Examination of some 100 Sheffield gardens is revealing biodiversity on a startling scale. Steve Farrar reports
Austin Brackenbury freely admits he has an unorthodox approach to gardening. In fact, to the untrained eye it appears that the wild, overgrown plot behind his home has never been sullied by human intervention. It is a small patch of wilderness in a quiet Sheffield street.
But this is not the product of uninterested neglect. On the contrary, the 76-year-old former signal worker knows almost every plant and creature that has made its home in this remarkable urban garden. If they are native British species, he actively encourages them.
Brackenbury is a thoughtful and perceptive natural history enthusiast. As we walk through the garden, he points out the birch tree that he planted in 1953 and the three species of shield bug he has identified living on it.
A pile of logs has been placed nearby to provide wasps with a source of nest-building material. The rotting stump of a fallen ash tree has been deliberately left as a haven for other creatures.
Understandably, Kevin Gaston loves Austin's garden. In the past two years, the head of the biodiversity and macroecology group at Sheffield University, and his colleagues have become very familiar with some 100-odd back gardens across the city.
There are 175,000 of them in Sheffield - 20 per cent of the urban sprawl. Nationwide, that makes for a significant, if fractured terrain, and one that will spread further with the anticipated home-building boom. Yet urban gardens are practically unexplored terrain for ecologists. Far more is known about the interaction of plant and animal communities on remote Atlantic islands than amid the mosaic of little green spaces a stone's throw from the scientists' homes.
The Biodiversity in Urban Gardens in Sheffield project, or Bugs, involves painstakingly logging plant and animal species - in particular insects with birds to follow - and cataloguing the habitats in a sample of 61 gardens. With the backing of the Natural Environment Research Council, Gaston's team hopes to gain an insight into what fauna and flora lurk unnoticed in cities, how the organisms relate to their environment and what garden features had the most impact. In another 35 gardens, they are testing simple ways in which the biodiversity could be improved.
All of the gardens, from tiny walled yards to virtual arboreta , have been volunteered for the scientists' scrutiny by their owners.
Brackenbury's garden is clearly at the extreme end of the spectrum. As field workers survey its contents, it emerges that even an enthusiast's home turf can contain a few surprises. When one of the experts announces that he has found 21 species of flowering plant in a square metre of lawn, Brackenbury looks impressed.
"Imagine what a difference it would make to the biodiversity of Sheffield if only a small proportion of gardens were like this," Gaston says. Brackenbury smiles and adds: "All you need is the courage to face up to your neighbours."
But it would be wrong to imagine that this small plot with its rampant undergrowth and thoughtful debris is a solitary wildlife oasis in an urban desert.
Across the city, the team has been turning up an enormous variety of wildlife in much more conventional settings. It seems even the most barren-looking gardens possess complex and flourishing populations.
Back in his laboratory, Gaston looks over a tray of insects. There are thousands of dead flies, wasps, bees and moths, all collected from a single site.
About a third of the gardens played host to black-and-white tent-like structures called malaise traps to collect flying insects. Beetles and other ground-active invertebrates, such as centipedes and millipedes, were captured in pitfall traps.
Specialists then spent hours sifting through the vast quantities of material, sorting them into groups and occasionally picking out rare species.
The quantity of data is enormous and is still being analysed.
However, preliminary results, released this week at the British Ecological Society conference at Warwick University, are starting to reveal some underlying patterns.
It has emerged that more mobile species, such as carabid beetles, are not especially concerned about the available habitats in an individual garden. They are capable of travelling further afield to find what they need.
One of the project's goals is to identify how these creatures move from garden to garden, utilising a green matrix that crisscrosses cities like the road network.
In contrast, more static, leaf-mining insects, such as moth caterpillars or fly larvae, are quite particular about their habitat. They require specific plants to live on, though of the 100 host plants identified by the team, 17 were not native to the United Kingdom.
There are, fortunately, easier ways to help wildlife than replanting.
Jean Helliwell, a 61-year-old retired civil servant, keeps a pleasant and tidy garden, though she insists it has seen better days. Dotted around it, however, are a number of unlikely garden ornaments - test rigs for the second part of the Bugs project.
Each had been chosen from the host of cheap and simple wildlife gardening tips offered in books and magazines and on television. Some have fared better than others:
- The small, plastic pond attracted midges and mosquitoes, with many eggs laid in the standing water
- The tub of stinging nettles was largely ignored by butterflies
- Bundles of plastic drinking straws, blocks of wood pitted with drill holes and short lengths of bamboo cane were visited by solitary bees and wasps. Eggs were laid inside, food was left for the larvae and then mud caps added to seal the chambers
- An upturned terracotta pot filled with furniture stuffing was snubbed by bumblebees as a potential nesting site.
The assessment work continues.
While the project will spawn a host of academic papers, Gaston is keen that the public is presented with the results when they emerge following the completion of fieldwork next year.
It might help customise the nation's gardens to boost biodiversity.
"If you could just redirect a proportion of the huge sums of money spent in this country on gardening to being more environmentally friendly, you could make a significant difference," Gaston says. "Each garden is a conservation possibility, a potential reserve."