New Zealand Journal of Ecology (1989) 12(s): 67- 96

Relationships between Moas and Plants

Research Article
I. A. E. Atkinson 1
R. M. Greenwood 2
  1. Botany Division, DSIR, Private Bag, Lower Hutt, New Zealand
  2. 107 Atawhai Road, Palmerston North, New Zealand.

Moas were common and widespread in pre-Polynesian New Zealand. They were most concentrated in the lowlands but ranged into the mountains with Megalapteryx didinus reaching at least 1800 m. This contribution examines niche separation between species of moa, the evolutionary effects of moas on plant species, and the effects of mammals and moas on New Zealand vegetation. The aim is to establish the extent to which mammals are ecologically equivalent to moas.
Within forest there was a high degree of coexistence between different species of moa. Their niches were separated by differences in feeding height, in bill shape, in gizzard development, and by other differences correlated with size. Femur circumference, as an index of weight, shows that moas were mostly in the range 20 to 200 kg.
It is suggested that herbivory by moas is responsible for the evolution of at least 11 kinds of growth characteristics seen in indigenous plants. Four of these, namely spiny tussocks, mimicry, reduced visual apparency, and divarication are discussed and suggestions are made on how hypotheses for such adaptations can be tested. These plant responses, together with adaptive responses among moas, are considered as an example of coevolution. Possible adaptive responses in moas are increase in the cutting power of the bill, increasing development of head and neck muscles, increasing development of the gizzard, and increase in height. Many other New Zealand plants have growth features including chemical characteristics which, although not originating as adaptations to moa browsing, nevertheless make them unpalatable to, if not ignored by, introduced browsing mammals.
There are both qualitative and quantitative differences in the feeding behaviours of mammals and moas. The ecological (and evolutionary) consequence of these is that selection pressures exerted on New Zealand plants by browsing mammals are not the same as those formerly exerted by moas. The preliminary conclusion reached is that mammals are equivalent to moas only in a limited site-specific sense, in particular, on lowland high- fertility sites such as river terraces. Elsewhere the effects of introduced mammals are either relatively minor or, in the montane and subalpine zones, are a major new effect on the vegetation, which is now much depleted from its pre-Polynesian state.