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.

A need for bees About 35 per cent of the world's crop production is dependent on pollinators such as bees, birds and bats, scientists estimated in the Proceedings of the Royal Society B: Biological Sciences in 2006. Another study the same year found many wild plants around the world weren't getting pollinated enough, especially those in areas with a lot of species, such as tropical jungles. The authors of the study published in Proceedings of the Natural Academy of Sciences said some could be at risk of extinction as a result. It's not just a problem in the tropics either — the diversity of bees and the flowers they pollinate have both declined significantly in Britain and the Netherlands over 25 years, reported a 2006 study published in Science

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."