Firefighters are tackling a blaze spreading across National Trust protected moorland near Huddersfield.
About 30 firefighters were called to Marsden Moor at about 1200 BST.
National Trust countryside manager Gemma Wren said birds, hares and vegetation could be wiped out due to the speed and severity of the fire.
She said four separate fires had started, possibly deliberately, engulfing more than 500 hectares (1,235 acres) of the land.
The moorland is designated a Site of Specific Scientific Interest.
Ms Wren said: "It's windy up there and the grass is dry so it's spreading very quickly.
"There will be breeding birds and mountain hares wiped out.
"Lots of people go walking up there so we're hoping no walkers get caught."
Ms Wren said men were seen leaving the area on quad bikes shortly before the fires took hold.
Six fire engines and a fire service "wildfire unit" were at the scene.
The National Trust scrambled a helicopter with a water bucket to help douse the flames and six volunteers are helping on the ground.