (3) The feeding by takahe on cut tussocks was largely confined to autumn and winter, when the starch content of these tussocks was higher than that of uncut tussocks. The combined effect of takahe feeding and the cutting induced a long-term decline in the biomass of the tussocks. Eight and a half years after cutting, the biomass of the cut tussocks was 64% less than that of uncut tussocks. It was estimated that it would take 20 years for the tussocks to recover, even if no further grazing occurred. (4) Pulling 25 and 50% of tillers from C. pallens tussocks in the manner that takahe feed caused no significant decline in tillering rates in a non-flowering year. However, severe cutting of tillers at the blade-sheath junction, with or without pulling, invariably caused a decline in tillering. (5) Competition for food with deer has played a major role in the decline of takahe since it was rediscovered in 1948. Intensive deer control operations since 1962, particularly in the early 1970s, reduced deer numbers appreciably. It is suggested that the long lag period between the decline in deer grazing pressure on the grasslands and the beginning of the recovery of the takahe population in 1982 reflects the 15-20-year recovery time needed for the preferred tussock species to recover. (6) To avoid long-term detrimental effects on takahe habitat, continued deer control is necessary because the rate of habitat degradation through deer grazing is much more rapid than the recovery time.