Nectar is an important source of food for many insects – especially in the city. As has now been shown, private city gardens are more important to insects than expected. Because in urban areas, an average of 85 percent of the nectar is produced in private gardens – mainly from non-native plant species. The design of the gardens therefore plays a crucial role in protecting insects.
Insects are essential for the pollination of most plants and an important food for countless birds and small mammals. But for some time now, researchers have noted a sharp decline in many important pollinator insects, including butterflies, beetles and bees. One of the reasons for the decline is lack of food, as more and more food sources for insects are disappearing due to intensive agriculture and the expansion of cities.
How much nectar is there in cities?
While data are already available on the remaining nectar resources in rural landscapes, such measurements are lacking for urban areas. That is why a team of researchers led by Nicholas Tew from the University of Bristol has now researched the nectar supply in cities and communities. “Although the amount and variety of nectar was measured in the countryside, it was not in urban areas, so we decided to investigate,” explains Tew.
To do this, the researchers first documented the number and quantity of flowers in the four British cities of Bristol, Edinburgh, Leeds and Reading. In addition, they took the flower nectar from more than 3,000 flowers of almost 200 plant species with a fine glass tube in order to determine the nectar production. They also determined the sugar concentration of the nectar with a refractometer, which measures how strongly the light refracts when it passes through a solution. They first compared the data with existing values for arable land and nature reserves, and finally they assessed the spatial distribution of nectar sugar within the cities.
When making a comparison, it was initially noticed that the amount of nectar sugar available did not differ significantly between the three landscape types. However, the researchers found that the variety of nectar varied depending on the location. “We found that the nectar supply in urban landscapes is more diverse, that is, comes from more plant species than on farmland and in nature reserves,” explains Tew. “And this urban nectar supply is decisively supported by private gardens.” This is surprising, among other things, because over 80 percent of non-native ornamental plants are planted in such gardens. As the new data reveal, contrary to earlier assumptions, they represent an important source of nectar for native insects in cities.
Three gardens already provide enough nectar for thousands of bees
And the gardens not only offer a great variety of plants, but are by far the largest urban food source for pollinating insects, according to the scientists. “Gardens are so important because they produce the most nectar per unit area and occupy the largest area in the cities we studied,” explains Tew. Because almost a third of the area in the cities consisted of house gardens, which is six times as much as the area of parks and 40 times as much as that of allotment gardens. According to the measurements, an average of 85 percent of the nectar is produced on the area of private gardens. In three gardens alone, an average of one teaspoon of the sugar solution was found daily, which the pollinators drink for energy. That is enough to care for thousands of bees, for example. “We expected private urban gardens to be an abundant source of nectar, but we didn’t expect the scale of production to be so overwhelming,” Tew said.
“Our results underline the central role that gardens play in promoting biodiversity in urban areas,” said the scientist. “It is critical that new housing developments include gardens, and it is also important for gardeners to try to make their gardens as good as possible for pollinators,” explains Tew. This includes, for example, that instead of paving or creating terraces, they plant nectar-rich flowers, whereby something should always bloom from early spring to late autumn. In addition, the researcher advises that lawns are mowed less often so that dandelions, clover and Co. can grow and that pesticides are avoided. “Gardens shouldn’t be looked at in isolation – instead, they’re a network of resources that provide valuable habitats and supplies when cared for with pollinators in mind,” says Stephanie Bird of the Royal Horticultural Society.
Source: University of Bristol, Article: Journal of Ecology, doi: 10.1111 / 1365-2745.13598