Latitudinal variation in the response of tidepool copepods to mean and daily range in temperature

Understanding the evolutionary potential of organisms to adapt to a changing climate, and the fitness consequences of temperature fluctuations, are critical to forecasting the future of biodiversity. Geographic variation among populations in life history response to temperature mean and variability offers one view of the potential for local adaptation to broaden the thermal niche. We used laboratory growth experiments to examine the effects of temperatures between 13 degrees C and 30 degrees C on five life history traits and the intrinsic rate of increase for 15 Tigriopus californicus populations distributed over 17 degrees of latitude. Different life history stages showed distinct latitudinal shifts in thermal response, while the temperature of peak population growth consistently declined with increasing latitude. In addition, high-latitude populations grew faster at optimal temperatures but showed steeper fitness declines at high temperature. To test geographic population variation in response to the amplitude of daily thermal fluctuations, we grew three northern and three southern populations and manipulated nightly low and daily high temperatures. We found the lowest fitness overall in the treatment with the highest mean temperature, and the treatment with the greatest variability showed high fitness despite an 8 degrees C greater daily range. Population responses to daily thermal variability were unrelated to latitude of origin. Our results indicate that trade-offs between adaptation to high vs. low temperature, and between growth and maturation vs. survival and fecundity, govern local adaptation along the latitudinal gradient. They also indicate that T. californicus populations can maintain fitness over a wide range of daily variability but are more sensitive to small changes in the mean temperature.