David Thomson studied the pollination of the glacier lily in the Rocky Mountains of Colorado over 17 years. The lily relies on queen bumble bees for pollination. (David Inouye/University of Toronto)

A 17-year study in a pristine mountain environment has found a 50 per cent decline in bee pollination, and suggests climate change may be to blame.

"To my mind, it gives some additional legitimacy to that concern that pollination systems are in trouble," said James Thomson, a University of Toronto ecologist, of his study of glacier lilies in the Rocky Mountains of Colorado, which rely on bumble bee queens for pollination.

Other studies have reported declines in honeybees in Europe and North America in recent years, and some have blamed the reduction on pesticides or the spread of parasites by human activity. In some cases, adult bees have disappeared from colonies full of honey and pollen — a phenomenon dubbed "colony collapse disorder."

Thomson's study showed a decline in pollination far from human agriculture.

Over nearly two decades, Thomson compared the fruiting rate of glacier lilies that were artificially pollinated by hand and those that were left to be pollinated naturally by bees. Over time, the fruiting of the naturally pollinated plants declined relative to the hand-pollinated plants.

Thomson said his is the first plant-based study that was long-term enough to show that kind of pollination decline and not just a short-term difference between the fruiting success of plants that were hand pollinated and those pollinated naturally.

Thomson said he saw no evidence that bees had declined at the study site over time: "My general sense from walking around in field is that there are as many as there used to be."

However, the lack of bee pollination was especially pronounced in the spring, leading Thomson to suspect it might be caused by the plants blooming earlier and earlier in the year, before the bumble bees become active after their winter hibernation.

The study found that when the lilies bloomed depended on when the snow melted, and the snow melt has been getting earlier. For example, in 2006, the bloom took place a month earlier than in 1995 — "a noteworthy displacement in a habitat where the entire growing season is considered to last only three months," the paper said.

"If the flowers are starting so early that the weather is really bad," Thomson said in an interview, "bees may have broken out of their hibernation, but they may not be working that hard visiting flowers."

Thomson cautioned that the effects seen in the glacier lilies might not be applicable to other plants. But he noted that the decline in pollination is something that scientists have been warning about based on bee studies and shorter-term plant studies.

"In general, we have a pretty shaky notion of what has caused the declines," he said.

He added that he hopes to see other long-term studies to find out how widespread pollination decline is.

"You can't look at questions about the effects of climate change with short-term studies," he said. "You simply need a pretty extensive time span to detect whether an effect is worth paying attention to or not."