Urbanized Invaders

Does urban evolution help or hamper invasive species?

Among biologist invasive species have been a well-documented problem for a long time. In the Netherlands ecologist are actively fighting the spread of Japanese knotweed, American crayfish and many more invasive species. A recent article by Borden and Flory (Borden and Flory, 2021) has suggested that the ongoing urbanization might further assist these invasive species in dominated the local ecosystem.

Cities can act as a launchpad for the spread of invasive species, through ports, airports or any other mode of transport cities are generally the first stop for an invasive species. Urban areas are unlike any natural ecosystem. They feature higher temperatures (a phenomenon known as the urban heat island effect), altered food availability, fragmented habitats, and intense human activity. These features create powerful selection pressures that can drive rapid evolution. Importantly, these selection pressures are very homogeneous between cities (Blackburn et al., 2011). Therefore any adaptation to life in any city will most likely also be beneficial for life in any other city. Making urbanized invaders well-adapted upon arrival.

The authors of the paper also highlight that there is a lot of what they call “ecological opportunities” in cities. This is a situation in which new resources become available or competition is reduced. Cities often remove native species from the landscape, whether through habitat destruction, pollution, or climate alteration. This creates gaps in ecological roles that can be filled by the urbanized invaders. At the same time, cities introduce new food sources and shelter opportunities: bird feeders, trash, ornamental plants, artificial ponds, and building crevices. Invasive species that can take advantage of these resources may evolve traits to do so more efficiently.

There are many examples of this, some are mentioned here below:

• House sparrows: Originally from Europe, these birds have colonized cities around the world. In some urban areas, they show changes in beak shape and stress responses, which are adaptations that help them feed on nuts from birdfeeders and cope with noisy, unpredictable environments.
• Asian tiger mosquitoes: These disease-carrying insects have spread rapidly in urban areas. City populations tend to be more tolerant of pollution and artificial breeding sites like gutters and containers, which aren’t common in natural settings. Furthermore they spread much more rapid due to high mobility between urban areas, even over great distances.
• White clover: A plant common in both lawns and wild fields, white clover has evolved different levels of a chemical compound called cyanide, which protects it from herbivores. In cities, where herbivores are scarce, urban clover populations tend to produce less cyanide — a sign of rapid evolutionary change.

These examples suggest that urban evolution may be common. Invasive species, often already genetically flexible and fast-reproducing, are especially good at taking advantage of these new selective pressures.

Despite growing interest, studies of urban evolution in invasive species are still relatively rare. The authors of the paper call for more research and monitoring of these urban invaders.

As the world becomes more urbanized, cities are becoming key battlegrounds in the struggle between native and invasive species. But they are also living laboratories, showing us evolution in real time. Invasive species may actually be some of the best examples of nature’s ability to adapt. And while their success can be troubling, it can scientifically also be very interesting.

 

Borden, J. B., & Flory, S. L. (2021). Urban evolution of invasive species. Frontiers in Ecology and the Environment, 19(3), 184-191

Blackburn, T. M., Pyšek, P., Bacher, S., Carlton, J. T., Duncan, R. P., Jarošík, V., … & Richardson, D. M. (2011). A proposed unified framework for biological invasions. Trends in ecology & evolution, 26(7), 333-339.