Charles Darwin would be so proud. After setting the scientific world alight with "On the Origin of Species," Darwin's last book in 1881 was the less dramatic "The Formation of Vegetable Mould through the Action of Worms, with Observations of their Habits". More than a century later, earthworms have been found to be the explanation for large mounds found in South American wetlands, which previously mystified those who saw them.
Known as surales, these mounds cover giant floodplains in Columbia and Venezuela. They can be up to 2 meters (7 feet) high with diamaters of 0.5 to 5 meters (1.7 to 17 feet). They are also regularly spaced, giving an impression of being constructed by some deliberate planning scheme, creating considerable puzzlement among visitors.
In PLOS ONE, a team including Professor José Iriarte of the University of Exeter reveal how the surales are formed, and the reasons for this regular patterning.
The surales are a response to the regular flooding of the earthworms' habitat. “Mounds are initiated when large earthworms feed in shallowly flooded soils, depositing casts that form ‘towers’ above water level. Using permanent galleries, each earthworm returns repeatedly to the same spot to deposit casts and to respire,” the paper reports.
“Over time, the tower becomes a mound. Because each earthworm has a restricted foraging radius, there is net movement of soil to the mound from the surrounding area. As the mound grows, its basin thus becomes deeper, making initiation of a new mound nearby more difficult,” the paper continues, explaining the regular placement.
Naturally, sometimes two worms will start mounds close together before one has become large enough to prevent the other taking off. In these cases, the authors report, the modest initial basin between them is filled in and the mounds coalesce, explaining the largest mounds.
Professor Iriarte taking advantage of a dry period to examine surales and the spaces between them. Professor José Iriarte
The very existence of the surales has hampered attempts to study them. The paper quotes a survey of the region observing that territory in which surales have developed is “difficult to traverse. If you are on foot, you have to decide whether to follow the endless twistings of the boggy ditches or to jump from mound to mound, both awkward expedients... I have heard stories of man and mule firmly stuck in a narrow, deep ditch between two sural mounds.”
Consequently, the authors note, the surales have previously been largely neglected for study, with their formation attributed to swelling and sinking of clays or the roots of strangely regular trees. Iriarte said in a statement: "This exciting discovery allows us to map and understand how these massive landscapes were formed. The fact we know they were created by earthworms across the seasonally flooded savannahs of South America will certainly change how we think about human verses naturally-built landscapes in the region."
Darwin, on the other hand, wouldn't be surprised. He spent years demonstrating that even in England earthworm castings build up soil, covering half-buried objects. The topic was so popular, Darwin's book initially outsold the work that made him famous.