Boris Johnson's partner Carrie Symonds said politicians have a "gigantic responsibility to make the right decisions" over the environmental crisis, in her first solo public appearance since moving into Downing Street.
Ms Symonds also condemned "cruel" trophy hunters, but expressed optimism that the number of people who care about the environment "far outnumber the people who don't".
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She addressed an audience in a marquee at Birdfair, which is described as "birdwatching's Glastonbury", at Rutland Water Nature Reserve.
"There are no simple answers to the environmental crisis this planet faces," she said.
"It is immensely complicated.
"There is no escaping the fact that politicians, business leaders and journalists have a gigantic responsibility to make the right decisions, to change the way they do business and report the truth about what is happening in the world.
"But so too do scientists, naturalists, campaigners, birdwatchers and all of us individuals.
"We all share this crowded little planet.
"We all have a duty to take care of it and we all have a massive role to play in doing so.
"And that means we all need to make the little changes that will help make a big difference.
"I'm far from perfect, but I try to remember to take a canvas bag to the supermarket, take my reusable bottle rather than buy plastic, and tonight I'm wearing a sustainable dress.
"I can't always do that, and I'm learning and I'm trying."
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She described her delight at seeing a puffin at Bempton Cliffs in the East Riding of Yorkshire, then her horror at seeing photographs in a newspaper of puffins "slaughtered by so-called trophy hunters on trips to Iceland.
"A trophy is meant to be a prize," she said. "Something you're awarded if you've achieved something of merit that requires great skill and talent.
"Trophy hunting is the opposite of that.
"It is cruel, it is sick, it is cowardly and I will never, ever understand the motivation behind it.
After her speech she took a seat in the front row for a question and answer session, which featured guests including BBC Springwatch presenter Chris Packham and entrepreneur Deborah Meaden, of Dragons' Den.
Tim Appleton, founder of the birdwatching conference, said ahead of the event: "We're absolutely over the moon that Carrie is supporting us and we hope she will continue supporting us and conservation for many years to come."
READ MORE: Johnson set for September showdown with MPs over Brexit
Public relations expert Ms Symonds, 31, resigned as director of communications for the Conservatives last year.
Today she is a senior adviser at Oceana, a US-based environmental campaign group, working with its marketing operation in London.
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