Coastal northwest American marten conservation

American marten. (Photo credit unknown.)

Just how many sub-species of American martens are living in California and Oregon? Well, there may be one less than experts thought, according to a 2009 study published in Conservation Genetics.

American martens (Martes americana) are slightly larger than a house cat and are carnivorous members of the Mustelid family. They live in boreal forests and are widely distributed across Canada, but they are also found in mountainous and coniferous forested areas in Washington, Oregon and northern California, as well as south through the Rocky Mountains to northern New Mexico. While the populations in Canada are largely contiguous due to the comparatively more intact forests there, populations in the U.S. tend to be fragmented. This is in part due to the animals moving into what’s called Pleistocene refugia — areas where the climate is more akin to the cooler, drier times of the Pleistocene, which are generally areas of higher-elevation — and also in modern times, habitat encroachment such as logging and development.

American martens also prefer mature forests, and much of the old-growth forests have been cut down. If memory serves, about 5 percent of our original old-growth forests remain here in the U.S. Because they are so tied to these dwindling habitats, and because they were historically over-trapped for their fur, the species has been on conservationists’ radar screens.

There are currently four sub-species of American marten recognized in the northwest. These are: M. americana humboldtensis, M. americana caurina, M. americana sierrae, and M. americana vulpina. You can find the ranges of these sub-species using this graphic: 

The sub-species called M. americana humboldtensis was thought extinct in Oregon and California until a single population was rediscovered in 1996, giving this group more focus from conservationists. The origin of this relictual population was up for debate, and one line of thinking posited that these animals had descended from the original population of M. americana humboldtensis used to describe the sub-species in 1936; while the other line of thinking posited that the marten was extirpated (driven out) of this range but then animals from one or more sub-species of marten re-colonized the historic range and then were re-discovered.

Mitochondrial DNA regions of mammals.

To tease apart the origins, a team of scientists studied samples of mitochondrial DNA from specimens of M. americana humboldtensis taken from the newly discovered population and they compared these to samples of mtDNA from museum specimens of this sub-species pre-dating its extirpation from the area. To be thorough, they also compared the museum specimen’s mtDNA to samples taken from the surrounding sub-species populations: M. a. caurina and M. a. sierrae. Mitochondrial DNA is passed down from mothers to offspring, so it provides scientists with a unit of genetically inherited material that can trace lineages back in time. Certain regions of mtDNA are prone to change, a lot actually. These are called hypervariable control regions. Studying them allows scientists to track evolutionary changes in a population or a species over generational time and to trace phylogenetic branching events. In this study, the scientists amplified base pair samples from three regions of the mtDNA: cytochrome B (cyt b), the control region, and tRNA-Pro. (Although this page is written with birds in mind, check out this resource for more about using mtDNA in animal genetics studies.)

To obtain their genetic samples, the researcher took a 2 mm square of nicked ear tissue from martens that were trapped alive. They also collected opportunistically from road-killed martens. They found ten mutations in the cyt b gene, and six in the control region, showing slight diversity across the sampled populations. They also found three haplotypes in the wild martens, and one of these same haplotypes was shared by the museum specimen from 1927. The haplotype contained by the museum specimen was also found in both the marten sub-species (M. a. caurina and M. americana humboldtensis) that span from coastal Oregon to coastal California. These sub-species were previously delineated mainly on morphological and range data. The genetic findings support the idea that the re-discovered martens in coastal California descended from a relict population that the 1927 museum specimen also came from. Due to a small sample size and problems with eight of the museum skins they tested producing testable base pair sequences, the researchers stop short of saying their results support erasing the sub-specific boundary between M. a. caurina and M. a. humboldtensis, but you sure get the feeling their interpretations lean to this conclusion. They write:

Despite the aforementioned sample size limitations, the occurrence of haplotype #2 in coastal Oregon and in coastal California populations, and its absence in the individuals from Oregon Cascades, suggest that coastal populations of California and Oregon are more similar to each other than to Oregon Cascades populations.

Their results also found that  M. a. sierrae (known from the Sierra Nevadas in central California) shared multiple mutations with M. a. nesophila of Alaska. This indicated that despite M. a. sierrae being geographically close to M. a. caurina and M. a. humboldtensis, it is in fact more closely related to populations in Alaska and probably split from the Oregon and coastal California sub-species long enough ago to indicate a complex past. They write:

At a minimum, this suggests that marten populations were not one large, genetically homogeneous population in the Pacific states. Past glaciations had profound genetic consequences on populations (Hewitt 1996). The relatively deep divergence between these clades, as indicated by the number of mutations and position (i.e., first), is presumably one of these consequences, suggesting divergence in separate glacial refugia. The close affinity of M. a. nesophila (southeastern Alaska) and M. a. sierrae (southern Cascades and Sierra Nevada mountains of Cali- fornia), to the exclusion of M. a. caurina (Oregon), indicates a possibly complex history of recolonization of northerly regions.

Taken on the whole, this means that the coastal sub-species of martens in Oregon and California are more closely related to each other than the Cascades populations in Oregon (known as M. a. caurina) is to coastal Oregon populations (also currently known as known as M. a. caurina – which is kind of problematic.)

So why on earth does any of this matter? Because, if they are correct it has implications for managing these populations to allow for gene flow between the coastal Oregon and coastal California populations. They report that there is currently habitat fragmentation threatening the division of the two coastal populations… which means that conservation groups had better get some corridor plans together to maintain the unique genetics of the northwest coastal martens.

NOTES:
Slauson, K., Zielinski, W., & Stone, K. (2008). Characterizing the molecular variation among American marten (Martes americana) subspecies from Oregon and California Conservation Genetics, 10 (5), 1337-1341 DOI: 10.1007/s10592-008-9626-x

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