The Martlet

A UVic PhD candidate is hoping nitrogen levels in tree rings will provide researchers with clues on the history — and future — of seabirds.

Steven McGehee’s research looks to adapt the work of UVic biology professor, Dr. Tom Reimchen, who found that concentrations of the nitrogen isotope nitrogen-15 (¹⁵N) in trees along the banks of salmon-bearing rivers provided accurate records of salmon levels in a given year.

“Salmon have large amounts of nitrogen in their body and because they are feeding in the sea, they have what’s called a stable isotope,” explained McGehee.

Reimchen and McGehee’s research focuses on two nitrogen isotopes, ¹⁴N and ¹⁵N, which are both used by most living organisms.

“The trees by the side of the river are going to have lots of ¹⁵N because it used all its ¹⁴N to grow and the ¹⁵N was extra, it didn’t need it, it just stored it. So you can look back historically and measure,” said McGehee.

Seabirds burrow under large trees to lay eggs and raise their hatchlings. McGehee’s hypothesis is that like salmon along a river bank, birds in these burrows provide the trees with nitrogen.

“They’ll burrow right underneath there and when they’re incubating their eggs they just defecate right in the hole. Babies hatch, babies continually go to the bathroom, they’re fed fish, they’re dropping that fish, they’re dropping feathers, eggshells, those holes are just lined with nitrogen.”

McGehee’s research has focused on three species of B.C. seabirds: the Rhinoceros Auklet, Ancient Murrelet and Cassin’s Auklet. He chose these three birds because they all have very different housekeeping tendencies.

“The Rhinoceros Auklet is a very dirty bird. It literally will just poop continuously in their hole, whereas the Ancient Murrelet, their holes are spotless,” McGehee said. “And the Cassian’s Auklet is sort of halfway in between . . . so it’s a really nice control.”

To conduct his research, McGehee goes to islands and takes tree core samples, either the diameter of a pencil or a thumb. A sample could contain 200 or more tree rings, so time and cost don’t allow McGehee to have each ring of each sample analysed. It takes about 15 hours in the lab to prepare just 50 rings from a single sample, and costs about $7 to have one ring analysed at the University of California-Davis.

“What I do is select a few years and see if the nitrogen corresponds to the size, and then assume that for the rest of those there’s ¹⁵N in there causing the larger size,” he said, adding that computer programs help eliminate variables like excessive rain fall that could increase growth in a certain year.

Results are still preliminary, and McGehee needs to sample more islands before any conclusions can be made. There are still questions around whether the burrows produce enough nitrogen to make a difference, but so far he’s optimistic the technique will work as well for seabirds as it has for salmon runs. If so, it would allow researchers and conservationists to find islands that once hosted seabirds and see about possibly restoring populations by removing introduced predators.

The research also offers the potential to provide researchers with a way to answer historic questions on seabird populations. Rarely do accurate reports go further back than 35 years, making it tough to evaluate declines in seabird numbers. McGehee’s research could change that.

“I really hope we can use this technique to, if nothing else, estimate as accurately as we can how many seabirds there were 200 years ago anywhere in the world, and how many 400 years ago before humans arrived; has there really been this big drop? You open up the books and generally they say as much as 90 per cent are gone at any given spot around the world.”