What is it about?

At a time when climate change is making many areas of the planet hotter and drier, it’s sobering to think that deserts are relatively new biomes that have grown considerably over the past 30 million years. Widespread arid regions, like the deserts that today cover much of western North America, began to emerge only within the past 5 to 7 million years. Understanding how plants that invaded these harsh deserts biomes were able to survive could help predict how ecosystems will fare in a drier future. An intensive study of a group of plants that first invaded emerging deserts millions of years ago concludes that these pioneers — rock daisies — did not come unequipped to deal with heat, scorching sun and lack of water. They had developed adaptations to such stresses while living on dry, exposed rock outcroppings within older, more moist areas and even tropical forests, all of which made it easier for them to invade expanding arid areas. The study by University of California, Berkeley, researcher Isaac Lichter-Marck is the first to provide evidence to resolve a long-standing evolutionary debate: Did iconic desert plants, like the stately saguaro cacti, the flaming ocotillos and the Seussian agaves, adapt to arid conditions only after they invaded deserts? Or did they come preadapted to the stresses of desert living?

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Why is it important?

The question has relevance today, Lichter-Marck said, because accelerating aridity due to climate change is challenging plants to adapt much more quickly than they have in the past. Already, about one-fifth of Earth’s land surface is desert. If adaptation to arid conditions was only possible for plants that had already evolved to deal with such stresses, then many today may not be equipped with an adequate genetic tool kit to survive. “If you think about aridity only as a stimulus to plant evolution, then in many cases people could say these plants are survivors, they are adaptable, and they will be fine. They will take advantage of these new conditions, and they will thrive,” said Lichter-Marck, who is also a National Science Foundation postdoctoral research fellow at UCLA. But the history of rock daisies suggests that “when the deserts emerged, those plants that had the necessary preadaptations to take advantage of new conditions were the ones that thrived,” he said. “Adding more aridification to the system doesn’t necessarily mean more rapid adaptive evolution will occur. There’s a limited source of lineages that can take advantage of new levels of aridity, and that is important for understanding the effect of climate change on biodiversity.”


“This is a clear empirical demonstration of what was originally Axelrod’s hypothesis — of a desert plant group originating in dry microclimates prior to the widespread emergence of desert habitats,” said Lichter-Marck. “What this means is that the strategies for drought tolerance that are so characteristic of desert vegetation might not actually represent responses to the dry conditions found in deserts. Instead, they could be traits that evolved earlier in association with much older and more stable dry microclimates, such as rock outcrops in tropical settings.” Preadaptation may be the key to the success of many desert plants, including cacti, which are known to inhabit rock outcrops or grow as epiphytes in the canopies of trees within tropical areas, though these large lineages would require a much more extended analysis, he said. A graphic with the words "The Climate Crisis: Justice and Solutions" A new Berkeley News series will examine how the campus community is confronting the climate crisis. Rock daisies, many of which live in specialized habitats that make them vulnerable to extinction, highlight the importance of conserving seemingly niche species. “A lot of the rock daisies are very specialized and tend to be very narrow in their distribution and might be seen as less significant to the survival of the ecosystem as a whole. In evolutionary biology and in conservation biology, specialized organisms with narrow geographic ranges are often considered vulnerable lineages and have sometimes even been called evolutionary dead ends,” he said. “An important implication here is that a group of ecological specialists growing on scattered cliffs in tropical habitats started this major radiation in the desert. So, it actually shows that specialists are not just these vulnerable lineages on the edge of extinction. They might actually be really important sources for innovation in evolution.” Lichter-Marck is currently extending his study of plants that grow on rock outcrops to Hawai’i, where many rare endemic species live only on the sides of steep mountains. Instead of scaling precarious cliffsides to reach rare specimens, however, he hopes to use drones.

Isaac Lichter-Marck
University of California Los Angeles

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This page is a summary of: Edaphic specialization onto bare, rocky outcrops as a factor in the evolution of desert angiosperms, Proceedings of the National Academy of Sciences, January 2023, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2214729120.
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