Exploring the concept of niche convergence in a land without rodents: the case of weta as small mammals

The distinctiveness of New Zealand’s large endemic orthopterans and lack of small mammals in our forest ecosystems led to the description of weta as ecologically equivalent to rodents in other countries. We review the use of this metaphor and the characteristics, such as diet and reproductive behaviour, given to support it. We note, however, that species are rarely specified when comparisons are made, thereby neglecting the ecological diversity of both weta and rodents. We suggest that if these taxa are to be compared, the details of their ecology are important and the scale of their influence in an ecosystem must be taken into account. We consider in particular the relevance of the ‘invertebrate mouse’ cliché in understanding evolutionary ecology in New Zealand and find it misleading. We show that reproductive potential and scale of change in population size differ greatly between mice and tree weta. We find that endothermic mice (Mus musculus) have a metabolic rate almost 20 times faster than ectothermic tree weta (Hemideina sp.), an intrinsic rate of increase some 275 times higher, and consume a high quality diet dominated by seeds and invertebrates and devoid of leaves, in contrast to tree weta diets. Comparative quantitative analyses of the influence of different animals on ecosystem services, biomass, nutrient cycling and energy turnover of forests in New Zealand and elsewhere will contribute to interpretation of the evolutionary history of the New Zealand biota.