There are about 20,000 known species of bee on the planet, and nowhere else is this diversity more concentrated than in southern Arizona along the US-Mexico border. Hundreds of bee species can be found in a patch of desert there about the size of Heathrow airport, meaning it has the world’s densest aggregation of bee species yet measured.
Unlike plants and many other organisms that see the highest diversity in the tropics, bees seem be most diverse in warm, dry regions around the globe. So when Robert Minckley at the University of Rochester in New York had the opportunity to study bee populations in the Chihuahuan desert at the US-Mexico border, one of his main goals was simply to count how many species were there.
Minckley and his colleagues targeted a site straddling the border, composed of a former cattle ranch in the Mexican state of Sonora and the San Bernardino National Wildlife Refuge in Arizona. From 2001 through to 2008, the researchers collected bees by leaving out traps in the desert scrub, then later identified the species.
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Out of tens of thousands of individual bees, Minckley and his team counted 473 species from a 16-square-kilometre area.
“That’s a tremendous number of bees,” says Minckley. The researchers estimate that 14 per cent of all North American bee species found north of the US-Mexico border call this region home, the vast majority of which live solitary lives and nest in the ground.
Some other locations in the US have logged more bee species, says Minckley, but these are in national parks, which are far larger, more environmentally variable regions thanks to bigger changes in elevation and habitat across their massive landscapes.
The researchers’ site at the border is a more homogeneous environment, fluctuating by only 120 meters in elevation, and is mostly home to creosote bush, mesquite and cactus plants. The next highest concentration of bee species in the US is found in Grand Staircase-Escalante National Monument in Utah, which has a much greater diversity of vegetation zones, including pinyon-juniper woodland and coniferous forests.
The incredible bee diversity on the border might have multiple causes. The site sits at the confluence of the Sonoran and Chihuahuan deserts, subtropical dry forests and the Great Plains – each of which has its own set of bee fauna.
Another contributor may be the local climate. There is a spring bloom and then a second bloom after the monsoon season starts, providing a twice-yearly flower bonanza for different bee species to feed from.
Karen Wright at Texas A&M University says the findings are “a wonderful achievement”, though not that surprising. Her own sampling project in New Mexico’s Sevilleta National Wildlife Refuge has yielded 372 bee species so far.
Compared with the stable tropics, deserts experience wild climatic swings: hot to cold, deluges to drought. It is possible that this variation has encouraged the evolution of new species, says Wright.
The dryness of these habitats may also help ground-nesting solitary bees, says Minckley. Ground nests are often plagued by fungus, which might be a bigger problem in the wet tropics.
Whatever the ultimate cause of this glut of bee species, documenting biodiversity in places like San Bernardino is important, says Melanie Kazenel at the University of New Mexico. Such research may help scientists “better understand where, when and why [wild] bee declines are occurring”, she says.
Journal Reference: The Journal of Hymenoptera Research, in press
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