American bison return to Cuatro Ciénegas after more than a century

In 2025, the American bison returned to an area of Coahuila in northern Mexico after being absent for more than a century. Three animals arrived first, followed by a further 44 brought to the El Santuario reserve in Cuatro Ciénegas. Within a few months, monitoring began to record grassland germination, increased insect activity and the presence of pumas, bobcats and peccaries. These are early indications that an environment is being restored as this large herbivore resumes ecological roles that had been lost.

Why did bison disappear from this part of Mexico?

Bison once ranged across vast areas of northern Mexico and the rest of North America. In Coahuila, hunting and changes caused by human settlement sharply reduced their numbers until the species vanished locally. Historical records point to an absence of around 160 years in the Cuatro Ciénegas region.

The reintroduction took place at the El Santuario reserve, a protected area of nearly 4,000 hectares in the Sierra de Menchaca. The 2025 operation was carried out in stages to establish a new herd able to remain in the area.

• Three animals were initially released in 2025;
• A further 44 arrived at the end of November that year;
• The larger group comprised 38 females and six males;
• Some of the animals came from the Janos Biosphere Reserve;
• The aim is to restore ecological processes that disappeared along with the bison.

How can such a large animal alter its surroundings simply by grazing?

Bison are regarded as ecosystem engineers because their effects extend well beyond eating grass. As they move, their hooves disturb patches of ground; by grazing, they remove dry vegetation and change the height of grasses; and their dung returns nutrients to the soil. Their fur can also carry seeds and provide nesting material for some birds.

What began happening just months after the reintroduction?

The reserve's monitoring programme detected changes quickly. According to those leading the project, grassland areas began showing fresh germination, while dung attracted dung beetles. These insects bury organic matter, redistribute nutrients and help form small channels in the ground.

A further outcome drew even greater attention. Cameras in the reserve again recorded animals that had not appeared there during previous years of monitoring, including pumas, bobcats and collared peccaries. Project leaders also reported birds collecting bison fur.

• Grasslands began to show signs of regeneration;
• Beetles started using dung left by the herd;
• Ground disturbance may support water infiltration;
• Cameras again recorded large and medium-sized mammals;
• Eight calves had already been born in the reserve by 2026.

Why does the return of pumas and bobcats not mean everything has already been restored?

The appearance of these animals is an encouraging sign, but it is still too early to claim that bison alone have rebuilt the entire Cuatro Ciénegas ecosystem. Predators and peccaries have extensive home ranges and may move through the area for many reasons. Demonstrating lasting ecological recovery will require years of monitoring vegetation, soil, water, insects, herbivores and predators.

• The project has had less than one full year of monitoring since the large reintroduction;
• Animals captured by cameras may simply be passing through the reserve;
• Vegetation changes must be compared across several seasons;
• Droughts, rainfall and fires also affect the environmental response;
• Full ecosystem recovery can take many years.

The aim is to let the ecosystem begin working for itself again

The significance of the reintroduction lies precisely in restoring processes that cannot be recreated merely by planting trees or scattering seeds. An American bison grazes, tramples, digs, carries material, fertilises the ground and continuously reshapes the landscape. When dozens of them repeat these behaviours each day, they create conditions that affect everything from soil microorganisms to insects, birds and large mammals.

The results seen at El Santuario are still the beginning of this experiment, but the herd has grown from 47 adults to include new calves and has started changing land that went more than a century without the species. The most promising aspect is not the idea that nature has fully recovered within a few months, but the recognition that returning one of the animals that helped create this environment has allowed natural processes that were absent to begin reappearing.