Hundreds of years ago, the American bison roamed freely across the widest natural range of any herbivore on the continent -- a vast habitat extending from northern Mexico across the United States to Alaska, and Canada.

Today, genetically pure buffalo -- as the species is also known -- subsist within four preserves in the United States, but also on a ranch in Mexico's northern Janos desert where the species was introduced a decade ago.

Weighing up to a tonne and as tall as 1.7 meters (5.6 feet) from the humps on their backs to their hooves, almost 200 bison live in a state of semi-captivity on the El Uno ranch, some 230 kilometers (150 miles) from the northern border city of Juarez.

Characterized by their distinctive humps, horns and dark coffee-colored fur, the herd, which is the only group of wild bison in the country, has grown from 23 individuals brought to the region from the US in 2009 to 140 adults and 44 calves born this year.

The goal, explained El Uno official Pedro Calderon Rodriguez, is to preserve the pure lineage of the species.

Having undergone a drastic decline in the 19th century, the American bison has partially recovered, according to the International Union for Conservation of Nature (IUCN) which today lists the species as "near-threatened" because of its dependence on conservation projects to survive in the wild.

Canada, the US and Mexico all list bison as both wildlife and livestock, according to the IUCN, and while commercial production has meant a significant rebound in numbers, such population growth does not contribute to the species' conservation in the wild.

According to Calderon Rodriguez, the fence on the US-Mexico border can inhibit the bisons' and other species' ability to roam freely in their historical habitat, one of the reasons they're considered at risk in Mexico.

El Uno is part of a network working to conserve bison which includes the four preserves in the US and is part of the Biosfera de Janos, which encompasses the grasslands and other species such as prairie dogs and certain types of birds that migrate across the continent each year.

Grazing in the brown desert under the blue sky, the herd has also contributed to the growth of pastures in the area. Among the most diverse and important ecosystems on the planet, the grasslands are also one of the most affected by human action.

"The grasslands need animals like bison to trample over them, they need their manure, their urine, their saliva, their activity. The disturbances created by the bison herds, the grazing -- it tills and fertilizes the land," said Calderon Rodriguez.

At birth, bison calves weigh some 20 kilograms (44 pounds). Adult females can weigh between 400 and 600 kilos and the males average between 600 and 900 kilos, though there are examples of individuals that surpassed a tonne, or more than 2,200 pounds. According to the Mexican environment ministry, various indigenous groups venerated the species for this large size and impressive presence.

They typically live 15 to 20 years.

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