The Duke of Cambridge says he fears the African elephant will have disappeared from the wild due to poaching by the time Princess Charlotte turns 25.
William told the audience of campaigners and policymakers at Time For Change - an event organised by the conservation charity Tusk, of which he is a patron - that he was "not prepared to be part of a generation that lets these iconic species disappear from the wild".
He said: "When I was born, there were one million elephants roaming Africa.
"By the time my daughter Charlotte was born last year, the numbers of savannah elephants had crashed to just 350,000.
"And at the current pace of illegal poaching, when Charlotte turns 25 the African elephant will be gone from the wild."
The Duke also warned that rhinos face extinction in "our lifetimes" during the speech 800ft (244 metres) above London at The View from The Shard on Thursday.
He said it was now time to send an "unambiguous message" that it is no longer acceptable to buy or sell ivory or rhino horn.
"We have the chance to say that ivory is a symbol of destruction, not of luxury, and not something that anyone needs to buy or sell.
"We have the chance to say that rhino horn does not cure anything and does not need a legal market."
William, who was speaking ahead of a major global conference in Johannesburg this weekend - the Convention on International Trade in Endangered Species (Cites) - added: "Materialistic greed cannot be allowed to win against our moral duty to protect threatened species and vulnerable communities."
Tusk has raised more than £30 million for projects in Africa since it was established 26 years ago.
The royal conducted a tour of the charity's projects in Botswana in 2010 with Prince Harry, and this April visited Tusk-supported programmes in the north of Kenya.
The illegal wildlife trade is now recognised as the fourth largest transnational crime by value, after drugs, arms and human trafficking.
It is estimated to be worth up to 20 billion US dollars (£15 billion) each year and is responsible for decimating populations of elephant, rhino and many other lesser known species.
Currently, one African elephant is killed every 15 minutes - which means the species could become extinct within 10 years.
Environment Secretary Andrea Leadsom was also at the event, the day after she announced a UK ban on modern ivory sales.
The ban will cover the sale of items dated since 1947 but the trade in "worked" items made before then will still be allowed.
She told the Press Association: "What we're trying to do is to send a huge signal to the world that the illegal trade and poaching of elephants is not acceptable and that's what's going on today, every day.
"So it's a start on our manifesto commitment and we're trying to send a very strong message at what is a very important time."
She said it was "absolutely crucial" to have a plan to "discourage" those who would benefit from the "appalling, violent, illegal trafficking (that is) destroying communities and really importantly destroying these iconic beasts - it's a total global tragedy".
The MP said the ban on modern day ivory sales is the "first step" to tackle the "difficult, international global issue" of poaching.
But former environment secretary Owen Paterson, who was also present, said the plans did not go far enough and thought that a 100-year ban would be stronger.
The MP for North Shropshire said: "It's a start but it's not enough. If you could farm your way out of this and have huge rhinoceros farms and elephant farms... but this isn't chickens, this isn't beef. You could never, ever, ever farm enough of these animals to satisfy demand so you have to stop using this material."
He praised the Duke's "strong, clear speech" in which he said "we must do more, we must do it faster, and we must do it better".
Twin sisters Tilly and Imogen Fitzjohn, 16, from Stowe School, spoke to the Duke after his speech.
Their father, conservationist Tony Fitzjohn, runs a national park in Tanzania where 27 black rhinos and two baby elephants are held, the girls said.
Tilly said: "I think it's really crucial for younger generations because we all really need to be aware of what's happening with wildlife conservation and make the change for the future. I think it's so important."
Imogen added: "Prince Harry came to stay with us and so Prince Harry has done most of the job with my dad - so we've had one Prince come to stay just to understand what we do and raise awareness."
The Duke of Cambridge also announced at the event that he will attend the third International Wildlife Trade Conference in Hanoi, Vietnam, in November, where he will engage with local people about conservation.