Introduction
An urban area can be defined as land which is irreversibly urban in character, consisting of permanent structures such as buildings, transport networks, amenity sites and other associated structures. They can vary a lot from each other depending on where they are found and they often include green or open areas that can be as beneficial to humans as to wildlife. (Pickett et al., 2001)
Growth of urban areas
During the 18th-19th century, when the Industrial Revolution occurred, the UK had a major growth of urban areas as people moved from the countryside to the cities to work. These areas have significantly grown throughout recent decades and it is predicted that they will increase in size even more due to human population growth. Urbanisation has had numerous effects on the natural environment and the biodiversity that resides in it. One of the reasons for this is the increased fragmentation of these natural habitats often causing deaths within various groups of animals that would usually inhabit them. In already developed countries 80% of the population lives in towns or cities and it is estimated that by 2030 80% of the people in developing countries will move to urban areas. (UNFPA 2007; Goddard et al., 2009) Urbanisation is affecting the ecology in various ways, degrading the water quality, causing disturbance to wildlife and transforming species composition by reducing the number of animals and increasing the number of threatened species (Table 1). (Gaston et al., 2004)
Gardens in the UK
In the UK, most “green spaces” are scattered in patches in urban areas and they can be interconnected in different ways using green corridors such as hedges, streams and tunnels. These green areas not only provide habitats for urban wildlife but they also provide other benefits such as the decrease of soil erosion and pollution but also the increase of well-being for humans. (Spellberg and Gaywood, 1993) The management and preservation of these areas has become more important as urbanisation further increases. Domestic gardens make up a significant fraction of what is considered “green space” and they are therefore considered important factors for the conservation of local biodiversity. Private gardens are usually relatively small, however, when grouped together they occupy an area of about 18-27% of the urban areas of cities and 87% of UK households possess a garden. (Gaston et al., 2004; Gaston et al., 2005; Gibbons et al., 2011)
Wildlife conservation
Many different species of mammals and birds inhabit domestic gardens in the UK and they all have needs which could be accommodated in various ways. If managed in the right way private gardens can become excellent habitats for both terrestrial mammals and birds in urban areas. Domestic gardens sustain a substantial fraction of the total UK population of some bird species. Some people are already contributing by making their gardens a more hospitable environment for animals through wildlife gardening, for example, there are at least 4.7 million nest boxes and 3.5 million ponds in gardens in the UK. (Davies et al., 2009) Another factor that has positively influenced the abundance of birds in certain areas is the number of households (12.6 million in the UK) that are feeding birds. The amount of bird feeders present in specific places depends on various factors, such as the size of the garden and also the socioeconomic level of the household. It has been observed that the presence of bird feeders was less in more socioeconomically deprived areas. (Fuller et al., 2008)
There can be both advantages and disadvantages for a mammal or a bird to live in an urban area, even if they inhabit a private garden that has been constructed using wildlife gardening techniques. If the owners of the garden place bird feeders and nest boxes this is an advantage for a bird as it has been provided with food and shelter. Birds, however, may encounter problems living in the garden, for example, if the owner possesses a cat this would place a predator in their habitat. Research conducted in the UK has indeed shown that cats mainly prey on bird species such as house sparrows, which are suffering population declines. (Baker et al., 2005; De Laet and Summers-Smith 2007). Green walls are another mechanism that can help preserve bird populations. These are able to provide shelter and nesting locations for urban birds and research has shown that avian abundance is greater on green walls compared to bare ones.(Hinsley and Bellamy 2000) Different bird species will use these walls in different ways such as shelter, nesting or feeding. They are a successful way of conserving birds as they allow them to use the upper end of the wall, where they are distant enough from ground disturbances such as predators or humans. Green walls are also appreciated by homeowners as they are aesthetically pleasing and they also provide insulation for houses.
Urbanisation has also caused domestic gardens to become places where disease transmission is higher for particular species (Table 1). This is due to fragmentation and the need to somehow connect all these green spaces to create advantages for wildlife, permitting animals to move safely and have larger territories. These connecting “green corridors” can, in some cases, enhance the spread of diseases. It is known that the red squirrel is in competition with the more recently introduced grey squirrel which has been responsible for reducing the numbers of the native squirrel. The grey squirrel has introduced in the UK the squirrel paramyxovirus, and food supply (squirrels often take advantage of bird feeders) in urban areas has increased squirrel contact rates, therefore disease transmission leading to more deaths. (Altizer and Bradley 2007)
Another issue with using private gardens as tools for conservation are the people themselves. Not everyone appreciates the presence of wildlife in their gardens, especially when some species are viewed as pests, such as foxes.
Being an urban animal is hard, generalist species are better adapted.
There is a chance that residential gardens could become an effective conservation tool for mammals and birds in the UK but it would require many changes and subsequently money to make them happen. The first thing that people need to do is that they can’t consider private gardens as singular entities or patches, they have to be seen as one big green area that needs to be connected so that fragmentation is minimised. Household owners who have gardens have to work together and build or restore direct habitat links. For example, since hedges are home to a significant amount of small urban mammals and birds, it would be sufficient that communities would use these plants as territory delimitations in their gardens instead of fences which block the movement of animals. If there is an impossibility to create direct links then people should at least try and build stepping stones so that colonisation distances are shortened. Appropriate wildlife gardening would improve the lives of many birds and mammals