Scitech | Honeybee decline a mystery

Cooperation vital to life on earth

You may not realize it, but mutual back-scratching is one of the most important dynamics in the natural world. Earth would not be a green planet were it not for the fungus mycorrhizae, which helps plants get nutrients from the soil in exchange for water. Flowering plants would never have evolved were there no insects to pollinate them in exchange for nectar.

So it’s unsurprising that a dynamic so important in nature is also important in human society. A 2006 report from The National Academy of Sciences estimated that, in North America alone, an estimated $15-billion worth of crops rely on honeybees for pollination. Without honeybees, the continent would have nothing to eat.

The problem is that honeybee populations are on the decline in North America. According to a Congressional Research Services report on the situation from August 2007, an estimated 651,000 to 875,000 of the U.S.’s approximately 2.4 million colonies were lost during the winter of 2006-2007. The cause of the bees’ decline is not yet known, although proposed explanations include climate change, genetically modified crops, parasites, pesticides, and the stresses of long-distance transport. Not surprisingly, these are all human-induced changes whose impacts are still unclear.

The mystery of honeybee decline ultimately stems from a lack of information on how the bees interact with the environment, and the National Academy of Sciences report emphasized the need for research into honeybee ecology.

“Effective conservation or restoration of pollinator populations requires comprehensive knowledge of their biology. Current knowledge is insufficient to inform conservation and management programs,” the report brief states.

Ants abandon acacias

But recent research in Science suggests that it may be difficult to understand the honeybee problem. The research, conducted by Dr. Todd Palmer of the University of Florida, shows how back-scratching relationships – or mutualisms – can be undermined in unpredictable ways. Palmer found that when large, African herbivores like giraffes and elephants are removed from their native savanna, it destroys the mutalistic interactions of other species.

Acacia trees grow in African savannas and “hire” ant colonies to guard them from herbivores. In return, the trees reward the ants with nutritious sugar-rich nectar and swollen thorns in which to live. Palmer’s study revealed that when the herbivores disappear, acacias sense that protection is no longer needed. As a result, they stop making nectar and thorns, and the ants leave. From his research site in Kenya, Palmer noted that while the ant-acacia reciprocity probably established itself over many millennia, once the giraffes and elephants were removed it took only 10 years to dissolve – the blink of an eye by ecological standards.

“Although the mutualism has likely evolved over extremely long timescales, it falls apart very rapidly,” he said.

But destruction of this mutualism did not result in a thriving forest of acacia trees. Palmer’s research found that, as the acacias start to ignore the ants’ needs, the ants start to ignore the acacia’s needs. They relax their guard and protect their home acacias about half as often, allowing wood-boring beetles to invade. Beetle invasions create new living space for the ant colonies, but at a deadly cost: the acacia often dies.

Palmer commented that the ultimate effect of removing herbivores from the ecosystem came as a shock to the researchers involved.

“If you had asked me 10 years ago, ‘What would happen if you took large herbivores out of the system?’ I would have answered, ‘I’ll bet the trees would be really happy!’” he said.

One for all, all for one

Many conservation plans aim to conserve single species, but Palmer feels that focus should be on the preservation of complete ecosystems. Without complete ecosystems, individual species can have trouble surviving.

“Without an understanding of species’ interactions, the unintended manipulations humans are continually causing could lead to potentially catastrophic effects,” he said.

The complex inter-relationships revealed by the Science paper show that without comprehensive knowledge, predictions and explanations about ecological relationships are hard to come by. It’s possible that the trend of declining bee populations in North America is due to a complex interaction like the one shown in Palmer’s paper, and will require equally rigorous research to understand.

Research must examine the many intimate and overlooked interactions of the natural world, and the role they play in holding it together. Tug on one strand in the web of life, and it might all unravel.