| 21 December 2020

Impact and management of small mammals

The majority of small mammals that have a significant impact on human activities or ecosystems belongs to Rodents, except for moles that belong to Soricomophs. In many contexts rodents represent, among Vertebrates, the most important species to manage. Their importance ranges from agricultural, zootechnical and silvicultural activities, to the urban and industrial areas, up to natural ecosystems, where they can cause significant impacts to native conenoses. However, despite the fact that they number over 2.000 species, making them the largest order of mammals, only a very small fraction of them can cause significant impacts.


Agriculture, animal husbandry, forestry

Over the millennia man has always fought against the damage caused by rodents to crops, sometimes so great as to compromise the entire harvest. Until the 19th century, Europe suffered periodic famines due to demographic explosions of rodents, especially voles, which compromised potato and cereal harvests. Today, in Western countries rodents are no longer able to have a significant impact on harvests, but they can still cause economic losses that are sometimes very significant for crops in the open field, in greenhouses and orchards. In less economically developed countries, these species can still have a significant impact on agricultural production, with consequences for the food supply of rural populations. Rodents are still of considerable importance in animal husbandry, particularly in the intensive sector, as they are carriers of more or less serious infections transmissible to domestic animals. Finally, rodents can sometimes have a considerable impact also in silvicultural activities, plundering forest sowings or damaging plantations created for the artificial renewal of the forest.


Impact on biodiversity

Small mammals have a cosmopolitan distribution and are present in all terrestrial ecosystems, from open areas to forests, from inland waters to high mountains, occurring with several species, often little known even to a specialised audience. However, the presence of species outside their original range can cause significant impacts on biodiversity, altering the balance in natural environments and threatening native species. In Italy there are 31 species of rodents, 8 of which are allochthonous: the House Mouse and the Black Rat, resulting from paleointroductions; the Grey Rat, the Muskrat, the Coypu, Finlayson’s Squirrel, the Siberian Chipmunk and the Grey Squirrel, coming from more recent introductions.  Among these, the House Mouse, Black Rat, Coypu and Grey Squirrel are included by the International Union for Conservation of Nature in the list of the 100 most invasive alien species, due to the ravaging impact they can have on native conenoses.


Impact on human health

Rodents throughout history have been among the most harmful vertebrates for human health, frequently acting as reservoirs or as secondary hosts of a number of zoonoses (diseases transmissible from vertebrates to humans).
A remarkable example of rodent transmitted disease causing the most significant epidemics worldwide between the 14th and 15th centuries is the plague or “black death”, transmitted to humans by flea bites infected with the Yersinia pestis bacterium coming from black rat individuals. Other examples of bacterial and viral zoonoses still widespread today are leptospirosis, salmonellosis, tick-borne viral encephalitis (TBE) and Lyme Borreliosis. The latter two are among the most relevant European tick-borne diseases, maintained by reservoirs often represented by rodents, such as Apodemus wood mice, the bank vole, the Norway rat, the edible dormouse and the garden dormouse.
Transmission of these infections to humans can occur by inoculation, in the case of ticks and fleas, ingestion, inhalation, direct or indirect (i.e. food and drink contamination) contact with faeces and urine.
Preventive diagnosis and an adequate control of zoonoses are still difficult to implement, especially in developing countries, where these diseases are more widespread due to poor hygiene conditions. Today, even for the most developed countries, zoonoses represent a major public health problem with significant economic consequences due to the aforementioned impacts on animal husbandry. Great concern is caused by the emergence of certain zoonoses (Hantavirus, Borreliosis) whose main reservoir is represented by rodents and which are influenced by factors such as changing climatic conditions and the transformation of natural environments (urbanisation, etc.), factors that can additionally create favourable conditions for the reproduction of the animal vectors.


Problems related to pest control

All these negative implications arising from the presence of rodents often entail a need to reduce damage using direct or indirect means of control. Pest control activities are most frequently carried out with the use of rodenticides. The lack of selectivity of these compounds raises the problem of the impact on non-target species, both those that accidentally feed on rodenticides (primary poisoning) and predators which, by feeding on rodents that have taken the active ingredient, may in their turn get poisoned (secondary poisoning). Another problem closely linked to the use of anticoagulants is that of the resistance, i.e. a decrease in the effectiveness of one or more active ingredients against a pest population. Recent studies allowed a deeper understanding of the genetic basis of resistance to anticoagulants, thus opening up the possibility of screening rodent populations prior to control intervention involving anticoagulants. Preliminary screening could therefore ensure a more focused treatment (optimal dosage, type of anticoagulant), assuring a greater effectiveness of the intervention and a lower environmental impact.
Alternative techniques include the use of traps and repellents, the latter generally having little effect in keeping rodents away. The use of contraceptives, although tried and tested on several occasions, has not offered satisfactory results to date.
Legislative issues occur in rodent control, as it is not always possible to control species that have a major impact, especially in the case of Coypu and Grey Squirrel that are protected by Italian laws. The control of synanthropic rodents not protected by law is also subject to restrictions concerning the use of rodenticide products. Finally, we must as well consider the high financial costs sustained to limit rodent damage, both in urban areas and in agricultural and industrial activities, and in eradication or containment plans aimed at alien species.

Associazione Teriologica Italiana ETR
Dipartimento di Biologia e Biotecnologie "Charles Darwin"
Università di Roma "La Sapienza" - Viale dell'Università, 32
I-00185 Roma (RM)