Oahu leads the Hawaiian Islands in the number of annual wildfires, which sometimes threaten homes, watersheds and native forests, University of Hawaii researchers have found. And the frequency of the fires is increasing, they said.

Clay Trauernicht and Creighton Litton of UH’s College of Tropical Agriculture and Human Resources were among a group of researchers who found that heavily populated areas in Hawaii were more prone to have wildfires.

“People don’t typically think of wildfire as a frequent disturbance on tropical islands,” said Trauernicht in a UH release Friday.

But between 2005 and 2011, there were about 1,007 wildfires statewide that burned an average of about 20,000 acres, the researchers found.

A 108-year history showed a more-than-fourfold increase in acreage burned annually statewide, they found.

While lightning is a significant cause of brush and forest fires on the mainland, the vast majority of wildfires in Hawaii are started by humans, Trauernicht and colleagues said.

On Oahu, the average number was about 600 wildfires annually — many were small, under 10 acres, Trauernicht said by telephone Saturday.

“Oahu is off the charts,” he said, adding that Maui averaged about 200 wildfires annually.

He said every once in a while, a large brush fire occurs, such as the Waialua wildfire in 2007 that scorched an estimated 6,700 acres. That one was in hilly, rough terrain that challenged firefighters.

Observers say in Hawaii, more homes being built near open brush land sometimes force firefighters to place themselves dangerously between the fires and houses.

Also, as agricultural companies in Hawaii abandon farm ventures, firefighters find themselves battling the fires alone, without the benefit of agribusiness bulldozers and other heavy equipment.

On Maui, Hawaiian Commercial &Sugar crews and bulldozer operators cut firebreaks, especially when an unscheduled sugar cane fire occurred.

Maui County Fire Services Chief Edward Taomoto said the absence of Hawaiian Commercial &Sugar, Hawaii’s last plantation, will be felt by firefighters. The company has said it will halt operations within a year.

“Once they’re gone, it’s going to be tough,” Taomoto said.

Last week, a brush fire burned about 200 acres and halted traffic periodically on Thursday and Friday on the main highway near Maalaea, Maui. It was extinguished at 9:10 a.m. Saturday.

Taomoto said Saturday that an investigation failed to find the source of the fire, but he suspects it was unintentionally set.

In the study published in Pacific Science, Trauernicht and colleagues say humans have increased the incidence of wildfires across the state not only by setting them — purposefully or accidentally — but by introducing fire-prone plant species, especially non-native grasses.

Trauernicht said incident commanders involved in fighting wildfires would like more training for firefighters. The report also quotes fire officials as expressing hope that more preventive measures could be taken, such as the development of more firebreaks and an increase in grazing to reduce the amount of fuel in grassy areas.

Co-authors of the report, “The Contemporary Scale and Context of Wildfire in Hawaii,” are Elizabeth Pickett of the Hawaii Wildfire Management Organization in Waimea on Hawaii island; Christian Giardina and Susan Cordell of the Institute of Pacific Islands Forestry, part of the Forest Service in Hilo; and Andrew Beavers of the Center for Environmental Management of Military Lands at Colorado State University.