Meet the robber of my peas, beans and sweetcorn

ANOTHER LIFE:THE RETURN OF MY canvas director's chair to its proper niche in the polytunnel is a declaration of faith in spring – at least, the season that arrives under plastic as soon as the sun starts to climb in the sky.

With a beaker of coffee and a jaunty bit of Bach on Lyric, I survey my little empire of vegetable order: broad beans edging into flower bud, spring cabbage swelling nicely, good growth already on the garlic, a positively awesome spread on the overwintered cauliflower.

New sowings wait in their little modules (spinach, lettuce, beetroot), and yogurt pots of mangetout peas cluster on upturned plastic buckets with slippery, unscalable sides – a nod to the wild world outside the tunnel.

Such a mild winter sustains both ground cover and the little creatures beneath it, among them that robber of my past sowings of peas, beans and sweetcorn, Apodemus sylvaticus, the "field mouse" of Ireland, the wood mouse of science. It does indeed love woodland seeds (a record 37 animals in one acre of yews in Killarney), but Apodemus is just as happy in a hawthorn hedge, snuggled in an empty bird's nest with a heap of haws and rose hips. It can also, regrettably, try to be a house mouse in cold weather, long tail stilled forever in the mouse trap under the sink.

An intriguing study of its prospects appears in the new issue of Tearmann, the Irish journal of agrienvironmental research. Such small mammals, often numbered among the rodent pests of farming, also have "a vital role in agricultural ecosystems", to quote Teagasc's Daire Ó hUallacháin and Donncha Madden.

Little mammals are good for biodiversity, they say, not least as prey for stoats and pine martens, kestrels and owls. ( Apodemusis the primary food of Ireland's red-listed barn owls.) They also help disperse the beneficial, growth-promoting mycorrhizal root fungi, improve soil fertility and woodland renewal, and control populations of insects – one small community was found to eat up to 6,800 per hectare per day.

A potential new home for mice and shrews is on banks of farmland streams and watercourses, formerly bare, often trampled and dunged upon, but now fenced against grazing by cattle and sheep. On the EU’s initiative, farmers recruited to the the rural environment protection scheme (Reps) have to fence off a one-and-a-half-metre margin and leave it free of fertilisers or sprays. In the agri-environment options scheme, which has succeeded Reps, the margins may be fenced even more widely – perhaps as much as 30m. Left alone, these ungrazed buffer strips can follow a natural succession of long grass and wildflowers to scrubby bramble and gorse, to sapling trees of alder and willow.

Ireland now has five small mammals. To the native field mouse, house mouse and pygmy shrew have been added the introduced bank vole and greater white-toothed shrew. (Our rats, it seems, are not small enough to qualify.) How should the new riparian habitats be managed to encourage them? The Teagasc researchers studied 42 such streamsides in the southeast and found that, if promoting small-mammal diversity is the aim, they’re just not doing their job.

Pygmy shrews, for example, protected under the Bern Convention, find most insects to eat in the moist world under long grass. Once scrub begins to dominate – commonly with the gorse – grass and herbs get shaded, so small mammals find less food and therefore become fewer.

Those live-trapped under scrub for this study weighed significantly less than those caught on grassy or woody banks. What is needed, it seems, is more diversity in the bankside vegetation, through periodical cutting, seasonal grazing, whatever promotes a “heterogeneity of habitats” and everything that lives therein.

The banks of streams may seem a fairly minor environment, but the recent report of the BioChange project showed how important such small ecosystems can be. BioChange, funded by the Environmental Protection Agency, mobilised more than 30 scientists for a multidisciplinary programme in Co Clare and south Co Galway, researching environmental change.

Its individual projects ranged widely, from the impact of seaweed harvesting on marine life to the potential growth of bracken in a warming world, from the spread of invasive species to the damage from peatland fires. It discovered ecological treasure, however, in unconsidered corners of the countryside – small, isolated fragments of wetlands quite outside the network of areas of conservation. To the scientists’ surprise, these marshy corners and ponds often have just as wide a range of wetland and aquatic invertebrates – water beetles, hoverflies and so on – as much bigger, protected wetlands. (They yielded, indeed, six species new to Ireland.)

Such small but rich reservoirs of life can help to sustain the biodiversity of the wider landscape. Locating and protecting enough of them will be the challenge.

Eye on nature

We saw a muntjac deer in headlights on the road in west Wicklow, at Rathsallagh. This is the first one we have heard of this side of the Wicklow mountains.

Jim and Iris Fox, Athy, Co Kildare

We spend six months of the year here, and when we see the swallows heading north, around the end of February, we say it is time we were on the move. We saw a flock of about 100 heading north on migration on January 15th.

Victor Fleming, Lanzarote, Spain

On January 24th in west Co Mayo, unseasonably early, Sue Cross saw a long, black, hairy caterpillar on the roadside near Bunlough beach. And, on the road to Roonagh pier, Bruce Vaughan saw a light brown caterpillar with dark brown stripes the length of its body, which was covered with short hairs.

Both caterpillars were members of the Arctiidaefamily. All members of that family hatch in summer and hibernate when small to emerge in spring and feed until they pupate, in May or June. The black one was the caterpillar of the garden tiger moth, and the light-brown one was the caterpillar of the clouded buff moth. They must have been fooled by the early mild weather.

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