"We've got a serious case of overgrazing by human herbivores, to put it in biological terms," said Martin Terry, professor of biology at Sul Ross State University in Alpine, Texas.

The US Government has allowed the use of peyote for strictly religious purposes only, in recognition of the fact that Native Americans have used the hallucinogenic cactus for centuries.

In the 1970s, the state of Texas licensed 27 peyote dealers.

There were many more before peyote was outlawed in 1967 -- now there are only three left.

Mauro Morales, one of the remaining dealers, refuses to take peyote as he recognises the precarious state of his profession.

"You have to make sure you don't have a problem with the law, you know?" he said.

The volume of peyote sold each year has dropped as supplies become harder to locate in the wild.

"There's still some peyote out there, but not like there used to be. It's getting kind of scary now," said Morales.

Several factors have contributed to the decline of the cactus,

As well as encroaching urbanisation, widespread 'root plowing' that scrapes away natural vegetation to replace it with grass for cattle grazing has devastated peyote's natural habitat.

Allied to this is the acquisition of wilderness by rich Texans to create hunting reserves, severely limiting the areas where peyote can be harvested.

One solution would be to cultivate the cactus -- but Federal law prohibits this.

Unless the government reforms the laws relating to peyote, dealers such as Morales will have to resort to more sustainable practices to preserve dwindling supplies.

His pickers cut the peyote in such a way that the plant can grow back.

"It comes back, but it grows slow," said Morales.

"It's hard to get enough medicine."