Urban invaders loving the heat

Guest post by Charly Géron, PhD student in The 3D Lab and lead author of a new study on how urban microclimates might facilitate plant invasions in cities.

They are submerging the news, movies, papers and talks everywhere. I am of course talking about the challenges the world has to face right now such as urbanization, climate warming and invasive species to only cite three. Anxiety is quite growing for a year now since we are directly affected by Covid 19 which can actually be placed in that last category (let me tell you that us humans can also be considered as belonging to that category 😉 ).

However, we don’t have to be frightened by these concerns, and we should look for ways to tackle them. My PhD has been an excellent way so far to better understand how they can interact with each other. Indeed, invasive species are listed among the biggest threats to the native nature but also to human activities. Historically, alien plant invasions have been mainly studied in natural environments focusing on their impacts on the local biota for example. It is only in the last few decades that alien plant invaders have received growing attention in cities. Urban environments are usually not perceived as containing major native species richness due to the habitat modifications by anthropogenic activities. 

Cities are incredibly connected via a dense transport infrastructure network, and they are hubs of exchanges of goods and people. This leads to a high number and proportion of alien plant species in urban areas, and especially of new comers. Moreover, one has for sure noticed how warm a city center can get in a heatwave compared to the fresh rural outskirts. This is one of the signs of the modified microclimatic conditions of the urbanized environments. They not only display higher temperatures but also drier soils due to the important use of materials such as asphalt or concrete characterizing our often – too – gray cities. It is also important to note that these microclimatic differences present along the urban-to-rural gradients are predicted to be more prominent in the coming years with climate change.

Ailanthus altissima taking over urban landscape

Cities then sound as the perfect laboratories to study emergent alien plant invasions. They not only display warmer and drier growing conditions, but they also concentrate newly arrived alien plant species. We decided to test the long lasting hypothesis that urban alien plant invaders are coming from warmer native climates. Indeed, the successful colonization of new environments by alien plant species highly depends on the match between their requirements and the local environmental conditions. Moreover, several observations and study have proven for other alien species from more favorable climates the clear link between warm cities and their establishment. For example, the persistence of alien aquatic animals locally depends on heated water effluents from cities in Germany. We focused on the European regions with a temperate oceanic climate termed “oceanic Europe”, which represents an area from the north of Spain to the south of Norway, through France, the UK, Belgium, the Netherlands, Germany and Denmark. We selected alien plant species that still have a limited distribution there, and we modelled their native range with a species distribution model framework, to visualize their native range climatic conditions. We analyzed their distribution in oceanic Europe along the urban-to-rural gradients using the percentage of built up area, with high values corresponding to highly urbanized areas.

Paulownia tomentosa at ease on top of brick walls.

We analyzed if the distribution of the selected alien plant species along the urbanization gradient in oceanic Europe was linked to their native climate conditions along 3 variables: the winter temperatures, the summer temperatures and the precipitation quantities, while taking into account the year of first observation in the wild. We found that more urban alien plants in oceanic Europe were coming from warmer or drier native ranges than the one currently found in oceanic Europe. A very good predictor of the distribution along the urbanization gradient of oceanic Europe was the annual mean temperature of the native ranges of the studied species, with the ones developing in more urban areas coming from warmer native ranges than oceanic Europe.

Mean urbanity (in %) of the studied alien plant species as a function of their native range climatic conditions. Mean urbanity of alien plant species as a function of: a) “precipitation” axis with high values indicating more precipitation; b) “summer temperature” axis with high values indicating higher temperatures, and c) the scaled year of first observation in the wild, ranging from 1683 (low values) to 2008 (high values). Each point corresponds to a species, colored following the main Köppen-Geiger climate class in which it was observed the most in its modelled native range. Full lines correspond to significant effects, while dashed lines correspond to non-significant effects.

We argue that despite the fact that alien plant invasions depend on a complex set of components, microclimatic barriers might be one of the reasons why some alien plant species thrive in urban areas, while others prefer rural environments. However, with the global changes listed earlier, the barriers that currently constrain numerous alien plant species to cities may be lifted. As cities are now recognized as hotspots for plant invasions, they could act as new potential “sites” for the seeding of future plant invasions, with the help of structures such as rivers or roads for their spread. We are now trying to disentangle what could be the underlying reasons of the preference of urban areas for some alien plant species, but this is for later.

Roads as paths for urban plant invaders?

The full story got published in Biological Invasions here.

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