Ecology: too many rats in the city?

Musophobes are horrified by them, while others adopt them as pets. Some people spend their lives studying them, others eliminating them. Rats have always coexisted with humans. For a long time, this rodent, especially the black rat (rattus rattus), was considered one of the worst banes of humanity, when the plague killed millions of people in a few years. It colonized granaries, where it found lodging and cover. Today, the black rat is mostly found in the countryside. Its cousin, rattus norvegicus, also called the brown rat or the Norway rat, has replaced it in big cities. This species’ omnivorous diet with a carnivorous tendency is perfectly suited to city trash. And these rats seem to be proliferating today in Paris, New York, London, Singapore and other cities. They are everywhere: in the streets where food waste is found, in sewers, in the underground rail system, in public gardens, and sometimes even at the bottom of our toilets! Are there really more of them than before or are they simply more visible? Are there any risks in coexisting with these rodents? What methods are there to get rid of it? Are rats useful in some way? An update on these figures of darkness…

Increasingly visible rats

Certain changes in cities have driven rats to the surface to mingle with humans.

No doubt about it: city dwellers are seeing more and more rats in their cities. But are they really proliferating? The question is more complex than it may seem. It is impossible to count all the rats to be found in the sewers, basements, cellars, gardens, etc. Instead, the factors that determine the size of a rat population are examined – specifically, the availability of food and places to build a nest. When one of these indicators increases, there’s reason to fear a proliferation. ‘Ten years ago, following a sanitation strike in Naples, we clearly witnessed a proliferation of rats,’ comments Pierre Falgayrac, one of the few experts in the reasonable control of rat populations. In Paris, as in most other cities, there has been no significant lasting increase in available food for rats. On the other hand, food scraps left in parks or on the terrace of cafés attract rats to the surface at night. As does the trash in the frame-held plastic bags that hover a few inches above the ground. Put in place as part of the Vigipirate anti-terror measures in France, these transparent trash receptacles are a boon for rats, whose sense of smell is 1,000 times more sensitive than ours, and 10 times more discriminating than a dog’s or a cat’s. In addition, extensive excavation and demolition work drives rats out to look for other burrows. Similarly, more frequent flooding of the Seine has forced many rodents to move. In the end, there are not necessarily more rats; they may simply be more visible.

Are rats necessarily harmful or could they be useful?

Some see rats as pests to be eliminated. Others think they are useful and that we must learn to live with them.

For a long time, the Norway rat was considered one of the greatest banes on the planet. Together with the black rat, it is known to be responsible for 5% and 10% of agricultural losses in the world, by eating seeds, crops and stocks, but more especially by contaminating food via its urine and its excrement. Yet, it is clear that in urban areas today the main impact of the brown rat is related to the fear it triggers, a fear that could adversely affect tourism and therefore the economy. ‘Visual discomfort’ is therefore the first argument cited by the City of Paris to justify its deratting campaigns, as animal rights groups, struggling to counter the negative reputation of rats based on persistent prejudices, have not failed to point out. In the fall of 2018, Paris Animaux Zoopolis launched a campaign with posters in the Paris metro stations declaring: ‘Rats are not our enemies!’ In fact, urban rats also offer benefits. They consume the equivalent of 10% of their weight per day. Relative to the estimated rat population in Paris (about 3 million), this would represent some 100 tons of waste per day. None of this trash needs to be handled by street sweepers, trash collectors or sewer workers. According to the specialist Pierre Falgayrac, ‘by circulating through the gutter gates and digging their burrows in the silt that accumulates under these gates, rats also counteract sewer clogging.’ But rats are not indispensable for this purpose: in Monaco, for example, there are almost no rats and the sewers, recently built with concrete pipes, work very well without them.

A boom in ‘generalist’ species

The more habitats are disturbed by humans, the more some species, such as the rat, the pigeon or the mouse, multiply.

At the expense of other species… As humans travelled and conquered new territories, they brought with them domestic animals (chickens, pigs, cows, etc.) but also a great many stowaways, such as ants, mice or rats. As a result, these species are found all over the planet, and they have gradually replaced native species. This phenomenon is called homogenization of biodiversity and it is even more striking in cities. Rats, pigeons, mice, sparrows, dandelions, ferns: the same species are found everywhere, whether you are in Europe, America or Asia. These so-called ‘generalist’ species tolerate noise, pollution and the presence of humans, unlike other more sensitive species whose populations are decreasing (skylarks, common cuckoos, etc.). A study published in December 2018 confirmed this finding again: generalist species do indeed benefit most from urbanization and they end up impoverishing local diversity. This is true in the city but also in all areas exploited by human beings. Unintentionally introduced into more than 80% of the world’s islands via human activities, the Norway rat, for example, is responsible for the extinction of many island species, especially among birds that lay their eggs on the ground. Deratting operations in some islands – for example in the islands of Molène and Ouessant – have led to a significant increase in plant and animal species.

Sources: M. Pascal et al., Invasions biologiques et extinctions (Éditions Belin, 2006) ; T. Newbold et al., PLoSBiol (Dec. 2018).

Can rat populations be controlled?

Stopping the proliferation of rats requires limiting access to food and making the construction of burrows impossible.

“We can live in harmony with rats, provided the city has a balanced biotope,” explains the expert Pierre Falgayrac, “which is not the case today in a number of cities and notably not in Paris”. In 2017 the City of Paris launched a campaign with a budget of 1.5 million euros to fight rat proliferation. While chemical products are the most widely used method of pest control, there are other ways to decrease rat populations in inner cities. First, reducing access to food is an effective means of control. In Paris, the frame-held plastic bags currently in use (as part of the Vigipirate antiterror plan) could be replaced by closed containers and trash collection could be rescheduled in the evening in the places most frequented by the rodents, given that rats are more active at night. Feeding other animals, especially birds, should be avoided since rats take advantage of this and eat the seeds and bread. In addition, the rats must be preventing from digging galleries or building nests, for example, by using concrete for sewers, tiling subway stations or plugging all openings with steel wool (particularly pipe penetrations). Finally, it is advisable to eliminate rats at work sites before construction, to avoid having entire colonies migrate in search of a new nest. In a different vein entirely, some crews, especially in New York, have not abandoned the use of ratter breeds, such as terriers, bulldogs, or Prague Ratters, used since the seventeenth century to catch rodents. It’s pointless, on the other hand, to use cats for the same purpose: a recent study has confirmed that they will not attack rats!

Source : M. Parsons et al., Front. Ecol. Evol., sept. 2018

What are the health risks today?

People primarily think of rats as animals that carry diseases, like the plague. But the reality today is less alarming.

Between 1348 and 1352, the ‘Black Death’ in Europe killed between 25 and 40 million people. In the late 19th century, this disease was discovered to be caused by a bacterium transmitted by rat fleas*. In the Middle Ages, the brown rat had not yet spread to Europe: it was its cousin, the black rat, that played the role of the ‘Grim Reaper’. But today, the brown rat is also a host of the Xenopsylla cheopsis, the incriminated flea. It can potentially transmit the disease, which continues to kill people: according to WHO statistics, there were 3,248 cases of human plague between 2010 and 2015, 584 of which ended in death. On the other hand, in Europe, the bacterium responsible for the plague, Yersinia pestis, has not been detected for decades. The main health risk related to the presence of rats is something else entirely: leptospirosis, a bacterial disease that is rarely serious and which can be treated with antibiotics. But if it is not diagnosed in time, leptospirosis causes renal, pulmonary and hepatic failure, or haemorrhaging, and leads to death in 5% to 30% of cases. In metropolitan France, the incidence of leptospirosis is today close to 1 case per 100,000 inhabitants. Lastly, the brown rat is also a carrier of viruses – hantavirus, which causes haemorrhagic fevers and cardiovascular problems (extremely rare in France) and the cowpox virus, which causes for skin infections (a few dozen cases a year) – and parasites.

According to another hypothesis, this bacteria is transmitted by human fleas and lice (K. Dean et al., Pnas 115 (6) 1304-1309, 2018). Sources: F. Ayral, Thesis 2015; L. P. Angley et al., Zoonoses Public Health. (Feb. 2018); Centre National de Référence pour la leptospirose, Rapport 2018; Santé Publique France.