The survival, longevity, diet and development of mountain stone weta Hemideina maori in the Rock and Pillar Range, New Zealand

Abstract

For ectotherms, environmental temperature is an important correlate of growth rate, development time and final body size, with the dominant trend being an increase in growth rate and a decrease in size at maturity as temperature increases. Approximately 99% of the species on earth are ectothermic, relying primarily on external sources for their body heat. Given the importance of body size in ecology and the desire to be able to forecast responses to climate change, there is a need to understand the effects of temperature and diet on body size in ectotherms, and how these environmental variables interact with genetic adaptation. In this respect, the mountain stone weta Hemideina maori is a species of particular interest, as it shows an increase in body size in both males and females, in association with increasing altitude and decreasing temperature. Little is known about survival and longevity of H. maori in its natural environment, but their relatively large size and flightlessness makes them an ideal candidate for mark-recapture studies. This thesis aimed to: ( 1) examine potential differences in the survival probability of adult alpine weta along an altitudinal and body size gradient, using mark-recapture methodology; (2) determine the diet preferences of adult weta at low, intermediate and high altitudes using vegetation quadrat surveys and faecal pellet analysis and (3) use a common-garden experiment approach to determine whether variation in body size could be attributed to variation in environmental temperature and/or genetic adaptation.

Adult weta at high altitudes were significantly larger than at intennediate and low altitudes, but there was no difference in monthly survival between weta at high and low elevation sites. The 1nonthly survival probability of adult weta at the high and low elevation sites fluctuated between 60o/o and 90o/o during summer and reached 95o/o over winter. Survival at the middle elevation site was lower, with this difference fluctuating between 25 and 40o/o during summer. The three highest ranking plant species in the diet of weta from all three altitudes have an unusually high lipid content, indicating that high lipid species may be an important diet item for H. maori. However, there was no difference in the diet preferences of adult weta at high, intermediate and low elevations, suggesting that food quality does not have a strong influence on body size. The results from the common-garden laboratory experiment revealed a significant altitudinal difference in growth rate, with high altitude weta having a faster growth rate, and low altitude weta a slower growth rate, under both warm and cool temperature treatments. If fast growth is an unavoidable consequence of high environmental temperatures, then slow growth will be selected for in low altitude weta if it increases the fitness of some other trait, such as larger adult body size and increased fecundity. At high altitudes, delayed maturity due to slow growth and cool temperatures increases the risk of 1nortality prior becoming an adult, and selecting for fast growth at high altitudes may represent a trade off between increased fecundity and minimising the risk of juvenile 1nortality. The results of this study are discussed in relation to the impact of global warming on alpine weta populations.