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Biodiversity: understanding nature to preserve it better

The animals with whom we share our cities

Benoit Pisanu, Ecology researcher at MNHN
On January 30th, 2024 |
4 min reading time
Benoit Pisanu
Ecology researcher at MNHN
Key takeaways
  • In the city, a wide variety of small mammals live in green spaces, including hedgehogs, field mice and shrews.
  • Knowing the density and distribution of these species will enable us to carry out behavioural ecology studies into the adaptations associated with living in an urban environment.
  • Invasive species, brought in and then released into the wild by man, can survive and harm other species.
  • Despite regulation plans, humans maintain these invasive populations by feeding them, to the detriment of other species that suffer from this cohabitation.

Paris has a diversity of small mammals that we didn’t know existed? 

Yes, we start­ed set­ting traps in sev­er­al Paris parks four years ago, because we had very lit­tle data on this bio­di­ver­si­ty in an urban envi­ron­ment, par­tic­u­lar­ly in the cap­i­tal. With doc­tor­al stu­dent Pierre Sachot, we dis­cov­ered that many pop­u­la­tions of these small mam­mals (hedge­hogs, field mice, etc.) lived in green spaces. Species such as field mice, shrews and some­times even voles pre­fer areas that gar­den­ers leave fallow.

We often talk about rats in the city, but are there many other rodents?

Yes, the wood mouse (Apode­mus syl­vati­cus) was known to live in the sub­urbs, in parks and gar­dens, but we were sur­prised to find it right in the heart of Paris. Less abun­dant in the cen­tre of the cap­i­tal, the farm vole (Micro­tus agrestis) was only observed in uncul­ti­vat­ed embank­ments – its pre­ferred habi­tat. There are also house mice, which are per­fect­ly adapt­ed to the under­ground, and of course rats.

What’s the point of drawing up this inventory of animal life in the city?

This inven­to­ry is a first step, which will lead to fur­ther research. Once we under­stand the pop­u­la­tion den­si­ties liv­ing in par­tic­u­lar places, we can, for exam­ple, car­ry out behav­iour­al ecol­o­gy stud­ies to see whether liv­ing in an urban envi­ron­ment leads to behav­iour­al and mor­pho­log­i­cal changes in species. For a long time (and it was Dar­win who ini­ti­at­ed this), we used islands as a field for study­ing evo­lu­tion and selec­tion process­es. We were able to observe that cer­tain plants lost their prick­les, because they had less need to pro­tect them­selves against the pres­sure of brows­ing (tak­ing of buds by ani­mals) than on the con­ti­nents. In the same way, some birds stayed on the ground because they no longer need­ed to take flight due to the lack of preda­tors. Will we see any changes in these ani­mals in the city: changes in their growth, their skele­ton, their phys­i­ol­o­gy? These stud­ies will enable us to exam­ine the effects of light, noise and chem­i­cal pol­lu­tion on these liv­ing crea­tures. For exam­ple, study­ing the poten­tial impact on their ner­vous sys­tems would enable us to deduce impor­tant infor­ma­tion for human beings.

Monitoring the health of these animals?

Some species can car­ry bac­te­ria or virus­es. It is there­fore impor­tant to know how they are dis­trib­uted across the coun­try. In this way, they pro­mote “nat­ur­al” bio­di­ver­si­ty in the city. Hedge­hogs are well known to gar­den­ers as aux­il­iaries, because they eat slugs and snails. Their pres­ence lim­its the use of cer­tain phy­tosan­i­tary prod­ucts, which are dan­ger­ous for humans and for bio­di­ver­si­ty in gen­er­al. Like hedge­hogs, shrews are insec­ti­vores – with a role that is often not very vis­i­ble, but is cer­tain­ly use­ful and has yet to be discovered…

Is the issue of regulating these populations particularly relevant in urban environments?

In the wild or in rur­al areas, all these small mam­mals have nat­ur­al preda­tors, in par­tic­u­lar cats and crows. Crows prey on rats, while rats prey on mice, insects and some of the waste pro­duced by humans… Nor­mal­ly, the dif­fer­ent pop­u­la­tions reg­u­late them­selves, but humans often dis­rupt these rela­tion­ships. The tawny owl is one of the rare noc­tur­nal birds of prey that can be observed in the heart of cities. In Mar­seilles, for exam­ple, the pro­tec­tion of the city’s nat­ur­al habi­tats helps to main­tain these preda­tors, which prey on rats. Anoth­er exam­ple: bats are such effec­tive nat­ur­al insec­ti­cides that we’re think­ing of cre­at­ing black cor­ri­dors, i.e. light-free urban routes that would allow them to move around the city to lim­it the nui­sance caused by mos­qui­toes and oth­er insects. As a final exam­ple, in the 12th arrondisse­ment of Paris there is a large pop­u­la­tion of wall lizards, which are great con­sumers of ants…

Are invasive species found in towns and cities?

Yes, cities are a breed­ing ground for these species, because there is a larg­er human pop­u­la­tion like­ly to release ani­mals bought on the oth­er side of the world! It’s hard to imag­ine what peo­ple can bring back from their trav­els or buy on the inter­net: snakes, birds, small mam­mals, insects… It’s not always easy for these ani­mals to adapt to the urban envi­ron­ment. Par­tic­u­lar­ly in Paris, which is a very sparse city com­pared with oth­er major Euro­pean cities such as Lon­don and Berlin, which have vast green spaces.  But the most resilient and adapt­able can actu­al­ly feel quite at home here [Editor’s note: the term « inva­sive species » refers to « liv­ing species intro­duced out­side their nat­ur­al habi­tat, whose pro­lif­er­a­tion caus­es dam­age to the envi­ron­ment in which they settle »]! 

Why are these species problematic?

Let’s take the exam­ple of the Pal­las squir­rel, which has been well stud­ied since it was intro­duced to Cap d’Antibes. This species, which orig­i­nat­ed in south-east Asia and was prob­a­bly brought back by a pri­vate indi­vid­ual from Tai­wan at the end of the 1960s, took around twen­ty years to cross the town of Antibes, and then anoth­er twen­ty years to cross the A8 motor­way that runs along the Côte d’Azur. This pop­u­la­tion is now invad­ing the region. These squir­rels attack the fruit of trees, gnaw­ing off their bark in 40 cm strips. They can destroy entire orchards. They also chew through tele­phone wires and irri­ga­tion sys­tems. This species is also a threat to the red squir­rel, the only tree squir­rel in West­ern Europe. The lat­ter is very sen­si­tive to the pres­ence of com­peti­tors. Since the species was clas­si­fied as “inva­sive with a high lev­el of con­cern” in 2014, it has been banned from sale. Despite this, peo­ple are often tak­en with this ani­mal and tend to feed it!

What can be done about it?

The most impor­tant thing is to explain to the pub­lic, using sci­en­tif­ic infor­ma­tion on these sub­jects, why this species of squir­rel is par­tic­u­lar­ly like­ly to cause dam­age, espe­cial­ly in rela­tion to the red squir­rel. In the south, local res­i­dents are recep­tive, as they have already suf­fered from oth­er inva­sive species such as algae on the coast and but­ter­flies on palm trees (par­tic­u­lar­ly those on the Croisette in Cannes). A plan to con­trol the squir­rel has been put in place, with trap­ping and shoot­ing oper­a­tions. In the Alpes-Mar­itimes, the species occu­pies too large an area (35 km21) for erad­i­ca­tion to be pos­si­ble; the only option is to con­trol the pop­u­la­tion. In Bouch­es-du-Rhône, the range is still lim­it­ed (a few hun­dred hectares), and erad­i­ca­tion would appear to be a real­is­tic objec­tive – but it is a mat­ter of urgency.

What effects might climate change have on these species in an urban environment?

This impact is dif­fi­cult to esti­mate. On the one hand, cities are heat islands where it can be dif­fi­cult to sur­vive. How­ev­er, in a con­text of drought, they are also the only places where these ani­mals will always find water. Para­dox­i­cal­ly, cities could become a reser­voir of bio­di­ver­si­ty for these species.

Interview by Marina Julienne

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