Matthew Nash
A critical review of the potential
suitability of residential gardens in the
UK as a conservation tool for four
contrasting taxonomic groups:
mammals, birds, insects and plants
Matthew Nash
INTRODUCTION
Over the last 50 years, the UK population has increased by 18.7% and the rate of this
population growth has been increasing (ONS 2014). To accommodate for the rising
population, more of the UK landscape must be converted into households. As a result of the
increasing population and house numbers, the UK landscape is becoming increasingly
urbanised. The UK definition of an urban area is a built up area that has a resident population
of over 10,000 inhabitants (Defra 2014). Figure 1 shows the population distribution across
rural and urban areas in England in 2009, the large majority of the population are shown to
now reside in urban areas. Urbanization is a global trend, urban areas were estimated to cover
4% of the total global land cover in 2000 (UNDP et al. 2000).
The fact that an increasing proportion of land area is becoming urbanised means that
an increasing proportion of green spaces available for wildlife are in the form of residential
gardens. A study which took samples of 250 randomly selected dwellings in the UK found
that 87% had gardens (Gaston et al. 2005). This means that although residential gardens are
typically small, due to their large numbers they make up a significant amount of green space
in urban areas. Residential gardens compose between 35-47% of urban green space which is
a large proportion of the available habitats for animals (Loram et al. 2007). The management
of these gardens can therefore be expected to have potential as a conservation tool for
wildlife in urban areas due to their combined size.
It is estimated that 82% of the UK population now live in urban areas and this high
proportion of the population living in urbanized areas has implications on local wildlife (The
World Bank Group 2014). Due to the fact that urban areas contain a large number of
buildings, they contain less green space than the other areas in the UK. The lack of vegetation
can detrimentally affect species of animals and birds who are dependent upon the species
richness of vegetation in the area (Mckinney 2008). The management of the many residential
gardens which, in total, compose a large proportion of urban green space could be a useful
tool in mitigating the negative effects of urbanisation on wildlife. Using residential gardens as
a conservation tool seems to be an increasingly relevant idea as the expansion of urban areas
is a trend which is set to continue (Liu et al. 2003).
Figure 1. Population figures for England by Rural/Urban area type.
(Reproduced from ONS 2014)
Matthew Nash
PLANT CONSERVATION
Since domestic gardens make up a large proportion of green space in urban areas, the choices
of which plant species are present in these areas can have effects on local ecosystem
functions. Urbanization is a primary cause of extinction for plant species and due to the
expansion of urban areas it is important to focus on what makes some plants survive in urban
areas while others don’t (Duncan and Young 2000). The most abundant plant species in UK
urban gardens are weedy natives, particularly herbaceous perennial species (Thompson et al.
2005). This may be expected from their ability to become established quickly and their
characteristic fast growth. The providence of habitat and forage by herbaceous plants is
largely seasonal as they die back in the winter.
The majority of plant species in urban gardens are alien species, one study found that
of the total 1166 species present in the surveyed gardens, 70% were alien, however alien
species were present in much smaller numbers (Smith et al. 2006). Plant conservation efforts
are commonly associated with the protection of native plant species since natives are known
to provide greater benefits to native animal species than alien species (Corbet et al. 2001).
Non –native species can have negative effects on native plant species through competition
and habitat modification (Manchester and Bullock 2000). The negative impacts of alien
species are listed as one of the primary threats to global biodiversity (IUCN 2000).
Research has found urban gardens to have similar levels of diversity to rural areas and
has found them to in fact be less homogenous despite the common feature of garden lawns
(Thompson et al. 2003). However, the larger proportion of alien species in urban areas may
mean that despite the similar level of diversity the plant species do not interact as well as
assemblages found naturally. There are worries that some invasive species such as the
Spanish bluebell Hyacinthoides hispanica, can out compete and hybridize with native
population and this can endanger them (RHS 2014). Urban gardens near woodlands with
native populations can help mitigate this problem by not growing Spanish bluebells and by
removing them if they are present in these gardens. Humans have the predominant influence
on what plant species are present in their gardens and can alter the composition with relative
ease. Urban gardens therefore have huge potential to aid plant conservation efforts as long as
owners are motivated and educated to view gardens from an ecological perspective.
INSECT CONSERVATION
The conservation of insects within urban gardens is a topic which may not be as warmly
welcomed by the public as other conservation projects and therefore may be harder to
achieve. This may be due to the common view of insects as unwelcome pests despite the fact
that only a very small proportion of insects are pests (Speight et al. 2008). Insects are the
main group of pollinators and therefore provide essential ecosystem services (Kremen et al.
2007). There has been a recent decline in the numbers of the European honey bee Apis
melifera which is a key pollinating species and this has raised conservation concerns
(Biesmeijer et al. 2006). Conservations of bees in urban areas is an important issue because
the species richness of bees has been found to decrease with urbanization, although some
species are aided by the availability specialist habitat features in urban areas (Bates et al.
2011).
It has however, been found that abundance of bees is more strongly linked to the
availability and diversity of flowers than the intensity of urbanization (Kearns and Oliveras
2009). This means that careful management of residential gardens to ensure that there is
Matthew Nash
diverse flower forage across urban areas could be a useful conservation tool. Bee abundance
is greater on non-native plant species than native species (Matteson 2007). It may therefore
increase the efficacy of bee conservation efforts to focus upon increasing the abundance of
non-native plant species in urban gardens however general biodiversity tends to be supported
best by native species (Smith et al. 2006).
It has been proposed that residential gardens primarily support generalist insect
species which do not rely on specific plant species as food sources. Garden butterflies are
typically generalist and this is because the variable nature of species composition across
residential gardens is not such a trouble for generalist species who have a wider range of food
sources (Bergerot et al. 2010). This, to some extent, brings into question the conservation
value of gardens. The generalist species, which have been found to be the most common
visitors, would have a better chance of finding food without urban gardens than the specialist
species which are currently benefitting from urban gardens to a lesser extent (Goulson et al.
2006). Research has also found that the diversity of native butterfly species decreases towards
urban centres whilst the opposite pattern is apparent in non-native species (Mckinney 2002).
It may then increase conservation efforts for butterfly species if urban gardens in near urban
centres aimed to support native butterflies with appropriate flora composition as they are
most affected in these areas.
BIRD CONSERVATION
The impacts of urbanization on birds, such as habitat fragmentation, may be expected to be
felt to a lesser extent than in other taxonomic groups due to their aerial mobility. Some
research has in fact found a positive relationship between species richness and urbanization.
However, these results are largely attributed to the presence of urban exploiters who are aided
by features associated with urbanization such as the availability of garbage (Chace and Walsh
2006). Research focusing on the abundance of birds along a gradient of increasing
urbanization found that some bird species decreased in abundance as urbanization increased
whilst other species were less affected (Bolger et al. 1997). A study which looked at what
makes certain species successful in urban habitats found that a combination of traits are
necessary, including diet and preferred nesting sites (Banker et al. 2007). By analysing the
available information on what makes bird species particularly adaptable or vulnerable to the
effects of urbanization, educated methods for conservation efforts can be put into place.
There is data that suggests that bird diversity is positively correlated with the number
of adult trees and the presence of heterogenic plant layers (Mason et al. 2007). A study
investigating the relationship between tree availability and bird diversity found a positive
correlation between the amount of tree cover and bird diversity in urban areas (Paker et al.
2014). This study also found that native birds preferred to forage on native trees whilst alien
birds preferred alien trees. The implications of these finding can be applied to conservation
efforts when trying to protect particular species as well as in broader efforts. Owners of urban
gardens can plant either native or non-native tree species in order to provide forage and
nesting sites for certain bird species known to visit the trees.
Diversity of bird species is supported by complex vegetation structure, there tend to
be more bird species in areas with the most complex vegetation (Sandstrom et al. 2006). This
finding suggests that if urban gardens are used for conservation goals, creating heterogeneous
gardens with both trees and shrubs would be likely to support the largest number of bird
Matthew Nash
species. However as some urban gardens are only small areas, it may not be possible for
some household owners to own trees due to their size. This may be a contributing factor to
the lack of suitable nesting sites for some species in urban areas. Birds in particular benefit
from involvement in wildlife gardening as well as the presence of gardens as green spaces. It
is estimated that in the UK, 12.6 million households provide supplementary feeding for birds
and that urban gardens provide at least 4.7 million nest boxes (Davies et al. 2009). The
presence of bird feeders in urban gardens has been found to have a positive effect of the
abundance of birds on a regional scale (Fuller et al. 2005).
MAMMAL CONSERVATION
Mammals in urban areas can be affected by a variety of factors associated with urban living.
They face challenges such as the mortality risks posed by roads and traffic and the barrier
effects caused by roads which impede the movement of urban animals (Rondinini and
Doncaster 2002). However some factors associated with urban areas can be beneficial such as
the presence of food subsidies (Saito and Koike 2013). Small mammals have been found to
be affected by urban disturbance and a lack of vegetation density which is typical in urban
areas (Dickman and Doncaster 1987). Domestic gardens which provide greater vegetation
density and less disturbance than surrounding areas can be used to mitigate the effects of
urbanization on small mammal populations.
Conflict between humans and urban mammals can cause problems for species whose
habits can cause damage to properties. An example of this is the management of badger setts.
It is more common in urban than in rural areas that wildlife advisers suggest total sett closure
of sets which are causing a problem as opposed to other less extreme management strategies
(Delahay et al. 2009). The high density of typically small, fenced-off gardens near to
buildings provides little room for badgers to create undisturbed setts and they are often
unwelcome. Gardens are typically quite small since urban areas need to accommodate a large
population. Small gardens separated by fences can impede the movement of mammals who
require larger areas of vegetation with little fragmentation, this therefore limits their
conservation value for some species (Bateman and Fleming 2012).
The red fox Vulpes vulpes has thrived in urban areas and is a frequent visitor of
residential gardens, even in areas of intense urbanization (Baker and Harris 2007). The high
density of red foxes and the lack of green spaces could mean that other, smaller mammals
may be subject to significant predation threats. A popular pet which is present in many UK
gardens is the domestic cat Felis catus. Cat predation is another form of threat to small
mammals in urban areas, it negatively effects populations of the species they prey upon such
as the wood mouse Apodemus sylvaticus (Baker et al. 2005). Cat predation is a difficult
factor to control, however it could be reduced by the choice of cat owners to keep cats inside
the house more if they are motivated to by conservation efforts.
CONCLUSION
Urban gardens provide valuable green spaces which aid species dispersal through built up
urban areas. They also provide temporary habitats for a wide range of species, the structure
and composition of flora in influence which species are likely to visit gardens (McIntyre et al.
2001). The way that household owners manage their gardens influences the wildlife that visit
and can therefore be an effective tool for conservation of biodiversity in urban areas. Projects
working towards conservation goals tend to involve a large range of stakeholders. It is
Matthew Nash
therefore helpful if the work has positive socio-political, health or economic benefits as well
as ecological impacts so a wide variety of people are motivated to help (Brown 2003).
A key issue with using residential gardens as a conservation tool is the fact that these
gardens are privately owned and their management can therefore not be regulated. It is
therefore of great importance to understand what motivates householder decisions to engage
in managing gardens for conservation in order to make residential gardens an effective
conservation tool in urban areas (Cook et al. 2011). Research carried out by the RHS found
that 70% of respondents would consider wildlife gardening, this would provide a large
amount of land for conservation efforts (Thompson et al. 2007).
Research has been conducted which suggests that 8% of species on the terrestrial
vertebrate species which are on the IUCN Red List are largely threatened by urbanization
(McDonald et al. 2008). This means that urban development is negatively impacting a
significant proportion of animals which we are trying to conserve. As urban areas continue to
expand residential gardens and public education on conservation matters are going to become
increasingly important in aiding conservation on local and global scales. In a typical UK city,
the area of gardens is larger than the area of parks and woodland combined, this gives
residential gardens a huge potential to aid conservation goals (Dunnett and Qasim 2000). The
crucial requirement for urban gardens to be used effectively for conservation is the
motivation of householders and providing available education of how to do so.
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