Exotic Dangers in the Urban Forest

In concrete jungles
nature’s pillars catch
the city’s heat,
foreign tones of green
that hug the sky with
firm, unrooted ease.
Washing ashen cloaks
with verdant, leafy lungs,
gifting shade
and life
and joy.
Hidden in our hubris
these unfamiliar buds
misstep the local beats,
delaying bursts
of slumbering boughs
and creeping feed –
a brooding doubt
to native, perching wings.

This poem is inspired by recent research, which has found that urban birds prefer native trees.

Urban trees offer essential ecosystem services like air purification and local temperature reduction, while also providing habitats for birds and insects. Globally, introduced, or non-native tree species are widely used in urban planning, with over a quarter of urban plant species thought to be non-native. However, whilst some advocate for non-native trees because of their tolerance to the harsh conditions of cities, it is essential to fully assess the impacts of their presence on the local ecosystem. For example, these non-native trees, also known as introduced, exotic, or alien species, do not have an evolutionary history with the local ecosystem, which can potentially cause issues for native wildlife.

To find out how different types of trees affect birdlife, researchers monitored 400 nest boxes of blue tits and great tits in five parks in the Swedish city Malmö over a seven-year period. The results, show that native trees (defined in the study as species that have been in the ecosystem for at least 700 years) provide more resources and are preferred by urban birds. Overall, the birds were found to be less likely to breed in territories dominated by non-native trees. Chicks reared in territories dominated by non-native trees weighed significantly less, compared to territories with fewer non-native trees. The researchers conclude that from a conservation perspective, specific native species known to host invertebrates, should be prioritized over and above non-native trees in management decisions regarding urban green spaces.