March of the Mule Deer

Every year in the American West, mule deer complete a long journey that begins in spring in their winter ranges at lower elevations and ends in higher elevations in their summer ranges. Their movement along this journey is carefully timed to take advantage of the emergence of the new, nutritious plant growth at the beginning of spring, which doesn’t start everywhere at the same time.

In mule deer country, spring starts first in lower elevations that are quicker to warm, then moves uphill across the landscape until finally reaching cooler, higher elevations. Mule deer follow this progression of spring and the resulting spring “green-up,” which scientists call “surfing the green wave.”

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In Nature, Timing is Everything

Mule deer aren’t the only ones that carefully time their seasonal activities. Timing in nature is crucial, even for people. Just ask a farmer or gardener how important it is to plant their crops on schedule.

Plants and animals have evolved to be at the right place at the right time and to avoid being in the wrong place at any time. They time things like winter dormancy/hibernation, flowering/reproduction and migration at the ideal times to ensure they preserve energy when resources like food are scarce, capitalize on resources when they are plentiful and to avoid dangerous conditions, like bad weather. There can be serious consequences for plants and animals if they don’t get the timing right.

Studying the timing of these seasonal biological activities is a field of science called phenology, which USGS partner USA National Phenology Network describes as “nature’s calendar.”

Nature’s calendar is not fixed; it fluctuates year-to-year. Plants and animals use signals from nature, such as nighttime temperature and precipitation, as alarm clocks for when to start their seasonal biological activities. Since weather patterns aren’t exactly the same every year, the alarm clocks plants and animals use will “go off” at different times from one year to the next.

Migrating animals take cues from nature to determine when they should start migration. For example, mule deer likely know it’s time to boogie when the spring green-up begins in their winter ranges. Timing for migrations is especially important because conditions at the beginning and end of their journeys can be vastly different.

Climate Change is Messing with Nature’s Alarm Clocks

As people add more greenhouse gases to our planet’s atmosphere, long-term weather patterns, or climates, around the world are changing. As a result, the timing of the signals that plants and animals use as an alarm clock is changing too.

In fact, phenology is so sensitive to climate change that it is an early warning sign that an area is being impacted by it. In many places, spring is starting sooner and fall is starting later. Seasonal precipitation patterns are also changing.

Unfortunately, plants and animals don’t all use the same cues from nature as their alarm clocks, so nature’s calendar isn’t just shifting because of climate change, it’s getting out of sync.

When Nature Sends Mixed Signals

In ecosystems, the seasonal biological activities of plants and animals are carefully timed to coincide with or follow the seasonal biological activities of other plants and animals, like an intricate chain of dominoes that is set off by different people in different places in the chain. An often-used example is the springtime nesting of birds that is timed to take advantage of an abundance of insects that have, in turn, timed their emergence to take advantage of the new plant growth in spring, which is timed to avoid killing frosts.

Nature’s climate-related signals like temperature and precipitation are changing, but plants and animals aren’t responding to those changes in the same ways. In the previous example, one change in the phenology of either the plants or the insects could lead to a disastrous nesting season for the birds.

When an ecosystem’s phenological dominoes don’t line up like they used to, this is called phenologic mismatch.

Migrating animals are especially vulnerable to phenological mismatch because climate change isn’t impacting every place the same or to the same severity. An animal might get the cues from a winter range signaling that it is time to start spring migration only to arrive in its summer range too early or late to take advantage of the resources that it migrates to take advantage of. Over time, scientists fear this will lead to population declines and even extinctions, unless the animals can behaviorally adapt.

For Mule Deer, Mixed Signals Don’t Lead to Missed Connections

Recently, USGS scientists had a unique opportunity to study a population of mule deer whose winter range in a Wyoming desert doesn’t produce consistent springtime signals for migration like mule deer experience elsewhere.

The study used movement data from 72 adult female mule deer over a 10-year period and found that deer in the study started migration 70 days before to 52 days after the wave of spring green-up, to which mule deer typically pace their migrations.

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Incredibly, despite these early or late starts, the deer all arrived at their summer range within an average 6-day window. How? Early deer took a more leisurely time, traveling slower and spending more time on layovers. Late deer put the hustle on and spent less time on layovers.

The study authors suggest that the deer’s ability to adjust their migration strategies en route—by slowing down or speeding up, for example—was a behavioral response to cues along the migration route. This would mean deer possess cognitive abilities to recognize where they are in space and time in relation to the wave of spring green up.

The findings of this study offer hope that some migratory animals, at least, can behaviorally adapt to compensate for phenological mismatch to connect with the resources they need to survive.