The rapid spread of an urban lichen

Is the urban landscape pushing lichen evolution or is it just providing space for opportunists?

Harold Timans

The urban environment creates a multitude of habitats that differ widely in their abiotic factors. In one street it can be scorching hot if it is exposed to the sun, but turn around the corner and suddenly there is shade and noticeably higher humidity. Some streets have broad verges in which all kinds of flowering plants can bloom, while others look like stone deserts where not much seems to live. Looking at it in detail, all kinds of species are actually living in these environments, sometimes showing very noticeable adaptations to the conditions of their niche.

Opportunists

The adaptations to their original niche can work out very well for a species when an environment that resembles its own niche is created frequently, or made possible by humans via urbanization. For instance, some rock-dwelling species of bryophytes in Western Europe that are adapted to acidification and sun exposure are now one of the most common urban species as cities provide plenty of stony substrates that endure long periods of drought and high temperatures in summer. There are numerous examples of species becoming ubiquitous in the urban landscape due to their pre-existing adaptations (for instance: Grimmia pulvinata & Bryum argentum). Although it is obvious from their proliferation that cities can provide lots of opportunities for certain species to thrive, these examples do not yet show signs of novel adaptations as a result of urban evolutionary processes. Or do they? Let’s take a closer look at one such ‘opportunist’.

Opportunist or adapted to the urban heat island?

In The Netherlands, a new species of lichen was discovered in 2014. It is Physciella chloantha, and in Europe, it has primarily been recorded in the Alps, the Mediterranean region, and some more continental locations. It forms pale lobes in messy rosettes and mainly reproduces vegetatively with soralia formed at the lobe ends. Although having been discovered in the country relatively recently, it was quick to find its way to many new locations, especially in cities. In the Netherlands, it seems to prefer warm and slightly humid environments on tree bark, although it can also be found in dryer locations. Although still quite rare, it can be found at quite a predictable rate in the biggest cities of the Netherlands when one knows what to look for.

When the distribution of the species in the Netherlands is compared to an urban heat island (UHI) map, which shows relatively increased temperatures compared with the non-urban surroundings, it becomes quite apparent that the localities seem to overlap nicely with the regions most affected by the UHI. Keep in mind that there is a sampling bias as not every city or region in the Netherlands has a local lichenologist. The current distribution model is far from complete (especially when considering that the species is spreading at a fast rate). Despite this, the species characterizes itself by being recorded regularly in the most urbanized or ‘warmer’ regions, like Amsterdam, Rotterdam, and Utrecht. It has also been recorded in urban areas of the far north (Groningen) and south (Limburg), without observations in the more rural areas separating these urban spaces.

Going into an unmapped urban region is always exciting because of species like Physciella chloantha. Finding a new locality for this species can always be predicted beforehand when investigating a new area, therefore confirming a trend. It usually results in one or more trees with Physciella present. This raises the question of whether this species is an urban environment opportunist, like the rock-dwelling mosses, or has it evolved to occur in a novel niche in cities? If it is the former, why then has it not been found before 2014? Was the climate too chilly up until recently for this species? And why does the distribution centre around urban heat islands while the mentioned mosses also occur commonly outside of the cities? Is it because of the dramatic improvement in air quality (SO2 pollution being reduced to hardly existing levels and Nitrogen-compound deposition decreasing at a steady rate for more than 20 years)?

Are cities the new Alps?

When the distribution on a European scale is considered, Physciella occurs mostly in the warmer places of montane or Mediterranean regions. Do certain city environments resemble conditions found in those regions? It seems likely. It would explain how another lichen called Flavopunctelia flaventior occurs very rarely, in The Netherlands primarily in cities, while its main distribution in Europe is centred around the Alps and almost completely absent from lowland Europe. These two species are not alone in this, as more species are showing this distribution pattern. Climate change might also be a factor pushing the dispersal of these species, providing just enough warmth in cities, for them to colonize these locations that were too cold up till recently.
Although many of these questions cannot be answered in a sufficient manner, it does provide opportunities for exciting urban evolutionary research. Are these ‘alpine’ species showing a novel adaptation making it possible for them to make cities their new home at a quick rate? Or does the urban environment simply provide space for these curious opportunists? I think I will not be the only one that is curious as to what the answers will be.

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References

GBIF.org

Verspreidingsatlas.nl

Wirth V, Hauck M and Schultz M (2013) Die Flechten Deutschlands. Ulmer Verlag Stuttgart.

Featured image: © mjpapay, some rights reserved (CC-BY)