Bee diversity
Twelve different species of bees swarming a flowery meadow. Etching by J. Bishop, after J. Stewart.
Credit: Wellcome Collection, CC BY 4.0
How many bee species are there? Wait a minute: honeybee, bumble bee, erhm… five? Five hundred? Five thousand? Not even close: the total is well over 20,000 – which means there are more species of bees than of birds and mammals combined.
There's no shame (nor surprise) for bee civilians like you or me in not knowing that. What is surprising, is that even scientists who specialise in bees didn't quite know how those species are distributed all over the world. Until now.
By combining and filtering more than 5.8 million public records of bee occurrences, a team of researchers from China, the U.S., and Singapore have built up the very first comprehensive picture of bee diversity worldwide. And that picture presents a few surprises, both for laypersons and specialists.
Bee ignoramuses will be surprised to learn that the United States is the throbbing heart of bee diversity. The U.S. has far more bee species than any other region on Earth. And by the fact that large tracts of Africa and the Middle East remain terra incognita, in terms of apiary diversity.
Counter-intuitive distribution
Relative bee species richness in the New World. Note the low density in the Amazon Basin.
Credit: Current Biology, open access
In general, there are more bee species in the Northern Hemisphere than the Southern and—confirming previous hypotheses–more in arid and temperate climates than in the tropics.
That goes against the common pattern in biology known as the 'latitudinal gradient', which predicts that species diversity (of most plants and animals) increases towards the tropics and decreases towards the poles. Bees are an exception, with a higher species concentration away from the poles (in what scientists call a 'bimodal latitudinal gradient').
To give that difference some visual immediacy, imagine a graph with one hump in the middle (i.e. the latitudinal gradient) versus one with two humps, one on either side of the middle (i.e. the bimodal latitudinal gradient). In other words: dromedary (one-hump) versus camel (two-hump).
It seems counter-intuitive that bees would thrive better in arid deserts than in lush tropical jungles; but that's because trees–the dominant vegetation type in the tropics–provide less bee food than the plants and flowers that grow elsewhere.
Much-needed baseline
Three ways of measuring species richness in the Americas: (A) richness of polygons, (B) sPCA and (c ) turnover. All suggest a large, distinct bee fauna in the southwestern U.S.
Credit: Current Biology, open access
Also, bees don't like it too wet, unlike their cousins the ants, whose populations peak in the humid tropics. The researchers think humidity may play a role in limiting bee distribution by spoiling pollen resources.
The relative absence of bees from the tropics has consequences for pollination, which in those regions is performed by a wide variety of alternative species: wasps, moths, and even cockroaches.
Previous datasets of global bee distribution were either inaccurate, incomplete, or difficult to interpret. This world map clearly establishes that bees prefer dry and temperate zones to wet and tropical ones. For bee scientists, it provides a much-needed baseline to predict the geographic distribution of bees and interpret the relative richness of species.
While much work needs to be done to fill additional knowledge gaps, this baseline is an excellent starting point, not just for greater understanding, but also for better conservation. Because bees are not just for making honey. In many countries, they're the top pollinator species. And they typically visit 90 percent of the leading crop types.
Carpenter bee (Xylocopa latipes) pollinating a flower in the Indian state of Kerala.
Credit: Charles J. Sharp (Sharp Photography), CC BY-SA 4.0
Yet over the past decades, bee populations have been crashing. In the U.S., honeybee populations have declined by 60 percent between 1948 and 2008. In Europe, 12 wild bee species are critically endangered.
That trend is potentially disastrous for agriculture. More than $550 billion in annual global crops are at risk from pollinator loss. The loss of bees as pollinators would lead to a collapse in crop yields and even entire ecosystems.
Better understanding bees increases our options for protecting them. This study will help pinpoint bee diversity hotspots in otherwise poorly examined parts of the world and help predict how bees will react to climate change–for example when certain regions will get wetter weather.
Protecting bee diversity is especially important and urgent in developing countries, where many of the knowledge gaps are located, and where many crops rely on native bee species for pollination.
Michael C. Orr et al.: 'Global Patterns and Drivers of Bee Distribution' is published in Current Biology.
Strange Maps #1060
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Bees have endured a disastrous half-century. In the winter of 2018, U.S. beekeepers reported losing 37.7 percent of their honeybee colonies. It was the largest die-off reported since the Bee Informed Partnership began its survey in 2006, yet in that decade, average winter losses of managed colonies were 28.7 percent. That's near twice the historic rate and part of a 50-year trend of declining species richness in wild bees and other pollinators.
That's bad news for the bees and also anyone who depends on the food generated through their labor. That is, all of us. According to the USDA, approximately 35 percent of the world's food crops depend on animal pollinators to reproduce, with some scientists estimating that "one out of every three bites of food we eat exists because of animal pollinators."
That many crops depend on pollination to reproduce is well-established; however, how much pollination proves a limiting factor to crop yield is less understood. If wild bee and managed honeybee populations continue to decline, will the amount of food available to feed us decline, too? That's the question a Rutgers-led team of researchers sought to answer.
From bee to farm to table
A bar graph showing the percentage of pollination limitation for the seven crops studied.
(Photo: James Reilly, et al/The Royal Society Publishing))
The research team selected seven crops to study: apples, almonds, pumpkins, watermelons, sweet cherries, tart cherries, and highbush blueberries. These were chosen because each is highly dependent on insect pollination for reproduction. The researchers then established a nationwide study across 131 U.S. and British Columbia farms. They selected only commercial farms in top-producing states—for example, Michigan and Oregon farms for blueberries. This way, their sample would represent the conditions and farming practices in which a majority of these crops are grown.
After collecting data on pollinator visitation rates and crop production, the researchers measured the data through three statistical models. They also analyzed the contribution differences between wild bees and managed honeybees as well as the economic value of the bees' service.
"We found that many crops are pollination-limited, meaning crop production would be higher if crop flowers received more pollination. We also found that honey bees and wild bees provided similar amounts of pollination overall," Rachael Winfree, a professor in the Department of Ecology, Evolution, and Natural Resources at Rutgers University-New Brunswick and the study's senior author, said in a release. "Managing habitat for native bee species [and] stocking more honey bees would boost pollination levels and could increase crop production."
Of the crops studied, apples, blueberries, sweet cherries, and tart cherries were hit hardest when pollination decreased. Watermelon and pumpkin yields weren't as limited by pollinators, possibly because these crops sport fewer blooms and flower in summer when the weather is less inclement. Almonds proved the outlier as the crop is the earliest bloomer yet not pollination limited. The researchers speculate that this is due to the almond industry's intense reliance on managed honeybees.
"Our findings show that pollinator declines could translate directly into decreased yields or production for most of the crops studied, and that wild species contribute substantially to pollination of most study crops in major crop-producing regions," the researchers write.
For the seven crops studied, the researchers estimate the annual production value of pollinators to be more than $1.5 billion. They also found that wild bee species provided comparable pollination, even for crops in agriculturally intensive regions.
Their findings were published in the most recent Proceedings of the Royal Society B: Biological Sciences.
Ecological and edible incentives
A protester shows a handful of bees that died by pesticides. The protest was held during the Bayer AG shareholder meeting in 2019.
(PhooMaja Hitiji/Getty Images)
The concern extends beyond these seven. Crops such as coffee, avocados, lemons, limes, and oranges are also highly dependent on pollinators and may prove pollination limited. If declining bee populations are tied to such yields, it could mean barer supermarket shelves and increased prices. While that may only be an annoyance to some, to poor and vulnerable communities who already struggle to secure salubrious, affordable food, such a deficit would present another barrier to the vital micronutrients necessary for a healthy life and diet.
Unfortunately, the threats to bees are numerous. Parasites, agrochemicals, monoculture farming, and habitat degradation all play a role, and neither stressor works in isolation. Sublethal exposure to neonicotinoids, an insecticide, can cause impairments in bees, while monoculture farming serves up a monotonous and unhealthy floral buffet. Both impede bees' immune systems, rendering them vulnerable to parasites such as Varroa destructor, a mite that can transmit debilitating viruses as it feeds on bees' fat bodies. And all of these stressors will likely be inflamed by climate change in the years to come.
Some have proffered mechanical solutions, such as Japan's National Institute of Advanced Industrial Science and Technology where technicians are developing robotic bees. These micro-drones are covered in gelled horsehair and have successfully cross-pollinated Japanese lilies. Other experiments include pollen sprays. However, the large-scale viability of tech-centric solutions seems questionable. After all, wild bees currently perform their ecological services pro bono and are as effective as managed honeybees. Any technological solution implemented in their absence would add to the agricultural costs and likely increase prices anyway.
Ecological amelioration will be necessary. To combat habitat fragmentation and strengthen biodiversity, many cities are implementing green-way strategies. For example, the Dutch city of Utrecht has decked its bus stop roofs with plants and grasses to create bee and butterfly shelters, while other cities are looking to foster bee-friend roadsides. And government initiatives incentivize farmers and landowners to adopt bee-friendly management practices. These solutions aren't only a matter of ecological conservation but also food security and public health.
