One of the ways we can help wild pollinators is by changing the mowing regime of urban green spaces
We like it when nature is tidy. Shortly cut, monotonous lawns, parks that look like football or golf courses instead of flowering meadows, became the standard in city parks. What we like to call an urban green oasis is, from a biodiversity standpoint, a green desert.
Cities are, in the first place, spaces where a majority of the human population lives, and their primary function is not to maintain biodiversity. City parks also have an important socio-cultural role. However, with increasing urbanization and numerous environmental pressures, such as pollution and climate change, it is becoming clear that the greening of cities is essential for a sustainable future and human health. Green cities are also becoming a clearly defined goal of global and European policies.
Wild pollinators are particularly interesting in this regard. Studies have shown that cities can support diverse wild pollinator populations. Pollinator diversity is sometimes, unexpectedly, higher in cities than in surrounding areas, especially if those areas are affected by agriculture. This is especially true for bees, but with careful management of urban parks, we can improve habitats for other groups of pollinating insects as well, such as hoverflies, butterflies, and moths. Cities can indeed become green oases, but they need minor changes in the way they manage their green spaces: primarily reducing the use of chemical pesticides and changes in the mowing regime.
The standard of a neat lawn is achieved by frequent, intensive mowing, which is scientifically proven awful for the environment. Frequent mowing warms and dries the soil, reduces the diversity and biomass of plants and animals, and prevents the flowering of plants that are an important source of pollen and nectar for pollinators. The moment the lawnmower passes through the park, pollinators, and many other animals are suddenly left without food and shelter. Research in European and American cities shows that a reduced mowing regime is environmentally and economically beneficial – leading to increased diversity of animal and plant species, reduced greenhouse gas emissions, and financial savings of as much as 36%.
For all of those reasons, we decided to test pollinator-friendly management practices in the city of Zagreb. Our collaborators for these activities are Public Institution Maksimir for the management of protected areas of the city of Zagreb, Zrinjevac Subsidiary, and Croatian Museum of Natural History, and we have the support from the City of Zagreb and the City institute for cultural and natural heritage conservation. We defined test plots in three city parks: Maksimir, Ribnjak, and Rokov perivoj. In those parks we will establish the so-called short-flowering meadow – an area mowed only 4 times a year, and a long-flowering meadow – a strip along the edge of the lawn or a larger area mowed only once a year. This is an example of a mosaic mowing regime – although all surfaces are cut, not all are cut at the same time, so at least part of the area is in bloom at all times.
This mowing regime is based on similar actions from All-Ireland Pollinator Plan, which we have adapted to our conditions. This Plan has been a huge success in Ireland, where its implementation started in 2015. For example, one of the unexpected results was an increase in the number of rare orchids, which began to flower on roadsides just a year after the reduction of the mowing regime. Ireland is a great example, but good stories can be found in other cities as well, such as the urban meadows in Berlin, the Olympic Park in London, and beyond.
In Zagreb, for a start, we expect to see more dandelions, clover, and daisies in bloom, all fantastic native plants and a good source of food for pollinators. Pilot plots are also our Cro Buzz Klima project areas, and we will continue to monitor the abundance and diversity of plants, bees, hoverflies, and butterflies, which we partially started last year. This will allow us to assess how these changes are affecting urban biodiversity and how to plan the management of the green spaces of our cities in the future.
P.S. If you belong to the part of the population that is wary of any bugs (useful to humans or not) and you are a little scared of all this, keep in mind that the increase in insect diversity (= larger number of different species) prevents pest outbreaks (= sudden increase in abundance of one species). Greater diversity means more natural enemies and a shift towards ecological balance. It also positively impacts the populations of other, larger predators (such as swallows, blackbirds, and other birds that feed on insects).
Last week we visited the natural history museum in Linz, Austria (Biologiezentrum Linz), which holds the largest collection of bees in Europe. The collection contains about 690 thousand individual bees, properly prepared and labeled, and permanently stored in entomological¹ boxes. Insect collections are just a part of natural history collections, which also include pressed plants in the herbarium, bones or stuffed bodies of vertebrates, fossil specimens in rocks, and similar.
Why is it important to visit the collection for our project? Museum collections² are essential for correct species identifications. To identify wild bees collected during last year’s fieldwork, we first used taxonomic keys³. We then took the train to Linz – together with 6 boxes of bee samples. In Linz, we compared the bees one by one with the material from the Museum collection, which was determined and confirmed by experts, in order to validate (or correct) our identifications.
Correct species identifications are basic data for further analysis. For example, in order to compare different areas and different habitats by their species richness and composition, we need to know what species we collected. Accurately identified samples are also important for establishing our project collection.
Another reason why a visit to Linz was so useful was meeting the museum staff and their collaborators, experts with years of experience in wild bees. They steered us in the right direction for some species, corrected our mistakes in others, pointed out the literature we didn’t know about, and in general helped with their advice and encouragement.
Museum collections all over the world are important tools of scientific discovery and nature conservation, which are sometimes insufficiently recognized. Collections enable us to reconstruct historical species ranges, track population dynamics of endangered species, and analyze morphological changes of particular species over time. They enable the discovery of new species and reconstruction of the evolution of life on earth.
By comparing historical data from collections with more recent data, we can study the impact of different anthropogenic pressures, such as climate change or land-use change, on wild pollinators and other living organisms. A famous example is the use of museum collections of bird eggs to determine temporal changes in eggshell thickness due to exposure to pesticides. This research was key evidence in proving the harmfulness and banning of DDT in the 1960s. The recovery of bird populations after the ban is one of the success stories of nature conservation and an example of the importance of museum collections.
1 – Entomology is a study of insects (from the Greek word entomon – insect).
2 – Museum collection – when you visit a natural history or any other museum you see only the part that is open to visitors, which is only a very very small part of the museum collections. Specimens selected for display are usually less than 1% of the total museum collection. Keep this in mind on your next museum visit.
3 – Taxonomic keys – expert publications used to identify species of groups. They are mainly based on morphology.
The majority of wild bee species are solitary, which means they don’t live in colonies like honey bees or bumblebees, but alone. Although most solitary bees nest in soil, some species nest in cavities in trees, stems, and twigs. These cavity-nesting bee species will consider renting a room in a so-called bee hotel.
Bee hotels are not new. We have been using them in agriculture since the middle of the last century, to improve pollination in orchards and other crop systems. Recently, bee hotels have become more broadly popular, mostly due to media attention given to the worrisome decline of wild pollinators across Europe and North America.
Whether bee hotels can help declining pollinators will depend somewhat on the context. In complex, natural habitats wild pollinators do not lack nesting resources. Setting up a bee hotel in such places will not do any harm, but it probably won’t have much of a positive effect either. In natural habitats, bee hotels can have an educational or research role. But in anthropogenic habitats that lack complexity, such as urban or agricultural areas, nesting resources are limited, so setting up a bee hotel can have a positive impact.
If you are planning to build a hotel, you should keep in mind the other important factor for bees: the availability of food. Most small solitary bees do not forage far from the nest, up to several hundred meters. If you are setting up a bee hotel in your garden, you can improve the floral resources very easily. Dandelions, daisies, and clover are great sources of nectar and pollen, so mowing less frequently is often the best thing you can do to help the pollinators.
Bee hotels can vary in design, but most are built from narrow horizontal tunnels or tubes, closed at one end. Each of these tunnels will serve as a nest for one female solitary bee, which will build individual rooms, one after the other, along the tunnel. In each room, the bee will lay a single egg that will develop into a larva, and supply it with food – a ball of nectar and pollen. Walls between rooms are built from mud, resin, small pieces of leaves and petals, or pebbles, the building material depending on the bee species.
Making a bee hotel is simple, but you need to keep in mind a few rules. We explain these in a short leaflet, which you can find at the end of this post. We created the leaflet together with students from Section for Hymenoptera of “Biology Students Association – BIUS“. Their section studies all Hymenoptera (including bees, ants, and wasps), but their main focus are wild bees, and they have practical experience in building bee hotels. If you are a biology student interested in wild bees (or ants or wasps), get in touch with them – they plan exciting activities for when it warms up.
To keep in mind:
If you are ambitious, it is better to build several small hotels than one large. Big hotels attract more predators and parasites.
Maintenance matters. Remove the used tubes and replace them with new ones each year. Replace drilled wooden blocks every two years.
Meadow flowers are not the only food source for bees – many trees and shrubs are rich in pollen or nectar.
One of the most common pollinators you can see in Croatia is the buff-tailed bumblebee, Bombus terrestris. This species is exactly what most people imagine when they hear the word “bumblebee”: a roundish black body with two yellow stripes and a white one at the end.
Bombus terrestris is one of the most abundant and widespread bumblebees in Europe, with as many as nine subspecies recognized. You can see it almost anywhere where some flowers can be found, including cities and farmland, from early spring to early fall. Like other bees, the buff-tailed bumblebee feeds on pollen and nectar. It belongs to the so-called short-tongued bumblebees, which means that it collects nectar from relatively shallow flowers, with short corolla.
Bumblebees are social insects, living in organized colonies. Their colonies are annual, meaning that at the end of each vegetation season the workers, males, and old queen die. Only new queens will survive, hibernate over winter, and establish a new colony in early spring. Bombus terrestris queens build nests underground, often in abandoned burrows of small mammals. The nests can grow large and contain up to 400 workers.
Buff-tailed bumblebee is one of the rare bumblebee species with a Mediterranean-centered distribution. Bumblebees evolved in the Himalayas, and they are primarily adapted to cold and temperate climates. Due to these cold-loving traits, bumblebees are most diverse in the mountainous and northern regions. They are also the groups of bees that are especially vulnerable to climate change, as they have a relatively low tolerance to heat stress. However, this is not the case for buff-tailed bumblebee, which can live in a variety of habitats and climates, from northern Europe to the shores of the Mediterranean, and which tolerates hot and dry conditions very well. In the Mediterranean region, the buff-tailed bumblebee queens enter summer aestivation instead of winter hibernation. Colonies are established in the fall, and they are often active all winter.
Bombus terrestris is the most common managed bumblebee species. Bumblebees are commercially reared for crop pollination, especially to pollinate greenhouses tomatoes due to their ability for buzz pollination. The annual bumblebee trade is estimated at 2 million colonies (estimate from 2016., for the IPBES report on pollinators). The global trade and high ecological adaptability of this species have unfortunately caused the buff-tailed bumblebee to become an invasive species outside its native range, in South America and Japan.
Can you mistake this species for some other? Yes. White-tailed bumblebee, Bombus lucorum, is very similar, as well as the other species of this complex (B. magnum and B. cryptarum). The stripe pattern is identical to buff-tailed bumblebee, but the yellow stripes in B. lucorum are slightly lighter, lemon yellow while in B. terrestris they are darker, golden-yellow. Sometimes even experts find it difficult to distinguish these species. Also, at first glance, the garden bumblebee, Bombus hortorum, may seem similar, as it has a black body with yellow and white stripes. However, the clear distinction is that Bombus terrestris has only two yellow stripes, while the Bombus hortorum has three. Biologist and photographer Steven Falk prepared some excellent, publicly available resources for distinguishing common species of bumblebees, which can be very helpful.
According to the latest estimates for the European Red List of bees, Bombus terrestris is not an endangered species, although many other bumblebee species are (Nieto 2014).
This October, we attended the SCAPE 2021 conference, which took place in Warsaw and Chęciny in Poland. SCAPE stands for The Scandinavian Association for Pollination Ecology, which has been organizing these conferences yearly since 1987.
We presented our project Cro Buzz Klima, met many nice people, and listened to 60 lectures covering the ecology and evolution of pollinators and plants, chemistry of floral scent and nectar, pollinator conservation, and pollinators in agricultural ecosystems.
In one of the plenary lectures, Massimo Nepi reviewed nectar neurobiology. Perhaps the best-known example is the discovery that caffeine in nectar improves bee memory so that caffeinated bees can find caffeinated flowers faster and easier. In addition to sugar and amino acids, nectar contains a number of other compounds. Some of these compounds protect plants from pathogens, and some are neuroactive molecules that regulate bee appetite, improve muscle function, or keep them calm. Such discoveries change our perception of the plant-pollinator relationship, as they suggest that plants may be active managers of the behavior of insects.
Research in Poland found that the pollen of trees, especially oak, is an important food resource for solitary bees in urban areas, where the availability of wildflowers is reduced. Such studies provide important data for the management planning of urban green spaces.
The meta-analysis of pollinator efficiency showed that honeybee, Apis mellifera, is not the most efficient pollinator, only average. Honeybees are still useful managed pollinators, but this research highlights that honeybees cannot replace wild pollinators or ecosystem services they provide.
The talk on mountain bumblebees revealed that at one locality and at a certain time bumblebees are picky eaters – they collect nectar and pollen from a small number of plant species, often not from the most abundant ones. However, when looking at data throughout the season and across the altitudinal gradient, bumblebees will collect food from different species, showing flexibility. Such analyses are important for predicting the adaptation of bumblebees to climate-related shifts in distributions of floral resources.
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