Nevada bookmakers took home $19.7 million in profits after this weekend’s Super Bowl, thanks to sports bettors. Maybe the unlucky gamblers on the other ends of those bets should have done some more hedging. Making a smaller bet against yourself to minimize your potential losses is a good strategy—and even brainless plants can do it.
Plants gamble not by laying down $20 on the Broncos, but by spending their resources to make seeds. If enough seeds land safely in the soil, sprout with the next rainfall, and grow into new plants, the bet pays off. But if the seeds or seedlings are, say, digested by a rat? The gambling plant goes broke.
In the desert, explains University of Arizona ecologist Jennifer Gremer, the way plants hedge their bets is by keeping some seeds dormant in the soil for an extra year, instead of letting them all sprout right away. In the short term, these plants sacrifice some of their reproductive payoff. But in an environment that’s changeable and hostile, setting some seeds aside is insurance for the long term.
Gremer and her colleague Lawrence Venable studied bet hedging in a dozen different plant species growing in the Sonora Desert. All the plants are annuals, meaning they live for less than a year: they sprout in the fall and winter, flower in the spring, and die with the start of the baking-hot summer.
The plants in the study were on 72 plots of land that scientists have been monitoring for over 30 years. This meant there were piles of data available on how well different plant species survive and how many seeds they tend to produce each year. The researchers could also deduce how many of those seeds sprout right away when summer ends, and how many stay hunkered down in the soil to wait for the following fall.
Sifting through these numbers, the researchers saw that plants with hardier seeds—those likelier to survive an extra year waiting in the soil—opted to invest more seeds this way. But plants with less hardy seeds let more of them sprout right away. In other words, the riskier the hedge was, the less they spent on it.
So far, a smart gamble. Then Gremer and Venable looked at other factors: How well do seedlings survive? How much competition do the plants usually experience from their neighbors? How variable is the weather? “They must hedge their bets against variability in both [living and nonliving] conditions,” Gremer says.
The scientists built a mathematical model that incorporated all these factors. Then they calculated, for each of the 12 species, the ideal percentage of seeds that should sprout right away. When they compared the model to what plants actually did, they found that it was a close match: each species hedged its bet by an amount that made sense for its own circumstances. The plant species were acting as if they’d done the math too.
Gremer explains that they described each plant’s strategy as an average fraction of sprouted seeds per year. “However, these fractions do vary a bit across years,” she says. Some of that variation comes from factors the plant can’t control (rain, temperature, rats). But a plant’s genes also affect this fraction, which is why different species can have different strategies—programming their seeds with a 60% likelihood of sprouting right away, for example, or 30%. And if the strategy needs to be tweaked in a given year, mother plants can even add their own influence in “fascinating” ways, Gremer says, such as adjusting the coat that surrounds the seeds.
Smart bet hedging lets these plants cope with changes in their habitat. It may already be protecting them from some of the effects of climate change, the authors say. Because the plants stick to a good gambling strategy, they can survive even in an environment more hostile than a 43-to-8 blowout.
Image: The team’s field site at the University of Arizona’s Desert Laboratory at Tumamoc Hill in Tucson, Arizona (photo by Kathy Gerst).
Gremer JR, & Venable DL (2014). Bet hedging in desert winter annual plants: optimal germination strategies in a variable environment. Ecology letters, 17 (3), 380-7 PMID: 24393387