Rodents in urban environments: ecology, prevention and control
Technical note on rodents in urban environments: ecology, prevention and control, with a focus on diagnosis, prevention and criteria applicable to professional pest management.
Urban environments are artificial ecosystems created by man in which only some invasive species turn out to be successful and, free from their natural predators, can become pest species. Synanthropic rodents are one of the best examples and, in fact, they are historically one of the main enemies of man, given that they cause significant economic losses due to damage and contamination of food and are epidemiologically responsible for numerous infectious diseases.
However, the increase in urbanization and poverty in some urban centers points to the existence of a new potential enemy: wild rodents. The few species of wild rodents that manage to survive in a highly transformed ecosystem such as a city, occupy relicts of riverside vegetation and/or green spaces in the urban fabric, such as large-scale parks and urban reserves.
These environments, although they were traditionally used by man for recreational purposes, in recent years, in the face of changes in the socioeconomic pattern, they have grown in occupation with the settlement of precarious housing and deprived neighborhoods. These conditions favor greater interaction between zoonotic pathogens that can be transmitted horizontally between populations of wild rodents, synanthropic rodents, man and their domestic animals, favoring the emergence or re-emergence of infectious diseases.
Rodenticides were and continue to be, due to their high effectiveness, the most used method in the world to control pest rodents. However, the increase in genetic resistance and greater concern for animal health have led to less and less acceptance of the use of reactive methods as the only control strategies. In fact, the exclusive use of toxins is currently justified only in cases where the infestation problem is clearly established.
However, this is not always the situation faced by both pest control professionals and researchers on these topics. It is indisputable that cities are very complex environments with a high rate of transformation that favors the invasion of opportunistic species.
The point is to consider when the environment offers the necessary resources of food and shelter so that a certain invasive species can colonize it, survive, reproduce successfully and become a pest and, consequently, disperse to other environments. In this context, it is plausible to understand why in order to act effectively in the face of a rodent infestation problem, it is necessary to know the characteristics of the species in relation to its biology and behavior, the characteristics of the environment and, fundamentally, the effect of said interaction, that is, the use that the species makes of that environment, since it defines the dynamics of its population: birth rate, mortality and dispersion.
The approach to managing pest rodents based on ecological knowledge of the species was first proposed by Hansson & Nilsson almost four decades ago. Despite the time that has passed and the very good examples in the rodent literature that show the important contribution of knowledge that such studies provide to the development of effective strategies, rodent control has traditionally remained oriented toward achieving a visible increase in mortality, without adequate attention to other demographic processes.
In fact, in environments with high availability of resources, food and shelter, the success of the use of rodenticides is very limited due to the high recovery capacity of pest rodent populations in a short time. Worse still, they can be ignored since the food offer can compete with the bait in its attractiveness and not be consumed, masking the real level of infestation that the environment presents in addition to putting non-target species at risk.
On the other hand, as previously mentioned, not only synanthropic rodent species are found in urban environments. The presence of wild rodents currently generates, at least for the City of Buenos Aires, a new challenge for population control. In this case, the main problem is that wild species, as they have different eating habits than invasive species, are not attracted to the toxic baits that are currently on the market.
Wild species are generally herbivorous, insectivorous and/or granivorous, being much more specialists in choosing their food items than invasive species, which in their dependence on man to survive present a more opportunistic behavior and a much broader diet, that is, omnivorous.
In this context, our research carried out in the City of Buenos Aires for almost 20 years aims to provide knowledge about the ecology of the species present in different environments, their requirements and life strategies, so that they can be used for making appropriate decisions either for the control of pest species or for the conservation of native ones.
Thus we have been able to observe that the presence and numerical dominance of each species of rodent present in different landscape units of our city, such as residential areas, slums and green spaces, is closely associated with the environmental characteristics of each landscape unit. And the most important thing is that this marked association with the environment is also reflected both in the pathogens and in the prevalence and abundance of intestinal parasites, many of them of zoonotic importance, recorded in the different species of rodents.
In our city the richness of rodent species increases as the degree of urbanization decreases. In its residential and industrial neighborhoods, Rattus rattus It behaves as a dominant species due to its high capacity to successfully exploit the aerial strata of the urban fabric, roofs, trees and power lines. These environments, compared to other less urbanized ones, are characterized by having a lower abundance and diversity of rodents and, likewise, the populations of R. rattus present, for having the lowest levels of infection of parasites of zoonotic importance compared to other species present in other landscape units.
Mus musculus and Rattus norvegicus, on the other hand, can coexist in sympatry both in the green spaces and in the emergency slums of the Autonomous City of Buenos Aires. However, in this last landscape unit M. musculus preferably selects the interior of the homes, mainly bedrooms and kitchens. These differences in the use of the microhabitat by Mus They are reflected in the level of infection since many parasites depend on certain environmental conditions to complete their life cycles.
Less contact with areas contaminated with larvae of certain parasites, such as permeable surfaces and vegetation, would explain their low prevalence in slums. It is likely that the pressure of competition and predation by R. norvegicus about M. musculus and the presence of domestic animals force the lauchas to seek refuge inside homes.
Rattus norvegicus, on the other hand, is associated both in villas and in green spaces with the presence of vegetated or bare soil, where it builds its underground burrows, and with the presence of water. So, R. norvegicus It is found preferentially occupying the peri-domicile in the villas and near river courses when these are present in green spaces. In this species, unlike R. rattus, the presence of antibodies to hantavirus and a greater prevalence and diversity of intestinal parasites have been detected, possibly reflecting their requirements in habitat use.
Finally, green spaces with large surfaces, such as large parks and urban reserves, are the environments with the greatest richness of rodents since, in addition to the synanthropic species mentioned, R. norvegicus and M. musculus, the presence of wild rodents such as Oligoryzomys flavescens, Deltamys kempi, Calomys musculinus and Cavia aperea, several of them involved in the transmission of pathogens to humans. Although reserves provide refuge for native species, this can generate a conflict between the need for control and the conservation of said species.
The results presented previously attempt to show in a very synthetic way how differences in the structure of the landscape contribute to explaining differences in the composition of the rodent community and the health importance that each species plays in its interaction with humans.
In summary, just as it is indisputable that the control and prevention of synanthropic rodents with chemical products in urban environments is the most effective and rapid method, there is also no doubt that a solid understanding of the specific behaviors of the species and their biology turns out to be the core of any rodent control strategy. Based on knowledge of the requirements of each species, for example habitat use of rodent species and population dynamics, the carrying capacity of the environment, that is, the availability of shelters and food, can be successfully managed and the epidemiological, economic and social impact reduced in a profitable and sustained manner without negatively affecting the environment.