Spring forward: warmer climates accelerate life cycles of plants, animals

Science News,  March 8, 2003  by Sid Perkins

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Ever since the winter solstice last Dec. 22, the days have been getting longer in the Northern Hemisphere and the noonday sun has climbed higher in the sky. These are natures' biggest cues that spring is nigh. As warmth gradually returns to the northern temperate latitudes, so do the birds that migrated south last autumn. Once back, they establish territories, make their nests, breed, and fledge their young. Meanwhile, bulbs and seeds sprout, trees bud, and insects emerge and start consuming the tender foliage. Plankton proliferates in lakes and ocean shallows, whereupon larval fish and seabirds begin their feeding frenzies.

Thus the cycle of life begins anew, but with a recent trend toward global warming, the cycle is changing.

Phenologists, who study organisms' responses to seasonal and climatic changes, have noted that the annual cycles for many creatures are beginning earlier on average, as global temperatures rise. Also, some heat-loving plants and animals have taken advantage of a warmer climate to expand their ranges toward the north and south poles or upslope toward higher elevations. Some organisms that thrive only in cooler have retreated from the heat.

On a regional scale--say, a forest--phenological effects on an organism may be masked by factors such as habitat loss, competition from non-native species, short-term variations of climate, or odd circumstances of local geography. On a global scale, however, scientists say the picture is clear: Global warming, regardless of its cause, is having discernable and generally detrimental effects on the planet's ecosystems. In the future, the winter solstice may presage a radically transformed set of biological responses.

THE HEAT IS ON In the past century, the average global temperature has risen about 0.6[degrees]C. That sounds like a small amount, but research on a wide variety of organisms shows that it's enough to drive major biological changes.

If a species responds to warming, it usually does so in one of several ways, says Terry L. Root, an ecologist at Stanford University. The population density of a species may change at locations within its normal range, or that range may shift, she notes. Or the timing of major events in the life cycle of the species-migration, flowering, or egg laying, for example--can accelerate or lag. Other changes, such as in body size or genetic variability within a species, might occur over longer periods, Root notes.

Significant problems can crop up when the intimately connected species in an ecosystem experience life cycle changes at different rates. Consider Europe's winter moth, Operophtera brumata, and the oak tree Quercus robur, which produces the young leaves that are the caterpillar's predominant food. Thirty years ago, the budding of the oak and the hatching of the caterpillars were synchronous, says Marcel E. Visser, a biologist at the Netherlands Institute of Ecology in Heteren. However, he notes, the past quarter century's trend toward warmer springs in Europe has disrupted that timing.

Precisely when the oak buds open is related to, but not solely dependant on, the spring's rise in temperatures. The specific hatching time of the winter moth's eggs is also related to rising temperatures as well as other factors, says Visser. The recent spate of particularly warm springs in some parts of Europe is causing the caterpillars to hatch 2 to 3 weeks before oak buds open. That's not good for the caterpillars, which typically can survive only 2 or 3 days--and absolutely no more than 10 days--without food.

A dearth of winter-moth caterpillars bodes ill for the small European bird Parus major, or great tit. This nonmigratory species--a common, widely studied bird that's similar to North America's chickadees--depends on winter-moth caterpillars to feed its fledglings. A 23-year study of great tits and winter moths at one site in the Netherlands revealed that by 1995, the early caterpillars were hatching about 9 days sooner and developing into moth pupae more quickly than they did in 1973. The birds' egg laying and hatching schedule hadn't changed dramatically, so the fledglings' caterpillar food source was disappearing just when the young birds needed it.

Visser and his colleagues recently conducted a broader analysis of great tits at 23 sites in six European countries. That study showed that some populations have been able to respond to climate change. In Britain, for example, the birds' egg-laying date has shifted earlier, so it now more closely follows the availability of food, Visser and his colleagues report in the Feb. 22 Proceedings of the Royal Society of London B.

SEA CHANGES Although ocean temperatures vary less from year to year and from season to season than air temperatures do, seabirds nevertheless can suffer from phenological shifts in the availability of prey. One spot that's been particularly affected in recent years is Triangle Island, the home of British Columbia's largest colony of seabirds.