High human population density negative for pollinators

Date:
August 25, 2020
Source:
Lund University
Summary:
Population density, and not the proportion of green spaces,
has the biggest impact on species richness of pollinators in
residential areas.

This is the result of a study from Lund University in Sweden of
gardens and residential courtyards in and around Malmo", Sweden.



FULL STORY ========================================================================== Population density, and not the proportion of green spaces, has
the biggest impact on species richness of pollinators in residential
areas. This is the result of a study from Lund University in Sweden of
gardens and residential courtyards in and around Malmo", Sweden. The
result surprised the researchers, who had expected that the vegetation
cover would be more significant.


==========================================================================
"We have found that, in cities, the higher the population density,
the fewer species of wild bees and hoverflies we find in gardens and residential courtyards. We also see that areas with enclosed courtyards
and tall buildings have fewer species of wild bees than areas with semi-detached and detached houses, even when there are large green spaces between the buildings," says Anna Persson, one of the researchers behind
the study.

It is believed that the result is due to two things, firstly, that
tall buildings and enclosed courtyards constitute physical barriers for
insects and secondly, that green environments in densely populated areas
often are insufficient for pollinators, as they may only consist of for
example a lawn and a few ornamental shrubs.

"Urban green spaces often look very different and the quality can vary a
lot. A space can be green and still be a poor habitat for pollinators. In multi-family areas these spaces are usually simplified and maintained
by an external contractor, compared to detached houses where there is
often personal engagement and a greater variation of both plants and
management practices," says Anna Persson.

Another interesting discovery the researchers made was that urban gardens contain different species of wild bees than those found in agricultural landscapes.

"Therefore, the city complements the countryside," says Anna Persson, who contends that this is important knowledge, particularly in regions with
intense farming, as this means that the city constitutes an important environment for the regional diversity of bees. It also means that
measures for the conservation of bees are needed both in urban and rural
areas, to reach different species.

For hoverflies, however, the result was different -- the species found in
urban areas were just a fraction of the species in rural areas, probably
due to the fact that hoverfly larval habitats are scarcer in the city,
for example, aquatic environments and plant debris.

Urbanisation is one of the main causes of biodiversity decline. This is
due both to urban land expansion and to denitrification through infill development.

The researchers wanted to study which factor affected the species
richness of pollinators to the greatest extent -- population density or vegetation cover.

In addition, they wanted to find out if the built urban form had
any effect on the species richness and what residential areas with
high diversity of pollinators look like. The study was carried out by
comparing species richness in areas with varying degrees of population
density and vegetation cover -- in total, forty gardens and courtyards
across nearly all of Malmo" were studied.

Researchers also made comparisons between gardens in the urban areas
and the intensively farmed agricultural landscape surrounding Malmo".

"Pollinators are interesting and important to study in cities as they
are crucial to the functioning of the ecosystem and, in addition, they
are necessary for us to be able to achieve good harvests in our vegetable gardens and community allotments," says Anna Persson.

She hopes the study will contribute new knowledge about how to plan
and build cities in a way that reduces their negative impact on species richness.

"We show that the urban form is significant. By reducing the physical
barriers between residential courtyards and by combining different kinds
of built environments it is possible to benefit pollinators. In addition,
we demonstrate that there is scope for improvements to the existing green spaces, particularly in areas with multi-family buildings. Green spaces
in these areas are often of low quality, both for biodiversity and for
human recreation. One way to upgrade them is to let them grow a little
more 'wild', with less intensive maintenance and more native plants,"
she concludes.


========================================================================== Story Source: Materials provided by Lund_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Anna S. Persson, Johan Ekroos, Peter Olsson, Henrik G. Smith. Wild
bees
and hoverflies respond differently to urbanisation, human population
density and urban form. Landscape and Urban Planning, 2020; 204:
103901 DOI: 10.1016/j.landurbplan.2020.103901 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825110716.htm

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