Acclimation effects on thermal tolerances of springtails from sub-Antarctic Marion Island: Indigenous and invasive species

Collembola are abundant and functionally significant arthropods in sub-Antarctic terrestrial ecosystems, and their importance has increased as a consequence of the many invasive alien species that have been introduced to the region. It has also been predicted that current and future climate change will favour alien over indigenous species as a consequence of more favourable responses to warming in the former. It is therefore surprising that little is known about the environmental physiology of sub-Antarctic springtails and that few studies have explicitly tested the hypothesis that invasive species will outperform indigenous ones under warmer conditions. Here we present thermal tolerance data on three invasive (Pogonognathellus flavescens, Isotomurus cf. palustris, Ceratophysella denticulata) and two indigenous (Cryptopygus antarcticus, Tullbergia bisetosa) species of springtails from Marion Island, explicitly testing the idea that consistent differences exist between the indigenous and invasive species both in their absolute limits and the ways in which they respond to acclimation (at temperatures from 0 to 20 °C). Phenotypic plasticity is the first in a series of ways in which organisms might respond to altered environments. Using a poorly explored, but highly appropriate technique, we demonstrate that in these species the crystallization temperature (Tc) is equal to the lower lethal temperature. We also show that cooling rate (1 °C min-1; 0.1 °C min-1; 0.5 °C h-1 from 5 to -1 °C followed by 0.1 °C min-1) has little effect on Tc. The indigenous species typically have low Tcs (c. -20 to -13 °C depending on the acclimation temperature), whilst those of the invasive species tend to be higher (c. -12 to -6 °C) at the lower acclimation temperatures. However, Ceratophysella denticulata is an exception with a low Tc (c. -20 to -18 °C), and in P. flavescens acclimation to 20 °C results in a pronounced decline in Tc. In general, the invasive and alien species do not differ substantially in acclimation effects on Tc (with the exception of the strong response in P. flavescens). Upper lethal temperatures (ULT50) are typically higher in the invasive (33-37 °C) than in the indigenous (30-33 °C) species and the response to acclimation differs among the two groups. The indigenous species show either a weak response to acclimation or ULT50 declines with increasing acclimation temperature, whereas in the invasive species ULT50 increases with acclimation temperature. These findings support the hypothesis that many invasive species will be favoured by climate change (warming and drying) at Marion Island. Moreover, manipulative field experiments have shown abundance changes in the indigenous and invasive springtail species in the direction predicted by the physiological data.