High fitness at the leading edge of a species range despite range-wide adaptive trade-offs in climate adaptation

Background/Question/Methods

Climate warming is having a severe impact on species and ecosystems. Studying population responses at climate extremes (range limits) is crucial for understanding the role of adaptation in determining distributions and for informing conservation actions under rapidly changing environments. I examined Mimulus laciniatus (cut-leaved monkeyflower), an endemic annual plant, distributed between 900 – 3300 m across the western slope of the Sierra Nevada and asked the following questions: What are the population responses at elevation range limits?  Is climate adaptation present across the species range and how might it affect distributional responses under continued warming climates? I sampled M. laciniatuswithin a large portion of its range spanning its full climate gradient (chaparral to sub-alpine zones).  Seed families were collected along three elevation-based transects (one in Yosemite National Park and two in the Sierra National Forest), each having 7-9 populations spaced at 200-400 m elevational intervals and plants were raised in greenhouses for several generations to reduce maternal effects. Experimental gardens were established near the high and low elevation limits and in a central elevation to estimate individual responses (starting from seed) at range limits and the extent of climate adaptation across the species range.

Results/Conclusions

There was clear, adaptive, elevational differentiation among populations.  The highest average fitness occurred at the highest elevation, consistent with a leading range edge under climate warming, whereas, the central and low gardens exhibit reduced fitness, consistent with a trailing range edge. Signals of climate adaptation were strongest at the two edge gardens, where individual fitness correlated significantly with elevation of origin.  There was no signal of climate adaptation in the central garden.  The top-performing populations at both climate extremes were populations occurring at that same climate limit. However, the “home” population in each garden was outperformed by other populations from similar elevations, suggesting adaptive differences in genetic variation and/or genetic mechanisms for climate adaptation between areas/regions.  Additionally, there were clear fitness tradeoffs between the highest and lowest populations in the high and low gardens.  In this system climate adaptation is present and apparently important for individual performance, yet future shifts in the spatial extent of the climate niche will likely change the importance of past climate adaptation in diferent parts of the range. Managing for robust climate-based redundancy among populations and high gene flow is critical for maximizing future distributions of restricted species under rapidly changing climates.