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.
According to the U.S Department of Agriculture, some 35% of our food crops depend on bee pollination. That means about one of out every three bites of food comes to us courtesy of insects including bees, butterflies, and beetles, or from birds and bats. With the world's pollinator populations in a snowballing state of decline, scientists are racing to discover what's causing this and how it can be stopped. In the case of bees, pesticides — especially neonicotinoids — are the leading suspect, and studies find these chemicals infiltrating a significant percentage of bee hives. (Pesticides also show up in our own food and drink.)
Research prior focused on the damaging effects these neurotoxins have on adult bees. Now a study of young bee brains, led by Richard Gill of Imperial College in London and published in Proceedings of the Royal Society B, shows that neonicotinoids do disastrous, irreversible damage to bees' neurological development.
Broken learning
The study involved introducing neonicotinoids to the nectar consumed by members of 22 Bombus terrestris audax (buff-tailed honeybee) colonies. The learning abilities of their offspring were then measured against a control group of young bees from colonies whose food supply had not been contaminated.
The test assessed the extent to which a bee could learn to associate a specific smell with a reward, which was a sucrose solution. The bees from the neonicotinoid colony consistently fared more poorly than the control population.
Image source: Mr. Meijer/Shutterstock
Tiny computed micrography (CT) scans
In hopes of identifying a structural explanation, the researchers stained the brain cells of 100 bees from the exposed colonies and took non-invasive micro-CT scans in a machine similar — albeit smaller — to those in which humans are medically imaged.
The researchers discovered a clear difference in brains of the young bees from pesticide-exposed colonies. A key brain area, the mushroom body, was found to be much smaller in these bees' brains than it was in those from control colonies. This makes sense, since this region is believed to be involved in olfactory learning and memory.
The tests and scans were performed three days after pupal hatching and again after 12 days. The substandard learning capabilities and mushroom body sizes had not been resolved by the second test, indicating to the researchers that the damage caused by the neonicotinoids was irreversible.
(A honeybee's life expectancy depends on its role. Drones live roughly 8 weeks, while sterile workers live for about 6 weeks in the summer or 5 months in the winter. A queen can live for a few years.)
Several views of the mushroom body
Image source: Gill, et al
Why this matters
The study's conclusion does not say definitively that the mushroom area is the only brain region impacted by pesticides. However, a smaller mushroom area is significant, explaining, as it does, the mechanism by which a bee's learning abilities and behavior may be impaired over the course of its life.
Gill says in a press release, "Worryingly in this case, when young bees are fed on pesticide-contaminated food, this caused parts of the brain to grow less, leading to older adult bees possessing smaller and functionally impaired brains; an effect that appeared to be permanent and irreversible."
In fact, after the young bees were returned to their colonies, researchers saw lower-than-expected colony growth two to three weeks after the subjects' reintroduction.
"If future generations of workers are predisposed to be inefficient functioning cohorts, this could lead to a density-dependent build-up of colony-level impairment increasing the risk of colony collapse." — Gill, et al
Illustration of *Bombus terrestris audax*
Image source: Duda Vasilii/Shutterstock
And then there’s the adult bees
In addition to the problems caused by the behavior of bees hatched with pesticide damage, it's not as if pesticide exposure necessarily abates later on. As lead author of the study Dylan Smith explains, "There has been growing evidence that pesticides can build up inside bee colonies. Our study reveals the risks to individuals being reared in such an environment, and that a colony's future workforce can be affected weeks after they are first exposed."
The study concludes that simply looking at the damaging effect of pesticide on adult bee population misses a significant, and more far-reaching, part of the story:
"Bees' direct exposure to pesticides through residues on flowers should not be the only consideration when determining potential harm to the colony. The amount of pesticide residue present inside colonies following exposure appears to be an important measure for assessing the impact on a colony's health in the future. " — Gill, et al
- Since 2006, the Bee Informed Partnership has conducted a survey on U.S. beekeepers. The most recent survey shows that the 2018 winter resulted in the biggest die-off since the survey began, with a loss of 37.7 percent.
- This die-off is part of a larger trend. Bee populations have been falling for decades.
- The reasons why are multifaceted and compound on one another.
Every year since 2006, the University of Maryland's nonprofit Bee Informed Partnership (BIP) conducts an annual survey to determine how many bee colonies were lost over the course of the year. The 2018–2019 survey asked 4,696 U.S. beekeepers to report how many colonies they lost, and the preliminary results of the survey suggest that things aren't looking so good.
Of the more than 319,000 managed bee colonies in the survey, 37.7 percent were lost over the winter. This represents the largest die-off since the survey began, and a full 7 percentage points higher than the previous year. Having fewer honeybees is more than just an ecological problem, it's also an economic one: Every year, honeybees contribute a nearly $20 billion value to U.S. crop production.
Bees are so integral to local ecosystems and economies that some states are paying residents to engage in bee-friendly practices. Minnesota, for instance, is paying residents to cover their lawns with bee-friendly plants such as creeping thyme, self-heal, and Dutch white clover; Virginia is giving away free beehives; and the vast majority of states offer tax exemptions for beekeepers. This is smart policy — without bees, grocery stores would be considerably emptier than they are now.
What's killing the bees?
This particular winter die-off is part of a much larger trend; honeybee populations have been in a major decline for the last 50 years. There are a variety of reasons, each of which interacts with and compounds each other.
First, bee habitats are disappearing or changing profoundly. Many wild bees are losing their habitats, but managed bee colonies are also made to live in habitats that aren't ideally suited to healthy bee populations. Many managed colonies exist on farmland or are brought to farmland to assist in pollination. As a result, honeybees feed on the pollen and nectar from just one or two kinds of plants. Biologist Dave Goulson and colleagues explained the impact of this in a research paper:
"If a human were to consume nothing but sardines one month, chocolate the next, turnips the month after, and so on, one could reasonably expect that person to fall ill. This may seem a frivolous example, but it is a reasonable parallel to the experience of some honey bee colonies, particularly those in North America that are transported back and forth across the continent each year to provide pollination for major crops such as almonds in California, blueberries in Maine, and citrus in Florida."
Photo credit: Matthew T Rader on Unsplash
During the wintertime, bees are often supplied a single food source by beekeepers as well, such as corn syrup. This monotonous diet has been linked to a weakening of bees' immune systems and to colony collapse disorder, in which the majority of worker bees abandon a colony along with its food, young, and queen.
A poor diet, in turn, makes bees more susceptible to parasites and diseases. There are several of these, but one of the most worrisome one is the Varroa destructor mite. The aptly named V. destructor can wreak havoc on bee colonies, primarily because North American bee species are not resistant to it. Varroa mites originated in Asia, and there they would have stayed if bee colonies were not shipped around the world.
V. destructor can be damaging to bee colonies first because they act as a vector for disease, especially deformed wing virus, an RNA virus that causes various physical deformities.
More directly, V. destructor also weakens bees by feeding on their fat. This becomes problematic since bees rely on their fat stores to survive the winter and also to detoxify pesticides. Without their fat reserves, bees are far more susceptible to the negative impacts of various toxic chemicals in pesticides, contributing to their struggles when trying to survive the winter.
What can we do to preserve bee populations?
Fortunately, there's plenty of ways we can strengthen bees' resilience to these challenges. For one, we can plant bee-friendly plants, such as Minnesota is encouraging its residents to do, or plant semi-natural flower fields around farmland. We can also reduce our reliance on pesticides by implementing integrated pest management practices, or IPM. IPM considers the use of pesticides as a last resort and acknowledges that the complete eradication of pests is not feasible nor worth the effort.
Finally, stricter shipping policies can help prevent the introduction of harmful parasites like V. destructor to bee populations with no resistance. There's plenty of actions we can take to help bolster bee populations. But if we don't take action, the 2018 winter won't be the worst one for U.S. bee colonies.
