Urbanisation is occurring at a rapid rate throughout the world, and it is widely regarded as being detrimental to biodiversity. To maximize the potential benefit of urban environments for wildlife, it is essential to understand the possible effects of different urban land uses on biodiversity.
Domestic gardens have the potential to play a crucial role in supporting urban biodiversity. In the U.K., residential zones can account for more than 60% of urban land area. Consequently, private gardens may represent a significant proportion of greenspace in a city. From work in Leicester, and elsewhere, it has been estimated that gardens typically constitute about a quarter of urban land areas.
Urban gardens will never act as substitutes for many semi-natural habitats, however, neither are they 'wildlife deserts'. Gardens can offer a rich variety of resources, such as a broad range of microclimates, plant species, and vegetation structures. They can also provide habitats, such as ponds, that may be increasingly rare elsewhere. The potential diversity of wildlife is illustrated by the long-term study of one suburban Leicester garden, which was managed sympathetically towards wildlife, where more than 2200 animal and plant species were recorded. Another study recorded more than 95 species of wild plant in a single garden.
Furthermore, gardens are not inhabited only by common species. The juniper pug is an example of a scarce moth whose natural foodplant is rare, but which successfully exploits ornamental junipers in domestic gardens. Likewise, the stag beetle lives in tree stumps and logs in its larval stage, but these are a rather scarce resource in many areas. Their presence in gardens supports stag beetle populations in south east England.
As these examples indicate, gardens may offer much potential as habitats for both common and rare species. However, there are very few detailed studies of garden biodiversity, and those that do exist are restricted to individual gardens. Such research cannot tell us how biodiversity varies between gardens. Studies at larger scales usually concern groups of organisms which can be observed by garden owners. Examples include the British Trust for Ornithology's Garden Bird Scheme and the R.S.P.B. / Wildlife Trusts' survey of thrushes and molluscs.
This type of study has proved very valuable but can not tell us whether the diversities of relatively well-studied taxa, (e.g. birds, amphibians) are representative of other, less well known, groups (invertebrates, fungi, and even native plants), which almost certainly form the largest component of garden wildlife. Clearly there is a need to understand what features of domestic gardens, and of their surroundings, affect biodiversity. However, until now these aspects have not been addressed systematically for a range of garden types.
Although the study of garden biodiversity is limited, there is public awareness that gardens can be managed to encourage wildlife. Much advice is available in books and the media on 'wildlife gardening'. Some popular and widely advocated approaches include pond creation, growing plants as nectar sources for insects, installing artificial sites for nesting or hibernation by birds, bats or insects, and neglecting corners of the garden, allowing grass to grow long, or dead wood to lie on the ground, for example.
If these activities are successful then, given the area of gardens in urban environments, such 'creative conservation' measures may play an important role in enhancing biodiversity, and hence in urban conservation more generally. Some enhancements are obvious - there is little doubt that ponds can attract species that were previously absent, or that bird boxes are used for nesting - however, the success of many garden manipulations in increasing the numbers and abundance of species remains unclear. Which methods are most effective, and in what circumstances, and over what time scales do they work?
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