Did ancient superbees squash diversity?

The recent discovery of several dozen extinct bee species in ancient amber deposits has paleontologists buzzing. They’re wondering if the very success of some bees’ social lifestyle provided an evolutionary bottleneck that led to today’s dearth of hive-dwelling species.

Entomologist Michael S. Engel of the University of Kansas in Lawrence thinks so. He describes the unexpected diversity of ancient bees and their demise in the Feb. 13 Proceedings of the National Academy of Sciences.

Eusocial insects, which live in colonies and have a social structure that divides household chores among different specialized castes, are dominant members of many ecosystems. Worldwide, there are more than 1,000 species of social wasps, 2,700 species of termites, and about 16,000 species of ants. Many scientists believe that these insects’ social lifestyle has fueled their ecological dominance and diversity, Engel says.

But that pattern doesn’t seem to apply to bees. Only two major groups, made up of about 380 species, are eusocial, Engel notes. Most of today’s 20,000 bee species, such as carpenter bees, are loners.

Modern eusocial bees represent only “the remnants of a once-great diversity,” Engel says. His analysis of fossils trapped within 45-million-year-old Baltic amber showed at least 38 species of eusocial bees, making up nine major groups.

“I never expected this staggering array of diversity,” Engel told Science News. “It blew me away.”

All these bee species, however, received a flick from evolution’s fickle finger and have since gone extinct. According to Engel’s research, most of the missing Baltic bees died out within a mere 10 million years.

This is an unusual extinction pattern for insects, says Conrad C. Labandeira, a paleobiologist at the National Museum of Natural History in Washington, D.C. Major insect groups typically have a much slower turnover rate, he notes.

The end of the Eocene, about 34 million years ago, was marked by dramatic geological changes, but Engel suspects other factors also drove the bee extinctions. He contends that an explosion of diversity among bees may have produced a “superbee”-an aggressively foraging eusocial competitor that swarmed over the landscape and eventually overwhelmed the others. Loner bees, which tend to fill different ecological niches, didn’t experience such competition and so were able to maintain a higher level of diversity.

Although the western honeybee, Apis mellifera, evolved far more recently, this eusocial species may provide a modern analog for what happened in the Baltic long ago. A mainstay of modern agriculture, this bee is one of only seven species of honeybee worldwide. Engel says that wherever A. mellifera has been introduced, native bees have suffered. Indigenous social bees are especially hard hit because they generally can’t compete against this honeybee for food and nesting spots, he says.

When the Baltic amber entrapped its victims about 45 million years ago, the world’s major land masses were closer together than they are today, Labandeira notes. With the warmer climate of that time, an evolutionarily superior bee could have spread unhindered among the continents, he says.