Urban invaders

How efforts to understand diversity in urban areas could change perceptions of biodiversity in cities and increase green spaces.

Studies about community assembly typically focus on local community properties along natural-rural-urban gradients and neglect regional processes. As such, it remains unclear which functional traits are filtered when going from a regional pool of potential colonists to local communities in urban habitats. Also, urban community assembly studies often focus on one or few urban habitats whereas a city is composed of a mosaic of different habitats including urban parks, gardens, brownfields, and densely urbanized habitats. Tackling these questions is difficult because it requires detailed occurrence data about several hundreds of species occurring in a region and their functional traits. By assembling data from several studies in the city of Zurich (Switzerland), national scale occurrence data of bees and carabid beetles, and several trait databases we were able to study in great detail the regional processes underlying community assembly in the city of Zurich. The studies in the city of Zurich were carried out in four different urban green types (parks, domestic gardens, allotment gardens, green roofs, and brownfields) and used standardized sampling methods (pitfall traps and colored pans). 

Image: Allotment gardens under pressure in the city of Zurich, Switzerland (Photo by David Frey)

Fournier, B, Frey, D, Moretti, M. 2020. The origin of urban communities: From the regional species pool to community assemblages in city. J Biogeogr.  47(3):615-629. 

Cities are very particular settings for community assembly as they are highly heterogeneous and include a diverse array of small habitats occurring close to each other. Cities are thus likely to impose specific constrained on species niches and dispersal. Understanding these constraints is important because more and more people live in urban environments worldwide. In addition, the climate in cities is several degrees warmer and dryer than in natural habitats. As such, understanding the regional processes of urban community assembly can help create a better environment for human populations and remedy the negative effects of climate change.

During this research, we were surprised by the extraordinary species and functional traits diversity occurring in the city and the important differences in community assembly processes among taxonomic groups and urban habitats. Most importantly, our study revealed that species occurring in a broad range of natural habitats could potentially colonize the city. Only specialist species from alpine habitats had very low chances of successful colonization. Also, we did not find neophyte or invasive species despite the high chances of human-facilitated dispersal in cities.

An important question arising after the completion of this work is how different are regional urban community assembly processes from those occurring in more natural environments. Cities are generally thought to have a negative impact on biodiversity. However, we are wondering how the filtering of species and traits differ between urban habitats and more natural environments. Answering this question can help change people’s perception of biodiversity in urban settings and encourage people to increase the proportion of green habitats in cities worldwide.

Written by: Bertrand Fournier (1), David Frey (2), & Marco Moretti (3)
(1) Concordia University; Department of Biology; Montreal, Canada.
(2) Swiss Federal Research Institute WSL, Biodiversity and Conservation Biology; Department of Environmental Systems Science, ETH Zurich.
(3) Swiss Federal Research Institute WSL, Biodiversity and Conservation Biology.

Additional information: @batgirl_susan, pteropus.net; @dj_lohman, lohmanlab.org

Pan trap in the city of Zurich (Photo by David Frey).

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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