Increased urbanisation may have a negative effect on the richness of moth species like this Vine’s Rustic, but it depends on what scale we consider richness (Image Credit: Patrick Clement, CC BY 2.0, Image Cropped)

Urbanization drives cross-taxon declines in abundance and diversity at multiple spatial scales (2019) Piano et al., Global Change Biology, https://doi.org/10.1111/gcb.14934

The Crux

You would think that the effect of building a whole lot of stuff on something’s habitat would have a negative effect on just about anything. But building a whole lot of human stuff (maybe let’s retain a modicum of science-ness and call it urbanisation) hasn’t always been shown to be necessarily bad for species. There are a lot of studies out there which show that urbanisation is can be a negative for biodiversity (which makes sense, since for starters it generally breaks up habitat patches and introduces a whole lot more pollutants). But there are also studies showing that urbanisation can increase biodiversity.

What results you get often depend on two things: what scale you’re looking at, and what type of organisms you’re looking at. Scale is crucial – if you look at the effects of urbanisation on too large a scale, you may miss extinctions in smaller regions and underestimate the effects of urbanisation. As are the species you’re observing – many are likely to react differently to urbanisation based on things like their dispersal ability or tolerance of nutrients. Today’s study looks at the effects of urbanisation on several different groups of taxa at two different spatial scales, to show exactly how much of a difference scale and species selection can make.

What They Did

The researchers selected 27 3km x 3km squares within an urban environment in Belgium, divided equally into 3 separate levels of urbanisation, with the highest all being located in city centres. 3 smaller (200m x 200m) plots (one in each urbanisation category) were then found within each larger plot. So for instance, within a plot that had a high level of urbanisation, the researchers would find a smaller plot that had lower levels of urbanisation, and one at each of the two other levels as well. This left them with a total of 27 large (landscape) plots and 81 smaller (local) plots.

Seven different types of terrestrial (uncluding beetles, butterflies, snails and moths) and aquatic invertebrates (rotifers and cladocerans) were then collected at various time periods over 2013 and 2014. A variety of different variables, incoporating different interpretations of species richness based on different scales, were then calculated.

What They Found

Whilst on a local level, increasing urbanisation showed a negative impact on every terrestrial invertebrate group, when species richness was considered on a landscape level, these trends often disappeared. Even for species groups like the butterflies, which showed clear reactions to urbanisation at a local level (even when just looking at those local plots within landscape plots of a certain urbanisation level), did not seem to react to changes at a landscape level.

Additionally, different species richness variables could show markedly different trends. Increase in species richness at landscape levels was often not related to increases at a local level. And whilst nearly all species groups showed negative reactions to urbanisation, it depended on which scale metric was used.

Did You Know: Urbanisation and Freshwater

This study also looked at freshwater species, and although it didn’t show an effect on their health, past studies have shown that an increase in urbanisation can often affect freshwater quality negatively. Soil provides a natural filter for water, so when water enters a river or lake in a more pristine environment it is often filtered of nutrients and vitamins. Yet when water empties directly from the atmosphere into a freshwater body through for example a stormwater drain, it doesn’t pass through soil and can thus cause a higher influx of nutrients and minerals than the local ecosystem is used to, potentially leading to eutrophication.

Problems?

Urbanisation here used a very strict metric – it referred almost exclusively to buildings. Roads, car parks, dams – all of these are also urban structures which are capable of affecting local species. However, incorporating more structures into our urbanisation metric here may lead to massive differences in the make-up of the plots. That may make results a lot more difficult to interpret.

So What?

Conservationists have some tough decisions going ahead. At the recent British Ecological Society Annual Meeting, Jonathan Chanse gave a plenary talk which focussed a lot on our perception of scale and what it will mean for managers. If we know that whilst some species disappear locally, they will still remain on a larger scale, does that affect our attitude to conservation?

This study doesn’t answer that question, but it definitely shows us that if we believe that urbanisation might not affect local ecosystems, we had better have tested out all possible scales and species groups before we act on that belief.