The last herd of bison is greening the American prairies.

The Lakota Indians counted their bison by the number of days it took for their herd to pass. In 1871, Colonel Richard H. Dodd came across one that stretched 24 kilometers across the Arkansas River and another 40 kilometers across. At the beginning of the last century, only a few hundred remained. After a century of protection and recovery of the American bison ( Bison bison ), there are 400,000 of them between the United States and southern Canada. That seems like a lot, but biologists consider them a functionally extinct species : 96% survive on private ranches, in herds of no more than 200 animals behind barbed-wire fences. Of the rest, only a true herd remains, still roaming freely. Now, a study of the last bison published in Science shows how the prairie greens with their presence. Like African elephants, they are true engineers or architects. But as with bridges or human buildings, without these animals, their landscapes fall apart.

The last herd of American bison that remain as ecosystem engineers, about 3,500 strong, is found in Yellowstone. Located between northern Wyoming, southern Montana, and eastern Idaho, it was the first national park created in the world . It covers an area of ​​about 9,000 km², ten times larger than the largest Spanish national park, the Sierra Nevada. The penultimate specimens of the species were brought there at the beginning of the 20th century. Only here do they continue to migrate long distances, in an area of ​​about 900 km², grazing from northwest to southeast on the grass that emerges as the snow melts. In the 1960s, humans stopped intervening in their fate, neither controlling their population nor that of their predators. They went from a huge open-air zoo to becoming bison again. This has allowed scientists to analyze their impact on the landscape, proving that it is their presence, not their absence, that maintains the few remaining grasslands.

Since 2015, a group of scientists has been monitoring the migration of bison. They have mapped out around twenty plots across the area to study how they fare free from the appetite of an animal that requires 4,500 kg of dry matter per year (these are data from ranching, not from the animals in Yellowstone). "Our work suggests that we need to approach bison foraging differently," recalls Bill Hamilton, a professor at Washington and Lee University (United States) and co-author of the Science study. "By losing them, we lost a different way for large herbivores to navigate and use the landscape. It's very different from the classic principles of grasslands with livestock," he adds. Grazing by domesticated animals, such as cows, sheep, or goats, can deplete the land if there are too many. "Our results suggest that, after analyzing biomass production, soil organic matter, and nutrient cycling, there is no evidence of overgrazing," adds Hamilton.

By comparing the soils along the fenced bison paths with the land open to their appetites, the researchers found that the nitrogen cycle accelerates where they grazed. The summarized process begins with the regrowth of freshly grazed grass. They found that it grows at the same rate as the grass in the fenced plots. They saw that the passage of animals strengthens the soil microbiome, which leads to more bacteria to oxidize ammonium, resulting in a greater amount of nitrogen, the main natural fertilizer. The more fertilized soil multiplied the yield per area: the plant protein yielded by the soil doubled, and even more so in the river valley grasses. “Bison accelerate the nitrogen cycle. Despite intensive use, the plants regenerate at the same rate, but they become 150% more nutritious,” Hamilton summarizes.

For the authors of this research, the Bison bison species as such would have become extinct, despite the 400,000 bison that existed, were it not for the Yellowstone herd: "The challenge we face is finding places large enough for the American bison to return and move again in large numbers," Hamilton concludes.

Without elephants there is no ebony

Thousands of miles from Yellowstone, in the jungles of Cameroon, another ecosystem engineer is disappearing, and with it, its forest. The African forest elephant ( Loxodonta cyclotis ) has been decimated for decades by poachers seeking its ivory. A recently published paper in Science Advances shows how its absence is complicating the survival of the African ebony tree ( Diospyros crassiflora ), whose jet-black wood is often seen in the form of various figurines at fairs and markets. In the last 30 years, the number of this pachyderm has been reduced by 86%, and it is now critically endangered, according to the International Union for Conservation of Nature's Red List . Their role in the forest is multiple: they recycle and distribute large quantities of nutrients, clear the understory, and disperse seeds. What happens if they're not there to do their job?

“Ebony still occupies a vast area in the forests of the Congo Basin, but we observed that the relative abundance of young trees has decreased by almost 70% in areas where the forest elephant has been exterminated,” Vincent Deblauwe, a researcher at the University of California, Los Angeles and first author of this work, said in an email. To see what was happening, Deblauwe and his colleagues selected four forests in the Cameroonian rainforest, two of which were already free of elephants, while the others had maintained an acceptable number of trees under protection. They inventoried the ebony trees in four plots of 400 hectares each and analyzed how they regenerated.

Where elephants still feed on ebony, which produces one of their favorite fruits, 47.2% of the trees were saplings. But where they have already been removed, only 15.1% were saplings. In other words, replacement by the next generation is being short-circuited. Furthermore, they found that kinship (measured by genetic distance) was 10.5 times greater in sites where no pachyderms remain. What they discovered is that, without elephants, this tree loses its best ally in seed dispersal. This explains both the difference in the number of saplings and the low genetic diversity. The ebony fruit is very large, as is the pod containing the seeds. This means that only elephants can swallow them whole.

The researchers believed that passage through the digestive tract facilitated seed germination. This, along with the fact that elephants could excrete seeds far from the mother tree, would explain what was happening. But they found no differences in germination success between the four forests. So the explanation must be somewhere else. And they found it in the pachyderms' dung. Once free of the pulp that could complicate germination, the seeds end up on the ground covered in manure. This has a double consequence: it provides them with basic nutrients, such as nitrogen, and protects them from rodents. To confirm this, they conducted a series of experiments. Seeds stripped of dung (those from the plots without elephants) were 8.5 times more likely to be eaten by the giant Gambian rat, a rodent that includes them in its diet.

“At the moment, adult ebony trees are not affected by the collapse of regeneration, but soon, the missing cohort of young trees will become a missing cohort of adult trees,” predicts Deblauwe, also a researcher at the International Institute of Tropical Agriculture in Cameroon. This means there will be less fruit and seed production, creating a double problem: the number of ebony trees will decline to the point of jeopardizing their survival, which in turn will make any attempt to recover the African forest elephant population fail because they no longer have ebony fruit to eat.