Backyard safari

Scientists in the north of England have uncovered a huge, secret wildlife haven, occupying some 33 square kilometres (13 square miles). The area is home to countless species of spider, insect, mollusc, lichen and moss, as well as bird and small mammal. It has more than a quarter of a million trees and innumerable shrubs and flowers. It also has about 25,000 ponds. In addition, there are 45,000 nesting-boxes, tens of thousands of compost heaps, and vast tracts of grass.

This wildlife haven is, of course, made up of people's gardens, those within the city of Sheffield. For the first time in this country, the network of urban gardens across a city has been surveyed as an ecological resource, revealing a large and important diversity of plants and animals. The survey should give important information about the role of gardens in maintaining biodiversity in urban areas, the interactions between organisms within them, and how they might be improved as a wildlife habitat. The information should be useful for urban planners to help them to decide the best way of ensuring that biodiversity flourishes in our towns and cities.

This unique audit has been carried out by ecologists from the University of Sheffield, led by Professor Kevin Gaston, together with his colleagues, Dr Ken Thompson, Dr Phil Warren and Dr Richard Smith. The three-year project was funded by the Natural Environment Research Council as part of its programme on urban regeneration and the environment.

"The project had three main objectives," says Professor Gaston. "We wanted to get some understanding of the garden resource – what area of the urban landscape is covered by gardens, and what ecologically important features are in them. We wanted also to understand the factors that promote biodiversity in gardens. And finally, we wanted to test some simple interventions that might increase and sustain biodiversity."

Gardens have received surprisingly little attention as an ecological resource. "They have fallen between the cracks in terms of scientific study," says Professor Gaston, "largely because they are not wild. If, as a scientist, you have the choice of going to the Serengeti or someone's garden, it's clear where your interest will lie." Another reason gardens have been ignored is that they are privately owned and so not under the responsibility of a single agency. However, with growing urbanisation and intensive agriculture leading to a decline in biodiversity, gardens are likely to become progressively more important, ecologically.

The first finding of the project was the sheer scale of the space occupied by gardens in a built- up area. Using maps and aerial photographs, the research team calculated that almost one quarter of the 143 square kilometres that comprise the urban area of Sheffield consists of gardens – some 175,000 green spaces. The researchers selected a range of types of garden to survey for their wildlife, from small backyards to near-palatial grounds. "We wanted a set of gardens that maximised the variation in terms of age, size, and proximity to the urban centre," says Dr Thompson. "We selected 61 representative examples, and each one was mapped in detail so we had a precise understanding of each garden's structure."

The next job was to survey a selection of types of organism. "There has been a lot of work on urban birds and mammals, so we decided to concentrate on the 'hidden' biodiversity," says Dr Warren. "We looked at a range of invertebrates such as snails, spiders, beetles, wasps, and crane flies. Many of these perform important functions in the ecosystem, such as pollination or predation." Mosses and lichens were also noted.

The research team then had to recruit a small army of naturalists, both amateur and professional, to identify specimens. "This generated a lot of enthusiasm," says Professor Gaston. "Sheffield has a thriving network of recorders who specialise in groups of organisms. But if they know a lot about what is in their own gardens, they tend to know less about what is in other gardens. It was an interesting exercise for them."

In fact, one of the unusual – and warming – aspects of the study was the extent to which the scientists relied on the cooperation of Sheffield's citizens, both naturalists and garden owners. "Many of the householders had little interest in the project initially, beyond plying us with tea," says Professor Gaston. "But as the project went on, they took more interest in what was, literally, in their own backyards."

After almost three years, the experimental part of the project is complete and researchers are analysing the vast amount of data that has been generated. "One important finding is that different groups of organisms respond to variations in their environment at different scales," says Dr Smith. "So some organisms are much more influenced by what is going on in the garden where they live, such as lichens and snails, while others respond to the wider environment – what is happening across many gardens – bumblebees and other flying insects, for example."

Unsurprisingly, the "structural complexity" of a garden – features such as shrubs and trees – seems to be important for biodiversity. However, the argument that manicured gardens with sculptured shrubs and billiard-table lawns are poor for wildlife seems to be without foundation. "It is clear that every type of garden plays a role," says Professor Gaston.