Counting frogs and tadpoles' teeth

ANOTHER LIFE: It was my 80th year to heaven and I walked abroad in a promised sunny spell, the surf on the shore calmed down at last, to see if the frogs had remembered my birthday. They got it spot on in 1997 ("garden pond full of frogs and first spawn laid") and again in 2005, but since then I have lost the great gatherings that February used to bring of 100 or more shiny heads, knotted in rapt amplexus, pumping out mounds of jellied eggs to glisten in the sun.

The fault is all mine, as I let what remained of open water succumb to creeping grasses, toppling rushes, fallen leaves, in the pond’s clear haste to become a fen. Hauling out this matted biomass every autumn is now beyond me, so that, at the imminence of spring, the algal scents said to summon frogs to their mating pools must be quite overwhelmed. I suspect also that the last few loyal habitues have been hoovered up by the badger that visits us on tour in February, testing his weight on the pond’s strawy mattress.

I do not, it turns out, need to feel at all guilty about depriving Rana temporaria of its breeding habitat. The Republic, at a new and highly rigorous calculation for the National Parks and Wildlife Service (NPWS), has about 165 million frogs, happily mating almost anywhere there’s water, and above all in the drainage ditches that edge the nation’s fields. They are probably the island’s most abundant vertebrate, next to the long-tailed field mouse.

This new and cheering estimate replaces the apprehension favoured a few years ago, as frog populations in many parts of the world were seen to suffer huge decline from fungal and viral diseases as well as a progressive loss of habitat.

Although no disease had reached Ireland, the steady obliteration of farm ponds and the sprawl of suburban development seemed likely to support the anecdotal reports of local scarcities. Despite two sampling surveys that seemed to find frogs where they should be, NPWS reported an “unfavourable” conservation status for this protected species.

Proof for this, however, was clearly limited, and a first national survey was designed with all the bells, whistles and arithmetic of the latest population algorithms. (Aspiring young scientists may care to download the frog report from the NPWS website to see the sort of test they could be in for.)

The fieldwork, too, most of it carried out by the nationwide network of NPWS wildlife wardens, was rigorous enough to need special training.

It was based on a simple truth – where there’s frogspawn, there’s frogs – but making sure of comparable inspections on squares of land and water truly representative of the current surface of the Republic needed a sampling programme of some complexity. Since the turn of the 19th century, for example, the numbers of farmland ponds had halved, especially in eastern cereal-growing counties like Wexford, but about 2 per cent of the Republic was still good frog-breeding habitat, ranging from lakes and canals to dune slacks and roadside ditches. Everything needed sampling, to a scientific plan.

Two springs ago, on 171 selected bits of the map 500m square, 405 water bodies were surveyed for spawn, with three return visits to check on frog numbers and accumulating spawn. Everything was carefully estimated, including dimensions and depth of the water. There was spawn in half the water bodies and three-quarters of the survey squares, with the greatest occurrence in Mayo, Sligo and Donegal.

Most obviously, the frogs of Ireland have adapted to the loss of ponds by moving to the ditches that drain our grassy fields: 86 per cent of them, no less, were breeding in these habitats (even in the west, where, confusingly, “ditches” are the banks above them, and the dug-out bits are the “drains”) and fewer than 5 per cent were in farmland ponds.

This is not exactly a novel discovery: the concentration of Irish frogs in ditches and at the sides of streams in bogs was noted by scientists three decades ago. Intensive study of the species has extended even to the variable number of a tadpole’s teeth. But the new survey makes Ireland one of the few countries with a robust headcount of Europe’s common frog.

With an average of about two dozen per hectare, we come nowhere near Finland’s teeming densities (up to 80 per hectare) or even those of good habitats in Scotland. But it’s a healthy population, even sometimes “explosive” in breeding compared with the frogs of continental Europe (a figure of 1,000 eggs per clump of spawn spans, like all averages, some wide extremes).

Frogs control slugs and midge larvae. The adults help feed our otters, badgers herons and kestrels. The tadpoles feed dragonfly larvae and great diving beetles – and goldfish.

Which is why, if you have fish in your pond, you won’t get any frogs.

Eye on Nature Your notes and queries

Can you settle a long-standing dispute? Does spring start on February or March 1st? Sinead Quinn, Wicklow town

Ireland follows the old Celtic calendar and has spring starting on February 1st. The Met Office here follows the calendar used by the rest of Europe, which has it on March 1st.

Recently in the fields of Clonliffe College I saw a bird of prey flying from tree to tree being mobbed by crows, magpies and seagulls. Was it looking for eggs or a nesting site for itself? Nicola Main, Fairview, Dublin

It was probably looking for eggs or small birds. Other birds will always mob a raptor that invades their territory.

A friend in Dalkey has been charmed all winter by the antics of a flock of bullfinches. Recently a flock of smaller birds appeared. They have red patches on either side of the head, red breasts, with flashes of white or yellow on wings, and dull brownish backs. Geraldine Monaghan, Parnell Road, Dublin

They were goldfinches.

* Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo, or email viney@anu.ie. Please include a postal address