RSPB Scotland said its Loch Garten reserve was home to 5,000 species, and insects, reptiles along with trees, lichens and mosses had been killed.
The charity’s James Reynolds said the wildfire had left a 6,177-acre (2,500ha) burn scar and the sight of it was “ugly and upsetting”.
He said: “This is still very much a live situation.
“There are smouldering points – hotspots – in the soil which is quite peaty and when the wind get up that reignites and there are flames again.”
Reynolds said it could be “weeks or more” before the wildfire was finally extinguished. “We are praying for rain,” he added.