The Impact of Fluctuating Temperature Regimes on Population Growth; a Comparison between Populations from Contrasting Ecotypes

Natural environments are characterized by daily and seasonal fluctuations in temperature, and ectotherms are expected to adapt to these fluctuations. Moderate temperature fluctuations often accelerate development rate as a result of the Kaufmann effect and possibly additional physiological adaptations. We examined the adaptive potential of ecotype-specific reaction norms by measuring the effect of temperature fluctuation on population growth of the soil arthropod Orchesella cincta from two distinct thermal habitats. We used a low and a high fluctuating temperature regime simulating field situations of either ecotype. We expected ecotype-specific responses in population growth rates under fluctuating regimes because in some ecotypes, extreme fluctuating conditions are more commonly encountered than in more buffered habitats. Number and size of springtails was measured after 8, 14 and 18 weeks and dry mass of the total population was estimated at the end of the experiment. Population growth increased with increasing amplitude of the temperature regime (T16°C < T12-20°C < T8-24°C). Mean body size was lower for populations that developed under the high fluctuating (T8-24°C) regime at all intervals, which was most likely caused by the high proportion of juveniles. Final biomass of populations that developed under the high fluctuating regime was twice as high as biomass of the populations that developed under a constant temperature. The strong and positive effects of fluctuating temperatures on population growth rate of Orchesella cincta are unlikely to be caused by the Kauffman effect, because thermal reaction norms are linear over the investigated temperature range; hence additional physiological mechanisms such as thermoperiodic stimulation may play a role. However, no ecotype-specific responses were found, indicating that earlier found variation in ecotype-specific reaction norms cannot be explained by temperature fluctuation in the original habitat alone.