A hard flower to crack

By Athayde Tonhasca

Brazil nuts are high up in the list of superfoods, a gimmicky but highly profitable market. For some internet gurus, the nuts protect you against inflammation, heart disease, diabetes and cancer; and, inescapably as superfoods go, they are loaded with ‘antioxidants that fight free radicals’ – a scientifically baseless but commercially catchy label. Hype aside, Brazil nuts are highly nutritious: they are loaded with proteins, carbohydrates, unsaturated lipids, vitamins and essential minerals such as calcium, magnesium, phosphorus and potassium. But their main claim to fame is to be one of the best sources of selenium, an essential element for a range of metabolic processes in our bodies. These nuts have their detractors because of the unlikely risk of selenium poisoning for those who over-indulge in them, and the danger posed by aflatoxins (carcinogens produced by certain fungi) when the nuts are not stored properly. The healthy aspects of Brazil nuts are clearly winning over the popular perception because the European and American markets keep growing steadily. Which is good news for the main nut producers Bolivia (the world’s major exporter), Brazil and Peru.

Assorted nuts, essential while watching a match on the telly © Melchoir, Wikimedia Commons.

The nuts are in fact seeds extracted from the hard, coconut-like fruits produced by the Brazil nut tree (Bertholletia excelsa). Named excelsa (high, exalted, lofty) by naturalists and explorers Alexander von Humboldt and Aimé Bonpland, this is one of the tallest trees in the Amazon region. Some individuals reach heights of 30 to 50 m, with trunks of 1 to 2 m in diameter – up to 5 m in older specimens. And they can live long lives: radiocarbon dating has identified some 800- and 1000-year-old trees (Camargo et al., 1994).

Brazil nuts are gathered from fruits fallen to the forest floor during the rainy season. The work, carried out by native people and small farmers, has one serious risk: the thick-walled fruit of the Brazil nut tree is 10-15 cm in diameter and weighs on average 750 g. A fruit of this size falling from about 7 m generates sufficient kinetic energy to fracture a skull and cause severe to fatal injuries (Ideta et al., 2021).

A Brazil nut fruit and seeds in their shells, and seeds ready for the market © P.S. Sena and Quadell, Wikimedia Commons.

Brazil nuts are harvested almost entirely from wild trees because tree cultivation has been largely unsuccessful. One of the reasons for the failure to turn the tree into a farm commodity is its pollination requirements.

The Brazil nut tree reproduces by cross-fertilisation, but its flower is not the run-of-the-mill, pollinator-friendly structure found in most plants. It has a curled extension – called a ligule – that forms a hood over the petals, which are pressed together like an inverted cup. Ligule and petals create a chamber that conceals stamens, stigma, and the nectaries. To access the nectar, an insect has to squeeze itself between the ligule and the tightly packed petals. If successful, the visitor may come out dusted with pollen and transfer it to another flower.

The inflorescence of a Brazil nut tree © Scott Mori, The New York Botanical Garden, Lecythidaceae – the Brazil nut family.

The flowers of the Brazil nut tree receive many visitors, including hummingbirds, moths, butterflies, beetles, and several bees. But getting into a flower for its nectar is not for the nimble or weak; only the largest and strongest bees can lift the ligule to reach the reproductive organs.  This select heavy-weight club includes bumble bees (Bombus spp.), Centris spp., Epicharis spp., orchid bees (Eulaema spp.), and carpenter bees (Xylocopa spp.). 

An E. meriana orchid bee (L) and a large carpenter bee (X. mexicanorum), two of the robust bees capable of handling a Brazil nut tree flower © Insects Unlocked, Wikimedia Commons.

In the central Amazon rainforest, the orchid bee E. mocsaryi and carpenter bee X. frontalis are especially important Brazil nut tree pollinators because of their abundance and frequency of flower visitations (Cavalcante et al., 2012). Watch X. frontalis hard at work. 

An E. mocsaryi orchid bee forces itself between the ligule and the tightly packed petals of flower of the Brazil nut tree © Cavalcante et al., Wikimedia Commons.

The large bees that pollinate the Brazil nut tree can fly long distances, which is important for maintaining its genetic diversity: trees typically grow in groups of individuals that are isolated from each other in the forest. These bees are solitary or semi-social, and none of them have been domesticated; so their survival depends on the natural habitats that supply nesting sites, food – one type of tree alone will not provide for the whole season – and other resources such as orchid fragrances, which are collected by male orchid bees.  

Other characters play an important part in the life of a Brazil nut tree: rodents. The fruit capsule is too hard for most animals that could be interested in the nutritious seeds – but not to agoutis (Dasyprocta spp.). Thanks to their powerful teeth, they can gnaw their way to the seeds. Sometimes an agouti can’t eat all the seeds at once, so the prudent animal takes them some distance away (up to 20 m) and buries them as a food reserve for leaner times. But it so happens that the agouti’s memory is not the sharpest; it often forgets the way back to the food cache. Or the agouti itself may become a meal for a large cat before returning to its seeds. Either way, the buried seeds germinate. This unintentional planting by agoutis, pacas (Cuniculus spp.) and the southern Amazon red squirrel (Sciurus spadiceus) is the only route to seed dispersal for the Brazil nut tree, thus is vital for its survival. Follow the exploits of a forgetful agouti in the forest.

A red-rumped agouti (Dasyprocta leporina) © Alastair Rae, Wikimedia Commons.

As one can imagine, these required interactions with bees and rodents don’t bode well for the future of the Brazilian nut tree. Amazonian ecosystems have been eroded away by the relentless march of deforestation and land conversion to agriculture and pastures. To make things worse for the tree, its wood is highly valuable (its felling is illegal, but that doesn’t stop illegal loggers). One of the consequences of the devastation is the increasing scarcity of Brazil nuts in Brazil, which helps explain why Bolivia took the lead as the main exporter. 

We may take the hard-nosed view that we can’t do anything about the plight of the Brazil nut tree, but that’s not quite right. One could be inquisitive and demanding about the origin of a nice piece of hardwood furniture for sale, or the juicy steak in a restaurant or supermarket freezer (much of exported South American beef comes from deforested areas). Or we could accept a life without Brazil nuts: after all, to us they are just comfort food. But the tree’s demise could be a portent. In the 1980s, doomsday prophet Paul Ehrlich and his wife Anne Ehrlich famously compared species’ roles in an ecosystem to rivets in an aeroplane’s wing. Aircraft manufacturers use more rivets than necessary to affix the wings, so removing a few of them would make little difference. But if they continue to be taken away, at some point a critical rivet is lost, and the aeroplane will crash. Similarly, how many species can you lose before an ecosystem fails? We don’t have an answer for that, or even know whether the Ehrlichs’ model is realistic. The eventual loss of the Brazil nut tree could be just one redundant rivet popped out of the body of biodiversity; or it could be a warning of bad things to come to bees, agoutis, the forest and its peoples, and, at the end of the queue, to us, sophisticated Brazil nut munchers. 

The Brazil nut tree is the symbol of the Amazon Forest. Its size makes it difficult to capture it whole in a photo © upper left: My Favorite Pet Sitter; lower left: mauroguanandi; rigth: Edsongrandisoli, Wikimedia Commons.

A pushy squatter on the march

By Athayde Tonhasca

In 2016, French researchers installed dozens of bee houses in twelve of Marseille’s public parks to monitor the local populations of solitary bees. In the following year, to the researchers’ consternation, most of the units had been taken over by the giant resin bee (Megachile sculpturalis) instead of mason bees (Osmia spp.), the usual bee house tenants. 

A female giant resin bee collecting pollen © Paula Sharp, Entomology and Nematology Department, University of Florida

The giant resin bee arrived in France via Marseille in 2008, probably as a stowaway from China, Japan or any of the other eastern Asian countries of its natural range. This bee nests mostly in wood cavities, so a shipment of timber was the likely means of entry. It spread quickly through France, northern Italy, Switzerland and South Germany; then it sneaked into Austria, Slovenia, Hungary and Spain. The European tour was not the giant resin bee’s first adventure outside its native area. It landed in the United States in 1994, probably through a port in the state of North Carolina. From there, it spread to most of the east coast estates. This newcomer is considerably larger than other solitary bees – females range from 22 to 27 mm – so it has been fairly easy to track its spread over Europe and America.

The spread of the giant resin bee in native areas (increasing year of records, from yellow to green) and in invaded areas (from yellow to red). Black dots are records with no available year © Polidori & Sánchez-Fernández, 2020. Global Ecology and Conservation 24 e01365

We may assume that the arrival of a bee can only be a good thing: the giant resin bee collects pollen from dozens of plant species, so it may join the local pollinating force. Unfortunately, it also adds to the roster of troublemakers that could cause havoc to other species and their habitats such as the honey bee (Apis mellifera) and the buff-tailed bumble bee (Bombus terrestris). Once outside their native ranges, these important pollinators are protagonists in well-documented cases of outcompeting other bees, endangering native species, transmitting diseases, and altering the local flora by pollinating aggressive weeds. 

Competition for nesting sites is one of the possible consequences of giant resin bee invasions. This bee nests in wooden structures, but it cannot chew through wood to excavate its own home. So it relies on pre-existing holes such as vacant beetle galleries, or openings in fallen or rotting wood. It will also readily take old nests of carpenter (Xylocopa spp.) and mason (Osmia spp.) bees. But empty cavities are not always available in sufficient number, and in these situations the giant resin bee resorts to brute force; in America, females have been caught pulling Eastern carpenter bees (Xylocopa virginica) out of their nests and making themselves at home. They will then build their own nest chambers with wood fibres, leaf fragments, mud, and resin.

The North American Eastern carpenter bee, a potential target of giant resin bee bullying © Bob Peterson, Wikipedia Creative Commons

In France and other European countries, several bees and wasps have been victims of nest usurpation such as the Mexican grass-carrying wasp (Isodontia mexicana). This wasp, itself an invasive species – and a recent arrival to Britain – captures grasshoppers and crickets and takes them to its nest to feed its larvae. But the wasp’s life plans can be ruined by a house-hunting giant resin bee, who will extract stored grasshoppers from the wasp’s nest and boot her out.

In Europe, Megachile lagopoda (L) and the Mexican grass-carrying wasp can lose their nests to giant resin bees © Gideon Pisanty (L) and pjt56, Wikipedia Creative Commons

Britain is not a home for the giant resin bee, but this is likely to change. It could arrive in a shipment of timber, or just come on its own: the English Channel is not much of a barrier for a bee capable of long distance flights thanks to its unusually large size. 

We can’t predict the consequences of such an invasion for our native bees. In America, the Eastern carpenter bee apparently has not been much affected, possibly because the life cycles of the two species don’t overlap completely. But in continental Europe, there is concern about the fate of several bees. The giant resin bee will take natural cavities or any manmade structure such as brick holes, metal pipes and plastic tubes, so bee houses are perfect for them. The observations in France suggest that the emergence of native bees from bee hotels is negatively correlated with the occurrence of giant resin bees. If that’s the case, bee houses may be not only detrimental to the native fauna, but also aid the spread of an invasive species.

A giant resin bee capping a bee house’s brood cell with resin and mud © Alonso Abugattas, Entomology and Nematology Department, University of Florida

If the giant resin bee becomes established in Britain, our plants may gain another pollinator, and our bees may have to deal with another hassle. Time will tell whether any of these outcomes are significant. As countries become increasingly connected by trade and travel, and the environment changes rapidly and unpredictably thanks to global warming, we will hear more stories such as the giant resin bee’s globetrotting. 

Small is ok

By Athayde Tonhasca

Gardeners, land managers, conservation groups and local authorities have been planting away to provide food for pollinators. Such commendable efforts have resulted in great swathes of flowering plants, often in the form of road verges, uncultivated crop margins, or un-mowed land strips. These large floral areas are beneficial and quite convenient for a range of pollinators, who can hop effortlessly from flower to flower to gather pollen and nectar. High plant density reduces foraging costs – in the form of energy expenditure – and increases visitation rates. Bumble bees for example are known to take advantage of high flower concentration.

A wildflower meadow: a profusion of pollinators’ food © Richard Croft, Wikipedia Creative Commons

But high flower abundance may have negative consequences. Visiting insects may not keep up with the task, so some flowers may receive an insufficient number of pollen grains – or none at all. Flower-rich areas attract lots of pollinators, who may compete with each other for resources: smaller, slower species may have hard times finding flowers not already cleaned out by more efficient visitors.

Smaller flower patches don’t attract many pollinators because the limited food supply may not be worth the effort of flying to them. But those that take their chances may be compensated by less competition and greater energy efficiency, as they are less likely to come across flowers depleted of nectar and pollen by a previous visitor. Being in a small group can be good for plants too: each one of them has a better chance of being visited.

So there are opposing effects at play. Larger flower patches attract more visitors, but pollinators may face hard competition, and the visitation of individual flowers may be compromised. Smaller patches can be costly to visit, but flowers may be more rewarding individually.

A variety of observations and studies at different locations and settings have demonstrated that in the case of bees, the balance tilts in favour of bigger:  the higher the floral abundance, the greater bee diversity and numbers. But not all bees follow this pattern: some small carpenter bees (Ceratina spp.) and halictid bees (family Halictidae), especially base-banded furrow bees (Lasioglossum spp.) seem to prefer isolated clumps of flowering plants over large swathes of habitat. This choice has been, at least partially, attributed to the need for escaping competition with the ever-present bumble bees.

A blue carpenter bee (Ceratina cyanea) (L) and a common furrow-bee (Lasioglossum calceatum) © Hectonichus (L) and Martin Cooper, Wikipedia Creative Commons

Only one species of small carpenter bee occurs in Britain, but there are around 34 species of base-banded furrow bees; the exact number is uncertain because they are difficult to identify and monitor. These bees visit a wide range of flowers, although their role as pollinators is not well understood. Elsewhere, they have been shown to pollinate melons and tomatoes, among other plants.

Extensive pollinator habitats are not always viable because space or resources are limited, as for example in urban settings. But smaller areas play their part by catering for the set of crowd-shy pollinators, which is likely to include species other than small carpenter and halictid bees: we have information about foraging habits of only a fraction of pollinating species. 

A small island of pollinators’ habitat in a private yard in Oakland, USA © Akos Kokai, Wikipedia Creative Commons

It also helps to keep in mind that most pollinators are highly mobile. For a bee in search of pollen or nectar, a small garden may not provide all it needs, but that islet of flowers may be a stopover on a journey over an archipelago of food sources. Small bees normally forage within 100 to 300 m from their nests (the bigger the bee, the further it goes), but if pressed by hunger, they can fly over 1 km to collect pollen or nectar. Larger species like bumble bees and honey bees make much longer foraging trips.

So if you ever question whether a flower box, a strip of flowers along a fence, or a tiny flower bed in a parking lot make any difference, the answer is yes.

A kitchen garden with a segment set aside for flowers © Ukiws, Wikipedia Creative Commons