Tim Appleton, founder of bird-watching conference Birdfair, said Ms Symonds will speak for five minutes on her support for conservation.
It will be her first solo event since moving into Downing Street.
The UK's "first girlfriend" will then appear on a Question Time-style panel alongside guests including Chris Packham, presenter of BBC SpringWatch.
Mr Appleton said the event at Rutland Water Nature Reserve has raised £40 million for global conservation projects in the 31 years that it has been running.
He described it as the "biggest ecotourism fair in the world".
"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," he said.
He said he anticipated her speech to cover why she supports conservation, in particular ocean projects.
Public relations expert Ms Symonds resigned as director of communications for the Conservative Party last year.
She is now a senior adviser at Oceana, a US-based environmental campaign group, working with its marketing operation in London.
A profile on the charity's site describes her as being "passionate about protecting the oceans and marine life".
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