Statues and Buildings Built to Last?

No way: some whittle and nibble and make the structure brittle  –  but who does?

Statues and monuments: will they last forever if not pulled down? Pigeons love most (but not all of them as Yukio Hirose of Kanazawa University has shown in a study that earned him an ig-Nobel prize in 2003) irrespective of which ruler or famous person they were meant to honour. But the corrosive content of the bird’s droppings on the famous person’s head or shoulders damages the statue (and with that also the person’s grandeur). Bird droppings provide nutrients to organisms like algae, bacteria, fungi and lichens, all of which occurring on metallic statues but being far more abundant and damaging on stone buildings and brickwork. Actually, a former botanist colleague of mine operated a small company, offering services to remove unsightly and potentially damaging algae and lichens from stone walls, buildings and roofs tops.

The first colonizers of stony materials, be they bricks or concrete, are usually some green algae or, given a sufficiently moist surface, cyanobacteria and even liverworts. The water in the cracks and crevices of the brickwork, at least initially, is of a very high pH making it a strongly alkaline environment. However, the colonizing lower plants and seepages of weak acids will ever so slowly lead to the release of some minerals and to minute holes, grooves, or cracks in the mortar. An increasingly uneven surface then serves plants and their spores (in case of ferns) or seeds (in flowering species) in two ways: spores and seeds cannot easily be washed or blown away and roots to anchor the young plants can find a grip without much trouble. Holes and cracks, moreover, can store rainwater, dew, pollen, dust and nutrients.

Once some food for animals is available, the next wave of colonizers will arrive. At first they will be tiny and consist mainly of some wingless springtails and some mites, possibly some spiders that hide in the crevices and build flat sheet webs that cling to the building’s wall. Depending on whether one deals with a north, south, west or east-facing side of the building, the colonization histories will, of course, differ. Birds are not likely to colonize the brickwork of a building, especially when one deals with straight and vertical walls, but if the wall’s surface is sufficiently rough, small birds may visit and briefly cling to the brickwork for three reasons: they may peck at the mortar to replenish their calcium intake or they may want to ingest some tiny stony fragments to store in their stomachs to help them with opening seeds they have swallowed or they find small insects in the cracks and amongst the plant growth on the wall. Their droppings and occasional feather downs left behind further enrich this environment nutritionally, so that ultimately bigger and stronger plants can get a foothold on the building and whittle away more of it. At the same time their droppings are corrosive, attack the mortar and cause further little chinks.

How about metal, concrete or brick constructions in the water? They have become more and more popular for underwater constructions because of the notorious damage caused by shipworms of the family Teredinidae to wooden structures. Therefore, are the former any safer from being attacked by organisms? Apparently not, for there are reports that the paddock Martesia striata bored through the outer solid lead sheath and subadjacent asphalt-impregnated insulation of an electric underwater cable off the coast of Florida. Holes of 6 mm in diameter and 2 mm in depth were reported. Attacks of cement mortar and concrete in the water are well documented and involve a variety of rock-boring molluscs, for example the bivalves Pholadidea penita, Platyodon cancellatus, Lithophaga plumula, and Botula falcata. We have to conclude: even seemingly unpierceable structures are not impervious to biological attack.

© Dr V.B. Meyer-Rochow and, 2021.
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