An increasingly recognized consequence of climate change is a higher variability in temperatures. Changes in magnitude and frequency of short-term temperature fluctuations are physiologically challenging for organisms living in Antarctic, Arctic and alpine ecosystems, which experience near-freezing temperatures throughout the growing season. In a controlled laboratory experiment, we investigated how temperature variability may affect the abundance and population structure of a model cold-specialist soil invertebrate, the free-living nematode Scottnema lindsayae. Long-term field-based records from the McMurdo Dry Valleys, Antarctica, showed negative correlations between frequency of freeze-thaw cycles and nematode abundance. To elucidate the underlying mechanisms, soil samples dominated by S. lindsayae were incubated under five treatments over a period of 8 weeks (the potential duration of a growing season): at constant temperature (-4°C, 4°C or 10°C), with large diurnal variability resulting in freeze-thaw cycling (-5 to 5°C), and large diurnal variability but no freezing (0 to 10°C). Living and dead nematodes were counted and identified to gender and life stage before, during (at 2 and 4 weeks) and at the end of the treatments. Individual nematodes were photographed with a micro-camera and measured to estimate body mass.
After 8 weeks, living nematode density had decreased and proportion of dead nematodes (a proxy for mortality) had increased under the freeze-thaw cycle and constant -4°C treatments compared to starting conditions and to the other treatments. The effect was driven by a selective loss of small-bodied individuals, which shifted the population body-size distribution towards large individuals; such a shift also occurred under the variable temperature treatment without freeze-thaw events. No significant changes in demography were detected, but high mortality of small juveniles would predictably slow down recruitment and population turnover, and therefore delay population recovery. The high susceptibility of small-bodied S. lindsayae to temperature variability (possibly related to the energetic costs of cryptobiosis) might be common among invertebrates that have evolved similar physiological responses to adverse temperature conditions. Therefore, our findings point to increased occurrence of freeze-thaw cycles as a potential threat to terrestrial invertebrates from cold ecosystems.