What urban nature really means for insect biodiversity
Parks and green spaces in cities provide health and wellness benefits to human inhabitants, but they're not necessarily beneficial for other urban dwellers—like insects. Researchers are investigating urban biodiversity with approaches such as 'bee hotels' to see how cities can better foster insect life.
Green spaces now cover about 40% of cities in Europe, where they have increased by about 38% in the past 25 years. However, biodiversity on the continent is plummeting, with only 23% of species and 16% of habitats thought to be in good health. Cities can contribute to the decline as they grow in size and new urban areas are created.
"Urbanisation can be a problem for insects or biodiversity in general," said Dr. Linjun Xie, a researcher at Durham University in the UK. "If an area gets urbanised and there is habitat loss, insects will be lost along with the habitat."
Cities can foster biodiversity as well though, so the pattern is not clear-cut. They are novel ecosystems that can be suitable habitats for certain species and drive rapid evolution or feature changes, allowing some types of insects to adapt. "It's not that cities are like a black hole that swallows all the biodiversity," said Dr. Marco Moretti, a senior scientist at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) in Birmensdorf. "There are a lot of dynamics and this is what researchers are starting to realise."
Green spaces, for example, can have varying effects on biodiversity. A small park wouldn't have the same impact as a large one, while the species of plants or flowers they contain would determine what types of insects could survive in them. How a green space is managed also plays a significant role. "If you cut the grass very often or if you use pesticides, then you will affect (insect biodiversity) a lot," said Dr. Moretti.
Dr. Xie and her colleagues have been looking at how urban nature, such as parks, rivers, green roofs and living green walls, contribute to biodiversity. They examined 976 nature-based initiatives in 100 European cities to get a better idea of their explicit goals and targets as part of the Naturvation project. "We also analysed how cities are working with nature-based solutions for biodiversity and what kind of measures were taken, not only to protect habitats but also to protect certain species and create habitat connectivity within a city," said Dr. Xie.
They found that where urban natural areas had been created with biodiversity in mind, it usually involved quantitative goals. One project at a large park in Bielefeld, Germany, for example, is aiming to protect 134 types of insects. Community gardens were most likely to take urban insects into account, where bees and butterflies were often considered when selecting plants, whereas larger green spaces like parks and rivers rarely had them in mind. However, they also found that most green areas didn't take biodiversity into consideration at all; only 351 of those they looked at had goals related to it. "If they don't consider biodiversity, that nature-based solution (such as green spaces) might not do any good to biodiversity at all," said Dr. Xie.
Furthermore, the team found that non-governmental organisations (NGOs) were more likely to lead initiatives that focussed on insects compared to governments. A project in Cardiff, Wales, called Urban Buzz, for example, run by a nature conservation charity, is aiming to turn Cardiff University into a bee-friendly campus. On the roofs of some buildings, they are installing bee hotels—structures containing tubes where bees and wasps can make their nests—specially designed for bees that lay their eggs in small cavities. Approaches such as setting up areas with loose soil for burrowing bees, are also being used to create 840 hotspots in England and Wales that encourage pollinators such as bees, butterflies and hoverflies.
Dr. Xie thinks that such projects can have a wider impact by raising awareness in the community, perhaps prompting land developers, architects and engineers to consider insect biodiversity when planning to build new infrastructure or renovating existing buildings. But she thinks that guidelines are also needed to help people implement recommend techniques, where maintenance is often a neglected aspect. "Who will take care of a project afterwards often isn't clear, whether it's by the developers or by the users," said Dr. Xie.
A better understanding of the specific factors that impact biodiversity in urban green areas should also help protect insects. Dr. Moretti and his colleagues investigated how green spaces in cities affect bats and nocturnal insects such as moths and caddisflies, which has rarely been studied, as part of the Bioveins project.
They focussed on the cities of Antwerp, Belgium; Paris, France; and Zurich, Switzerland, which are different in size and in terms of light pollution, which can disturb nocturnal animals. Species that are sensitive to light, for example, may not be able to survive, while more hardy species can still become stressed by light, resulting in a decline in numbers. "The idea of this paper was to assess (how urban areas might affect) the interaction between nocturnal insects and bats because bats prey on nocturnal insects," said Joan Casanelles-Abella, a Ph.D. student at WSL. "We wanted to see what was influencing these two groups."
The team found that light pollution had a negative impact by reducing bat and insect diversity and the effect was noticeable in Antwerp and Paris where there is a lot of artificial light. They think that improving the types of lighting used in cities can help. Using intelligent lighting with motion sensors that only turn on when a person is approaching, for example, can cut down on unnecessary illumination as well as investigating different types of light, such as LEDs, as some may disturb animals less. "Thinking a bit more in terms of what light is needed where, is the solution," said Dr. Moretti.
The project is also investigating how bees and wasps are faring in the same three cities, as well as in Poznan in Poland and Tartu in Estonia. Dr. Moretti and his colleagues set up bee hotels to obtain more detailed information about these insect populations. "You're not only obtaining data on species richness or abundance, which is the typical information that is reported in these kinds of studies, but you also get information on how many bees (and wasps) are born, how many have died and how many have parasites in their nest," said Casanelles-Abella. "All of this is important because it tells us a bit more about the fitness of the population."
In addition, they are also doing genetic analyses of pollen consumed by bee larvae to better understand their diet and the role of different types of vegetation. Preliminary results show that one bee species relies heavily on pollen from wind-pollinated trees, which was not expected to be a main source in highly-urbanised areas.
Casanelles-Abella thinks that this type of finding can help guide conservation efforts in cities. "Instead of saying put more plants, we are trying to estimate what kind of plants we should put and what is the importance of, for instance, trees, exotic plants and so on," said Casanelles-Abella. "I think this is useful for management."
With more forethought, cities could provide new opportunities for biodiversity.
Threats exist in rural areas too, where environmental factors such as climate change can also harm insect life. In cities, however, there is more scope for making change happen. "People have the capacity to act and if they are eager to preserve and restore biodiversity, they can actually mobilise many resources to do that," said Dr. Xie. "And so I think it's definitely possible (for cities to become places where biodiversity can thrive)."