How does wildlife find garden ponds?

here, at the bottom of the emptied pond, wriggled an unexpected cadre of eels. This body of water was huge, originally designed as an outdoor swimming pool by whoever lived in the house previously. But the former owner soon deemed it too cold to swim in. So they had filled it with ornamental goldfish instead.

When Lydia Massiah, now a former teacher and aspiring writer, moved to the property years ago, she and her family decided the pond was far too large for the garden. So they began the process of draining it. Someone came to take the goldfish away, she recalls. But she never thought she would find eels hiding in the pool’s darkest depths.

"They would have crossed a road and come across two fields from the river," she says, still marvelling at how they had slithered over land, which eels are known to do, and climbed into the pond, which was also raised 2ft (0.6m) above ground level. Once discovered, they soon vanished, presumably back towards the river, Massiah adds.

It is arguably this mysterious property of water in gardens – how it attracts wildlife so successfully – that makes ponds such special things. Massiah and her family made sure to keep another, more ecologically balanced, wildlife pond intact in the back garden in the hope that it would continue to benefit local species.

Not all ponds are biodiverse, of course – but the recent wildlife gardening craze has thrust them into the spotlight. Even people with limited outdoor space are encouraged to get involved, with several environmental charities recommending mini versions made with half-sunken washing up bowls. There’s an increasing awareness that – when thoughtfully designed – they can act as lifelines that boost the resources available to native plants and animals.

According to the Wildlife Trusts, there are around three million garden ponds in the UK. In the US, researchers estimate that there are millions of artificial ponds, though some of these are agricultural rather than recreational. How important are they really for wildlife, though? How do animals actually locate them? And what should you think about when designing a pond to support local species?

"It is like magic," says Jenny Steel, an author and plant ecologist, referring to the power of ponds to conjure up wildlife. "The first big pond I made myself, within something like 12 hours of filling that pond up, there were dragonflies."

Besides the frogs and aquatic insects that many hope to see in their garden ponds, sometimes even more unusual visitors drop by these watering holes – from tawny owls to grass snakes or even kingfishers.

Steel reminisces about watching swallows and house martins swoop and soar above her garden ponds on summer evenings, picking off airborne insects on the wing. You don't need a gargantuan lagoon, either. On her patio, Steel has a small pond in part of an old barrel and even this attracts animals. "That little barrel pond is used all the time by birds," she says, describing how they love to bathe in it.

It does raise the question of how these creatures detect ponds from afar. Some people have observed that you can simply take the "build it and they will come" approach. But how does nature know?

Many flying animals look for the light or sound reflected by bodies of water. Researchers in Hungary have studied dragonflies' attraction to the polarised light that bounces off water’s surface. Depending on species, these insects seem to prefer dark or light ponds to lay their eggs in, and possibly select this by looking for differences in the polarised light reflected by watery habitats. So powerful is the draw of this light that some dragonflies may even get confused. One 2007 study suggested that they can mistake the reflective surfaces of polished gravestones in cemeteries for ponds.

Birds also sometimes confuse the polarised light reflected by asphalt for water.

Other animals likely use their sense of smell to sniff out their next aquatic abode. Experiments suggest that amphibians such as salamanders can detect water in darkness by scent alone.

"My advice is always that any water is better than no water at all in a wildlife garden," says Steel.

But you don’t have to leave all the work to nature. If you are thinking of establishing a wildlife pond, or making an existing pond in your garden more wildlife-friendly, there are lots of things you can do to maximise your chances of both attracting and benefitting native species, says Steel. And it's worth making the effort.

For one thing, it's important to try and use rainwater to fill the pond up rather than tap water because of additives such as chlorine, and contaminants including nitrates, which may harm wildlife. Nutrients in tap water can also be problematic since they encourage the excessive growth of algae, adds Steel.

Steel also stresses the importance of including a variety of native aquatic plants, and perhaps some terrestrial vegetation along the margins of the pond, which will benefit all kinds of species. Crucially, it's important to add some plants that will grow underwater and help to oxygenate the pond – they'll do this automatically as they photosynthesise. Two classic examples are shining pondweed, which is native to North America and Europe (though there are several species, so make sure you get a native one), and hornwort, also native to both continents.

Plants and animals need oxygen, so ensuring that there is a good supply of it will help the pond's ecosystem as a whole. It's also wise to avoid having fish in your pond if you want to support a diversity of different species. While fish are often important in natural freshwater ecosystems, in small ponds they can easily dominate and eat too many of the other inhabitants.

Matt Hill, a freshwater ecologist and senior lecturer in geography at the University of Huddersfield in the UK, stresses that establishers of ponds ought to avoid siting them in places that are very heavily shaded, since that will deter many species. And he suggests that people prioritise variability in their pond designs wherever possible – which means building some deeper areas, say up to 1m (3.3ft) or so, but also shallow banks since these are used by creatures such as aquatic insects. He says there is no problem in adding stones around part of the pond but you should avoid completely encircling it, to help smaller wildlife get in and out.

"The key message for me is that variability is to be expected – and also to be encouraged. You want different environmental conditions," he says, noting that your pond life might differ from your neighbour's. Some people might notice an abundance of snails, others will have high numbers of dragonflies. The important thing is that, dotted around a whole district or county, hundreds or even thousands of differing ponds will serve a broad swathe of plants, insects, amphibians, mammals and birds, says Hill. Together, they should benefit nature right across the area.

Hill and colleagues have studied the wider impact that garden ponds might have on biodiversity. In one paper, published last year, the researchers counted the different species in a total of 50 garden and non-urban ponds in Oxfordshire. The rural ponds had a noticeably larger diversity of species, 172 to the garden ponds' 99, but that doesn't rule out their significance. It's still a large enough selection of plants and animals to suggest that garden ponds could act as refuges or stepping stones for wildlife in urban landscapes, the authors wrote. Plus, the most diverse example of the garden ponds they studied punched above its weight and was comparable in terms of biodiversity to the non-urban ponds.

In separate research, published in 2016, Hill and colleagues assessed data from multiple earlier studies to quantify the number of invertebrate species – such as dragonfly nymphs – across 800 or so ponds in the UK, 230 of them in urban locations. They found that there was a similar variety of species and invertebrate families represented across different types of ponds, again highlighting garden ponds' potential as wildlife havens.

"I would say they are under-used as a biodiversity resource," says Hill.

The benefits of ponds may be broader, even, than that. As Pippa Johnson, ponds officer for Cheshire Wildlife Trust, notes, watching wildlife dart and dash around the surface of a pond can give endless enjoyment. She herself has childhood memories of observing frogspawn hatch in her family’s garden pond.

"They’re good for nature but they're also good for us," she says. And she agrees with Hill that having a variety of different elements in a pond will offer the greatest assistance to wildlife.

However, it's not all about the pond itself – the rest of the garden habitat surrounding any body of water is also worth considering. The Wildlife Trusts suggest maintaining a patch of unmown grass near the pond for young amphibians and butterflies and perhaps a compost heap, which animals such as grass snakes may lay their eggs in. Also, animals might find it easier to get into the garden in the first place if the habitat is bordered by hedging rather than walls or a solid fence.

The truth is that no-one knows what wildlife they'll get in their pond until they build it. And while maximising nature's chances by design is worthwhile, it’s also OK to experiment, says Steel.

"It is a subject that you can overthink," she adds. "The water itself is the important part." Next door to her garden, for instance, lies a pond in a field that has formed quite naturally after someone dug a hole in the ground many years ago. The pond gradually filled with rainwater and wildlife eventually took it over."It's now an amazing pond – and nobody has done anything to it," says Steel.