Milder winters have prompted growing numbers of some birds, such as the Canada goose, to skip migration altogether. The birds that don’t migrate are in danger of being wiped out by a sudden cold spell.
PHOTO: ISTOCKPHOTO/PAUL TESSIER
A newly hatched chick waits with hungry mouth agape for a parent to deliver its first meal. A crocus peaks up through the snow. Rivers flow swiftly as ice breaks up and snows melt. Sleepy mammals emerge from hibernation, and early frog songs penetrate the night.
Spring awakening has long provided fodder for poets, artists and almanac writers. Even for a notoriously fickle time of sunshine, rain and temperature swings, some old-fashioned seasonal wisdom was consistent enough to be passed down through generations. The first blooming of a specific flower, for example, could traditionally signal when to find certain fish running the rivers, when to hunt for mushrooms, or when to plant crops. The timing of such seasonal events is coordinated in an intricate dance — a dance underappreciated, perhaps, until something jolts it out of step.
With global average temperatures up 0.5 degrees Celsius since the 1970s, springtime warming is coming earlier across the Earth’s temperate regions. A number of organisms have responded to the warming temperatures by altering the timing of key life cycle events. The problem, however, is that not all species are adjusting at the same rate or in the same direction, thus disrupting the dance that connects predator and prey, butterfly and blossom, fish and phytoplankton, and the entire web of life.
The timing of seasonal biological events, otherwise known as phenology, has been tracked in some places for centuries. Japan’s much-feted cherry tree blossoming has been carefully recorded since before 1400. The trees showed no clear trend in timing until the early 20th century, when they began to bloom earlier, with a marked advancement since around 1950.
The meticulous records of Henry David Thoreau help us gauge how spring has changed in Concord, Mass., since the mid-1800s. Comparing his notes on over 500 species and subspecies of plants with modern surveys and records in between, researchers found that springtime blooming advanced by an average of one week over the past 150 years as local springtime temperatures rose.
The plant varieties that advanced their timing appear to have thrived over the years, while others declined in numbers. The varieties left behind include asters, mints, orchids, lilies and violets. Some native plants advanced their blossoming dramatically: the highbush blueberry by three weeks and the yellow wood sorrel herb by a month. Yet these native plants may be the exception rather than the rule. On average, non-native invasive plants advanced their bloom by 11 days more than natives. With exotic invasives appearing to adapt more quickly to warming temperatures, the concern is that they could outcompete some native plants, leading to their disappearance.
Earlier springs and later autumns mean longer growing seasons — as long as plants do not succumb to a surprise late cold snap or wilt in the peak summer heat. In Germany, apricot and peach trees now bloom more than half a month earlier than in 1961. Apple trees in the northeastern United States moved up flowering by eight days between 1965 and 2001. Apple trees require chilling time before they flower, and warmer winters have been tied to smaller harvests. Earlier spring blooming has lengthened pollen seasons in some places by weeks. Allergy sufferers beware: This trend is likely to get worse as the planet gets warmer.
A longer growing season could benefit some crops, such as the sugar beet. For other foods, however, including important cereals such as rye, the increased early-season temperatures could hurt yields by pushing plants to devote more energy to vegetative growth than to the seed that we eat. The premature warming also elevates the risk of damage from late frosts. In 2007, for example, a warm March in prime U.S. agricultural regions pushed spring into gear early, only to be followed by unusual cold in April. The damage to the nascent crops exceeded $2 billion.
Exactly how these changing plant communities will interact with pollinators and foragers that may or may not be changing at the same pace remains unanswered. Members of the animal kingdom are responding to warming in different ways. A quintessential early bird, the American robin, now sometimes makes an even earlier springtime debut. In the Colorado Rocky Mountains, where robin migration is not just south-to-north but also up to higher elevations, the birds have responded to warming in their wintering grounds by traveling to their high-altitude summer breeding grounds two weeks earlier in 2009 compared with the early 1980s. In some years the robins arrive long before the snow has melted — making it far more difficult for this early bird to catch the worm.
For pied flycatchers that breed in the Netherlands, migration timing from their West African wintering grounds has not changed, but earlier spring warming has caused the birds to breed about as soon after their arrival as possible. Unfortunately, their caterpillar food has been able to respond even more strongly, advancing hatching in one woodland by an average of 15 days over two decades, while the birds only advanced by 10 days. At sites where the caterpillar populations still peak somewhat late, flycatcher populations have dropped by 10 percent, but where the caterpillars have advanced hatching the most, flycatcher populations have plummeted about 90 percent.
Across Europe as a whole, populations of birds that did not advance their migration time along with earlier spring warming have shrunk since 1990. Short-distance migrants seem to be faring better than those traveling long ways. Milder winters have even prompted growing numbers of some birds, such as the Pacific brant and Canada goose, to skip migration altogether. Like the early crops, however, the birds that stay are in danger of being wiped out by a sudden cold spell.
As evidenced by the caterpillars in the Netherlands, short-lived insects have some of the fastest life cycle responses to global warming. In Central Europe, where almost every summer since 1980 has been hotter than the long-term average, warming has allowed some species of butterflies and moths to become active earlier and actually add an extra generation in the year — something not seen among those species in records dating back to the 1850s. If predation does not increase, a population explosion could overstress the plants that the butterfly and moth caterpillars eat.
High-altitude mountain pine beetles in western North America present a similar case. In warmer weather, they can complete their life cycles in one year instead of two. Insects that once were active for just two weeks a year now can be found flying for up to six months, leaving devastated forests in their wake. Earlier springs and milder winters are also linked to an increased incidence of tick-borne encephalitis and other diseases spread by insects that do well in the warmer conditions.
In addition to or instead of adjusting life-cycle timing, some organisms have responded to warming temperatures by shifting their geographical ranges, often poleward or upward. Bird and butterfly range shifts averaging 6 kilometers per decade have already been observed, with some species moving quite faster. There are, of course, limitations to all these adaptations. Species that are even more mobile can gain only so much altitude before running out of mountain or can travel only so far before becoming blocked by pervasive human development.
Some wildlife take their timing cues from environmental factors other than temperature. The snowshoe hare, for instance, appears to rely on changes in day length to signal when to transform its coat color from winter white to summer brown. While day-length patterns are the same from year to year, snow in the hare’s Montana wilderness habitat now melts up to a month early. If hares are not able to speed up their coat change, they will be in trouble: A stark white hare on bare ground is a remarkably easy target. And as go the hares, so go the lynxes that feed almost exclusively on them.
Tinkering with an incredibly complex and interconnected system is fraught with risk. These mismatches are just some examples of how a hotter world is a world unlike any we have known. It is still too early in this global experiment to tell which creatures will be the climate winners and losers, but the signs indicate that the losers will be the majority. Turning down the global thermostat by cutting greenhouse gas emissions is the only way to avoid the risk of throwing nature further out of sync.
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