Refuge Notebook • Vol. 8, No. 24 • June 30, 2006

American Marten: A mammal barometer of global climate change by Andy Baltensperger

Global climate change is increasing recognized as marten rest under the wood, which keeps them dry, a powerful force reshaping the environment in recent while the snow overhead insulates them from cold decades. Climate change raises many new questions temperatures outside. which challenge traditional views of wildlife manage- A layer of fresh snow at least eight inches deep ment, such as how will altered climate affect various is sufficient for maintaining resting site temperatures wildlife species, how will it change ecosystem func- between approximately 10 and 32℉, even when out- tions, and ultimately, how should we manage ecosys- side temperatures approach -40℉. This critical depth tems within this constantly shifting climatic frame- increases linearly, however, as the snow layer be- work? comes older and denser with thawing and refreezing, The American marten (Martes americana) is one which is occurring more frequently during Kenai win- species whose strict physiological requirements in ters nowadays. If snow depths and densities do not winter make it especially for monitoring for climate meet insulative thresholds, marten have to capture ad- warming. Marten are very sensitive to environmen- ditional prey simply to stay warm or they will freeze to tal conditions and should be a good indicator species death. Thermodynamic stress and unavailable prey are for detecting environmental change. This sensitivity both related to inconsistent snow-cover and are prob- however does not necessarily allow a straight-forward ably highly significant factors controlling the distribu- prediction of the marten response to climate change. tion of marten on the Kenai. On the Kenai Peninsula, marten are most abundant If snow depths are in fact limiting marten distribu- within the denser, wetter forests of the eastern Kenai, tion, there are several alternate scenarios which may and they extend westward along forested drainages play out as the climate becomes warmer. Under one through the Kenai Mountains to Skilak Lake. They scenario, climate warming may actually increase win- are not typically found within the black spruce dom- ter snowfall the Kenai, in which case suitable winter inated Kenai lowlands where canopy cover is sparse marten habitat may expand into the lowlands. On the and where snow depths are generally lower than in other hand, despite any potential increase in snow- the Kenai Mountains. fall, an accompanying increase in winter thawing may For an animal which traditionally inhabits bo- destroy the insulative capacity of snow for marten in real forests, marten are physiologically rather poorly many areas of the Kenai. A third, long-term scenario equipped to deal with winter. Marten have long, slen- involves significant warming to the extent that win- der heat-radiating bodies, poorly insulative fur, low fat ter lows no longer fall below 0℃. In this case marten reserves, high metabolic requirements, and do not hi- would better optimize their heat conservation by rest- bernate. All of these factors combine to make survival ing above the snow, when it is present. Under such during cold winters a struggle for marten. extreme warming conditions (unlikely in the near fu- The reason marten have been able to exploit boreal ture), snow cover would cease to be a requirement for forest habitats so successfully is their ability to seek marten survival on the Kenai. out thermodynamically efficient resting sites. In win- There are other factors in addition to snow-cover ter, because of their physiological vulnerability to cold, which should be considered. Marten tend to pre- marten must rest under an insulating layer of snow fer older, closed-canopy forests, which have more in order to stay warm. Pieces of course-woody debris prey, more course woody debris, and better protec- (downed trees, logs, stumps, etc) protrude up through tion from predators than do open-canopy forests. On the snow and create pockets and tunnels where marten the Kenai Peninsula, mature forests include hemlock, can easily access the subnivean zone (space between white spruce, Lutz spruce, and Sitka spruce forests, but the snow surface and ground). In the subnivean zone, generally exclude areas of black spruce which tend to

USFWS Kenai National Wildlife Refuge 43 Refuge Notebook • Vol. 8, No. 24 • June 30, 2006 lack a closed canopy. Recent research on the Kenai in- cates that marten and shrew body sizes have increased dicates an upward movement of tree-line at a rate of slightly in size over the past 100 years, which suggests approximately one meter per year since the 1950s. As a positive response to greater food availability due to closed-canopied forests continue to move upward into climate warming. areas of alpine tundra, marten habitat should expand It will be interesting in the coming decades to mon- upward into higher elevations already having suffi- itor exactly how marten populations respond to a host cient snow-cover. of hanging factors such as snow-depths, temperatures, Over the next two winters as part of my gradu- tree-line, and prey availability as their habitat changes ate thesis research, I plan to examine marten distri- with our warmer climate. bution patterns and their relationships with average snow depth across the Kenai. My working hypothe- Andy Baltensperger is a graduate student at Col- sis is that marten have been restricted from dispers- orado State University, currently working on his MS ing into the Kenai lowlands and other areas of suit- thesis on the Kenai National Wildlife Refuge. Previous able habitat by the lack of sufficient snow-cover. A re- Refuge Notebook columns can be viewed on the Web at cent study of museum specimens from the Kenai indi- http://www.fws.gov/refuge/kenai/.

44 USFWS Kenai National Wildlife Refuge