University of San Francisco, California, United States
The Climatic Variability Hypothesis (CVH) posits that organisms in more climatically variable environments should be able to handle a wider range of conditions. Underlying the CVH is an assumption that variable environments select for wider niche breadths, while stable environments select for maximized performance under a smaller range of conditions — a breadth–performance tradeoff. The CVH was conceived to explain latitudinal gradients in range size, but the hypothesis and its underlying assumption can be tested more broadly. Due to marine influence, coastal areas typically experience smaller temperature fluctuations relative to inland areas. According to the CVH, coastal organisms should have more narrow thermal niches because they experience a smaller range of temperatures. We tested the CVH in a novel setting by comparing thermal performance curves of coastal and inland perennial populations of Mimulus guttatus. In a growth chamber experiment, we measured relative growth rate of leaf area on plants from three coastal and three inland populations under eight temperature treatments. We used these data to build thermal performance curves for each population. We then compared the breadth and height of these curves among populations to test the CVH and the breadth–performance tradeoff.
We did not find support for the CVH — coastal and inland populations did not differ in thermal breadth, with coastal populations having wider thermal breadths than expected. Despite this, we found evidence for a breadth–performance tradeoff — populations with smaller thermal breadths had higher maximum performance. Surprisingly, the two most-inland populations occupied the opposite ends of this tradeoff. Temperature variation, mean annual temperature, and mean maximum and minimum temperature did not explain the patterns of thermal performance we found. Our unexpected results raise new questions about the drivers of thermal niche evolution in M. guttatus.