Washington - The circle of life is closing in on the king of the jungle.When Minnesota dentist Walter Palmer killed Cecil the lion, the Internet exploded with outrage. But scientists who have studied lions say the big cats have been in big trouble for years.They've watched the African lion population shrink by more than half since 1980 and dwindle even faster in East Africa, where lions used to be most abundant. They've seen trophy hunting like Palmer's - promoted as a way of raising cash to preserve wildlife populations - fail to live up to its promise. And even more importantly, they've seen lion habitat shrink and many beasts killed by local residents because of conflict with livestock and agriculture.When humans and lions clash, the king of the jungle usually loses."We should be very worried," said Oxford University lion researcher Hans Bauer, who is based in Ethiopia. "The numbers are clear. They are in dramatic decline."Numbers decreasingExperts estimate there were about 75 000 African lions in 1980; now there are between 20 000 and 32 000. Last year the United States Fish and Wildlife Service proposed placing African lions on the threatened-but-not-endangered list. On its red list of species in trouble, the International Union for the Conservation of Nature calls the lion "vulnerable”, one step away from endangered.The number of lions in East Africa dropped 59% between 1993 and 2014. Lion counts in West Africa fell 66% in the same time period; lions there "are on the brink of extinction, they are desperately rare," said famed Duke University conservation biologist Stuart Pimm.Only in the southern part of Africa are lions' numbers rising, slightly, because of efforts to protect them. And that's where Cecil was shot."The reason Cecil was becoming iconic was that it lived in a national park; it lived in protection," Pimm said. He said if hunters can lure out of the park and kill even Cecil - legendary in Zimbabwe, known for his majestic black mane - "it does not bode well" for other lions.That's why even though hunting isn't the main cause for the lions' decline, it splits the conservation community more than any other factor, Pimm said. Some see it as a way to provide money for conservation — just as duck hunters do in the United States — other see it as ineffectual, too costly and even unethical."Hunting in Africa is a complex issue," said Pimm. "Kenya does not allow hunting of any kind and Tanzania sets aside more of its land for hunting than it does for ecological parks."Hunting schemeA decade ago, top lion researcher Craig Packer and his team came up with a way to allow limited trophy hunting of lions and not hurt their dwindling numbers. If only certain, older, unattached lions, identifiable by nose colour, were hunted in specific ways, the practice could be sustainable. His team even published a guide on telling the age of a lion by nose colour to help trophy hunters go after lions in a sustainable way."It led to me being kicked out of Tanzania," said Packer, on the phone from a game preserve in South Africa. "In Africa it's a business. It's very cynical and very corrupt."Other scientists say his vocal anti-hunting advocacy got him in trouble.Bauer takes a more nuanced position on trophy hunting. Studies show hunters pay as much as $50 000 to governments and guides, with some of the money going to conservation while the rest boosts the economy. In theory, Bauer said, "there's a lot of habitat in Africa where lions exist because of trophy hunting." While it removes individual lions, "it preserves habitat."But Bauer added, "it's very often poorly managed as in the case of Cecil where a lion gets lured out of the habitat. This type of mismanagement happens much more than hits the news."Bauer and his Oxford colleague Claudio Sillero said as bad as trophy hunting can be - estimates of lions killed each year range from 600 to more than 1 000 - habitat loss and conflicts between lions and locals over livestock and agriculture are bigger problems.