Ponderosa pines, mountain chickadees and western pine beetles migrated, have long traveled together

By Jeff Mitton

As the most recent glaciers subsided, ponderosa pines move north from their southern refuges. But it was not a solitary journey, for the pines brought along a community of dependent species.

Ponderosa pine had two southern glacial refuges, an eastern diffuse refuge on the mountains of New Mexico and Arizona and a western refuge in the San Bernadino Mountains of Southern California.

Ponderosa pine forms a foundation for an entire community of insects and, to a smaller extent, mammals and birds. So when the pines migrated with the glacial cycles, other species moved with them, including the western pine beetle, a relative of the infamous mountain pine beetle.As the glaciers began to wane about 16,000 years ago, populations from the eastern refuge moved north, reaching the Grand Canyon about 14,000 years ago, Colorado's southern boundary around 10,000 years ago and the foothills west of Boulder only 5,000 years ago.

Native Americans living beneath the Flatirons would have watched ponderosa pines arriving from the south and climbing higher into the mountains. Pines from the eastern and western refuges met in Montana just 1,000 years ago, about the time that Leif Erikson was establishing a Viking settlement in Newfoundland.

Pines from the western refuge move north through California, Oregon and Washington and east through Idaho to Montana.

Although pines from the eastern and western refuges meet in Montana, they have little opportunity to exchange genes, for the populations are small and widely spaced.

So the eastern and western ponderosa pines retain their distinct forms and adaptations and we recognize those differences by calling them subspecies — Pinus ponderosa ponderosa (western) and P. p. scopulorum (eastern). In addition to their adaptations to different environments, their mitochondrial genomes are distinctly different.

I think of ponderosa pine as more than a species of tree, for approximately 100 species of insects depend on it for food or a site for mating or a place to live.

Ponderosa pine forms a foundation for an entire community of insects and, to a smaller extent, mammals and birds. So when the pines migrated with the glacial cycles, other species moved with them, including the western pine beetle, a relative of the infamous mountain pine beetle.

Western pine beetles, Dendroctonus brevicomis, are specialists on ponderosa pine, meaning that they cannot breed without their host tree. So when ponderosa pines migrate, the beetles necessary follow.

Scott Kelley, who received his PhD at the University of Colorado working on the genetics of many species of bark beetles, discovered that western pine beetles living on the different subspecies of ponderosa pines have distinctly different mitochondrial genomes, just as their hosts do.

Judging by the magnitude of the genetic differences between eastern and western beetles, it appears that they have not exchanged genes for millions of years and although they appear morphologically similar, they should be considered separate species.

Mountain chickadees nest in cavities in conifers, and although they commonly use ponderosa pines, they may use other species at higher elevations.

Some behavioral studies suggest that if they build nests in other conifers at higher elevations, during exceptionally cold years, they migrate downhill and spend winters in ponderosa pines. Thus, they are somewhat dependent on ponderosa pines.

Given that mountain chickadees hunt for insects on ponderosa pines, I have no doubt that they feed on western pine beetles whenever they are available. In an ecological sense, these three species are intricately intertwined.

Mountain chickadees, like ponderosa pines, have eastern and western subspecies, and the subspecies' boundaries in the trees and chickadees are nearly coincident.

In addition, the mitochondrial genomes of the eastern and western chickadees are so different that evolutionary biologists estimate that they have not exchanged genes to any substantial extent for about a million years.

As consequence of their shared histories, ponderosa pines, western pine beetles and mountain chickadees share essentially the same pattern of genetic differentiation across their ranges.

Apparently, they have been traveling companions for at least a million years, through numerous glacial cycles.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

March 30, 2015