Sunday, November 25, 2007

Forest birds, beetles and koa trees: size matters

If you replant a koa forest, how soon does it become useful habitat for native creatures?

It seems to be a matter of, if you build it, they will come. But both age and location also matter, according to research by Steve Goldsmith of Austin College in Texas.

(Photo: 'Akiapōlā'au on a tree. U.S. Fish and Wildlife Service.)

Goldsmith worked in the Hakalau Forest National Wildlife Refuge, where more than 30,000 acres of a native forest sweep up and down from an elevation a mile high on the windward slopes of Mauna Kea.

Some of that forest has never been logged, but other sectors were converted a century or more ago to pasture and are being replanted in koa in other species.

Goldsmith's work was on the density of beetles in the koa trees. The beetles, while they are pests to the koa trees, are a part of the native ecosystem, and they're a food source for native birds, notably the 'akiapōlā'au, whose Latin name is Hemignathus munroi.

These yellow native forest birds are remarkable in part because their beaks form two separate tools. A stout, short lower beak is used woodpecker-like for excavating trees, while their longer, slender and curved upper beak is used for probing and pulling out food items, like beetles.

Among their prey are a pair of longhorned beetle species found only in Hawai'i, Plagithmysus claviger and Plagithmysus varians. These bore into dead branches of koa trees.

The goal of Goldsmith and fellow researchers Hayley Gillespie and Cole Weatherby was to determine how the age of the forest affected the population of the beetles. Their work was published in the September 2007 edition of The Southwestern Naturalist.

They studied young koa plantations that were planted 3 to 8 years earlier, middle-aged plantations with trees 12 to 15 years old, and then compared those with ancient trees that formed the canopy in native forest.

The result, perhaps predictable, was that the older, bigger trees have more dense populations of beetles.

Goldsmith, in an email, said “that the trees of the intact forest harbors the most beetles (per branch), that the trees of the older plantations have fewer beetles per branch (but still substantial numbers), and that the trees of the young plantations have the fewest beetles per branch.”

To a certain point, the fact that bigger trees have bigger dead branches accounts for the difference, although there is a point where getting a lot bigger doesn't seem to make a great deal of difference, he said.

Also, the beetles seem to be gregarious critters. Branches tend to have either a colony of them, or none at all. The Goldsmith team did not find many branches with solitary beetles.

In a second article in the same journal, Goldsmith noted that beetles change density with elevation.

The koa trees on the high, colder slopes have fewer beetles than lower slopes. Goldsmith said both climate and seasonal changes appear to be at play in the density difference by elevation.

Goldsmith credited the management team at Hakalau Forest National Wildlife Refuge for improving the habitat for native creatures.

“The folks at the refuge deserve a lot of credit and recognition for their hard work to preserve what is left of Hawaiian montane forest and its biota,” he said.

Scientists have long known that for many native bird species, a mature forest has more value than a young one, not only for things like insect food, but things like the presence of cavities that can be used for nesting.

© 2007 Jan W. TenBruggencate

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