Where the Wild Things Are

Urban aliens and threatened near-naturals: Land-cover affects the species richness of alien- and threatened species in an urban- rural setting (2020), Petersen et al., Scientific Reports, https://doi.org/10.1038/s41598-020-65459-2

The Crux

Land-use changes (in particular, urbanisation and everything related to it) have huge effects on biodiversity patterns – some habitats can support populations of many different species, others cannot. This seems intuitive on a large scale (think a rainforest vs. a large, industrialised city) and on a small scale (a small patch of concrete vs. a patch of soil in a forest), but what about on a medium scale, more relevant to management organisations? How different species of plants, animals and fungi are distributed in space on such a meso-scale is far more relevant to everyday management, compared to say a global distribution, or the organisation of a 10 x 10 metre quadrant.

Today’s authors (myself and my current supervisors) looked at how species richness changes with land-cover on a municipality scale. We also looked at whether these patterns differ if one considers the total number of species, threatened- or alien ones, and whether animals, plants and fungi react to concrete vs. forests in the same way.

What They Did

We used species records from the Global Biodiversity Information Facility (GBIF) and digitised land-cover maps (find out more about them in last week’s post) to make statistical models to investigate if and how species richness among different groups of organisms (plants, fungi, birds and other animals) differed between the different land-cover types in 500 x 500 metre grid cells in Trondheim Municipality (Norway). The land-cover in each grid cell was classified based on the dominant, fine-scale land-cover within the grid cell – some examples are “Urban/developed area”, “Cultivated land”, “Coniferous forest” and “Open marsh”.

We included data on whether the observed species were registered on the Norwegian Red List or Alien Species List, to test if these groups (or all species in total) react to different land-cover types in the same way.

What They Found

Threatened species, alien species and overall species richness do not depend on the same land-cover types, and they are not distributed similarly across space. The mechanisms determining total species richness in an area turned out to be highly complex, depending both on land-cover, the heterogeneity of land-covers in a grid cell, and whether the ground faced north or south. Additionally, the response to these variables differed depending on what kind of organism was in question. As an example, the number of bird and plant species responded positively to grid cells with heterogenous land-cover, whereas the opposite was the case for non-avian animals and fungi. Generally, urban/highly developed areas showed lower biodiversity than did more natural areas.

The number of threatened species depended on the land-cover type in a grid cell, with near-natural areas having the highest number of species for all organism groups. Surprisingly, none of the land-cover variables could help predict where the most alien species were found, meaning that the number of alien species (on this spatial scale) is only determined by where they were first introduced.

Did You Know?

“Everything is related to everything else, but near things are more related than distant things” (Tobler, 1970).

This is Tobler’s First Law of Geography and it describes what is known as Spatial Autocorrelation. Whenever scientists are making statistical tests or –models (whether that is correlating people’s height to their bodyweight, or the number of species observed in a grid cell on a map), the most fundamental assumption required by these tests is that the observations are independent of each other. As an example: Person A’s height and weight are not influenced by (i.e. dependent on) the height and weight of Person B. However, once we start working with either time or space, this becomes a problem. If you find many species in one grid cell, you are likely to find many species in the neighbouring grid cells as well – not necessarily because these grid cells are “better” than grid cells far away, but simply because teleportation still only exists in science fiction. This breaks with the fundamental assumption of most statistics, and researchers have to be very careful to account for this effect whenever they make statements about anything involving spatial (or temporal) aspects.


Using species occurrence records from online portals is always tricky – these are a mix of observations by laymen, museum specimens collected in structured surveys by professors, and everything in between. This makes the quality of the datasets highly fluctuating. Likewise, some areas have been investigated a lot more closely than others: easily accessible areas close to people’s homes have a lot more records than do remote, inaccessible areas. This could potentially over- and underestimate the importance of these areas and habitats, respectively.

The lack of correlation between any land cover types and where alien species pop up make tracking their introduction crucial to preventing their spread (Image Credit: Tanja Kofod Petersen, CC BY 2.0)

The lack of correlation between any land cover types and where alien species pop up in cities like Trondheim make tracking their introduction crucial to preventing their spread (Image Credit: Tanja Kofod Petersen, CC BY 2.0)

So What?

Where we find high (and low) numbers of species, and why we find them there is obviously important, especially as cities and other human constructs have a major impact on these patterns. Investigating patterns of biodiversity on a spatial scale relevant for managers could help them make better, informed decisions on how to deal with these issues.

The most surprising and important finding of this study is the lack of correlation between any of the included predictors and the number of alien species in a grid cell; it seemingly only matters where they were first introduced. As urban areas are generally acknowledged to be hotspots of alien introductions (see some of the other posts on alien species here on the blog), this makes the need for proper monitoring and management of urban areas even more important.

Tanja Petersen s a PhD candidate at the Norwegian University of Science and Technology. She studies the effects of urbanisation, land-use and land-use changes on biodiversity, focussing on threatened and alien species. She uses records from the Global Biodiversity Information Facility (GBIF) and offical land-cover maps to track the patterns and changes over time and in space. Check out her previous articles at her Ecology for the Masses profile here or follow her on Twitter @NeanderTanja.

Title Image Credit: Tanja Kofod Petersen, CC BY 2.0 (Image Cropped)

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