Some 700 people yesterday were evacuated from areas near the blaze, which is sweeping through a wooded, mountainous area in La Palma, one of the lesser-known, unspoiled islands of the Spanish archipelago off the coast of Africa.
Emergency services said a further 1,800 were being moved from their homes yesterday night as the blaze showed no sign of abating.
"The fire has not come under control yet," Fernando Clavijo, regional president of the Canaries, told reporters.
The ranger, Francisco Jose Santana, died while working to put out the flames, said Spain's UGT union of which the 54-year-old was a member.
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"He leaves a wife and five children," it added.
A spokesman for the Guardia Civil police force told AFP a 27-year-old German man was arrested Wednesday afternoon, suspected of having caused the fire.
The man, whose identity has not been revealed, "told officers he accidentally provoked the blaze when he defecated... And set fire to the toilet paper he used for that," the Guardia Civil added in a statement.
Some 200 people are fighting the blaze, as are helicopters and planes, a spokeswoman for the Canary Islands' emergency services said.
Weather conditions are not making their task any easier, as high temperatures and strong winds fan the flames, she added.
Acting Prime Minister Mariano Rajoy sent his condolences to the park ranger's bereaved family.