By Sarah Locke
Many people look at the sea of dead and dying trees stretching across Alaska and the southern Yukon and see a natural disaster. After all, the infestation of spruce bark beetles is the largest and longest outbreak of its kind in Canada, and at first glance, it is hard to see anything positive about trees with red needles.
But look at this landscape with different eyes and you might see something else: fantastic habitat for woodpeckers for one, and a natural system taking care of itself, even if it does not look so pretty for awhile.
“The key point is that these outbreaks are a natural disturbance, a natural component of the system,” says Katie Aitken, a biologist with the Canadian Wildlife Service. And for Aitken and her colleagues, the infestation in the Yukon also presents a stellar opportunity to look at how these cycles run their course.
In 2007 Aitken—along with Fiona Schmiegelow, a renewable resources professor at the University of Alberta, and Canadian Wildlife Service colleagues Pam Sinclair and Shawn Taylor—began studying how the spruce bark beetle infestation has affected birds in the Yukon’s boreal forest.
The outbreak, which began in the 1990s, is thought to be on the decline, but little is known of its impacts, especially in the Yukon. “There was a lot of anecdotal evidence,” says Aitken. “For example people in Haines Junction were saying there were a lot of woodpeckers, but it was all anecdotal.”
After fire burns a forest, or beetles invade, insects arrive to take advantage of the stressed trees, and woodpeckers follow, particularly the Black-backed and American three-toed species. The insects provide plenty of food, and dying trees are prime sites for building nesting holes.
Even species such as Yellow-rumped Warblers, which prefer nesting in live trees, have been shown to adapt. Studies in Alaska found that the warblers switched to nesting in dead trees in beetle kill areas, or found the few surviving live ones.
“So species richness does not change too much,” says Aitken.
She also points out that these natural cycles are important for controlling future insect outbreaks. Studies on spruce bud worm infestations in the boreal forest have shown that they occur in regular cycles.
“It is known that birds like warblers are very tied to spruce bud worm cycles, and studies have shown that certain birds can help to control those populations,” says Aitken.
As well, climate change can affect the range of certain species, and birds might shift their range away from where spruce bud worm outbreaks occur.
“You might get southern warbler species moving north, and then they would not control the spruce bud worm outbreaks as well.”
Aitken says that salvage logging of beetle-killed trees, if done improperly, can disrupt these natural cycles, removing habitat for birds such as woodpeckers which could help control future outbreaks.
“When you have a natural disturbance, people see it as a catastrophe and want to go in and fix it by removing the dead wood. The initial response is to clean it up and try to get some timber value out of it,” says Aitken. She notes that people also raise concerns that dead trees could increase the fire hazard.
“But a lot of studies have shown that salvage logging disrupts the natural process, and if you leave it untouched, the animals will deal with it. The species are adapted to dealing with these disturbances, and things work themselves out. It is not something that needs to be fixed.
“With woodpeckers it has been shown time and time again that they help prevent insect outbreaks in forests. When bark beetles cycle through an area, the birds can help prevent really large outbreaks. They can regulate and lengthen the period between outbreaks. But what has happened in the southwest Yukon is that the beetles increase so much that woodpeckers cannot control the outbreak.”
The beetle epidemic in the Yukon is believed to have peaked in 2003, and now is declining. Aitken says the territory offers a rare opportunity to study how natural cycles progress in a large beetle-kill area where little logging has taken place.
In 2007 and 2008, biologists surveyed birds in the Haines Junction area, and found that the numbers of woodpeckers declined between one-third and one-half from one year to the next. As this is the only snapshot they have of bird numbers, it is difficult to assess how significant this change is.
This year, biologists are conducting a similar study in the Teslin area, but plan to resurvey the Haines Junction area in 2011. Bird surveys are conducted in June when most birds are nesting. But as woodpeckers nest earlier, the biologists used electronic trickery to see how many woodpeckers were out there.
When woodpeckers set up nesting areas, they choose a really nice tall tree to drum on and establish their territory, explains Aitken. They biologists broadcast tapes of woodpecker calls and drums to provoke the woodpeckers to respond.
“The woodpeckers get really angry when they hear the tapes, so you get a better idea of who is out there. It worked really well as we did get a lot of response,” says Aitken.
For more information on the study, contact Katie Aitken at Katie.Aitken@ec.gc.ca
This column is coordinated by the Northern Research Institute at Yukon College with financial support
from Environment Yukon
and Yukon College.