Global Warming and the Decoupling of Seasonal Life Schedules

Main source: Grossman, Daniel, 2004, "Spring Forward", Scientific American, January issue, pp. 84-91

An article in the January 2004 issue of the Scientific American (cited above) tells of a man who, living in the vicinity of Oxford, England from 1954 to 2001, systematically recorded the first-flowering dates of hundreds of local plant species, spring arrival times of scores of birds, late-summer departure dates of butterflies, and other seasonal information for his home area during every year of that entire 47-year period. In 2001, his son decided to analyze these records in relation to the evidence of global warming, so he compared the flowering dates for each decade. Remarkably, he found no consistent patterns from the 1950s to the 1980s, but when he compared the 1990s decade with those of the previous 4 decades, he found that 385 plants flowered an average of 4.5 days earlier, and that some 60 species flowered on an average of fully two weeks earlier. This is an extremely sudden change for a single decade, but a number of recent studies show similarly rapid changes in life patterns around the world.

These rapid changes are breaking the links among food chains, and straining the fitness of some creatures with their habitats. For example, Great Tit birds in the Netherlands today are continuing to lay their eggs at the same time as they did back in 1985, but during the same interval mid-spring temperatures have risen 2 degrees Celsius and the quantity of Winter Moth caterpillars with which these birds feed their chicks now peaks two weeks earlier than it did in 1985 (when it peaked just when the Great Tit hatchlings needed these caterpillars the most). In the 1990s and today, by the time most chicks have hatched, the caterpillar season is waning and only the earliest chicks are getting fed.

Moreover, the Winter Moth's caterpillars must hatch at exactly the time when the oak leaves on which they feed come open -- if the insect hatches more than five days before the buds burst, it will starve, or if it hatches more than two weeks too late (since by then oak leaf tannin makes the leaves inedible to the caterpillars). Bud burst is in fact now occuring about 10 days earlier than it did 20 years ago, while the caterpillar's hatchings have changed even more, so they must now wait about 8 days for food.

Marcel Visser, leader of the Netherlands Institute of Ecology team studying the population dynamics of the Great Tit, notes that so far this decoupling between links in the food chain has not produced a noticeable decline in Great Tit numbers, but if it continues to grow there can be no doubt that the populations will begin to drop, and he adds that "if we go to other food chains we'll find the same thing."

For those of us living in the Sonoran Desert, this should be alarming news. While we Associates of Saguaro Juniper lack the capability to make similarly detailed studies for our area and its creatures, we would be wise to try to develop projects that could contribute to understanding what similar kinds of changes may be happening around us. A data bank of some kind that compares -- for example -- flowering schedules of some of our most significant species -- as devised by McGinnies for the 1966-1985 period for our part of the Sonoran Desert (see this link) -- would be a good start, against which to measure changes that may eventually involve our local fauna. Maybe we need to set up some standard chart locations with lists of the species we want to monitor.

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