“Lagging adaptation to climate change supersedes local adaptation to herbivory in an annual monkeyflower”
Nicholas J. Kooyers, Jack M. Colicchio, Anna B. Greenlee, Erin Patterson, Neal T. Handloser, and Benjamin K. Blackman (Special Feature on Maladaptation)
Adaptation lag! Elevation-matched California monkeyflowers populations have higher fitness in Oregon sites than Oregon monkeyflowers
Lagging behind: Monkeyflowers cannot keep pace with current climate change
In the western US, climate change is causing the timing of growing seasons to shift. Spring is starting earlier on average, and consequently populations are experiencing seasonal conditions more characteristic of habitats to their south. In addition, precipitation is falling more often as rain rather than as snow, which deprives plant populations in mountainous areas of the snowmelt that serves as a dependable water source into summer.
A team led by Nicholas Kooyers and Benjamin Blackman examined how well annual populations of the common monkeyflower, a widespread species in the western US, are adapting to cope with these environmental shifts. At both low and high elevation field sites, they find that local Oregon populations are less well adapted to their current environment than populations sampled from >500 miles south in California at equivalent elevations. Thus, their results are consistent with what is known as an adaptation lag, a pattern where local populations fare worse in current climates than populations whose historical climates better match current climates.
What gave the California populations the advantage? The team found that they possessed several trait differences that fostered success in earlier growing seasons through timing reproduction to occur at more favorable times at each site. Oregon plants did have at least one home court advantage though. They sustained less damage from herbivores than the more heavily chewed on California plants, suggesting future years at these sites may also feature mismatches between Oregon herbivore communities and California-like growing seasons.
Although they did not thrive as well as the California populations, the Oregon populations are not yet facing a truly dire threat from the changing climate. They still produced sufficient seed to sustain positive population growth rates even in the drastically early year when the team completed their study. Nonetheless, that the team observed patterns consistent with an adaptation lag even in this species, which has all the qualities like a short generation time and vast reservoirs of genetic variation that should favor rapid adaptation to changing climates, is dismaying for the many species that lack such evolutionary advantages.
While native populations are often adapted to historical biotic and abiotic conditions at their home site, populations from other locations in the range may be better adapted to current conditions due to changing climates or extreme conditions in a single year. We examine whether local populations of a widespread species maintain a relative advantage over distant populations that have evolved at sites better matching the current climate. Specifically, we grew lines derived from low and high elevation annual populations in California and Oregon of the common monkeyflower (Erythranthe guttata), and conducted phenotypic selection analyses in low and high elevation common gardens in Oregon to examine relative fitness and the traits mediating relative fitness. Californian low elevation populations have the highest relative fitness in the low elevation site and Californian high elevation populations have the highest relative fitness in the high elevation site. Relative fitness differences are mediated by selection for properly timed transitions to flowering with selection favoring more rapid growth rates at the low elevation site and greater vegetative biomass prior to flowering at the high elevation site. Fitness advantages for Californian plants occur despite incurring higher herbivory at both sites than the native Oregonian plants. Our findings suggest a lag in adaptation causes maladaptation in extreme years that may be more prevalent in future climates, but local populations still have high growth rates and thus are not yet threatened.