A national scale inventory of resource provision within domestic gardens

Abstract

The human population is increasingly disconnected from nature due to urbanisation. To counteract this phenomenon, the UK government has been actively promoting wildlife gardening. However, the extent to which such activities are conducted and the level of resource provision for biodiversity (e.g., food and nesting sites) within domestic gardens remains poorly documented. Here we generate estimates for a selection of key resources provided within gardens at a national scale, using 12 survey datasets gathered across the UK. We estimate that 22.7 million households (87% of homes) have access to a garden. Average garden size is 190 m2, extrapolating to a total area of 432,924 ha. Although substantial, this coverage is still an order of magnitude less than that of statutory protected areas. Approximately 12.6 million (48%) households provide supplementary food for birds, 7.4 million of which specifically use bird feeders. Similarly, there are a minimum of 4.7 million nest boxes within gardens. These figures equate to one bird feeder for every nine potentially feeder-using birds in the UK, and at least one nest box for every six breeding pairs of cavity nesting birds. Gardens also contain 2.5–3.5 million ponds and 28.7 million trees, which is just under a quarter of all trees occurring outside woodlands. Ongoing urbanisation, characterised by increased housing densities, is inevitable throughout the UK and elsewhere. The important contribution domestic gardens make to the green space infrastructure in residential areas must be acknowledged, as their reduction will impact biodiversity conservation, ecosystem services, and the well-being of the human population.

Full text

A national scale inventory of resource provision for
biodiversity within domestic gardens
Zoe G. Davies
a
, Richard A. Fuller
a,1
, Alison Loram
a,2
, Katherine N. Irvine
b
,
Victoria Sims
a,3
, Kevin J. Gaston
a,*
a
Biodiversity and Macroecology Group, Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
b
Institute of Energy and Sustainable Development, De Montfort University, The Gateway, Leicester LE1 9BH, UK
ARTICLE INFO
Article history:
Received 3 September 2008
Received in revised form
2 December 2008
Accepted 6 December 2008
Keywords:
Backland development
Backyard
Biodiversity
Conservation
Green space
Urban planning
ABSTRACT
The human population is increasingly disconnected from nature due to urbanisation. To
counteract this phenomenon, the UK government has been actively promoting wildlife gar-
dening. However, the extent to which such activities are conducted and the level of
resource provision for biodiversity (e.g., food and nesting sites) within domestic gardens
remains poorly documented. Here we generate estimates for a selection of key resources
provided within gardens at a national scale, using 12 survey datasets gathered across the
UK. We estimate that 22.7 million households (87% of homes) haveaccess to a garden. Aver-
age garden size is 190 m
2
, extrapolating to a total area of 432,924 ha. Although substantial,
this coverage is still an order of magnitude less than that of statutory protected areas.
Approximately 12.6 million (48%) households provide supplementary food for birds, 7.4
million of which specifically use bird feeders. Similarly, there are a minimum of 4.7 million
nest boxes within gardens. These figures equate to one bird feeder for every nine poten-
tially feeder-using birds in the UK, and at least one nest box for every six breeding pairs
of cavity nesting birds. Gardens also contain 2.5–3.5 million ponds and 28.7 million trees,
which is just under a quarter of all trees occurring outside woodlands. Ongoing urbanisa-
tion, characterised by increased housing densities, is inevitable throughout the UK and
elsewhere. The important contribution domestic gardens make to the green space infra-
structure in residential areas must be acknowledged, as their reduction will impact biodi-
versity conservation, ecosystem services, and the well-being of the human population.
2008 Elsevier Ltd. All rights reserved.
1. Introduction
For the first time in recorded history, over half of the global
human population live in urban areas and are, therefore,
becoming progressively disconnected from the natural world
(Wilson, 1984; Pyle, 2003; Miller, 2005). In developed countries
this proportion is much higher and, on average, is expected to
rise to 84% by 2030 (United Nations, 2007). In fact, urban areas
are presently expanding at a faster rate than any other land
use type (Meyer and Turner, 1992; McKinney, 2002), with
approximately 4% of global land cover being defined as urban-
ised (characterised by high human population densities or
0006-3207/$ - see front matter 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biocon.2008.12.016
*Corresponding author: Tel.: +44 (0)114 2220030; fax: +44 (0)114 2220002.
E-mail addresses: z.davies@sheffield.ac.uk (Z.G. Davies), r.fuller@uq.edu.au (R.A. Fuller), Alison.loram@basc.org.uk (A. Loram),
kirvine@dmu.ac.uk (K.N. Irvine), victoria.sims@atmosconsulting.com (V. Sims), k.j.gaston@sheffield.ac.uk (K.J. Gaston).
1
Present address: Spatial Ecology Lab, The Ecology Centre, The University of Queensland, St. Lucia, QLD 4072, Australia.
2
Present address: The British Association for Shooting and Conservation, Marford Mill, Rossett, Wrexham LL12 0HL, UK.
3
Present address: Atmos Consulting Ltd., Waen Farm, Nercwys Road, Mold, UK.
BIOLOGICAL CONSERVATION 142 (2009) 761–771
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significant commercial or industrial infrastructure; United
Nations Development Programme et al., 2000).
Whilst the world’s population is growing increasingly iso-
lated from biodiversity as a result of urbanisation, evidence
is rapidly accruing as to the advantages that people gain from
experiencing and interacting with nature (Katcher and Beck,
1987; Kaplan andKaplan, 1989; Irvine andWarber, 2002; Maller
et al., 2005). The personal and societal benefits to human
health and well-being are wide ranging, but include quicker
recovery rates from ill-health (Ulrich, 1984), self-reported gen-
eral health (Stilgoe, 2001; de Vries et al., 2003; Maas et al.,
2006), longevity (Takano et al., 2002), stress-relief ( Moore,
1981; Leather et al., 1998; Parsons et al., 1998; Stigsdotter
and Grahn, 2004), reduced mental fatigue (Hartig et al., 1991;
Kuo, 2001), opportunities for reflection (Herzog et al., 1997;
Fuller et al., 2007), degree of social interaction (Sullivan
et al., 2004) and reduced crime rates (Kuo and Sullivan,
2001). Similarly, lack of access to green space has been asso-
ciated with higher levels of depression and anxiety (Macintyre
et al., 2003).
Improving urban environments for biodiversity will not
only be beneficial to human individuals and communities
inhabiting those areas, but will also be advantageous for bio-
logical conservation. In developed regions where intensive
use of the wider landscape, particularly through agriculture,
has resulted in population declines of species, urban areas
are becoming increasingly important for sustaining regional
abundances. Indeed, substantial proportions of the popula-
tions of some previously widespread and common species
now occur in urban environments (e.g., Beebee, 1997; Gregory
and Baillie, 1998; Mason, 2000; Bland et al., 2004; Peach et al.,
2004).
A variety of options is available to increase the environ-
mental quality of urban areas and, thereby, the degree of
interaction between people and nature, including creating
green networks and corridors, developing urban forests,
improving the management of public parks, and encouraging
householders to participate in ‘wildlife gardening’ (e.g., Baker,
1997; Tyrva
¨inen, 1997; Good, 2000; Savard et al., 2000; Beebee,
2001; DEFRA, 2002). Wildlife gardening can be broadly defined
to encompass any actions conducted in private or domestic
gardens to increase their suitability for wildlife, and thus in-
cludes the provision of a diversity of resources (e.g., sub-
strates, food, breeding and overwintering sites). A
significant proportion of urbanised areas comprise domestic
gardens (Loram et al., 2007), so one of the attractions of such
an approach to improving urban environments is the poten-
tial for mass participation by householders. The multiple
ownership of the domestic garden resource should be viewed
as an opportunity to be exploited, rather than the complica-
tion that it commonly presents in the context of other land
uses (Lepczyk et al., 2004; Gaston et al., 2005a; Warren et al.,
2008).
This said, the extent to which wildlife gardening activi-
ties are undertaken, and hence the level of provision of
important resources to enhance biodiversity, remains poorly
documented (but see Cowie and Hinsley, 1988; Lepczyk
et al., 2004; Gaston et al., 2005b, 2007; Buczacki, 2007). In
the UK, an assortment of estimates has been published
for different regions, most frequently in the popular media,
but seldom with any indication of how they were origi-
nally derived (e.g., Baines, 1985; Moss and Cottridge, 1998;
Owen, 1991; Bevan, 2001; Packham, 2001; Ryrie, 2003; Burton,
2004; Glue, 2006). In this paper we seek to redress this
situation, generating estimates for a selection of key re-
sources provided within domestic gardens across the coun-
try. This is the first time that such a comprehensive dataset
has been assembled to address this issue at a national
scale.
2. Materials and methods
2.1. Urban and suburban household/garden surveys
Twelve datasets were used to generate estimates of the
domestic garden resource in the UK (Tabl e 1). Eleven of
these were collected as part of seven recent projects that
have been conducted within urban and suburban areas
throughout the country (Fig. 1). They are highly representa-
tive of the UK population as approximately 90% of people
live in towns or cities (ODPM, 2001), with over 40%
distributed in London and other major cities (DETR, 2000).
Some of the surveys were household focused, whereas
others were more specifically aimed at garden users
(Tab le 1; the survey methods for the unpublished data
can be found in Appendix A of the Supplementary
material).
2.2. Survey of English Housing
The remaining dataset used in the calculations of garden re-
sources across the country was the Survey of English
Housing (SEH). It is an annual survey of approximately
20,000 households, stratified across the counties of England.
The SEH covers all types of household, from those in rural
areas and small towns through to city centres, therefore
accounting for potential variation at regional scales. It is
conducted for the UK government’s Department for Com-
munities and Local Government (formally part of the
Department for Transport, Local Government and the Re-
gions) by the National Centre for Social Research. The pri-
mary purpose of the survey is to collate reliable
information on the main features of each household and
the opinion of the respondents in relation to their personal
housing circumstances. In its entirety, the survey consists of
approximately 800 questions, comprising a core of factual
questions that remain largely unchanged from year to year
(e.g., regarding factors such as tenure, housing costs, hous-
ing history), in addition to a set of questions on attitudes
and intentions that are revised annually (see http://
www.esds.ac.uk/ for details).
The 2001/2002 survey (National Centre for Social Research
and Department for Transport, Local Government and the Re-
gions, 2004) included a small set of questions investigating
the degree to which households participate in wildlife garden-
ing activities. Respondents were asked: (i) whether they had
access to a garden or patio area and, if so, (ii) whether they
provided food for birds, (iii) if they had a nest box and (iv)
whether the garden contained a pond.
762 BIOLOGICAL CONSERVATION 142 (2009) 761–771

Table 1 – Characteristics of each household/domestic garden survey included in this study, and the relevant garden resource data extracted. Details of the survey
methodologies for unpublished data can be found in Appendix A of the Supplementary material.
Survey name Year(s) of
survey
Area surveyed Survey methods (variables) Survey strategy Type of households
surveyed
Number of households/
gardens surveyed
(response rate, if
applicable)
Resources recorded
(measurement)
Reference
BUGS 1 (a) 2002 Sheffield GIS analysis (garden areas) Random All residential
households
250 households Garden (presence/absence)
Garden area (m
2
)
Gaston et al. (2005b)
BUGS 1 (b) 2002 Sheffield Telephone questionnaire
(garden features)
Random All residential
households
250 households (85%) Garden (presence/absence)
Nest boxes (presence/
absence) Ponds (presence/
absence) Trees >3 m tall
(count)
Gaston et al. (2005b)
BUGS 1 Extension 2002 Sheffield Telephone questionnaire
(garden features)
Random All residential
households
500 households (83%) Garden (presence/absence)
Supplementary bird food
provision (yes/no) Nest boxes
(presence/absence) Ponds
(presence/absence) Trees
>3 m tall (count)
K.J. Gaston et al.
(unpublished data)
BUGS 2 (a) 2005 Belfast Cardiff
Edinburgh Leicester
Oxford
GIS analysis (garden areas) Random Detached, semi-
detached and terraced
households
3643 gardens Garden area (m
2
)Loram et al. (2007)
BUGS 2 (b) 2004–2006 Belfast Cardiff
Edinburgh Leicester
Oxford
Garden visits (garden
features)
Informally stratified
sample, representative
of the range of
households, from a pool
of volunteered gardens
Detached, semi-
detached and terraced
households with rear
gardens
267 gardens Rear garden area (m
2
)
Supplementary bird food
provision (yes/no) Bird
feeders (presence/absence)
Nest boxes (presence/
absence) Ponds (coverage m
2
)
Trees >3 m tall (count)
A. Loram et al.
(unpublished data)
CityForm Project (a) 2004 Edinburgh Glasgow
Leicester Oxford
Sheffield
GIS analysis (garden areas) Census All residential
households
84,136 households Garden area (m
2
) R.A. Fuller et al.
(unpublished data)
CityForm Project (b) 2004 Sheffield GIS analysis (tree coverage) Census of 149 km
2
area All residential
households
Not determined Tree coverage (m
2
) R.A. Fuller et al.
(unpublished data)
CityForm Project (c) 2005 Edinburgh Glasgow
Leicester Oxford
Sheffield
Postal questionnaire (garden
features)
Random, stratified by
city area (inner, middle
and outer)
All residential
households
4381 households (37%) Garden (presence/absence)
Supplementary bird food
provision (yes/no) Bird
feeders (presence/absence)
Nest boxes (presence/
absence) Ponds (presence/
absence)
Gaston et al. (2007)
British Urban Domestic Cat
Survey
2004–2005 Twenty urban areas
across England and
Wales (Fig. 1)
Telephone questionnaire or
household visits (garden
features)
Random census of
twenty 1 km
2
squares
All residential
households
1000 households (33–
71%)
Garden (presence/absence)
Supplementary bird food
provision (yes/no) Nest boxes
(presence/absence)
Sims et al. (2008)
Sheffield Urban Domestic
Cat Project
2003–2005 Sheffield Telephone questionnaire or
household visits (garden
features)
Census of two 1 km
2
squares (one middle
and one outer city area)
Detached, semi-
detached and terraced
households
2782 households (61%) Garden (presence/absence)
Supplementary bird food
provision (yes/no) Bird
feeders (presence/absence)
V. Sims et al.
(unpublished data)
Valuing Open Space
Questionnaire
2006 Sheffield Postal questionnaire (garden
features)
Random, stratified by
socioeconomic
grouping
All residential
households (those
without access to a
garden were excluded)
2016 gardens (33%) Bird feeders (presence/
absence) Nest boxes
(presence/absence) Ponds
(presence/absence)
K.N. Irvine et al.
(unpublished data)
Survey of English Housing
(SEH)
2001–2002 England Household visits (garden
features)
Random, stratified by
Government Office
Region and
socioeconomic
grouping
All residential
households
19,913 households (67%) Garden (presence/absence)
Supplementary bird food
provision (yes/no) Nest boxes
(presence/absence) Ponds
(presence/absence)
NCSR and DETR (2004)
BIOLOGICAL CONSERVATION 142 (2009) 761–771 763

Fig. 1 – The location of the urban and suburban areas sampled in seven of the household/domestic garden projects (Table 1),
the relevant survey data from which were collated for this study: (a) the UK cities targeted in six of the projects: B, Belfast; C,
Cardiff; E, Edinburgh; G, Glasgow; L, Leicester; O, Oxford; S, Sheffield; (b) the 20 1 km ·1 km areas sampled, in England and
Wales, for the British Urban Domestic Cat Survey: Bas, Basingstoke; Bd, Bridgend; Bs, Bristol; Bt, Bath; Ca, Cardiff; Ch, Chester;
Con, Conisbrough; Cov, Coventry; Cr, Croydon; Ep, Epsom; Gr, Greenwich; Ol, Oldham; Pl, Plymouth; Po, Pontypool; Pr, Preston;
Re, Reigate; Ru, Ruislip; Ur, Urmston; Wo, Woking; Wt, Walton-on-Thames.
764 BIOLOGICAL CONSERVATION 142 (2009) 761–771

2.3. Estimating the extent of the garden resource
All surveys were weighted equally, although we acknowledge
that each individual survey will potentially have associated
biases. However, a broad range of research questions moti-
vated the different studies and a variety of methodological
techniques were used to collect the datasets. As such, there
is no reason to suspect that the garden resource estimates de-
rived from the surveys will be systematically biased. In
addition, the purpose of this paper is not to produce a highly
accurate figure for each resource, but to provide approximate,
robust and transparent estimates that can be used to
assess the value of domestic gardens to biodiversity
conservation.
Fig. 1 (continued)
BIOLOGICAL CONSERVATION 142 (2009) 761–771 765

The garden resources examined were supplementary bird
feeding, provision of nest boxes, and the occurrence of ponds
and of trees taller than 3 m. For each dataset, the proportion
of households and/or gardens in the survey providing each re-
source was calculated, in addition to the average of any re-
corded quantitative measure (e.g., garden area, number of
trees). Data were then averaged across all studies to generate
a mean proportion and confidence interval for each resource
type (see Appendices B–F).
The number of households in the UK with access to a gar-
den was calculated by multiplying the mean proportion (gen-
erated using all surveys with relevant data) of households
with a garden by the number of households recorded in the
most recent UK census conducted in 2001 (Table 2 ). Subse-
quently, the area of all gardens across the country was extrap-
olated using the average garden size, which was recorded in
some of the surveys, and the previously estimated number
of gardens across the country. The number of households/gar-
dens in the UK providing each resource (e.g., supplementary
food for birds, a pond) was calculated using the same simple
assumptions; all household-based estimates were scaled up
using the 2001 census data, and all estimates of resources
specifically within gardens were extrapolated using the num-
ber of UK gardens derived above. The garden resource esti-
mates are hereafter presented as whole numbers, but were
calculated using exact figures.
3. Results
3.1. Garden sizes and total garden area
On average, 87% of households had a garden area associated
with the dwelling (Appendix B). Mean garden size, excluding
zero values within each survey, was 190 m
2
(95% CI = 173.0–
207.8). Extrapolating to the 26,159,440 households in the UK,
this equates to 22,738,563 (95% CI = 21,768,735–23,708,391)
domestic gardens with a total area of 432,964 ha (95%
CI = 393,391–472,537).
3.2. Supplementary food provision for birds
The average proportion of households and, more specifically,
households with a garden providing supplementary food for
birds were 48% and 51% respectively (Appendix C). Scaling
up, using the known number of households in the UK, this
gives an estimate of 12,581,718 (95% CI = 10,469,000–
14,694,435) homes across the country participating in bird
feeding activities. Similarly, using the proportion of house-
holds with a garden providing food for birds and the esti-
mated number of gardens in the UK, the extrapolated
number of gardens containing supplementary bird food is
11,640,921 (95% CI = 9,907,575–13,374,266).
The discrepancy between these two values is likely to be
due to the approximate nature of the calculations. However,
it is also possible that it reflects the provision of bird food
by householders living in flats, who may have access to an
outside space (e.g., a balcony or roof terrace) or a bird feeder
attached to a window in order to attract avian visitors, but
were excluded from the garden surveys.
From the surveys, a mean of 28% of households and 23% of
households with gardens specifically used bird feeders for
supplementary food provision. Assuming that each house-
hold/garden only has one bird feeder, which is likely to be a
conservative assumption, it is estimated that there are
7,376,105 (95% CI = 5,240,886–9,511,324) bird feeders associ-
ated with households at a national level and 5,299,590 (95%
CI = 3,132,608–7,475,571) bird feeders within gardens across
the UK.
3.3. Nest boxes
On average, 16% of households had at least one nest box
(Appendix D). Extrapolating this to the number of households
in the UK gives a minimum estimate of 4,305,621 (95%
CI = 2,658,848–5,952,394) nest boxes across the country. This
figure is consistent with the 4,710,632 (95% CI = 3,869,926–
5,551,337) nest boxes derived from a mean proportion of
21% of gardens containing a nest box.
3.4. Ponds
The mean proportion of households with a pond was 10%,
equating to 2,543,201 (95% CI = 1,487,803–3,598,599) ponds in
the country (Appendix E). The garden surveys, on average, re-
corded 16% of gardens containing a pond, giving a total of
3,531,118 (95% CI = 2,496,205–4,566,031) ponds in UK gardens.
The mean size of a garden pond was 1 m
2
(95% CI = 0.5–1.5).
Scaling up to the 3,531,118 gardens in the UK with a pond, this
corresponds to a total area of standing water of 349 ha (95%
CI = 158.5–540.3).
3.5. Trees
On average, 54% of gardens contained one or more trees taller
than 3 m, equating to 12,200,863 (95% CI = 9,846,431–
14,555,296) gardens across the UK (Appendix F). The mean
number of trees within a garden was 2.4 (95% CI = 1.8–2.9),
generating a national estimate of 28,730,986 trees within
domestic gardens (95% CI = 22,196,978–35,264,995). One study
examined the extent of tree cover in gardens, reporting that
11% of garden area was tree-covered; this figure translates
to a national tree coverage of 47,402 ha.
4. Discussion
4.1. Gardens
In the UK, we estimate that 87% of households, or 22.7 million
homes, have access to a domestic garden. These figures are
Table 2 – Number of households across the UK according
to the most recent census conducted in 2001.
Country Number of households
England 21,262,825
Northern Ireland 1,311,860
Scotland 2,308,939
Wales 1,275,816
UK 26,159,440
766 BIOLOGICAL CONSERVATION 142 (2009) 761–771

comparable to the 85% of households in the UK that have a
garden according to English Nature (2004), but is somewhat
higher than the outdated estimate of 80% of households in
Britain quoted by Hessayon and Hessayon (1973) and subse-
quently cited in other ecological publications (e.g., Owen,
1991; Buczacki, 2007).
The average size of a garden in the surveys was 190 m
2
,
equating to an area of 432,964 ha across the country. Hessa-
yon and Hessayon (1973) give a mean area of 186 m
2
for gar-
dens which is highly consistent with our calculation, as is
the frequently quoted figure of over 400,000 ha of gardens in
Britain (e.g., Baines, 1985; Ryrie, 2003; Moss and Cottridge,
1998). To illustrate the extent of this area, it is equivalent to
the US state of Rhode Island and is larger than the English
county of Suffolk. To a householder, the area of their individ-
ual domestic garden may seem limited, particularly for those
living in terraced accommodation in urban centres, but the
sum of all these gardens contributes substantially to the over-
all amount of green space in urban areas across the country.
For example, Loram et al. (2007) found that domestic gardens
accounted for approximately one quarter of urban land cover
within five UK cities. Moreover, the total contiguous area, and
heterogeneous nature, of all the gardens in a residential street
or neighbourhood is of considerably more relevance to spe-
cies requiring large areas over which to fulfil their niche
requirements, than any one individual plot (e.g., Cannon
et al., 2005; Knight et al., 2005). As well as providing habitat
in their own right, domestic gardens may facilitate species
moving between areas of public green space (e.g., parks)
and/or the wider countryside, therefore increasing habitat
connectivity at a landscape-scale (Ferna
´ndez-Juricic, 2000;
Ferna
´ndez-Juricic and Jokima
¨ki, 2001; Rudd et al., 2002).
Nonetheless, the national coverage of domestic gardens is
still an order of magnitude less than the 4.7 million hectares
of statutory protected areas throughout the UK (S.F. Jackson
and R.A. Fuller, unpublished analyses). In addition, these esti-
mates of garden coverage may include large areas covered by
temporary structures, such as garden sheds or greenhouses,
or areas that have been converted to hard standing (e.g., con-
creted, paved, decked). Indeed, this is a growing phenomenon
(Goode, 2006), and a recent report has suggested that nearly
half of all households in North East England have paved over
the majority of their front gardens to create off-road parking
(RHS, 2007). Furthermore, the area of domestic gardens
throughout the UK is likely to decrease as a result of increas-
ing urbanisation. For example, the human population in Eng-
land alone is projected to grow by a further 7%, from
49 million individuals in 1998, to 52.4 million by 2021 (DETR,
2000). In response, new houses are being built at an increased
density of 40 dwellings per hectare, up from 25 dwellings per
hectare before 2002 (ODPM, 2006). Meanwhile, existing gar-
dens are frequently being built upon (commonly referred to
as backland development or ‘garden grabbing’) and thus pro-
portional coverage of green space within residential areas
seems likely to decline in the coming years (Goode, 2006).
4.2. Supplementary food provision for birds
Approximately 12.6 million households, or 48% of homes
across the UK, were estimated to be participating in supple-
mentary food provision for birds. This compares well to a poll
conducted in 2003 on behalf of the Royal Horticultural Society
that showed that half of all households with a garden provide
food for avian visitors (Moss, 2000).
Of the 12.6 million households supplying supplementary
food for birds, 7.4 million specifically use bird feeders. The
sheer scale of this resource estimate can best be illustrated
by an undoubtedly simplistic calculation. There are
134,602,702 (SE = 3,751,514) breeding birds in the UK and
67,734,400 (SE = 2,165,000) of these belong to species that
commonly use bird feeders (R.A Fuller, unpublished analyses
using national population estimates compiled by Baker et al.,
2006). Therefore, across the country, there is approximately
one bird feeder for every nine potentially feeder-using birds.
In recent years, bird feeding has developed from just
throwing food scraps out of the back door in winter to becom-
ing a multi-million pound industry (Moss and Cottridge, 1998;
Moss, 2000). Indeed, the British Trust for Ornithology (2006)
estimates that the total annual expenditure on outdoor bird
feeding in the UK is £200 million. The quantity of food held
by specially designed bird feeders across the country can be
roughly calculated as a typical small bird feeder contains
approximately 350 g of bird seed (R.A. Fuller, unpublished
data). Conservatively assuming that each household recorded
as using bird feeders only has the one, there is a standing crop
of 2580 tonnes of bird food. However, although this illustrates
the extent of the potential resource, up to 80% of feeders are
believed to be empty at any one time (Toms, 2003) and Gaston
et al. (2007) found that 36% of bird feeding activity was carried
out less frequently than once a month.
Moreover, supplementary feeding may not be universally
beneficial to all avian populations. Despite some evidence
that birds do not become dependent on food resources pro-
vided by householders (Brittingham and Temple, 1992), and
that levels of bird feeding are positively correlated with the
abundance of urban birds (Fuller et al., 2008), the possible
negative consequences still remain to be fully investigated
(Jones and Reynolds, 2008). These may include reliance on
an unpredictable resource, a reduction in diet quality, in-
creased predation pressure, loss of natural foraging behav-
iours (Brittingham and Temple, 1992; Robb et al., 2008) and
an increase in the number and abundance of exotic species
(Daniels and Kirkpatrick, 2006).
4.3. Nest boxes
At a national level, we estimated that there is a minimum of
4.7 million nest boxes within domestic gardens. This figure is
likely to be an underestimate as it assumes that each house-
hold only has one nest box, as none of the surveys collated
data on the number of nest boxes per household.
A review of 46 studies by Newton (1998) determined that
the availability of nesting sites can limit breeding bird densi-
ties for hole-nesting species. This can be a particular problem
in urban areas, where public land managers remove dead and
decaying trees and, along with them, the potential opportuni-
ties for nesting (Moss, 2000). As a result, it has been recom-
mended that nest boxes should be provided in urban parks
in order to increase the colonization by a greater variety of
cavity-nesting birds (Jokima
¨ki, 1999). Of the 134,602,702
BIOLOGICAL CONSERVATION 142 (2009) 761–771 767

(SE = 3,751,514) breeding birds in the UK, 53,098,498
(SE = 1,929,443) belong to species that may commonly use gar-
den nest boxes as breeding sites (R.A. Fuller, unpublished
analyses using national population estimates compiled by
Baker et al., 2006). Using our conservative estimate of the
number of nest boxes in UK gardens, this approximates to
one nest box for every six breeding pairs of cavity-nesting
birds in the country. Even accounting for the fact that some
nest boxes will be structurally unsuitable, have been placed
in inappropriate locations, or not be in a serviceable state of
repair, this is still an impressive resource.
4.4. Ponds
Across the UK, we estimate that there are between 2.5 and
3.5 million ponds in domestic gardens, equating to 349 ha
of standing water. This corresponds to the over 2 million ci-
ted by Bevan (2001). To put these figures in context, the 1996
Lowland Pond Survey estimated that there were only 228,900
lowland ponds within the wider countryside of Great Britain
(Williams et al., 1998). Such ponds are defined as a small
water body between 1 m
2
and 2 ha, which holds water for
all or part of the year, and collectively includes pools, marl
and brick pits, bog pools, kettle holes and lagoons (Rouen,
2001). Following this definition, and excluding those within
gardens, they comprise 97% of the standing waters in the
country, but only 14% of the total surface area (Bailey-Watts
et al., 2000). Nonetheless, meaningful comparisons between
the numbers of ponds in gardens and those in the wider
countryside are difficult because most garden ponds are
generally much smaller (with a mean surface area of only
1m
2
).
In the wider countryside, many ponds are threatened by
natural succession, land drainage and development (Boothby
and Hull, 1997). Despite the total surface area of garden ponds
being so small, the resource is fragmented over a wide distri-
bution and provides a haven for many amphibians, inverte-
brates and plants in residential areas, including some of
specific conservation interest (e.g., the common frog Rana
temporaria and common toad Bufo bufo;Beebee, 1997; Williams
et al., 2000). Indeed, almost all ponds are used by aquatic
organisms, irrespective of age or structure, as long as the
water quality is maintained (Pond Action, 1994; Linton and
Goulder, 2000; Wood et al., 2003), and they do not have to be
large to be of high conservation value (Oertli et al., 2002). Gar-
den ponds also provide birds with drinking water and a place
to bathe, in addition to supporting a variety of invertebrates
which are a possible food resource (Moss, 2000; Burton,
2004). The important habitat heterogeneity associated with
domestic garden ponds is often lost due to the removal of veg-
etation and silt (Biggs et al., 1994). A balance therefore needs
to be found between maintaining the pond and preventing
succession without over managing the resource (Linton and
Goulder, 2000).
4.5. Trees
Over 12 million gardens across the country are estimated to
contain at least one tree. Of these gardens, the average num-
ber of trees in each was approaching 2.4, giving a total esti-
mate of 28.7 million trees. According to the Forestry
Commission (2003), there are just over 123 million live trees
occurring outside woodland in Great Britain (England,
89,217,000; Scotland, 18,576,900; Wales, 15,334,000) and a fur-
ther 2 million dead trees. Domestic gardens therefore contrib-
ute just under a quarter of this total.
Trees in residential areas can play a number of important
roles. First, they contribute to ecosystem services such as
microclimate regulation and air filtration (Bolund and Hun-
hammar, 1999). Second, garden trees may include rare species
that have either disappeared, or been severely depleted, in the
wider countryside, often as a result of changing land use (e.g.,
old varieties of fruit trees). Finally, they may provide impor-
tant habitats for wildlife. For instance, the species richness
of some taxonomic groups increases in urban areas in rela-
tion to the volume of available vegetation (Dickman, 1987;
Jokima
¨ki and Suhonen, 1993; Savard et al., 2000) and trees
provide nesting sites, food and refuge from predators for
many species (Ferna
´ndez-Juricic and Jokima
¨ki, 2001).
Although the utility of trees within domestic gardens as hab-
itats, particularly in urban centres, will frequently be limited
by an array of factors such as human disturbance, stress as a
result of pollution or heat, and whether they are exotic or na-
tive species, this is still a potentially substantial resource for
biodiversity that has, so far, largely been overlooked in the
conservation literature.
5. Conclusions
The potential importance of domestic gardens to biodiversity
has been acknowledged in the popular media for many years
(e.g., Hammond, 1974; Baines, 1985; Good, 2000; Packham,
2001; Ryrie, 2003; Buczacki, 2007). This assertion is supported
by a small number of studies that have demonstrated that
gardens play a substantial role in maintaining, and enhanc-
ing, biodiversity. They have included intensive investigations
of the biodiversity of individual gardens (e.g., Owen, 1991;
Miotk, 1996), the presence and abundance of particular tax-
onomic groups in one or more gardens (e.g., Morley, 1944;
Barnes, 1949; Davis, 1978; Dickman, 1987; Rapoport, 1993;
Vickery, 1995; Cannon et al., 2005; Daniels and Kirkpatrick,
2006; Smith et al., 2005a–c; Osborne et al., 2008), and the
occurrence of multiple taxonomic groups across many gar-
dens (e.g., Saville, 1997; Thompson et al., 2003, 2004; Gaston
et al., 2005a). However, further research is required to better
understand the complex interactions between wildlife, both
native and exotic, and the habitat features provided within
gardens. Knowing how best to manage these resources must
be a priority if we are to maximise the retention of biodiver-
sity within domestic gardens in the future. To date, informa-
tion on the magnitude of the resource provision has also
been lacking (Gaston et al., 2005b; Loram et al., 2007; Buc-
zacki, 2007). This paper is the first to illustrate the sheer
scale of the contribution that gardens make to possible hab-
itat provision in a national context. Planning strategies
which neglect the role of gardens within estimates of green
space, particularly those in urban areas, undervalue the ex-
tent of the resource. This is particularly pertinent as further
development and urbanisation throughout the world is likely
to continue unabated.
768 BIOLOGICAL CONSERVATION 142 (2009) 761–771

Acknowledgements
This work was funded by the Engineering and Physical Sci-
ences Research Council (through the CityForm research con-
sortium) and the Countryside Council for Wales, Department
for Environment, Food and Rural Affairs, Environment and
Heritage Service, English Nature and Scotland and Northern
Ireland Forum for Environmental Research (through the BUGS
II project). K.J. Gaston holds a Royal Society Wolfson Research
Merit Award. We also wish to acknowledge the National Cen-
tre for Social Research (data collectors), Department for Trans-
port, Local Government and the Regions (sponsor) and the UK
Data Archive (data distributor) for providing the Survey of Eng-
lish Housing (SEH) 2001/2002 dataset. Crown copyright mate-
rial is reproduced with the permission of the Controller of
HMSO and the Queen’s printer for Scotland. These organisa-
tions bear no responsibility for the data analysis or interpreta-
tion of findings stated within this publication. MasterMap data
were kindly supplied by Ordnance Survey, by license through
the CityForm Consortium. We are grateful to members of the
CityForm Consortium, S. Berry (Natural England), N. Burke
and D. Lewis (Sheffield City Council), R. Davies, I. Fishburn,
C. Gascoigne, A. Holt, S. Jackson, M. McKenna (Environment
and Heritage Service), J. Tratalos, R. Schaible (Department of
Agriculture and Rural Development) and K. Watts (Forest Re-
search) for collecting and/or supplying data. We also extend
our thanks to M. Dallimer and S. Davies for suggestions and
constructive discussion in relation to the manuscript, and to
four anonymous reviewers for their comments.
Appendix A. Supplementary material
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.biocon.2008.12.016.
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