Left: Artificial solitary bee nest structures Right: Investigating invertebrates on nettles using a "pooter"
The results confirmed that breeding populations of solitary bees and wasps might be increased by provision of simple, inexpensive nests. However, nests for bumblebees were conspicuously unsuccessful, despite trying various different recommended designs. The results also suggested that dead wood would be valuable if left undisturbed for a sufficient duration. Even within the short period of the study, the fruits of four fungi and a slime mould appeared on the wood, and it provided shelter for invertebrates and amphibians.
Ponds, though in some cases used by frogs, were very slow to build up invertebrate communities of any diversity by natural colonization. However, if established artificially, even small ponds could maintain their populations over several seasons. Planting nettles for larvae of nettle-feeding butterflies appeared to be a waste of time, perhaps because of the occurrence elsewhere in the wider urban landscape of larger patches than most garden owners would be prepared to tolerate.
The broad conclusion is that while some widely-recommended kinds of ‘wildlife gardening’ may be very effective, others have a low probability of success on the time scales and spatial scales likely to be acceptable to many garden owners.
BUGS 1 conclusions
The results of the BUGS project confirm that gardens are a unique and important feature of urban systems, probably harbouring the bulk of biodiversity in such areas. There are a variety of features of gardens that seem to have positive effects on biodiversity, many of them under the control of garden owners; if even a modest proportion of the huge effort invested in garden management were directed at enhancing these features of gardens then the effects could be considerable. However, the greatest value will come when the quality and quantity of garden space is also maximized at the neighbourhood scale, which requires both planners and garden owners to take the biodiversity value of gardens to heart.
BUGS 2 Project
The first Sheffield BUGS project had three key elements. It provided information about the scale and nature of the garden resource in that city. It also provided detailed information about the plant and animal biodiversity in 61 Sheffield gardens, and whether some frequently recommended techniques for increasing biodiversity in gardens actually work.
Much of that work, specifically that concerned with trapping and identifying invertebrates, was both expensive and extremely labour-intensive, and is unlikely to be repeated. But it was clear that expanding other parts of the project to a larger (national) scale would be very valuable. One obvious outstanding question is whether the gardens of other UK cities, in terms of their quantity, nature and floristic composition, are similar to those of Sheffield.
The Biodiversity in Urban Gardens 2 project was carried out over three years (2004-2007) in five cities: Leicester, Oxford, Cardiff and Belfast and Edinburgh. It had four main components:
1. To characterise the size distributions of gardens, and patterns of spatial variation in size, shape and connectivity, and the relationship of these to housing type, using aerial photography and Ordnance Survey mapping.
2. To characterise the features of about 50 sample domestic gardens from each city, including trees and mature shrubs, lawn areas, ponds and compost heaps.
3. To determine the floristic composition and diversity of the same sample gardens.
4. Using the same sample gardens, householders were asked to complete a brief questionnaire about their gardening activities, including for example weeding, lawn mowing, dead-heading, watering and use of fertilisers and pesticides. Householders were also asked about attitudes to wildlife, and which specific (vertebrate) species they had observed in their gardens (from a list of 28 species).
The scale of the resource
The proportion of the urban area of each city covered by domestic gardens ranged from 21.8 % to 26.8 % (so Sheffield, at 23 %, is typical). In a random sample of at least 500 houses in each city, 99 % had gardens, the mean areas of which ranged from 155 m2 to 253 m2 (so once again Sheffield, at 173 m2, is not unusual). Not surprisingly, garden area was closely associated with housing type; taking median values across all five cities, detached houses had approximately twice the garden area of semi-detached houses, which in turn had approximately twice the garden area of terraced houses. As in Sheffield, relatively small gardens contributed disproportionately to the total garden area of each city, being more numerous than larger gardens. Perhaps surprisingly, there was no obvious relationship between garden size and distance to the edge of any of the cities.
As in Sheffield, garden size played an overwhelming role in determining garden composition. Larger gardens contained more different kinds of land-use, and several land-uses (including cultivated borders, mown and unmown grass, trees >3 m, uncultivated areas, vegetable patches, and, to a lesser extent, ponds and compost sites) were more likely to be found in large gardens. Larger gardens also had proportionately larger areas of cultivated border, unmown grass, uncultivated land and vegetable patches. Larger gardens are also more likely to support a greater number of taller trees and large shrubs, and therefore to possess a disproportionately greater extent of vegetation cover >2 m in height.
On the other hand, buildings and non-vegetated features (e.g. patios, decking, areas of gravel, ponds, sheds, greenhouses and garages) were more likely to maintain a relatively constant absolute size, and therefore proportionally decreased with increasing garden area. In its effect on garden features, house age was much less important than garden size.
All these results are broadly consistent with those from Sheffield. Therefore, given the positive relationship between the extent of taller vegetation canopies and invertebrate species richness and abundance found in Sheffield, larger gardens should generally provide more potential habitat and thus greater opportunities for wildlife. But note that this is not an effect of garden size per se, but an indirect one of larger gardens being more likely to contain some (but not all, e.g. ponds) wildlife-friendly features.
The entire garden flora consisted of 1056 species. Numbers of plant species recorded in individual gardens ranged from 7 to 157 with a mean of 58 across all five cities. Of the total flora, 30% of species were native and 70% alien. These proportions are similar to those found in Sheffield, but the total number of species, and species per garden, is lower. A major reason for this is that the Sheffield survey also included lawns, which contain several species (many of them native) that tend not to be found elsewhere in the garden. For this reason, the Sheffield BUGS and BUGS 2 floristic surveys are not strictly comparable. Thus, for example, although a high proportion of the plant species recorded most frequently in BUGS 2 were native, this tendency was not as marked as in Sheffield.
Floras of gardens in the five cities were similar in most respects, but a conspicuous feature of the results was that Belfast gardens were less species-rich. The most diverse were in Leicester, followed by Cardiff, Oxford, Edinburgh and then Belfast. The graph below (adapted from the BUGS XII paper listed below) shows species accumulation curves, in which the total number of plant species found is plotted agains the sample number (quadrats). The red lines show the upper and lower bounds for gardens. For the same number of quadrats, Leicester gardens had roughly 50% more species than Belfast gardens. This may in part be because Belfast had many small paved yards rather than cultivated borders, as to some extent did Edinburgh.