School of Biology

Into the Urban Expanse:

Gardenlife

gardens as a reservoir for arthropod biodiversity
by Grant Brown

Have you ever wondered how many species you have living in your garden? Which factors determine what makes a good garden for wildlife, and how we can make our gardens as wildlife-friendly as possible?

There is a growing interest in gardening to encourage wildlife, but surprisingly few scientists have looked at what's actually happening in our gardens. Of the studies that have been carried out so far, most have focused on birds, mammals, and butterflies. What about the some of the invertebrates which some people might find 'less appealing' - arthropods like beetles, spiders and springtails?

These creatures play a vital role in many important processes such as decomposition, cycling nutrients, and regulating pests. But despite their contributions to these beneficial and essential tasks the arthropods are seldom considered when it comes to making decisions about managing conservation in the urban environment.

To find out more about the arthropod sepcies in our gardens, staff in the School of Biology launched a project called Gardenlife. Over the next three years this project will study a number of gardens both in the United Kingdom in St Andrews, and, in South Africa at Stellenbosch. We will be looking at the relationship between biodiversity and the age and size of gardens, and also at how different practices in garden management affect the range and scope of species found in gardens.

We hope that the 'Gardelife' project will provide better understanding of the potential that gardens have as reservoirs for biodiversity of these important animals in our towns and cities. Results will help to inform future policy on sustainable urban expansion and allow planners to design in a way which can foster biodiversity in 'the urban expanse'.

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The Urban Expanse

Urbanised areas presently cover between 2.8 - 4% of the earth's surface and more than 50% of the global human population now dwells in cities and towns. The rate of urbanisation in many developing countries is increasing rapidly and is expected to eventually contain the majority of the urban population globally, with important implications for human welfare and biodiversity maintenance. [ refs: 1-5 ]

Whilst the total land area currently occupied by urban environments is relatively low on a global scale compared to other habitat types such as agriculture, there is increasing evidence to suggest that the rate of urban sprawl is increasing at a rapid rate in excess of any other land use - with few signs that rates will decrease towards 2030. [ refs: 3,6,7 ]

The physical expansion of cities and towns tends to outpace increases in human population density, so that a new development often uses more space per occupant than older constructions closer to the urban core, further hastening urban sprawl and exasperating impacts from altered land use. Changes to pre-existing landscapes during urbanisation are often rapid, intensive and usually permanent. Urban development has been linked to changes in local climate, nutrient cycles and changes to habitat complexity, which can manifest in high rates of local extinctions [4, 5, 8].

A significant component of the urban landscape in these regions is in the form of green space areas. These are often diverse in function, and include such areas as playing fields, parks and public or private gardens, as well as brownfield sites and habitat remnants of pre-existing land use prior to urbanisation.

Urban green spaces have been labelled as key places of interaction between humans and nature, bringing positive influences on public attitudes and perceptions. Despite this, green space provision is often under threat from development, and the limited temporal data suggest that per capita green space provision has declined over recent decades. [ refs: 3,9-11 ]

Gardens can represent a substantial proportion of the green space within urban regions, and in Scotland represent around 30% of the land within the urban expanse. In UK cities, the proportion of garden habitat within cities is variable, but ranges from around 10-25%, with a UK average of 13%. [ refs: 2,12 ]

Gardens and biodiversity

Gardens used to be seen as ecologically barren.  Work by researchers like Jennifer Owens has done a great deal to change this viewpoint.  Over a thirty year period Owens monitored biodiversity in her suburban garden in Scraptoft Lane, Leicester. [refs: 13 ]

The total richness for all organisms in her garden peaked at 2,673 species, with the majority of diversity within invertebrate and plant taxa. Certain taxa, such as Opiliones (harvestmen) were well represented with almost half of the UK species recorded. Other taxa, such as ground beetles, were less well represented with less than 10% of the UK fauna recorded within the garden plot. Overall however, species richness was surprisingly high. [refs: 13 ]

Similar research was conducted by Ken Thomson and colleagues, as part of the Biodiversity in Urban Gardens in Sheffield project. [ refs: 14 ]

Initially focusing on 61 gardens around Sheffield, this work later expanded to include other major UK cities such as Edinburgh and Oxford. Species richness was not as spectacular compared to the levels reported by Jennifer Owens but they did highlight several interesting avenues for further research. Of interest to us was that taxa with similar life histories could respond differently to the same garden characteristics and that the commonly encouraged 'wildlife friendly' garden modifications often had a lesser than anticipated effect at the scale of the individual garden. [ refs: 13,15,16 ]

It seemed that whilst sympathetic garden management could promote wildlife to some extent, there were factors constraining the maximum biodiversity that lay outwith the control of individual land owners. We wanted to identify these and gain understanding of their relative influence to better inform future conservation and planning policy.

Specifically, we chose garden age, area and management as factors of potential interest.
Garden and urban planning have varied in design through time, and it would seem likely that older gardens will differ in various ways from younger gardens.

One obvious difference is location - as towns and cities tend to expand from the outskirts, the newest properties will generally lie on the fringes of an existing urban region. Older gardens may experience less migration if they exist in the urban core, or younger gardens may be limited by lack of mature vegetation. We need to find out more to undersatnd how garden age and other variables such as size, maturity of vegetation, and garden management may relate to biodiversity.

This study aims to extend our understanding of these relationships in order to inform future planning policy and raise the profile of arthropod conservation in general.

 

 

Finding out more

The Garden Life project website

BUGS

Royal Horticultural Society 'Wildlife and the Gardener'

A short article on Jennifer Owen and her work

References

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2. UNFPA, State of world population 2007. Unleashing the Potential of Urban Growth. 2007.
   
3. Davies, Z.G., et al., A national scale inventory of resource provision for biodiversity within domestic gardens. Biological Conservation, 2009. 142(4): p. 761-771.
   
4. Grimm, N.B., et al., Global Change and the Ecology of Cities. Science, 2008. 319(5864): p. 756-760.
   
5. Gaston, K.J., Urbanisation, in Urban Ecology, K.J. Gaston, Editor. 2010, Cambridge University Press: Cambridge.
   
6. McKinney, M.L., Urbanization, Biodiversity, and Conservation. Bioscience, 2002. 52(10): p. 883-890.
   
7. Miller, J.R., Conserving Biodiversity in Metropolitan Landscapes: A Matter of Scale (But Which Scale?). Landscape Journal, 2008. 27(1): p. 114-126.
   
8. Luck, G.W. and L.T. Smallbone, Species diversity and urbanisation: patterns, drivers and implications, in Urban Ecology, K.J. Gaston, Editor. 2010, Cambridge University Press: Cambridge. p. 88-119.
   
9. Miller, J.R., Biodiversity conservation and the extinction of experience. Trends in Ecology & Evolution, 2005. 20(8): p. 430-434.
   
10. Fuller, R.A. and K.J. Gaston, The scaling of green space coverage in European cities. Biology Letters, 2009. 5(3): p. 352-355.
    
11. Davies, R., et al., City-wide relationships between green spaces, urban land use and topography. 2008, Springer Netherlands. p. 269-287.
    
12. Greenspace Scotland. 2009. State of Scotland's Greenspace 2009. [online] Accessed 19th August 2011.
    
13. Owens, J., Wildlife of a Garden: A Thirty-year Study. 2010, Peterborough: Royal Horticultural Society.
    
14. BUGS. Biodiversity in Urban Gardens - University of Sheffield [Online]. Sheffield. Available: http://www.bugs.group.shef.ac.uk/index.html [Accessed 19th August 2011.
    
15. SMITH, R., WARREN, P., THOMPSON, K. & GASTON, K. 2006a. Urban domestic gardens (VI): environmental correlates of invertebrate species richness. Biodiversity and Conservation, 15, 2415-2438.
    
16. SMITH, R. M., GASTON, K. J., WARREN, P. H. & THOMPSON, K. 2006b. Urban domestic gardens (VIII): environmental correlates of invertebrate abundance. Biodiversity and Conservation, 15, 2515-2545.