Written By Ken Thompson
Reviewed by Steve Head
Research shows that gardens are among our most wildlife rich - or “biodiverse” habitats.
What is biodiversity?
Biodiversity is the whole variety of life on Earth, along with the habitats they depend upon. It embraces plants, animals, fungi, single celled creatures and bacteria. It also includes the genetic variation within species, and the great number of varieties and cultivars that humans have selected for food and pleasure. Biodiversity has grown through evolution over hundreds of millions of years, despite mass extinctions, major climate variation and changing patterns of continents. It is now considered globally threatened by human activities, destroying habitats and in other ways. Biodiversity loss is seen as a major threat to ecosystems of the world, including those that support human life.
Measuring biodiversity is rather complicated and contentious, since it would ideally be the measurement (impossible in practice) of all the genetic diversity within the area of study. In practice we usually quantify biodiversity as species-richness, or the number of species living in an area, habitat or climatic zone. Ecological diversity is a bit different, and it brings in concepts of relative population abundance, so habitats with the same number of species, but where a couple of species are hugely dominant, are less ecologically diverse than ones with more evenly spread population numbers. This latter approach hasn’t been applied extensively in gardens yet.
Biodiversity in gardens
We now know that ordinary gardens are extremely rich in species. This may seem odd, because most gardens are small and highly managed, and we also only see about 40 species of birds in gardens (6.7% out of 596 species on the British list). The biodiversity in gardens lies in the invertebrates, the insects, molluscs and other animals without backbones. Until quite recently, ecologists were unaware of this. In 1966, one of the great founders of modern ecology, Charles Elton wrote:
“Broadly speaking the Domestic habitats (buildings and gardens) are in the direction of biological deserts, or at any rate very unnatural surroundings, though this fact is a bit concealed by the multitude of exotic plant species that inhabit our gardens and parks, yet have so few animals attached to them.” (Charles Elton 1966 The Pattern of Animal Communities. Methuen, London. pp77-78)
The embarrassing truth is that no-one had ever looked seriously at garden wildlife, so of course few species had been recorded. This changed when Dr Jennifer Owen described her studies in her quite ordinary Leicester garden. Learn more about the remarkable story of her work and how it has been validated and extended, in Ken Thompson’s account "Jennifer Owens's studies".
The total included only the invertebrate groups Owen could sample and identify. Adding in the many groups (such as most flies and soil creatures) she couldn’t study, and assuming a similar proportion of the British species of these groups live in gardens, her results suggest there could be up to 8,450 species of insects in her garden. She also presented evidence that the great majority of the animals she found were feeding in the garden, and that, depending on the group, between 20% and all of them were resident breeders. As our leaflet "Jennifer Owen's studies" shows. later work in Sheffield and several other cities has broadly confirmed Jennifer Owens findings.
Owen’s remarkable total is primarily a tribute to the intensity of her collecting and observation in one place over a long period. Few nature reserves have remotely such comprehensive lists, and a thirty year study in some could yield comparable diversity. Nevertheless, her work, backed up by later studies, shows a single garden can hold exceptional biodiversity in a tiny area.
There are few places studied as carefully as Owen's garden. The Yorkshire nature reserve Malham Tarn has a list of fly species far in excess of the number recorded in Owen’s garden. But Owen was only able to study one group, the hoverflies, and remarkably, found more species of them in the garden than in the nature reserve, which shows how good gardens can be for groups of animals that find the habitat particularly suitable for them.
The 157 hectare
Monks Wood National Nature Reserve has been closely studied and has recorded many more species than Owen's garden, especially
beetles which are very diverse in woodlands. However, Owen recorded more bees and wasps in her garden than were found in
the 20,000x larger nature reserve, because gardens are such excellent places for these insects.
The 157 hectare Monks Wood National Nature Reserve has been closely studied and has recorded many more species than Owen's garden, especially beetles which are very diverse in woodlands. However, Owen recorded more bees and wasps in her garden than were found in the 20,000x larger nature reserve, because gardens are such excellent places for these insects.
How do gardens sustain great biodiversity?
Jennifer Owen considered this question, and came up with some answers that still seem reasonable. In summary, we can suggest the following factors:
Download our summary "Why are gardens so rich in species" to find out more about how these factors affect garden wildlife.
Changes in garden biodiversity
Unfortunately, the wildlife of Britain is generally in steep decline, and that of gardens is following the same trend. Download "Changing Garden Wildlife" for some facts and figures on declining - and a few increasing - species. With increasing pressure on the countryside for housing and growing food gardens may become increasingly important for wildlife.
Value Judgements in Biodiversity
People tend to value biodiversity in different, not always consistent ways. We all prefer species we see as beneficial or pretty or exciting, like honeybees or songbirds or eagles, to those that bite or damage crops, like wasps or rats or cabbage butterflies. For too many years, conservationists concentrated on preserving birds and larger mammals, and only relatively recently have invertebrates (except butterflies) become valued.
Like stamp collectors, we get very excited by really unusual plants or animals, and in the past conservationists expended huge efforts preserving rare charismatic species like ospreys, lady’s slipper orchids and swallowtail butterflies, even though they have never been major players in the ecology of Britain. Almost by definition (think of the phrase “common or garden”) species found in gardens are widely distributed, non-specialist types, that don’t require unusual soils, climate or food plants, or need low disturbance. Equally, these are the species that are common enough to really matter in British environments, and when these start to decline (as they are) we need to worry.
In Britain in particular, we distinguish between native and non-native biodiversity, so that natives (which arrived under their own steam after the ice age only 12,000 years ago) are thought “proper” and important, but non-natives (which arrived later and mostly with human help) are on principle considered improper aliens, or downright dangerous. That is except for those we happen to like, which include pretty arable flowers such as corn-cockle, brown hares and snakes-head fritillaries, all of which are introductions. See the page Native or Non-Native? for more on this contentious issue.
Gardens are a real object lesson in valuing biodiversity, because they contain “useful” and “weeds/pest” plants and animals, and bags of non-native flowers, fruits and vegetables alongside the native species. And they are the most biodiverse small habitats in Britain.