Ecosystems are built from complex relationships between many different living things that rely on each other for survival. But the foundations of an ecosystem are non-living: water, sun, soil nutrients, and climate together determine which group of living things a landscape can support. And when these abiotic (non-living) factors shift, effects can ripple out to individual species, cascade through communities, and reshape entire ecosystems. Like ourselves, Yosemite’s species have three options when confronted with change:
Adaptation: Over many generations, individuals genetically better-suited to survive in a changed environment will be more successful and reproduce more, slowly altering the species to better fit its environment. Many species in Yosemite are adapting to climate change by adjusting their behavior or phenology (patterns of biological timing).
Shifting ranges: If change occurs too quickly for a species to adapt, it may seek an environment better-suited to its traits—often by moving to higher elevations or more northern latitudes. Mammals, birds, insects, and even forests in Yosemite are now found in different places than they have been historically, forcing other species that rely on those animals and plants to respond as well.
Disappearance or extirpation: If a species is unable to adapt or move, it may be stranded in an inhospitable environment, and populations may begin to drop.
Climate change is not the only source of environmental change in Yosemite. Shifts in fire management, human modification of Yosemite Valley’s hydrology, and nitrogen deposition originating far outside the park have all rocked the park’s abiotic foundations. However, researchers are observing climate-related responses in many of Yosemite’s species, which in turn force other members of the ecosystem to adapt, move, or put their own survival at risk. Because each species responds differently, climate change is ultimately expected to replace current living communities with novel combinations of animals and plants.
What would you do if summer temperatures in your hometown were getting uncomfortably hot? Some of us might opt to spend more time inside—modifying our behavior to adjust to changes in the environment. Although most animals and plants don’t have access to air conditioning, other climate-linked changes in behavior and phenology have been observed in Yosemite.
Ahead of Schedule
Scientists working in northern and central California found that songbirds are arriving in the state at different times than they did historically. Eight of the 21 species surveyed are clocking significantly early arrivals—some by a margin of weeks or months. Researchers tested for connections between the birds’ new arrival times, natural short-term climate cycles, and long-term climate change. For the majority of the 21 species, climate change was linked to early arrival in at least one location. And when one life event shifts, the rest of an organism’s life cycle may be affected—across the US, some bird species are nesting earlier due to warming temperatures.
Out of Sync
However, other living things rely on different cues to set their biological clocks. Seasonal changes in daylength trigger the start of long-distance migrations in locusts, butterflies, moths, and some songbirds. Most plants, on the other hand, time flowering and leaf-out using both temperature and daylength—raising the risk that flowering plants and their pollinators could fall perilously out of sync with each other. Meanwhile, Yosemite’s ground squirrels rely on their own internal rhythms, hibernating seasonally regardless of daylength or temperature cues. The potential loss of synchrony is one of the greatest threats to Yosemite’s ecosystems, where many organisms time their life cycles to align with those of their neighbors.
What if staying indoors to beat the heat just wasn’t enough? While many of us might not opt to move up a mountain or over to the next town north, some living things in Yosemite are doing exactly that.
The Grinnell Project
From 1911–1920, a group of scientists led by Joseph Grinnell surveyed around 1,500 square miles from the San Joaquin Valley through the Yosemite wilderness to Mono Lake, trapping, measuring, and taking careful notes on small mammals of the Yosemite area. In the early 2000s, researchers revisited the exact locations of previous observations within Yosemite to see what had changed. By comparing their observations with the 2,000 pages of detailed notes left behind by the original Grinnell Survey, researchers found that 17 of the 30 species have moved to higher elevation. Some expanded into new territory—the pinyon mouse is now found over 3,400 feet higher than it was a century ago, quintupling its range. On the other hand, 10 of the 17 upward-bound species saw their overall range shrink as they were forced out of historic low-elevation habitat. And a handful of species, like the alpine chipmunk, have collided with the ultimate barrier to upward mobility—the top of the mountain.
What’s behind these changes? Grinnell Project researchers attribute the upslope movement primarily to human-caused climate change—specifically, the over-5°F increase in nighttime low temperatures. Other climate-linked shifts (changes in fire patterns, the timing of seasons, plant phenology, and community composition) may also play into the Grinnell Project’s findings. Although all natural communities change over time, rapid change can challenge the ability of living things to adapt. The Grinnell Project found that while many short-lived, fast-reproducing mammals were able to expand their ranges, long-lived, slow-reproducing species were more often trapped in dwindling windows of livability.
Small mammals aren’t the only ones heading for higher ground. More butterflies are now found in the High Sierra than they were in the 1970s, according to a study conducted north of Yosemite. At the same time, some high-alpine specialists are declining in abundance. Yosemite also conducts an annual butterfly count, but data can be highly variable depending on temperature, rainfall, and wind on the day of the census.
Although forests themselves can’t just pick up and move, warmer temperatures can stress and kill individual trees at lower elevations while raising the upper cusp of survivability. In the foothills north of Yosemite, the lower edge of the ponderosa pine forest retreated upslope by over 1,700 vertical feet between 1850 and 1996, leaving hundreds of square miles of foothills treeless. Meanwhile, repeat photography between the early 1900s and mid-1980s shows an increase in forest density near treeline and tree invasion into Yosemite’s subalpine meadows, including Tuolumne Meadows. Historic sheep grazing along with hydrological changes caused by the construction of Tioga Road are likely responsible for some of this encroachment; however, similar meadow loss is taking place throughout the Sierra Nevada.
While some mammals, trees, and insects are beating the heat by moving uphill, birds have another option—simply head north. Between 1975 and 2004, northern range boundaries for songbirds across the US advanced by an average of 0.9 miles every year. Even when compensating for the northern boundary shift, researchers found the centers of abundance within songbirds’ ranges shifting north as well.
When species respond to change in different ways, effects can cascade unpredictably across the landscape. One study in Arizona found that less snow meant more winter browsing by herbivores (in this case, deer and elk), which damaged broadleaf trees and caused declines in local songbird populations that used those trees to nest and avoid predators. Here in Yosemite, aquatic predators are more active in warmer temperatures, which may be one cause for widespread declines in frogs and toads over the past few decades. Warmer waters also promote aquatic parasites, and have been linked to copepod outbreaks in foothill yellow-legged frogs north of Yosemite.
These complex relationships make it difficult to predict the future. However, ecological models allow scientists to identify broad trends likely to impact Yosemite’s landscapes:
Non-native plants and animals could become more common: changing conditions may allow the survival of species not currently found in the park (as many as 25 new species of mammals and seven species of birds), while eliminating native ones (up to seven species of mammals and eight species of birds).
The lower edge of the conifer forest may creep upward from its current elevation of 3,000 feet, transforming low-lying areas of the park from pine-cedar forests into oak-chaparral scrubland.
Advancing snowmelt and late-summer drying of streams and meadows may alter plant phenology and community composition, changing the wildlife that the ecosystem can support.
Intensifying drought may stress plants, increasing their vulnerability to harmful insects and causing widespread tree die-offs.
Increasing fire activity may lead to permanent ecosystem turnover, as changing conditions prevent historically present species from returning to burned areas.
Climate change has already led to range contraction in almost half of studied land animals and plants in North America. According to the Intergovernmental Panel on Climate Change, most plant species cannot shift their geographical ranges fast enough to keep up with current rates of change. While many shifts underway in Yosemite are subtle, their combined effects over time is likely to fundamentally alter the living tapestry that sustains everything from black bears to giant sequoias.
IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)] (2014): 67.