26th May 2006 at 01:00
Creating a freshwater habitat can kick-start a fascinating ecosystem and give your pupils inspiration all year round, says Ben Aldiss

A you looking for a low-cost, low-maintenance resource that would provide your school with ideas and opportunities throughout the year? A pond could be the answer. Many schools have them already, but they are often under-used, largely because of a failure to appreciate their value across the curriculum: they're not just for pond-dipping juniors or older biology students.

Creating a useful and attractive pond is an adventure in itself (see "Making a pond" overleaf), but the real joy comes later, when its full potential is realised. Not only is a pond a great educational tool, with careful siting and minimal maintenance it can also be a huge asset to the school landscape. But an overgrown and neglected pond can reflect badly on the school, so it's worth looking after.

Freshwater ecosystems provide homes for an abundance of colourful and fascinating wildlife, much of which is adapted for seeking out and colonising new habitats remarkably quickly. After a flurry of activity in March when amphibians converge in their hundreds to spawn, pond-life begins to flourish in earnest as the threat of frosts recedes - in southern Britain usually at the beginning of May.

Taking up residence

Whether you allow your pond to become colonised naturally, or you plant it up to give it a start, you will be amazed at how fast new species arrive.

Planting up Ponds, a leaflet from Pond Conservation, points out that, left to colonise itself, a new pond will be as diverse as one 50 years old after just three seasons.

Colonising plant species may not be obvious at first, but seeds can arrive and begin to germinate within days of establishing a spring or summer pond.

The fluffy fruits of greater willowherb are transported by wind, others stick to feathers or fur, or in mud attached to birds' feet. Yet more appear in the droppings of visiting wildfowl.

Flying insects can spot water surprisingly easily and they will check out a new pond, even one so recent it contains very few plants. Water beetles, dragonflies and bugs will even lay eggs, despite the apparent lack of food for their offspring. The newly hatched larvae and nymphs are very tiny and initially feed on microscopic organisms such as algae, diatoms and protoctistans that abound within days of a pond being filled with water. By the time they have increased in size and are searching for larger prey, these predatory arthropods turn their attention to each other, unwittingly creating the first strands of what will become a food-web of immense complexity.

Classification masterclass

For students of biology, ponds provide the perfect lesson in classification. Apart from the echinoderms (starfish and sea urchins), British freshwater habitats include representatives of every major taxonomic group in the animal kingdom. Many of these creatures are very small and can be scooped up in an average-sized net, so studying them requires only the minimum of equipment (see page 10).

The tadpole is just one of the fascinating organisms worthy of a closer look. The life history of this amphibian is astonishing, reflecting in a moment the immense achievement of evolution. A lot of internal and external reorganisation needs to happen for the tadpole to convert from a wholly aquatic herbivore with gills to a largely terrestrial carnivore with lungs, an astonishingly rapid metamorphosis.

Frogs, toads and newts are not the only vertebrates associated with ponds.

Among the reptiles, the impressive grass snake will glide stealthily into the water in search of amphibian prey, though most school grounds are much too busy to harbour these wary creatures. Fish can cause problems in small ponds by drastically reducing biodiversity and hence educational value, so it is possibly a good idea to exclude them from your plans. Most species, such as roach and rudd, are predatory - as is the common goldfish - and will quickly consume the larger and more interesting insects and other arthropods. Others, such as carp, are more omnivorous and can wreak havoc by stirring up the roots and mud of a carefully planted pond.

Mammals tend to be shy and many of them are nocturnal, so are unlikely to be seen by pupils collecting specimens. Signs of otters and water voles can be found by the sides of rivers, streams and dykes in suitable areas, but ponds are only frequented by these animals if they form part of an interlinked system of waterways, such as in the Broads of East Anglia.

By contrast, birds will make use of a pond, however public, from the moment it fills with water. Providing it has at least one shallow, gently sloping side, a pond will be visited by birds of many species as they fly in for a wash, while passing ducks are quite likely to drop down to investigate.

Artistic inspiration

From a teacher's point of view, the most rewarding aspects of pond study are provided by the plants and invertebrates. A carefully planted pond can provide endless inspiration in all seasons for art lessons and descriptive writing (see "Ponds and creative writing", page 10). Leaf shape and size, flower colour and position in the water vary widely between species, giving plenty of scope for artistic talent. The reedmace grows tall and straight, marching out from the shallows into deeper water, while other emergents, such as the attractive blue-flowered brooklime, barely clear the surface.

Some have white flowers, but contrast in the structure of their leaves: arrowhead, water plantain and frogbit are all highly distinctive. So, too, is the water soldier:its brittle, sawtoothed leaves appear out of the depths in the spring, looking like pineapple-tops as they rise up in serried ranks, only to sink below the surface again with the onset of autumn.

Your school pond can reflect the full spectrum of colours without the need to resort to exotic imports. Britain's wildflowers include a dazzling array of species adapted for life in or around fresh water - try arranging the flowers of the white water lily, yellow iris, purple loosestrife, water forget-me-not, flowering rush and broad-leaved pondweed together in a vase.

These colourful plants are pollinated by insects, so your school pond will attract an assortment of bees, butterflies, flies and beetles on sunny days. Some of these will inevitably overshoot and land in the water, probably to meet a grisly end. Lurking below are legions of invertebrate predators, poised to take advantage of such mistakes.

Pondskaters and their allies, all beautifully adapted for moving on the surface, also move in for the kill. With long legs spread wide to distribute their body weight, these insects also secrete an oily substance that prevents them from getting wet and being dragged below the surface.

Acutely sensitive to the faintest vibration, these creatures respond immediately to the distress signals given out by their prey as it struggles to overcome the forces of surface tension.

The pondskaters co-operate quickly to subdue even the largest of insects by injecting poison with their sharp beaks, through which they inject enzymes before sucking up the liquefied remains. Whirligig beetles are also predators on the surface-film, but they tend to specialise on mosquito larvae, pouncing on them as they approach the surface to recharge themselves with oxygen. As they whirl around in dizzy circles like frantic bumper-cars, they simultaneously view the world above and below the water, using eyes divided into two types - one that peers through the murky depths, the other for vision in the clarity of air. At the slightest sign of danger from above, whirligigs dive and scatter, before bobbing up again to their favoured patch when the coast is clear.

Thanks to their nimble feet and water-repellent hairs, many spiders can walk on water if necessary, though only two in Britain are specially adapted for life in or on ponds. One, the raft spider, is quite spectacular. Our largest native species, it is also one of the rarest and, with its dark brown and cream markings, perhaps one of the most attractive.

Living at the edges of pools and ponds in peaty areas of southern England, this impressive spider hunts by sight and by feeling for vibrations on the water surface with the sensitive hairs on its front legs. It can grapple large prey and will unhesitatingly dart beneath the water to haul out a stickleback.

Though not so large, the water spider is even more impressive, being the only spider known to spend the greater part of its life under water. This spider spins a silken shelter attached to water weeds and then proceeds to fill it with air by returning time after time to the surface to grab a bubble. Once its "diving-bell" is fully charged, the spider can live there in an atmosphere that continually readjusts as carbon dioxide released by the spider diffuses into the water and is replaced by oxygen. Dashing out to capture any shrimps or mayfly nymphs that vibrate the trip-wires radiating from its lair, the water spider occupies an unusual niche in this underwater world.

Among the most numerous of the organisms living in the pond are the millions of tiny crustaceans that swim around in shoals. These filter feeders provide food for the smaller predatory insects. Two of the commonest are water fleas, or daphnia, and the little one-eyed cyclops, both of which are eaten by insects of all kinds. Adaptations of legs, mouthparts and body-shape enable each species of insect to hunt in its own peculiar way and help it avoid becoming the food of something else.

The bugs all have sharp beaks which they use to pierce their prey, whereas the beetles have toothed mandibles that work from side to side. Dragonfly nymphs have an elaborate hinged "mask" that shoots out from under the head and grasps the prey with its two claws, before retracting and placing it in reach of the mouth.

From seriously scary to simply fascinating

The scariest predator is the great diving beetle. Superbly adapted in every way, this large and long-lived insect, shiny black with golden edges to its body, is beautifully streamlined and has powerful wings. On landing in the water it immediately submerges and swims strongly using its strong hind legs, which are fringed with hairs to act as paddles. Its eyes, too, are second to none and it hunts its prey by sight, immobilising it with its first and second pair of legs before ripping it to pieces with its sharp mandibles.

If the adult is a fearsome predator, the larva is an even meaner adversary.

Growing to a length of about seven centimetres, the great diving beetle larva has a legendary appetite and will attack anything that moves within reach: even fish and frogs are not safe. Unlike its parent, the larva lacks jaws, but has an enormous pair of hollow fangs instead, which it sinks into the flesh of its unfortunate victim. Enzymes are then poured out to digest the prey and a powerful pump in the head of the larva then sucks up the resultant soup. Being semi-transparent as well, the entire internal workings of this morbidly fascinating creature can be seen in action.

The humble flatworm, by contrast, appears to be a most innocuous beast.

Gliding sedately over the sediment at the bottom of the pond, it seems the epitome of harmlessness, yet appearances can be deceptive. The moment it detects an animal - even one similar in size to itself - something extraordinary happens: the worm's stomach suddenly extends from a hole in the centre of its body, engulfs the animal and disappears inside the flatworm again.

Ponds are full of surprises - some gruesome, others simply fascinating - but they all add up to a great teaching resource.

Ben Aldiss is an ecologist with a PhD on wasps. He is an expert adviser for the Heritage Lottery Fund and has set up the School and Farm Wildlife Programme. He lectures in conservation management at Otley College, teaches biology at Thorpe House School in Norfolk, and runs an A-level biology field course in the Pyrenees


Studying pond animals

* Nets of various qualities can be bought from Griffin Education, but plenty of small creatures can be caught easily by dipping a jam-jar on a piece of string into the pond. If the contents are poured into a white tray (also from Griffin), any animals can easily be seen and transferred to a cavity slide topped with a coverslip, then viewed with a microscope (preferably binocular). If the microscope has a video attachment you can project the image of the live animal onto a whiteboard or screen.

www.griffineducation.co.uk * The Field Studies Council produces an excellent identification key to freshwater invertebrates in its AIDGAP series www.field-studies-council.orgpublications

Ponds and creative writing

Numerous poets have been inspired by water and the plants that grow in it, eg Charles Kingsley (The Water Babies), and Percy Bysshe Shelley: And nearer to the river's trembling edge There grew broad flag-flowers, purple, pranked with white, And starry river buds among the sedge, And floating water-lilies, broad and bright.

* The Field Studies Council produces an excellent identification key to freshwater invertebrates in its AIDGAP series www.field-studies-council.orgpublications


* Choose a sunny spot - shade and fallen leaves can prevent water plants from growing. Ideally, site it close to a source of fresh water such as a stream, spring or ditch. Bigger ponds are more diverse, so make it as large as possible. Mark out an irregular outline if you want a natural-looking rather than a formal pond.

* Dig the pond so that it has at least one gently shelving side - wildlife can then use it to drink and wash.

* Give the pond irregular depth contours for a greater diversity of animals and plants

* Dig to at least 60cm deep in one part. This is vital, as it ensures the water does not heat up too much in summer or freeze throughout winter.

* Measure the length and width of the pond using a flexible tape-measure and add an extra metre to both dimensions to allow for the necessary overlap of the pond liner.

* Smooth the contours (if too sharp, the plastic liner will stretch), remove surface stones and cover the surface with about a centimetre of builders' sand.

* Buy the thickest black butyl pond-lining sheet you can afford (thinner ones will last as long, but are more prone to damage by sharp objects).

Colourless polythene, though cheaper, becomes brittle on exposure to sunlight, even under water.

* Lay the liner loosely over the pond surface so that the weight of water will not stretch it.

* Fill with water (if you want to know the capacity of your pond and your water is metered, check the meter reading at start and finish).

* Pile some of the earth you excavated along the sides of the liner where it emerges from the pond, then trim off excess liner with scissors.

* Carefully tip some of the excavated earth into the pond on all sides to make a muddy bottom. Add some gravel in places to vary the habitat.

* Carefully place large stones near the shallow edges to provide stepping stones for animal life.

* Tie water plants to small rocks or add small rocks to the root ball and wrap with plastic mesh before dropping them in. Floating species can just be dropped into the water.

* Unless you want natural colonisation, plant the pond edges with plants of your choice by digging small holes in the ridge of earth, then cutting the liner to allow roots to grow through.

* Animals will arrive by themselves.

* Avoid treading in your pond, throwing in stones or breaking the ice in winter -all can result in holes in the plastic liner.

* Top up your pond with tap water only when really necessary - the chlorine in it gradually disperses, but will discourage some small creatures from colonising.

* If you want a "natural" pond, avoid buying exotic species of plants or animals - many plants sold for ponds are naturally-occurring British species, or more colourful varieties of them. Ideally, ask permission from landowners to take specimens from established wildlife ponds nearby. Be sure that plants bought from garden centres are free from alien species.

Many exotic species spread will rapidly displace native ones.

* If you can afford it and your pond is big enough, lay some weatherproof boardwalk on one side and install a pier. This allows for regular pond-dipping with minimum damage.

* Don't forget to write a comprehensive risk-assessment.

* For more details download Great Crested Newt Conservation Handbook from Froglife www.froglife.org

For leaflets or a copy of The Pond Book visit www.pondstrust.org.uk

The QCA's latest document Work-related learning at key stage 4 - Science has a case study entitled 'Students work together to create a school pond' www.qca.org.uk

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