The thermal effects on selected life history traits in an arctic and a temperate population of the Collembola Hypogastrura viatica.

A set of life history traits were studied in order to compare the thermal responses of two populations of the surface living Collembolan Hypogastrura viatica, from the highly contrasting climates of north east Svalbard and the southern coast of Norway. This was done by analyzing hatchling size, growth pattern, development, age and size at reproduction and mortality of the two populations over a range of temperatures (from 10 to 20 or 25˚C, depending on trait). The goal was to study how these traits co- vary and if they represent a general difference in strategy between the two populations involved. The results were also compared with those from a similar study on the soil living Collembola Folsomia quadrioculata. The hatchling size of the two H.viatica populations were not notably affected by temperature, and differed only at 10˚C, where the temperate population was distinctly larger in size. However, in terms of growth, both populations displayed decreasing growth rate and increasing asymptotic size with decreasing temperature, meaning that both populations responded in accordance to the Temperature Size Rule. The arctic population grew slower and reached a larger asymptotic size than its temperate counterpart at all temperatures, as predicted by the Bergman Cline. Moreover, the arctic population reproduced later and at a larger size than the temperate one. In terms of mortality, the temperate population was less sensitive to heat stress and therefore experienced lower mortality at the highest temperatures. The overall trend of the temperate population was to speed up growth and developmental rates at the expense of size, in addition to being more heat tolerant. This is consistent with the potential time limitations of a fixed one-year life cycle, and its relatively warm habitat. By comparison, the opposite trend of selection of large body size rather than increased developmental rates was favored in accordance with the more flexible life cycle of the arctic population in this species. The population specific differences found in H. viatica were the opposite of what was found in the two corresponding populations of F. quadrioculata. This may be caused by differences in life cycles and mobility, which enables local adaptations in F. quadrioculata in a totally different way than H.viatica. The latter, with a much larger dispersability showed signs of large scaled macro climatic adaptations. Hence, adaptations to climate may vary greatly between species of ectotherms.