The ancestor of your stir-fry might be growing closer than you think. Wild field mustard, or Brassica rapa, is an unassuming plant that reaches a couple of feet tall and produces clusters of small, yellow flowers perched atop spindly green stalks. It is the nearest wild relative to such produce aisle hits as turnips, napa cabbage, and bok choy. Outside of farm fields, feral forms of the plant are found worldwide thanks to human introductions, and these free-growing B. rapa varieties are edible, too. The plant has likely been grown, bred, and eaten by people for thousands of years.
But scientists have noticed some recent changes in B. rapa populations unrelated to intentional human breeding. Two studies, one from 2007 and another from 2018, compared field mustard plants grown from seeds collected and stored during different years in California. The researchers found that seeds produced before and after periods of major drought in the region developed into plants with notably different traits. Among other changes, seeds collected after droughts tended to grow into plants that bloomed earlier.
Earlier flowering is an “escape” strategy for plants, as Steven Franks, a plant ecologist at Fordham University and one of the researchers involved in the field mustard studies, told Earther. Drought selected for plants with earlier flowering times because those were the ones able to reproduce most successfully in dryer years.
By studying the changes in B. rapa, Franks and his co-researchers had documented evolution, likely in response to human-caused climate change, in real-time. Climate change is increasing the frequency and intensity of drought across the American West; a study from earlier this year attributed 42% of the West’s current megadrought to anthropogenic climate change.
Franks also attributed the field mustard findings to luck. He said it was “fortunate just that we had those seeds available.” The old, pre-drought seeds had been collected by researchers with an uncanny amount of foresight and stored in a seed bank.
There are different types of seed banks out there. Some, like Project Baseline, which grew out of those B. rapa studies, are created with a specific research intent. Others are meant to capture a wide diversity of rare or ecologically important wild plants, which are used for biological conservation projects. Then, there are seed banks that attract a wide amount of media attention, like the Svalbard Global Seed Vault, which aim to bolster the resilience of our future food supply by storing backups of crop seeds and their close relatives.
Current conditions on Earth — the shifting climate, invasive species, pollution, habitat destruction — make every type of seed bank invaluable. We’re losing plant species 500 times faster than the historical extinction rate, according to one 2019 study. Agriculture is becoming more difficult in many regions because of factors like extreme weather, soil depletion, and disease.
Seed banks do more than just catalogue. All the stored genetic information they contain can be used to create better crops to feed more people or to bring a plant back to its native range and restore an ecosystem. But those same conditions also pose challenges for seed banks. How can a strategy intrinsically based on pausing the biological clock make sure that it’s keeping up with the future? If climate change and other factors are pushing plants to evolve faster, do seed banks need to evolve, too?
Wild seeds stored in freezers for restoration purposes don’t undergo the same evolutionary process as wild seeds allowed to grow into plants on the ground, as was evident in the studies of B. rapa. The seeds that experienced drought directly changed. Their counterparts kept in a facility didn’t.
“Seed banking efforts are huge, but the best case for conservation is to keep things where they are,” said Elizabeth Leger, a plant biologist at the University of Nevada, in a phone call with Earther. Banking seeds means those sequestered plants are both literally frozen and also figuratively frozen in time. They “miss out on the opportunity to evolve in response to temperature, or invasive plants, or fires,” Leger said. “It has pluses, in that you have saved something. But also, minuses in that that thing is no longer a wild plant, and it can’t do the things that populations do to evolve in response to change.”
Leger has seen this sort of change firsthand in her own research. In a 2017 study, she studied how native plants in the arid Great Basin were shifting to compete with cheat grass, an invasive species. The native plants were doing energy-intensive things like producing more seeds, or growing their roots faster, where cheat grass was present. Cheat grass was forcing the native plants to act more like an invasive species by being so dominant itself. If the native plants wanted to persist, they had to try to beat cheat grass at its own game. Native seeds stored away from cheat grass would not have developed like those exposed to the wild environment.
That’s not an argument to give up on seed banking, according to Leger or any of the experts I spoke with. Rather, it’s a case for doing even more of it — just alongside conservation of natural habitats. The more seeds that are collected from more locations and with more frequency, the better the chance you have of capturing a snapshot of useful adaptation. “It’s only valuable to the degree that we keep it up,” said Leger.
But just because some plants theoretically adapt in response to human-caused changes doesn’t mean all species or populations can, or that evolution is enough to stave off disaster.
“One of the things that we’re very concerned about with climate change is the [rate of] adaptability of species,” said Julie Etterson, an evolutionary biologist at the University of Minnesota. Etterson works with Franks as part of Project Baseline, the seed bank that emerged from the field mustard studies and is dedicated to supporting research into contemporary plant evolution in the face of rapid environmental change.
Some of Etterson’s earliest academic research was focused on the question of whether or not plants could broadly evolve fast enough to keep up with the climate. Through that work, “I just came to terms with the fact that I think, generally, the answer is no,” Etterson said. Even where plants did demonstrate adaptive traits, the overall loss of a species’ numbers often left the community more vulnerable to collapse. In her view, the future of conservation through seed banks isn’t just one where species are brought back to where they once were, but also one where people actively work to relocate species into suitable environments as those ideal habitats shift. “You know, assisted migration, moving species north with [rising temperatures],” Etterson offered as an example.
And the need for this type of work is apparent. Plants are already being pushed out by climate change. In those two wild field mustard studies, what started as a hopeful sign of plant adaptation and resilience became a portrait of plant limits. The first 2007 study compared seeds collected in two places in California in 1994 and 2004 (pre and post-drought). The researchers determined that post-drought B. rapa bloomed sooner, and that those post-drought plants were doing OK. In the 2018 follow-up, Franks and his co-researchers added in two later generations of seeds collected in 2011 and 2014 from the same locations. They also observed earlier blooming among drought-exposed plants.
But in that second study, the scientists noted something else: The plants grown from more recently stored seeds had lost “fitness.” They produced fewer seeds of their own and seemed less well-equipped to thrive. They had some adaptations to dry periods, but in other ways, they were worse off for having had to change at all.
The researchers hypothesised that the level of ongoing drought had exceeded the limit of what the plants could manage. In the meantime, that local wild field mustard community had lost genetic diversity as its numbers shrunk, making the whole population less likely to be able to evolve in response to the next challenge. Eventually, that lineage of B. rapa could peter out entirely. The plant isn’t native to California, so it’s not a massive biological tragedy — but losses like this are a major concern for crop-focused seed banks.
Food-minded seed banks race to collect unique, heirloom seed varieties or crop wild relatives (like B. rapa) from populations spread all over the world before they disappear. They aim to have enough genetic diversity preserved that more resilient crops could be bred, or even directly genetically modified, to manage the stress of continuing environmental shifts. But climate change is speeding up and seed collecting is slowing down, said Hannes Dempewolf, a senior scientist at Crop Trust, the UN-funded nonprofit that helps manage the Svalbard Global Seed Vault.
“I think that we’re far away from reaching [our] goals,” he said. “I think we’re losing a lot of diversity every day from farmer’s fields, still. And there’s very little being done to actually halt that loss.” While it’s hard to get collecting permits, he said, the more difficult part is funding and resources.
Leger pointed out that a single collection trip to get wild seeds from just one population can cost $US5,000 ($6,941) to $US10,000 ($13,882). “It’s really a lot,” she said. The process sometimes involves trekking far distances, and then the timing kicks into play: just a week or two off, and there might not be any seeds for collection at all.
After the seeds are collected, the costs keep accruing — and it’s not just the cost of keeping things cold. Dempewolf explained that the most expensive part of seed-banking is often “growing the seeds out” once they get old. Seeds lose viability with time, regardless of how frozen they are, so everything held in a well-managed seed bank is closely monitored to see how gracefully it’s ageing.
If samples from a collection of seeds start to fail germination tests, the seed bank will plant the whole collection and glean new, fresh seeds from that. This process takes a lot of land and a lot of work. The plants are isolated as they grow, to try to preserve the genetic integrity of the original collection. Seed banks even have to provide their own pollinators to get the next generation going. Nonetheless, it’s an imperfect process. Some of the initial genetic diversity is inevitably lost. Collections can only be grown out a few times before inbreeding starts to get in the way. And the more seeds you bank, the more plants you have to be prepared to grow.
Both the expense and the imperfection prove an important point: seeds were not meant to stay frozen in time. Seed banking is a useful tool, but it cannot be the only tool for the future of food or biodiversity.
We need more seed banks, and seed banks need more resources, but we also need land preserved in its natural state. And, more than anything else, if we want to guarantee a verdant, viable planet full of diverse plant communities, we need to do our best to mitigate climate change. Humans need to slow things down, so that everything else can catch up.
This story is part of Covering Climate Now’s ‘Food & Water’ joint coverage week.