Introduction to mammals
David Wembridge, People’s Trust for Endangered Species Reviewed by Steve Head
Mammals (except humans) are much less conspicuous in gardens than birds, because most of them are secretive and mainly active at night. Many species are regarded as nuisances by gardeners, so perhaps it’s just as well they are rarely seen.
How many species?
The Garden Mammal Survey in 2001
recorded 43 of the 58 or so wild, terrestrial mammal species in Britain and Ireland. More typically, about two dozen species are collectively recorded in surveys of gardens, including a third of the 18 formerly designated as priority species
under the UK Biodiversity Action Plan.
Individual gardens are home to fewer species, but six are recorded in a fifth or more of gardens: bats, red fox Vulpes vulpes, grey squirrel Sciurus carolinensis, hedgehog Erinaceus europaeus, mice, and voles. On average, between three and four wild mammal species are commonly recorded in gardens (not counting similar species, such as bank and field vole, or bat species, separately) and when different urban green spaces are compared, gardens support among the highest number of mammal species .
We are covering the mammals in five pages:
Studying garden mammals
Generally, wild mammals are unobtrusive: most are crepuscular or nocturnal, and many are prey for other mammals or raptors, so do their best to keep out of sight. Observing mammals therefore requires some patience.
Sitting out on summer nights will enable you to see bats, and possibly foxes and hedgehogs (which you often hear before you see). For rodents, you can use sticky “traps” that retain hair which can be identified under a microscope. Special traps, such as the expensive Longworth Mammal Trap, catch mice and voles harmlessly and provide food and shelter until you release them. Shrews die so quickly when deprived of their food that they must not be caught, so traps for mice and voles must have a small 12mm hole to let the shrews escape.
For all mammals however, the modern approach is to use “trail cameras”. These use infra-red detectors, and fire an invisible infra-red flash to photograph passing mammals. You will probably take a lot of photos of next-door’s cat.
Garden mammal surveys
Gardens are ideal places to study mammals, situated (literally) on our doorstep and subject to less disturbance than busier, public spaces. The People’s Trust for Endangered Species (PTES) has run a survey of mammals in gardens and other green spaces in the built environment each year since 2003, supported by thousands of volunteers. This citizen science approach is about the only way to monitor mammals in gardens that, as privately owned spaces, are otherwise inaccessible. See our page on Garden Surveys
Two other surveys (run separately by the RSPB and British Trust for Ornithology) currently record mammals in specifically urban habitats but were set up primarily to record bird species.
Surveys gather information about where species occur (their range) and the numbers of individuals (the size of a population). Each survey is a snapshot of a population at a particular instant – in the month or season that the survey was carried out or over an individual year.
To understand how a population is changing, repeated surveys are needed, and to identify the long-term trend of a population – whether it is increasing or declining, for example – repeated surveys over several years are necessary. Currently, PTES’ Living with Mammals and BTO’s Garden BirdWatch are the only surveys of garden mammals that have run over a long enough period to identify trends in populations.
For most common mammals, populations have shown little change over the last ten or so years, but for one, WHAT there is evidence of a steep decline. A report by the British Trust for Ornithology1 found that five separate surveys between 1996 and 2010 showed declines in either urban or rural populations. In gardens and urban green spaces, Living with Mammals, suggests a fall of a third between 2003 and 2012.
Given the importance of gardens to many mammals (including ourselves), recording the species that make up our local biodiversity is important on a bigger scale: in conservation efforts and assessing the health of towns and cities.
There is evidence that actions of gardeners such as building a log pile or growing plants that produce fruits, nuts or seed-cones, can make a difference to urban mammals. The findings of Living with Mammals
suggest that the presence of berries or seeds, or of a shed, pond or woodpile, is associated with a greater abundance of species. Similarly, a study by Phil Baker and Stephen Harris of residential gardens in Bristol2
found that an increase in the number of wildlife-friendly habitats in a garden and/or the number of food-bearing plants is associated with an increase in the proportion of gardens used by hedgehogs and mice. For hedgehogs, at least, the value of gardens might just be a saving grace.
You can find a recent review of the Living with Mammals findings by David Wembridge and Steve Langton here
Finding out more:
Two concise guides to mammals by PTES:
Britain's Mammals a concise guide. 2010 David Wembridge and Clare Poland Bowen. Whittet Books
Urban Mammals: A Concise Guide. 2012 David Wembridge. People's Trust for Endangered Species Guides Whittet Books.
1. Toms, M.P. & Newson, S.N. (2006) Volunteer surveys as a means of inferring trends in garden mammal
populations. Mammal Review 36:309-317
2. Baker P.J., Harris S. (2007) Urban mammals: what does the future hold? An analysis of the factors affecting
patterns of use of residential gardens in Great Britain. Mammal Review 37: 297-315