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Many animals are herbivores, which means they get all their nutrients from eating plants. American pikas are cute rabbit relatives that eat plants in the mountains. But alpine winters are harsh, so pikas spend their entire summer gathering and storing plants to eat under the winter snow. Just like people, pikas in Colorado have a favorite food: a plant called alpine avens. This plant species is a special pika snack because it contains natural preservatives called phenolics, which keep the food fresh all winter. We studied how climate change is affecting this important feature of the pika’s favorite meal. Alpine avens contains more phenolics now than it did 30 years ago, so they preserve better in storage. But there is a catch: these preservatives can be hard to digest. Studies like this help us start to understand the many complicated ways that climate change affects herbivores like pikas. 

Climate change in mountain regions has exposed high-elevation species to rapidly changing temperatures. Although climate exposure can be reduced in certain microclimates, the quality of microclimatic refugia might also degrade with climate change. The American pika ( Ochotona princeps ) often inhabits high elevations, and is considered climate-sensitive due to its narrow thermal tolerance and recent extirpations in some warmer portions of its range. Pikas behaviorally thermoregulate by taking refuge in the subsurface microclimates found in taluses and other rocky habitats, where daily thermal fluctuations are attenuated and somewhat decoupled from free-air temperatures. Changes in microclimate might reduce the efficacy of this behavioral thermoregulation. This study compares recent (2009–2021) subsurface temperatures at a long-term pika study site with a rare instance of historical (1963–1964) data from the same location. We also place historical and recent microclimates in context using long-term data on free-air temperatures from the same area. Recent free-air temperatures were often warmer than historical records, and subsurface temperatures exhibited even stronger warming between periods. Temperatures measured in the talus were often dramatically warmer in recent records, especially at the deeper of two subsurface sensor placements in this study. Winter months showed the greatest changes in both talus and free-air temperatures. Differences between historical and recent microclimates were not explained by the precise placement of sensors, as recent temperatures were similar across a wide variety of subsurface placements, and temporal changes in free-air temperatures at the historical study site were also reflected in data from nearby weather stations. Together, these results suggest that subsurface microclimates important for pika thermoregulation have changed over the past few decades, perhaps even faster than observed changes in free-air temperatures. The generality of these results and their potential ramifications for ecosystem processes and services should be explored. 

Abstract Climate change is increasing temperature, decreasing precipitation, and increasing atmospheric CO₂concentrations in many ecosystems. As atmospheric carbon rises, plants may increase carbon‐based defenses, such as phenolics, thereby potentially affecting food quality, foraging habits, and habitat suitability for mammalian herbivores. In alpine habitats, the American pika (Ochotona princeps) is a model species for studying effects of changing plant chemistry on mammals. To survive between growing seasons, pikas cache “haypiles” of plants rich in phenolics. Although they are toxic to pikas, phenolic compounds facilitate retention of plant biomass and nutrition during storage, and they degrade over time. Alpine avens (Geum rossii, Rosales: Rosaceae) is a high‐phenolic plant species that comprises up to 75% of pika haypiles in Colorado. Here, we tested the hypothesis that contemporary climate change has affected the nutritional value of alpine avens to pikas in the last 30 years. Specifically, we compared phenolic activity, nutritional quality, and overwinter preservation of plants collected at Niwot Ridge, Colorado (USA), in 1992 to those collected between 2010 and 2018, spanning nearly three decades of climate change. Phenolic activity increased in alpine avens since 1992, while fiber and nitrogen content decreased. Importantly, overwinter preservation of plant biomass also increased, particularly on windblown slopes without long‐lasting snow cover. Previous studies indicate that pikas at this site still depend on alpine avens for their winter food caches. Higher phenolic content in alpine avens could therefore enhance the preservation of haypiles over winter; however, if pikas must further delay consuming these plants to avoid toxicity or invest extra energy in detoxification, then the nutritional gains from enhanced preservation may not be beneficial. This study provides important insights into how climate‐driven changes in plant chemistry will affect mammalian herbivores in the future.