Diet selection of European rabbits (Oryctolagus cuniculus) in the semi-arid grasslands of the Mackenzie Basin, New Zealand

There have been no previous quantitative studies of European rabbit (Oryctolagus cuniculus) diet in New Zealand, despite the prolonged and significant impact these introduced pests have had upon both pastoral and conservation values of this country's semi-arid grasslands. Knowledge of rabbit diet selection will allow us to better assess the current and potential impacts of rabbits. Diet selection was assessed on three replicate sites in the Mackenzie Basin, South Island of New Zealand, during the period December 1996 to March 1997. Selection ratios (Manly et al., 1993) were used to quantify diet selection, using Scott's (1965) height-frequency technique to estimate available vegetation and rabbit stomach contents analysis to estimate diet composition. Rabbits were generalists, feeding on a mean of 11 plant species per stomach sample and 33 plant species per site. The single largest dietary component was introduced Hieracium species; these comprised 32 to 39% of the diet composition at the three sites, which was proportional to their availability. Over all sites combined (n=90 rabbits), six plant species were positively selected, there was no selection for 17 species, and 24 species were negatively selected; this pattern was consistent across the sites. The negatively-selected species were sometimes common in the environment but never comprised an important part of the diet. In contrast, the positively selected species made up only 11% of the available vegetation but comprised a large part of the diet (44%). Sex and age of rabbits did not influence their diet selection. The nutritional composition of rabbit diet was very similar to the nutritional composition of the available vegetation. Of a range of nutrients measured in the vegetation, magnesium was the only one found to be significantly correlated with diet selection (negative correlation). The determinants of rabbit diet selection in this habitat therefore remain unclear. Selective grazing by rabbits will have implications for plant species diversity and richness, but further research will be required on plant community dynamics before these effects can be predicted.