Nature 2012 August

Water water everywhere…

This year, the unseasonably heavy rainfall over the past three months or so is having some interesting, albeit temporary effects on our local wildlife. There’s just room for three of my thoughts on this…

It has been a bumper season for those pesky biting insects, Horse flies, that often plague us at the height of summer. In Elizabethan times they, and their relatives, were collectively known as Gad-flies and Shakespeare borrowed the name to refer to a satirical gentleman whose words could sting another. The most common of the three species of horse fly that attack mammals, and often described as the most obnoxious of all insects, also goes by the name of a ‘Clegg’!

The explosion in their numbers is a consequence of the large amount of standing water around: not just established ponds but ditches or even waterlogged soil. Horse flies lay their eggs on leaves or stones close to water. On hatching, the larvae fall into the water. Their food are any suitable invertebrates; from water fleas to earthworms, snails and slugs, on which they prey voraciously. The adult flies emerge in the early morning ready to start flying at midday and need to feed immediately.

Whilst male flies seek out pollen and nectar the females need to find a source of fresh blood to get sufficient protein quickly in order to reproduce. One large meal will suffice. The flies rest up in shady places close to where their prey passes. With large eyes they can identify movement of a suitable animal. Often this will be a cow, horse or deer and, though some species only seek out birds or even frogs or toads, the flies are not that selective and as we know to our cost, will seek out the bare skin offered by us humans.

Unlike other insects that sting such as wasps, horse flies are able to pierce the skin and find a blood vessel without inflicting immediate pain for the unfortunate host. They are able to make an incision without drawing attention to themselves by using their serrated scimitar-like and razor sharp mouth parts. Saliva acts as an anticoagulant and they suck up their meal through a straw-like proboscis. Though the bites cause infections and can spread disease in livestock; apart from an irritating itch and a small scar they are, in this country at least, nearly always harmless to humans. On the upside, large numbers of such flies have been a boon for the likes of house martins, swallows and swifts, which can be seen swooping low over meadows where such large slow flying flies are easy prey.

Other benefactors of the increased rainfall have been the animals and plants which inhabit our ponds. The earlier extended period of drought would have had a significant if temporary impact on the ecology and productivity of these ponds. However, by their very nature, ponds go through periods when the species are put under major stresses, so the powers of recovery are all the more impressive. Amphibians such as the common frog and toad and the palmate newt can survive in the muddy borders of ponds or can migrate and return in better times. In dry years few, if any, tadpoles or efts (newtpoles) will make it to adulthood, but with water and food levels recovering this year, populations should also recover. Many pond invertebrates can survive drought buried in the substrate at the bottom of the pond. Meanwhile plants are adapted to survive as seeds or rootstock.

There are all sorts of ponds. At one end of the spectrum is Pallett’s Pond (Cholesbury Road) which is rich in a wide variety of flora and fauna and looks like a pond all year round. Roadside ditch ponds, such as the one almost hidden under trees in Oak Lane, will provide a home to a range of more resilient species that are able to contend with pond levels which wax and wane over the seasons and light and oxygen deficits which would be fatal to some of those in Palletts. At the other end of the spectrum are the dew ponds, such as the one on Cholesbury Common, with its ephemeral invertebrates and plant species that thrive on the margins where other plants dare not go. Such ponds may not look pond-like for much of the time. Rather it appears as just a slight depression in the ground holding a small amount of water for a few winter months. This year everything is topsy- turvy. Having been almost dry for a year, including all last winter, it has almost filled up this summer as the water table rises to engulf its shallow bowl. Perhaps the most important but admittedly not very exciting species growing there is water purslane, which is a rarity across the Chilterns and Bucks. For all ponds, including those in gardens, it is important to keep the area leading away from the pond as natural as possible to provide a habitat suitable for creatures to escape or hide nearby. Water consumption by trees can be awe inspiring. One mature Oak may take up over 15 gallons of water an hour in hot weather and over 300 gallons a day. On cooler days take-up can be less than 60 gallons a day. Meanwhile Beech trees happily operate on a range between 20-60 gallons a day. Whereas hardwood trees take up next to no water during winter months, coniferous trees need a constant supply of water year round and can suffer from winter drought. All trees can survive for a period on half the normal uptake but water deficit will, over time, result in leaf and limb loss, and early seasonal leaf fall. Prolonged periods of drought can be withstood but will shorten the life of trees. Though surface water levels may have improved over recent weeks, providing a welcome respite to our stands of beech trees, unless all this water has a sustained and positive effect on the water levels within the chalk aquifer, the decline in the health of our beech plantations, which has become more noticeable in recent years, will sadly continue.

I hope you have enjoyed reading this ‘Rime of the Ancient Naturalist!’