Page 42 - ALG Issue 2 2019

pollinators
in cities
Insects such as bees and hoverflies are important pollinators of many
wild flowers and crop species. The global value of the services provided by pollinators for crop production is estimated at between US$235 billion and US$577 billion per year. There has been a lot of publicity over the last few years about the threats to pollinators and concerns about their declines. The major threats include climate change, intensification of farming methods, disease and changes in
land use (e.g. loss of natural habitat to farmland or urbanisation). However, several recent studies have suggested that urban areas could be good places for pollinators. The Urban Pollinators Project team, led by the University
of Bristol in collaboration with the Universities of Edinburgh, Leeds and Reading, carried out a three-year study to find out more about the pollinator communities found in urban areas in the UK.
In the first stage of the research we looked at pollinators in and around
12 towns and cities across the UK, with the aim of comparing pollinator communities in urban areas to those in neighbouring farmland and nature reserves. We found that there were
no differences in the numbers of pollinators, or the number of pollinator species, between the three landscape types. We also found that there were more bee species in the urban areas compared to the nearby farmland, which suggests that urban areas could be particularly good for bees.
In the second part of the project we looked in more detail at pollinators in four cities: Bristol, Reading, Leeds and Edinburgh. Our aim here was
to find out which parts of towns
and cities are better for pollinators. Urban areas are a complex mosaic
of different land uses and habitats so we first mapped all of the different land uses in the four cities. We then selected 360 sites (90 per city) which we surveyed for pollinators over two years. Nine land uses were included in the study: allotments, cemeteries and churchyards, residential gardens, public parks, other green spaces (e.g. playing fields), nature reserves, road verges, pavements and man-made surfaces (e.g. car parks and industrial
estates). At each site we carried out surveys along fixed ‘transect’ walks of 100 m in length, recording all insects visiting flowers and noting the insect species and which plant species they were visiting.
In total, we recorded 5,000 individual insects from nearly 350 different pollinator species visiting flowers from 326 plant species. Surprisingly, bees were not the most common insects
on flowers in our study. Around one third of all insects recorded were bees; these included bumble bees and solitary bees, as well as honey bees. Honey bees were responsible for only 7% of all recorded insect visits, whereas bumble bee visits were more common (20% of all visits). Almost all of the other insects recorded visiting flowers were flies, with much smaller numbers of visits from beetles and butterflies. Hoverflies made up a quarter of all visits and other flies 30% of visits. The flies were an amazingly diverse group, with more than 200 different species recorded. Several recent studies have demonstrated that flies are important pollinators for crops and wild flowers.
We found that numbers of bees were higher in allotments and gardens compared to most other land uses. Numbers of hoverflies were similar
in allotments, gardens, cemeteries, nature reserves and parks, although higher in allotments and gardens compared to other greenspaces, verges and pavements. Numbers of other flies were similar across most
of the land uses studied. We also carried out analyses that estimated the effects of changing the area of the different land uses in a city, modelling how increasing the area of each land use affects pollinator community robustness at a city scale. Robustness is a measure of how a community responds to species loss; robust communities can withstand some extinction whereas species loss in fragile communities leads to a cascade of other extinctions. Our models predicted that increasing the area of allotments resulted in the greatest increase in pollinator community robustness at a city level. Overall,
our findings suggest that allotments are good places for bees and other
We found that numbers of bees were higher in allotments and gardens compared to most other land uses
pollinating insects, something that might not surprise most allotment gardeners! And although allotments make up a very small proportion of urban areas (less than 1% of the cities we studied), our study suggests that increasing the area of allotments
has a much bigger positive effect on city-wide pollinator communities than increasing the areas of other urban land uses.
42 Allotment and Leisure Gardener