A critical review of the potential suitability of residential gardens in the UK as a conservation tool for two contrasting taxonomic groups: mammals and birds.

Shona Redman

Word Count: 1497

INTRODUCTION

Alongside the current rapid human (scientific names in Appendix 1) population growth,

urbanisation is increasing (Gering & Blair 1999; United Nations Population Fund 2007).

Between 2001-2011, total human population and the percentage living in urban areas

increased in England and Wales, resulting in a human density higher than the natural carrying

capacity of the environment (Office for National Statistics 2004, 2013: Figure 1).

Figure 1. A comparison of key population statistics in England and Wales between 2001 and 2011: (a) total human population; (b) percentage of population living in urban areas; (c) percentage of land classed as urban; (d) human density in urban areas. References: Office for National Statistics (2004, 2013).

To

house the

increasing urban population there is a current trend in the UK to build low-cost, high density

housing with small gardens (Baker & Harris 2007), typically in areas of high biological

diversity and endemism; this hugely affects native wildlife (Garden et al. 2006). However,

residential gardens constitute a considerable portion of urban green space (Loram et al.

2007). This essay will discuss the feasibility of using gardens as a conservation tool for rare

and declining UK bird and mammal species.

CONSERVATION CONCERN SPECIES

In the UK, there are 18 terrestrial mammal and 59 bird species of conservation concern,

whose numbers or ranges are markedly declining (Joint Nature Conservation Committee

2016a, b), primarily due to urbanisation causing habitat loss and fragmentation of rural

environments (Table 1). This forces species into urban areas (Hof & Bright 2009), where

residential gardens could increase and stabilise populations of species which possess the

2001 20110

102030405060708090

100

Total population (in millions) Urban population (%)Urban area (%) Urban human density (per ha)

Year of census

N

typical characteristics of urban residents i.e. small-medium sized, phenotypically plastic

generalists, tolerant of, and tolerated by, humans (Møller 2010a; Evans et al. 2011: Table 1;

Box 1). However, these areas are not suitable for some conservation concern species because

they create insurmountable problems e.g. lack of specific habitat (Table 1).

Table 1. Characteristics of selected UK Biodiversity Action Plan species, threats to their populations and their suitability for conservation within urban residential gardens. References: Joint Nature Conservation Committee (2016a, b).

UK BAP Species

Habitat requirements

Diet Threats Found in urban areas?

Benefits of gardens Drawbacks of gardens Suitable for urban garden conservation?

Additional references

Water vole Riverine systems, fens, and swamps. Especially areas with dense vegetation and a slow current.

Herbivorous - Loss of habitat- Predation by the non-native American mink - River quality

Occasionally - Protection from mink- May have suitable habitats e.g. dense vegetation and ponds- Large source of food- Do not roam far (<300m) so suitable area needed is relatively small- Voles liked by homeowners

- Predation by domestic cats - Vegetation normally kept tidy- Normally do not have a water body and unlikely that a garden pond will be large enough - Most gardens do not back onto a river or stream- Require a contiguous habitat; suitable gardens would need to be adjacent

Possibly Angold et al. (2006);Baker & Harris (2007);The Wildlife Trusts (2016a, b)

Wildcat Woodland for shelter, open pastures for hunting

Predatory – rabbits, voles, mice

- Hybridisation with, and exposure to new diseases from, feral domestic cats- Habitat loss and fragmentation

No - May have mice to hunt and refuse to scavenge in

- High density of domestic cats so hybridisation still likely- Not large enough (territories in wild up to 18km2)- Probable negative human perceptions, especially owners of small pets - Lack of suitable habitat

No Kilshaw (2011)

Pine Marten Woodland with above-ground den sites

Predatory – small rodents, birds, poultry, game, insects

- Habitat loss & fragmentation- Persecution- Secondary rodenticide poisoning- Predation by red foxes

No - Possible breeding sites: den boxes, old/damaged trees, and roofs- May have birds, rodents, and insects to hunt

- Red foxes common in, and around, urban gardens- No woodland habitat (is a specialist)- Sensitive to human disturbance - Homeowner perceptions: possible threat to small pets

No Birks et al. (2005)

Dormouse Woodland and hedgerows

Fruits e.g. berries

- Habitat loss & fragmentation- Climate change

Yes - Hedges common and planting more would only be a relatively small modification- Would provide a large enough habitat and food-bearing plants could be planted - Gardens close to woodland would be most beneficial- Could cut small holes in fences to allow connectivity between hedgerows, or replace fences with hedges

- Unlikely to provide the variety of trees and arboreal pathways needed- Most will not be close to woodland- Predation by cats-‘Mice’ not liked by homeowners

Possibly Baker & Harris (2007)

Polecat Arable fields, marshes, along riverbanks

Predatory – rabbits, amphibians, game, poultry

- Hybridisation with feral ferrets - Persecution- Secondary rodenticide poisoning

Occasionally - Protection from hybridisation and maybe persecution

- Lack of food- Area not large enough - Possible negative homeowner opinions, especially owners of small pets - Exposure to rodenticides may increase

No The Mammal Society (2016)

Red Squirrel Woodland Seeds, nuts, flowers, and berries

- Competition and spread of squirrelpox virus from grey squirrels

Occasionally - Liked by homeowners so likely to be provided food and care- Nesting sites in trees or nest boxes - Could climb over fences to reach other gardens

- No protection from grey squirrels and feeding red squirrels can attract more grey squirrels- Unlikely that tree density would be high enough to provide a suitable habitat

Yes, but unlikely to be effective unless grey squirrels are controlled

Thomas et al. (2003);Baker & Harris (2007)

effectivelyBats Woodland and

parkland with hedgerows, and buildings

Insects - Habitat loss & fragmentation, including destruction of roosts in buildings

Yes - Roosting sites in roofs- Liked by homeowners and tolerant of humans- Can fly to avoid threats from domestic animals and to move between gardens- Could provide a safe ‘stop-off’ between two more suitable habitats- Plants can attract insects for the bats to feed on- Provide ecosystem services by eating insects – reduces need for homeowners to use insecticides

- Would need to be surrounded by suitable habitat for foraging – possible on urban fringe but unlikely in city centre. - Can create noise and smell which homeowners may not tolerate after a while.

Yes Baker & Harris (2007)

House sparrow

Farmland and urban areas, close to humans – especially in barns and house roofs

Seeds Rural – changing agricultural practices reduce food availabilityUrban- unknown but probably loss of garden habitat due to housing crisis. Could also be predation and pollution.

Yes - Already abundant in urban gardens so they must be suitable- Lots of homeowners feed passerines- Suitable habitat could be created by planting trees/bushes in each garden- Homeowners likely to change gardening practices e.g. stop use of chemicals- Avoid the effects of fragmentation by flying over barriers

- Cat predation - Cannot stop building of more houses (but can plan for species during development)

Yes Chamberlain et al. (2007); Davies et al. (2009);Fuller et al. (2009)

Herring gull Shorelines and urban areas, close to water bodies e.g. lakes

Fish - Changes to, and decline in, commercial fishing – less food available - Avian botulism

Yes - Can live away from coast - Can nest on roofs- Avoid the effects of fragmentation by flying over barriers

- Serious lack of habitat (no large water bodies/shorelines) - Would concentrate individuals, could spread botulism - Disliked by homeowners- Lack of natural food - Cat predation (although unlikely)

No Mitchell et al. (2004)

Box 1. The effects of urbanisation on European hedgehogs and the current use of residential gardens in their conservation.

European hedgehog case study

Less than 1.5 million hedgehogs remain in the UK (Wembridge 2011) because populations have declined by a third in the last 10 years (Johnson 2016). Evidence increasingly suggests that hedgehogs are surviving better in urban areas than rural areas (Doncaster et al. 2001; Young et al. 2006) due to several reasons.

Urban hedgehogs negatively affected by:

Roads and fences - Cause fragmentation of habitat, impeding movement (Baker & Harris 2007; Wembridge 2011),

especially as hedgehogs avoid crossing large roads (Rondinini & Doncaster 2002; Baker et al. 2003).- Road deaths are common from hedgehogs crossing small roads (Dowding et al. 2010a).- Smaller gardens from low cost housing increase perimeter of surrounding fencing.

But positively affected by:

Absence of predators- Badgers rarely found in urban areas because they are sensitive to human disturbance and create

human-wildlife conflict through damage caused to buildings and gardens by sett building (Delahay et al. 2009; Bateman & Fleming 2012; Trewby et al. 2014).

- Badgers can rarely access residential gardens, enabling hedgehogs to use them as a refuge from badger predation (Doncaster 1992; Ward et al. 1997; Huck et al. 2008; Roper 2010).

Abundance of food- Opportunistic feeders so can exploit anthropogenic food sources (Hubert et al. 2011).- Increased temperatures and rainfall in urban areas increase abundance of invertebrate prey (Dowding

et al. 2010a). Liked by humans (Baker & Harris 2007)

- Provide hedgehog boxes for nesting (Hof & Bright 2009; Loram et al. 2011).- Ecosystem services- an insectivore so reduces pest species and the need for homeowners to use

insecticides.

Current conservation practices:

Urban hedgehogs frequently use residential gardens but the fragmentation of this potentially invaluable habitat is the main concern for these populations. To make gardens more accessible to hedgehogs, the Hedgehog Street campaign has been launched by the People’s Trust for Endangered Species which encourages homeowners to cut holes in their garden fences to allow free movement of hedgehogs (Wembridge 2011; Johnson 2016). However, the success of the campaign depends on how many householders engage and how the participating gardens are spatially related to each other (Johnson 2014).

This is a well-studied case of how the exploitation and slight alteration of residential gardens could help conserve a species in urban areas, with little impact on human life.

DISADVANTAGES OF URBAN AREAS

Profound ecological changes occur along an urban gradient which only select species can

tolerate (McKinney 2006). Human disturbance severely affects the prevalence of

disturbance-sensitive and actively persecuted species (McKinney 2002; Baker & Harris

2007), some of which adapt their behaviour to avoid humans but this can reduce their ability

to locate and capture prey (Ditchkoff et al. 2006). Birds are less affected by these problems

because they can fly away from threats and are generally liked by homeowners.

Companion cats and dogs exist at high densities, and create disturbances and new

predation pressures which affect the utilisation of gardens by wildlife (Baker et al. 2003). The

effects of dogs have only been investigated in rural areas where they cause substantial

disturbances on birds (Banks & Bryant 2007) and mammals (Mainini et al. 1993; Reed &

Merenlender 2011). Cats exert greater predation pressures than dogs because they roam

freely and have retained their tendency to hunt (Adamec 1976), with an estimated minimum

predation rate of 18.3 prey cat-1 year-1 (Thomas et al. 2012). The most common prey are small

rodents and house sparrows, whose populations are likely diminished by this predation

(Baker et al. 2005, 2008; McDonald et al. 2015).

Roads and fences cause habitat fragmentation, which decreases habitat size and

carrying capacity of the environment, and increases mortality risk from crossing roads (Baker

& Harris 2007). Small mammals are ten times less likely to disperse between habitats

separated by large roads (Forman & Alexander 1998); if these habitats become isolated,

genetically distinct populations, vulnerable to local extinction, can form (Underhill & Angold

2000; Wembridge 2011). Fragmentation is not a problem for birds and bats because they can

fly over barriers and have high dispersal abilities. Road mortality can hugely decrease

populations; for example, over 1600 birds and mammals were killed during four years on the

Long Point Causeway (Ashley & Robinson 1996). In Great Britain, nearly 250 billion vehicle

miles are travelled annually, although urban roads only receive around 35% of traffic flow

(Department for Transport 2016).

Traffic and other anthropogenic activities increase pollution in urban areas. Birds rely

on audial and visual communication so sound and light pollution affect their reproductive

success (Table 2). Population effects can also be seen, e.g. reduced breeding density near

roads or high noise levels (Reijnen et al. 1995).

Table 2. The effects of chemical, light and noise pollution on birds and other urban species

Type of pollution Examples Effects Species affected ReferencesChemical Rodenticides

InsecticidesMolluscicidesHeavy metals e.g. leadOestrogens

Reduced prey Second-hand poisoning Bioaccumulation Endocrine disruption which can

result in reduced immune function and changes to song repertoire

Oxidative stress

Rodents; Predatory species whose prey are targeted by, or are exposed to, these chemicals

Baker et al. (2003);Blanchoud et al. (2004);Chandler et al. (2004);Gorissen et al. (2005);Markman et al. (2008);Dowding et al. (2010b);Wembridge (2011)

Light Street lampsSecurity lightsCar headlights

Altered timings of behaviours e.g. birds start singing earlier in the day which can increase reproductive success: (1) clutches laid earlier, (2) males get more extra-pair females (3) get cuckolded less

Altered physiology Reduced activity periods or foraging

ranges for nocturnal species but increased periods for diurnal species – may impact on longevity

Increased predation risk/prey visibility

Disorientation of night-migrating species

Nocturnal species; Species with circadian rhythms, especially birds

Miller (2006);Navara & Nelson (2007);Kempenaers et al. (2010)

Noise CarsHumansMachineryFireworks

Altered song structure, e.g. shorter, and sung faster with atypical elements

Interruption of vocalisations by ambient noise – can affect survival and reproductive success

May disorientate animals if panicked Changes to stress hormone levels Divergence in sender-receiver

dialogue Masking of warning calls Masking of hunger calls from chick

to adult. Results in fewer young, lower body mass and fewer fledglings

Species that rely on vocal communication e.g. birds

Ditchkoff et al. (2006); Slabbekoorn & den Boer-Visser (2006);Hu & Cardoso (2009);Mockford & Marshall (2009);Nemeth & Brumm (2010);Payne et al. (2012);Schroeder et al. (2012)

Urban areas lack natural food resources and nesting sites within their modified habitat

patches, so fail to provide specialists with sufficient resources unless they can adapt their

behaviour and diet (Doncaster et al. 1990; Hinsley et al. 2009). Paucity of natural food

reduces reproductive success in urban birds, compared to their rural counterparts, whilst

increasing energy expenditure (Hinsley et al. 2009; Chamberlain et al. 2009). However,

urban areas experience increased temperature and rainfall which increases invertebrate

abundance and plant growth, providing adequate food for insectivores and herbivores (Oke

1982; Dowding et al. 2010a). Humans can supplement food; this is a major benefit of urban

areas.

ADVANTAGES OF URBAN AREAS

Several benefits exist to species in urban areas, if they can overcome the disadvantages.

Anthropogenic food sources supplement natural food and are abundant throughout the year so

provide food security, especially to species liked by humans (Baker & Harris 2007; Davies et

al. 2009). Feeding birds can prevent the need for migration (Plummer et al. 2015), and

increase survival and reproductive success (Arcese & Smith 1988; Robb et al. 2008), unless a

temporal mismatch is created between availability of resources and hatching of chicks

(Hinsley et al. 2008, 2009). Additional problems arise from the nutritional inadequacy of

anthropogenic food (Heiss et al. 2009), and the high density of individuals at feeding sites

which exacerbates disease transmission, but decreases predation risk (Smith & Engeman

2002).

Urban areas also provide anthropogenic nesting and denning sites such as roofs and

bird boxes (Gaston et al. 2005, 2007). Indoor nests typically have lower predation rates and

increased brood sizes than natural outdoor nests (Møller 2010b), but communal roosts

facilitate disease transmission (Smith & Engeman 2002). Some mammal species readily nest

near humans by adapting their behaviour e.g. stone martens (Herr et al. 2010), but others only

do so if natural sites are limited and population densities are high e.g. red foxes (Newman et

al. 2003).

Natural predators struggle to find food or are afraid of human disturbance so are

seldom found in urban areas, creating refuges for some species (Gering & Blair 1999; Møller

2012), for example, the endangered San Joaquin kit fox is limited to a few urban

conurbations in California, correlating with areas of coyote absence (Nelson et al. 2007). A

UK example is hedgehogs’ use of residential gardens as a refuge from badgers (Box 1),

exemplifying how gardens can help conserve a species.

GARDENS AS A CONERVATION TOOL

Urban residential gardens have huge potential as a conservation tool because they constitute

35-47% of green space, although the majority are small (<400m2) (Loram et al. 2007).

Wildlife-gardening can improve garden attractiveness to a range of species and is already

undertaken by homeowners to improve well-being, sometimes resulting in neighbourhood

mimicry (Goddard et al. 2013; Cox & Gaston 2016). This can increase the success of

conservation practices, especially if they only require a small cost or task.

Gardens are heterogeneous which enhances their suitability for many species by

providing food-bearing plants and shelter vegetation, which can attract species that do not

benefit from typical anthropogenic feeding e.g. non-granivorous birds (Baker & Harris 2007;

Fuller et al. 2008; Evans et al. 2009). Fragmentation hardly affects birds because they can

fly, but planting trees could improve garden connectivity for them and small scansorial

mammals, but would not help larger mammals, the species worst affected by fragmentation.

Other problems within urban areas can be minimised, e.g. bell collars reduce cat

predation by up to 53% (Gordon et al. 2010). However, owners that perceive cats as harmless

to wildlife may not participate (McDonald et al. 2015). Birds can evolve to avoid predation

e.g. give alarm calls and stay tonically still when caught (Møller & Ibáñez-Álamo 2012), or

sing at higher levels (Møller 2011). Traffic-induced mortality could be reduced by placing

trees or posts along verges to increase a bird’s take-off height. Some mammals adapt to

become most active after midnight e.g. hedgehogs (Baker et al. 2007; Dowding et al. 2010a),

and smaller species can avoid mortality by fitting beneath cars (Ford & Fahrig 2007).

Current conservation practices focus on reconciliation ecology which modifies

gardens to encourage use by both humans and wildlife (Francis & Lorimer 2011). These

techniques include the installation of green roofs and walls which can support a range of

vegetation and invertebrates, providing additional food for mammals and birds (Francis &

Lorimer 2011). Green roofs also provide habitat for birds, being used by black redstarts to

adapt to urban life (Grant 2006). However, most houses lack a flat roof, capable of supporting

the substrate, so would require extensive work.

CONCLUSION

Urban residential gardens would be suitable for conserving several conservation concern

species, especially small bird generalists which can exploit small habitat patches, avoid the

effects of fragmentation, and are fed by humans, but are severely affected by pollution.

However, large specialists are not suitable for conservation in gardens because they cannot

overcome the lack, and fragmentation, of habitat. More homeowners will undertake

conservation practices, increasing their success, if they only require a small task focused

towards a well-liked species.

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Appendix 1. Scientific names of species mentioned in the text

 Common name  Scientific nameHumanWater voleAmerican minkDomestic cat WildcatRabbitPine martenRed fox Dormouse Polecat Ferret Red squirrel Squirrelpox virusGrey squirrelHouse sparrowHerring gullAvian Botulism European hedgehogEurasian badgerDomestic dogStone martenSan Joaquin kit foxCoyoteBlack redstarts

Homo sapiens Arvicola terrestrisNeovision visonFelis catus Felis silvestrisOryctolagus cuniculusMartes martesVulpes vulpesMuscardinus avellanariusMustela putoriusMustela putorius furoSciuris vulgarisParapoxvirusSciurus carolinensisPasser domesticus Larus argentatus Clostridium botulinumErinaceus europaeus Meles melesCanis lupus familiarisMartes foinaVulpes macrotis muticaCanis latransPhoenicurus ochruros