High altitude ecology: Perliminary observations of alpine grasshoppers in a habitat modified by deer and chamois
- Forest Research Institute, Wellington
Four brachypterous grasshoppers, Paprides nitidus, Brachaspis collinus, B. nivalis and Sigaus villosus, occur above timber-line at Cupola Basin, a 3,000 acre tributary catchment of the Travers River, Nelson. Samples collected between January 1965 and April 1966 were measured to obtain information on population structure, overwintering and reproductive biology, and correlation between weight and length. Data for Brachaspis females are analysed and presented in this paper to illustrate the general bionomics. Population densities were calculated on plots of about 100 sq. ft. area. Biomass (lb./acre) was also estimated using the product of weight of each type of grasshopper recognisable in the field and their estimated numbers.
Brachaspis females appear to hibernate in instars III-VII, as adults, and as eggs in the soil. They mature about three years after hatching. Adults mate and lay weather is suitable. If laying occurs early in the season eggs hatch without a winter diapause. Such unusual longevity and flexibility of the life cycle are believed to reflect adaptation to the erratic nature of the New Zealand alpine climate: Snow may fall at any time of the year, the winter snow-pack may form between April and July and melt as early as September or as late as December. Winter temperatures are relatively mild.
Populations of Brachaspis weigh less than one lb./acre in dense grassland and up to 29lb./acre where scree forms a high proportion of total cover. A population of 15lb./acre is calculated to require over 30% of total leaf matter produced per annum on eroded sites, a result which suggests Brachaspis is a critically-important herbivore. The implications of this hypothesis are discussed. Modifications of the habitat by introduced ungulates, and recent changes in the pattern of snow-cover are considered as possible causes of the critical status of Brachaspis indicated by the calculations.