An uphill hike through Yosemite

Michael Peterson

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​Presently snow-covered and always picturesque, Yosemite National Park is an icon of the Sierra Nevada Mountains. Successive storms have dumped feet of snow on Yosemite National Park and the Sierra Nevada Mountains of California, giving it the appearence a winter wonderland. Yet, even with current snowstorms, it was only a short time ago when the lack of snowpack in the Sierra Nevada Mountains was highlighted as an important problem for water resources in California and a major consequence of increased temperatures and severe droughts. While we enjoy a year of snow, the warming temperatures over the past century in these mountains are concerning for the animals that live there.

Among the beautiful granite vistas live many small mammals. No, not pigmy elephants, or baby mountain lions, but those scurrying animals that you might glimpse on a hike. Marmots, chipmunks, voles, squirrels, bushy-tailed woodrats, and pikas; the type of alpine-adapted animals who appear and disappear among the rocks, logs, and trees. These are the cute animals you might try to catch with your camera, but whose quickness leaves you with only digital photos of random rocks that you will clog up your hard-drive back at home. Surprisingly, these small mammals may be one of our best indicators of the consequences of climate change for high-elevation ecosystems.
 
The main concern is that air temperatures will continue to rise and some animals who are adapted to cold temperatures will move to higher elevations to avoid those warm temperatures. But, animal and plant species living near mountaintops can only move to higher and higher elevations until they reach the peak. At that point, there is nowhere to go. Herein lies a big question for conservation biology and climate change science. Is this happening and what are the consequences? Are our alpine adapted plants and animals moving to higher elevations? Can we project how much longer they will be able to do this before they reach the top? How physiologically tolerant are they if they reach the peak?
 
Much of how we approach conservation biology depends on how we set baselines. How many species are there in an area? Is that number increasing or decreasing or stable? For each species, whether it be dragonflies or elephants, how many individuals are there, and how does that number compare with something in the past? All these questions require some sort of historical baseline. And in the case of our small, charming pikas and marmots, we need a baseline as well. How can we tell that they are moving to higher and higher elevations if we do not know where they were before? 
 
Much like the early surveys of C.H. Kennedy for dragonflies and damselflies, an early 1900s zoologist and Professor at UC Berkeley named Joseph Grinnell saw great value in the description of species and their natural histories. His work across western North America, and particularly in California, provide incredible baselines for scientists today. This includes, his transect through Yosemite National Park.
 
A transect is a common field biology approach to figure out which species of plants and animals live in a particular region or area. You start by establishing a line that cuts through your area of interest. You then walk that line an record what plants or animals you can see, or trap along that line or within a pre-arranged distance of that line. Because trees do not move, their presence, size, age, and type can be recorded by sight. For mobile animals, traps are necessary. The strength of this approach is that you can compare the animals found along that transect among years.
 
The Yosemite Transect is an exceptional baseline in part because it is relatively old among biological surveys. Similar in timing to C.H. Kennedy’s survey of dragonflies and damselflies (see previous article, ), Grinnell’s transect survey took place from 1914-1920, making it about a century old. The range of human activities in California has changed dramatically over that century, but because the Yosemite Transect occurs in Yosemite National Park, it lies on land protected since 1890. This makes it not only an old baseline, but a baseline without confounding impacts of local human activities, which is particularly important as scientists assess biological changes associated with global climate change. The Yosemite Transect is also incredibly thorough. Over 4,000 specimens were collected, with more than 3,000 pages of accompanying field notes, and even several hundred photographs.
 
Even more important, the Yosemite Transect ranges dramatically in elevation. Lace up your boots and pack your extra chocolate bars because this transect runs from 60 meters above sea level to 3300 meters above sea level. This is the key to understanding whether animals are moving to higher elevations because the presence or absence of species at increasing elevations can be compared to what it was in the early 1900s.
 
In the early 2000s, nearly a century after Grinnell first walked the Yosemite Transect researchers led by Craig Moritz from the UC Berkeley Museum of Vertebrate Zoology re-surveyed the same transect. Many species found only at lower elevations a century before now had expanded their ranges to include higher elevations. Species previously found at high-elevations showed contracted ranges. On average, the upper elevation limit of small mammal species had risen 500 meters, meaning that animals are indeed moving up the mountain. While this may have been expected, it does not bode well for those adapted to those mountain peaks.
 
So, what options do biologists have when it comes to pikas? Are there arguments for assisted-migration of pikas to other mountain areas and would that policy be wise? At what point are those scenarios worth the risk?