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  Yahoo News 16 Oct 07
Biodiversity said to be key to healthy forests: study

Forests planted with a diverse species of trees will be better able to withstand pest infestation than those that are sown plantation-style with just one species, a study released Monday said.

A diversity of trees will support a greater range of insects than a single species, ensuring that there are more predators to keep down the numbers of a pest that, unchecked, could decimate a swath of woodland in an outbreak year.

"Mixed forests have a greater flexibility than plantation-style forests," explained Eldon Eveleigh, an entomologist with the Canadian Forest Service in Fredericton, New Brunswick.

The findings have implications for the management of forestry lands, and also commercial plantations.

Eveleigh and colleagues studied three patches of the Arcadian Forest in the eastern Canadian province of New Brunswick as part of an effort to examine how biodiversity could protect forests from pest damage.

They looked at three sections of forest: one was almost entirely composed of balsam fir, which is a favourite target of a moth called the spruce budworm - one of the most destructive native insects in the northern spruce and fir forests of the eastern United States and Canada.

A pest outbreak occurs once every 35 years, and once it has begun, it usually continues until the larvae consume much of the available foliage.

The other two plots were varying mixtures of balsam fir and hardwood species such as birch, maple and deciduous varieties.

The Canadian researchers found that the budworm thrived in the plot that was almost entirely balsam fir, laying twice as many larvae per square meter than in either of the two other plots during a peak reproductive year.

The results were devastating, with tree mortality averaging 65 percent in this plot - almost three times higher than the mortality rates seen in either of the other two chunks of forest.

Separately, the scientists also noticed that as the abundance of budworms increased, so too did the numbers of other plant-eating insects or parasites that feed on the moth. The so-called "birdfeeder" effect continued on up the food chain, with other higher-order insects flocking to the area in search of more plentiful food sources.

However the abundance of these higher-order predators was much greater in the plots with several species of tree, suggesting that it takes multiple species of trees to support a cross-section of insect life.

This is important because ecologists think that the influx of these various insect species is what allows many ecosystems to remain stable and survive periodic fluctuations in the populations of one or other parasites.

"The greater the diversity of these generalist insects, the less the impact of herbivores, such as the budworm, on the trees," said Eveleigh.

The study appears in the Proceedings of the National Academy of Sciences.

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