American Society of Naturalists

Shifting Precipitation Regimes Influence Optimal Germination Strategies and Population Dynamics in Bet-hedging Desert Annuals

William S. Cuello, Sebastian J. Schreiber, Jennifer R. Gremer, Pete C. Trimmer, D. Lawrence Venable, Andrew Sih: Read the article

Water is essential for life. Yet, this precious resource is becoming less reliable in response to global warming, jeopardizing the survival of all the species on Earth. Desert plants are particularly vulnerable to these changes because they already experience extreme heat and drought. At the same time, their ability to survive in such extreme conditions could also enable such plants to be more resilient to changing weather patterns. A recent paper by Cuello et al. highlights the importance of current strategies for resilience found in desert plants in determining their future survival in the face of climate change.

In highly fluctuating environments, like the desert, it is hard to predict what future conditions will look like. Therefore, it is generally best not to “place all your eggs in one basket,” since you are never sure what the future will bring. This strategy of spreading risk is called bet-hedging. Many desert plants have adopted this strategy in the form of delayed germination. To avoid the risk of one bad year killing all their newly sprouted seedlings, they will save some of their seeds by keeping them dormant for one or more years. The idea is that, given the highly fluctuating climate, a good year will soon come, and the dormant seeds can then sprout and thrive, offsetting the losses of bad years. The dormant seeds essentially become insurance against bad years.

While desert plants have learned how to succeed when facing an uncertain future, current climate predictions suggest rain will continue to become less abundant and even less reliable. Will delayed germination be enough to help desert plants survive these unprecedented conditions?

To answer this question, Cuello et al. used 30 years of data on 10 species of Sonoran Desert annual plants to develop mathematical models that showed how the abundance of each species changes under historical versus predicted future rainfall patterns. Unlike previous studies, they also examined the potential benefits of evolving an ideal germination strategy in response to changing conditions. More specifically, the models allowed each species to instantly change (1) the proportion of seeds that remained dormant and (2) the length of time they spent dormant. In other words, species were allowed to change how they hedged their bets in a way that maximized their future survival based on the new rainfall pattern. In doing so, they provided an optimistic prediction on how well species could potentially fare in these scenarios.

Overall, Cuello et al. found that all plants exhibited lower abundance under the new predicted rainfall patterns. However, contrary to what may be expected, the average quantity of rain had greater survival consequences than its reliability (or predictability). Therefore, species appear to be more resilient to changes in the consistency of rainfall than to an overall decrease in rainfall, which is more likely to lead to lower population abundance and extinction. This is probably due to the fact that these species are already adapted to a highly variable environment, but still require a certain amount of water to survive. Indeed, bet hedging is a way “to mitigate the risk of increased variance rather than to buffer against overall decreased rainfall.” In some species, evolving an ideal germination strategy did grant additional resilience to new climates, but it provided only a small advantage.

Interestingly, the trait that often conferred the most resilience was a higher survival rate of dormant seeds. In other words, the more resilient species were often those with more viable seeds in their seed bank. After all, the hedge is only as good as the safety it brings. Therefore, dormant seed survivorship could serve as an important indicator of population resilience to climate change. However, Cuello et al. uncovered another surprisingly successful strategy: capitalizing on wet years instead of maximizing water use efficiency. The authors conclude that species can show resilience to unpredictable rainfall by employing alternative strategies so long as enough of their invested (or dormant) seeds in the seed bank survive.

Alas, though plants have options that increase resilience, the future of Sonoran Desert annuals is dim if climate change continues. This study predicts that a pattern of less and more inconsistent rain will decimate population abundance. Even the evolution of ideal germination strategies seems to provide only a small buffer against moderate shifts in rainfall. Unfortunately, this means that, on its own, delayed germination is not a sufficiently resilient strategy to save populations, especially those experiencing more drastic climate shifts.

Xochitl Ortiz-Ross is a PhD candidate in Ecology and Evolutionary Biology at UCLA, where she investigates how early life experiences influence behavior and life history strategies in wild mammals. Her research integrates behavioral ecology with conservation and animal welfare, emphasizing the cumulative impacts of early stressors. Passionate about fieldwork, she values the field research community fostered at field stations and advocates for inclusive and safe field research practices. She is dedicated to making scientific research accessible, as she believes that discoveries gain value when they are shared, and that everyone deserves to feel included in scientific discourse. As a field ecologist, Xochitl loves observing animals in their natural environments and this is how she spends her free time—often documenting nature through photography. At home, she likes to crochet while listening to an audiobook.