A survey and assessment of grasshoppers as herbivores in the South Island alpine tussock grasslands of New Zealand

Abstract

Abstract A detailed assessment of grasshopper grazing pressures has been made for 10 alpine tussock grassland sites throughout the South Island of New Zealand. All sites were selected as reputedly high-density sites based on questionnaire returns from 61 observers, and a capture-recapture census of adult grasshoppers was made at each site. Standardised census procedures and timing permitted sites to be compared with a reference site where species biologies and population behaviour had been investigated in detail over a 4-year period. The comparisons were made using a behavioural grazing model that had been developed for populations at the reference site, given five major variables between survey sites: population density, consumption level of the population members, vegetation composition, percentage living ground cover, and site aspect. Variations in grazing pressure of up to 25-fold were demonstrated between sites, and these reflected the combined impact of all five variables rather than any particular one. All comparisons were necessarily calculated in relative terms, because appropriate data on vegetation productivity and the available crop are at present unavailable for any alpine tussock grasslands. However, evidence is given of the cumulative vegetation damage of low-volume grazing by grasshoppers, and this is shown to be serious in some grasslands. A general assessment of the importance of grasshoppers as herbivores is given for four classes of tussock grassland throughout the South Island.