By Alex Kirby
BBC News Online environment correspondent
A familiar feature of Christmas in many British families, the Brazil nut, faces a threat from too-intensive harvesting.
Too many nuts now could mean none soon (Image: Claudia Baier)
A report in the journal Science says populations of trees picked heavily over many years produce very few young trees, threatening the species' future.
The authors say it may be necessary to control the numbers of animals which rely on the nuts, including the agouti.
But they say people should not stop eating Brazil nuts, because the trade helps to protect the Amazon rainforest.
Brazil nuts, strictly, are not nuts at all, but seeds, up to 25 of which are packed tightly inside a hard woody fruit the size of a large grapefruit.
They are the only seed crop traded internationally which have to be collected from the wild. Attempts to grow Brazils in artificial plantations have failed, because the trees produce fruit only in the forest.
No time to breed
They can reach 50 metres (164 feet) in height and 16.5 m (54 ft) in circumference at breast height.
The authors of the Science report studied 23 Brazil nut tree populations in Amazonia, in Brazil, Bolivia and Peru.
They found populations extensively harvested over some decades were dominated by older trees, with very few younger trees in evidence, suggesting the normal regeneration cycle had been disrupted.
Each fruit holds up to 25 nuts (Image: Carlos Peres)
When they ran computer models to predict population trends for the next two centuries, the patterns they found were "highly consistent" with their observations.
The report's main author is Dr Carlos Peres, a tropical conservation biologist in the School of Environmental Sciences at the University of East Anglia, UK.
He says: "The clear message is that current Brazil nut harvesting practices at many Amazonian forest sites are not sustainable in the long term."
He believes his team's observations and simulations show the dwindling numbers of older trees in persistently over-exploited areas are not being adequately replaced by young trees.
The trade is significant, with more than 45,000 tonnes collected annually in Brazil alone, worth more than $33m.
The team recommends close monitoring and careful management of the trees to avoid an eventual collapse of the industry.
It suggests a possible annual nut quota, a replanting scheme using seedlings grown in nurseries, and a rotation system to leave areas unharvested from time to time.
Perhaps controversially, it says it may be necessary to control seed-eating animals and large herbivores, to give the seeds the best chance of germinating.
Possible candidates include tapirs, brocket deer and agoutis, large rodents which play a crucial role in sustaining the Brazil nut trees.
Dr Peres told BBC News Online: "Most of the fruit lie where they fall on the forest floor until they're attacked by termites or fungi.
Agouti: Indispensable nut devourer (Image: Honolulu Zoo)
"But agoutis will gnaw them open - it can take them the best part of an hour - and then bury the seeds to eat later.
"One agouti can scatter and bury a lot of seeds, and eventually it will eat most of them, but not all. So they're very important seed dispersers - but also seed predators.
"If you shoot a few agoutis, you'll get more seedlings. We've got another study, using agoutis with radio collars, to learn more about their role.
"What we don't want is for people to conclude that they should stop eating Brazil nuts.
"They provide an income for millions of people in Amazonia, and stopping the trade would undermine a relatively benign use of large tracts of the forest.
"That would leave people without any income and open the forest up for more damaging uses like ranching and logging."