By Athayde Tonhasca
With a girth (Equatorial circumference) of over 40,000 km and a land mass of more than 148 million square kilometres (29% of the total; the remainder is water) planet Earth may seem like a home roomy enough to accommodate its many land-based creatures. But these figures are misleading, because all forms of terrestrial life are confined to a slim layer between the top of trees’ canopies and the bottom of aquifers. Every physical, chemical and biological process necessary for life happens within this wafer-thin coating. Gail Ashley labelled this living skin ‘the Critical Zone’.
The narrow Critical Zone has an even narrower core, which is responsible for the vital water, carbon, nutrient and decomposition cycles: the soil – which is also the growing medium for the majority of plants and countless other organisms. Soil sustains life on the planet, but is also shaped by living beings such as ants, termites, beetles, earthworms, millipedes, woodlice, mites and nematodes. They degrade organic matter and help create humus, and also shuffle soil around: the uprooting of trees displaces and turns lumps of earth, moles dig and burrow, ants and termites build earthen nests above ground. This form of ecosystem engineering is known as bioturbation, which is the subject of ichnology: from the ancient Greek íkhnos (footprint), it is the study of existing and fossilized tracks and excavations made by animals. Ichnology was an obscure and fringe scientific field until Charles Darwin had a go at it. Unsurprisingly, his endeavours had enormous repercussions.
In 1837, Darwin visited his uncle and future father-in-law, Josiah Wedgwood, who suggested that earthworms were responsible for the slow burial of chunks of marble scattered around his property (Huxley & Kettlewell, 1965. Charles Darwin and his World. Viking Press, New York). That titbit of domestic chitchat stirred Darwin’s scientific imagination, so much so that he conducted observations and experiments with earthworms on-and-off for over 40 years. His efforts culminated in his last book, published about six months before his death: The Formation of Vegetable Mould Through the Action of Worms, with Observations on their Habits. Darwin didn’t think much of it: “I have now [1881] sent to the printers the manuscript of a little book on The Formation of Vegetable Mould through the Actions of Worms. This is a subject of but small importance; and I know not whether it will interest any readers, but it has interested me.” (Barlow, 1958). He was wrong: the book was a huge success, selling as many copies as On The Origin of Species (Feller et al., 2003).
Darwin was largely responsible for changing the perception of earthworms from garden pests to major contributors to the formation and ecology of soils. Since then, other ground-living organisms have been identified as contributors to soil morphology. Among them, ants and termites are considered particularly important simply because they are spectacularly abundant; both groups comprise a huge chunk of terrestrial animals’ biomass.
Ants, termites and a few other ground-dwelling insects such as dung beetles transport and rearrange soil particles, affecting soil structure and the cycling of water and nutrients. So they rightfully have received a great deal of attention as ecosystem engineers. But one group is absent from the select club of bioturbation agents: bees.
Most of us are familiar with honey bees and bumble bees, and we may assume that other bees are like them – but they are not. Of the 20,000 or so known species of bee in the world, most (~80%) don’t live in colonies; they are solitary, that is, each female constructs and provisions a nest by herself. And around 60% to 80% of them are fossorial (from the Latin fossor for ‘digger’), meaning animals adapted to digging and living underground. These bees are known as mining bees or miners. Each female’s nest consists of a tunnel that may branch into cells. For some species, tunnels can be 10 mm wide and up to 0.5 m deep. The female will stock each cell with pollen and lay an egg on it; the larva will feed on the pollen until it is ready to emerge as an adult. Collectively, mining bees (mainly from the genera Andrena, Anthophora, Amegilla, Eucera, Halictus, Lasioglossum and Melitta) make up the most important group of crop pollinators (Kleijn et al., 2015), despite spending most of their lives underground.
Most mining bees, like those in the genus Colletes, produce a resin that becomes a transparent, waterproof film when exposed to the air. Female bees brush this glandular secretion on the walls of the brood cells to protect them against excess moisture and possibly against pathogens. This feature explains why these bees are known as plasterer bees, cellophane bees, or polyester bees. Other species line their nests with petals, leaves, pebbles or other materials. Besides protecting the brood, these home improvements help to uphold the nest structure, so that air and water keep flowing along the tunnels long after the emerging bees are gone.
A solitary mining bee is no match for the digging capacity of termite or ant colonies, but the term ‘solitary’ is deceiving. Each bee builds her own nest, but many species nest close to each other, perhaps to take advantage of relatively scarce good spots. These nest aggregations can be massive: the heather colletes (Colletes succinctus) can reach concentrations of 80,000 tightly packed nests along a 100-m stretch. These gatherings give the impression that bees are swarming: watch them going at full tilt.
Heather colletes aggregations may seem overcrowded, but they are sleepy villages when compared to those put together by Calliopsis pugionis: they can reach over 1,600 nests/m2 (Visscher & Danforth, 1993). Mining bees’ relentless burrowing and tunnelling produce one important by-product: enormous volumes of spoil.
In temperate areas, earthworms can deposit 10- 50 t/ha of castings (soil-enriched poo) on the soil surface annually, while ants and termites move 1- 5 t/ha of soil, reaching 10 to 50 t/ha in some instances (Wilkinson et al., 2009). These figures do not impress the alkali bee (Nomia melanderi), a prodigious soil engineer in its native deserts and semi-arid areas of the western United States. One gigantic colony, estimated to house around nine million bees, dug out 96 t of soil to the surface in one year. Much of this earth is taken away by wind and rain, which would result in a loss of 4 cm of soil surface in 50 years (Cane, 2003). In Japan, Andrena prostimias deposited 27 t/ha of soil in a temple’s garden. The volume does not seem that impressive until we learn that the excavation was completed in one week (Watanabe, 1998).
Bees are hardly ever considered soil organisms, but that’s a gross oversight. Thanks to their burrowing activity, mining bees are likely to contribute to nutrient cycling, water storage, soil structure and atmospheric composition: their inclusion in the roll of bioturbation agents is much justified. And you thought they only contributed to ecosystem functioning by being great pollinators.
Scottish pollinators 