Flowering plants are now better pollinated in urban areas compared to rural ones, a new paper reports. But it's likely because insects are struggling in the countryside.
A team of researchers from the German Centre for Integrative Biodiversity Research (iDiv), the Martin Luther University Halle-Wittenberg (MLU) and the Helmholtz Centre for Environmental Research (UFZ) reports that test flowers in urban areas were better pollinated than plants in the countryside. The findings are quite surprising as the team also recorded less diversity in pollinator species in the city.
This discrepancy is attributable to the larger number of bees living in urban settings, they conclude.
"Urban people are constantly changing their environment. Finding your way around is a challenge that bees are particularly well-equipped to deal with due to their highly developed orientation and learning skills," says the head of the study, Prof Robert Paxton from Martin Luther University Halle-Wittenberg (MLU) and the German Centre for Integrative Biodiversity Research (iDiv).
"Flies and butterflies obviously find this more difficult."
Human expansion is often associated with habitat destruction, but the reality isn’t so black-and-white. People don’t ‘destroy’ habitats per se, they transform them; the shape we transform them into may suit our needs very well, but not do much good for wildlife. Previous research has shown that conversion of natural areas into built landscapes affects virtually all native species, including insects.
Biodiversity overall plummets in this scenario but some insect species seem to thrive in such habitats. That being said, we know precious little about how urbanization impacts the ecosystem services insects provide, such as pollination -- that's what the team set out to understand.
For the study, they selected flower-rich, inner-city locations such as parks and botanical gardens in nine large German cities (Berlin, Braunschweig, Chemnitz, Dresden, Göttingen, Halle, Jena, Leipzig, and Potsdam) and compared them with similar areas in rural areas around the same cities. Here, they analyzed local flying insect species and species diversity using pan-traps. They used potted red clover plants as a reference for pollination, recording all insect visits to the flowers 20 times a day for 15 minutes. By the end of the experiment, the team also numbered how many seeds these plants produced -- which determined how successful the pollinators were.
Pollination success rates were, surprisingly, higher in cities. Plants in urban settings were visited more often than those in rural settings, and by a greater number of insects overall. In rural settings, pollinator diversity was higher, as was the biomass of individual insects, especially flies and butterflies — in essence, pollinators in the countryside were more varied and plumper. Apart from bees, however, no species seemed very keen on visiting the red clover. Three out of four of the recorded visitors in the study were bumblebees, followed by honey bees at a frequency of 8.7%.
The team argues that wild bumblebees and honeybees made such a show of force in cities because they outcompete other pollinators in this setting. Cities, it turns out, are a pretty nifty place for bees. They can find a range of suitable habitats to settle in, like soils, dead wood, or wall cavities. The large variety of flowering plants in parks and gardens also ensures they have a reliable food supply. Finally, bees are likely better able to cope with the highly dynamic city life than other groups of insects.
Why pollinators in rural areas fared so poorly is still unclear. Rural areas should, in theory, provide living ideal conditions for these species. The diverse habitat structures in these areas ensure a reliable source of food, ample opportunities to nest, and help the insects orient themselves. The team suspects that it has to do with agriculture: the flower strips, grasslands, forest, and hedges species rely on in rural settings are wiped away in an intensive agricultural landscape.
"I was really shocked at how consistently poor the pollination performance in agricultural land was," says Paxton.
"Other studies have shown that wild bees and bumblebees are particularly susceptible to pesticides. This could also explain why their diversity is greater in the city, where pesticides play a lesser role."
The team recommends that we take the needs of bees into account when planning out landscapes, both in cities and in the countryside. And we should definitely take care of bees and other pollinator species because they directly underpin our ability to produce food.
"What would our urban green spaces be without flowers?" asks lead author Dr. Panagiotis Theodorou. "The number of urban vegetable gardens and orchards is growing, but without pollinators, no fruit will ripen there."
"If agricultural land degrades further, cities could serve as a source of pollinators for the farmland surrounding them."
The paper "Urban areas as hotspots for bees and pollination but not a panacea for all insects" has been published in the journal Nature Communications.