Time to mow the pond

THE memo to myself had, been waiting on the kitchen blackboard since New Year's Day: "Mow The Pond!" - and as soon as the frost…

THE memo to myself had, been waiting on the kitchen blackboard since New Year's Day: "Mow The Pond!" - and as soon as the frost retreated, I got down to it.

Those for whom the garden pond is mere ornament, a designer kit of blue leaved hostas, bamboos, two red water lilies and five goldfish, may find the message strange. Habitues of this column, however, will recognise that what once seemed to me a dramatic intervention in the natural life of our pond has now become a necessary annual chore.

It is in the nature of all ponds to want to fill themselves in and grow a tree, instead: it is their idea of ecological progress. A deliberately "wild" pond, like, ours, furnished with chosen plants from stream and lake and adding, all by itself, several I hadn't considered, is already well on in succession towards becoming a swamp.

By the end of each summer, when the last freshly hatched dragonfly has spread its wings and flown, the water has quite disappeared between the crowding stems of rushes and water horsetail, the curling leaves of bogbean and water lily. As autumn wears on and the foliage crumples, grasses and mosses creep out across to weave a final layer of camouflage across the water.


If the pond were big enough to have a swamp zone, a floatingleaf and emergent zone, a submerged plant zone and a free floating plant zone - that is, if it left some water, some reflections, some glitter and ripples, between the leaves at the middle - I might rest content. But my pond doesn't seem to have a middle: it is all edge. Hence, once a year, it gets mown - clipped right down below waterlevel, that is, with long handled shears.

The urgency of the blackboard memo, with its exclamation mark, is readily explained. If, by the end of January, the mattress of vegetation is not hacked back and hauled off to the compost heap, the frogs will come up from the mud in February and climb out to spawn all over it. Last year, with a late spring, I did indeed forget, so that March found me shovelling great heaps of jelly back into the water and hoping not to decapitale too many frogs with the shears.

No hint of Rana temporaria was there to trouble me last week, however; they were all safely sleeping, still, between the bog bean roots. As I chopped away, the scent of water mint wafted up, delightfully, among the inimitable, dark green smells of a pond disturbed in winter.

When the tadpoles hatch in spring, lashing the water into froth with 10,000 tiny tails, I must lift some out in a jar for a closer look. A new research paper* by a pair of American biologists finds that tadpoles in different Irish habitats have enough physical variations to be matched to their locations by "a stepwise discriminant analysis" (uh-huh) which defines for example, the arrangement of their rows of teeth.

It was John Kelly Korky (the Kelly may be significant) of Montclair State University, New Jersey who came over a few springs ago and collected 748 tadpoles from 17 different sites in 15 counties of Ireland, labelling each jar carefully with the specific bog pool wayside ditch, flooded wheel rut garden pond, housing estate marsh etc., that they came from. Later with fellow researcher Robert G. Webb, of the University of Texas at El Paso, he got down to work.

Not for Korky and Webb the mere "anecdotal and speculative" discussion of regional differences in tadpoles: they are into "univariate analysis of covariance", if not your actual "powerful multivariate statistical analysis". Already, tadpole species from Mexico, Rana montezumae, and from Nebraska, Rana pipei, had been dissected beneath the microscope, measured with ocular micrometer, and generally taken apart and totted up.

Now it was the turn of Rana temporaria: body length, tail length, tail height, tail musculature height, dorsal and ventral fin height, interocular and internarial width - all these with the Cenco calipers, and then under the microscope for the really small stuff, like teeth. A total of 25 variables (not including, it is confessed, corneal diameter or mouth width) were recorded for each of 26 tadpoles from five selected sites.

And yes, different kinds of habitat produce different types of tadpoles. Irish tadpoles in the south, for example, are more likely to munch their algae (or each other) with four upper tooth rows, and those in the north, with three; a dietary explanation seems likely.

Does any of this matter? No ecologist would need to ask: almost any new facts about geographic variations in a species are worth knowing and filing away. The variability of colour in adult frogs is thought to be managed by hormones, reacting to what the frog sees of its habitat. But Irish frogs seem to remain dark brown, olive green and light gold (or, ash once seen by Frank Mitchell, sooty black) no matter how long they stay in the dark.

Tadpoles are generally more monochrome, black and grey, with pink, iridescent dots. Those in streams usually have blotchier tails than those in ponds, but the study by Korky and Webb produced a surprise: tadpoles netted from a garden pond in Malahide were not only the longest, but also the palest, and had the blotchiest tails.

Paleness in tadpoles may be a camouflage against predators in still and cloudy water. But the Americans still find the Malahide specimens "enigmatic". More may be revealed in their third paper, still to come, which analyses the tadpoles ultimate statistics - the geographical variation of their DNA.

Michael Viney

Michael Viney

The late Michael Viney was an Times contributor, broadcaster, film-maker and natural-history author