Commonly, when thinking of mammals, one refers to the great African herbivores or the great carnivores, such as lions, tigers and bears. Actually, only a surprisingly small number of people are aware that about 90% of mammal species have a body weight of less than 5 kg, or that out of the 5,418 species currently known to belong to this group, 2,700 are in the order of Rodents. Thanks to a very large adaptive radiation, this group of mammals has colonized most of the environments of our planet: from deserts to the Arctic tundra, from forest ecosystems to open areas, from the plains to the highest mountains (in Italy the snow vole, Chionomys nivalis, is present at an altitude of more than 4,000 m a.s.l.), up to colonizing even the anthropized environments (e.g. rats, Rattus spp. and house mice, Mus musculus).
The success of this group of mammals is mainly attributable to their ecological and biological characteristics, such as a great reproductive potential (both in terms of number of young per litter and number of litters per year) and a rather short average life span, which generally does not exceed two years. These characteristics allow them to adapt quickly to environmental changes and to rapidly colonize new habitats. In addition, the small size of their body allows them to occupy several microhabitats and to use many trophic resources hardly usable by other vertebrates. On the other hand, having a small body involves a greater expenditure of energy and body heat (due to the high surface/volume ratio), requiring a regular and abundant food intake: the common shrew (Sorex araneus), for instance, needs a daily amount of food equal to 80-90% of its body weight. Small mammals play a crucial role in regulating the processes that support natural ecosystems. They are fundamental “nodes” at the base of the trophic webs as they are a relatively abundant and widespread prey, indispensable for the sustenance of many other vertebrates. There are many predators whose diet is based almost exclusively on this group of mammals. Among these are birds, such as barn owls (Tyto alba) and tawny owls (Strix aluco), other mammals, such as beech martens (Martes foina), martens (Martes martes) and foxes (Vulpes vulpes), reptiles, such as vipers (Vipera spp.), Aesculapian snakes (Zamenis longissimus), European whip snakes (Hierophis viridiflavus) and many others.
Rodents, being primary consumers, feed almost exclusively on plant parts, in particular seeds, fruits, flowers and fungal spores. In addition, some species, such as squirrels (Sciurus vulgaris) and wood mice (Apodemus spp.), are known to move and bury acorns and other fruits, in order to hoard food and to create scattered caches which are essential for their winter survival. Their diet, and especially the latter behaviour, makes these animals important agents in the dispersal of seeds and fungal spores, actively contributing to the renewal and expansion of many habitats. The trophic pressure exerted by rodents on habitats can be so relevant that their presence locally affects the floristic composition and favours an increase in the richness of some plant species.
Insectivores generally have a greater food specialisation than rodents, relying almost exclusively food sources of animal origin. The ecological role of insectivores is therefore that of small predators (secondary consumers) of insects and larvae and whose activity mainly takes place in close contact with the soil, in the leaf litter of woods, down to the underground environment. This large predatory activity can affect the structure of soil invertebrate communities: it generally produces a reduction in dominance with an increase in species diversity and richness. The impact of insectivores on soil invertebrates is however more complex than simple direct predation and includes, among other effects, the continuous contribution of organic matter that this component provides to the trophic chain of decomposers.
With their significant excavation activity, many small mammals also favour the vertical circulation of nutrients in the soil, allowing thus the oxygenation and mineralization of the organic matter in the soil and increasing its water retention capacity. To give an idea of the excavation ability, some studies have shown that a mole (European Mole) can dig up to 6 kg of soil in twenty minutes of excavation. In addition, the catabolites produced by small mammals, as well as the decomposition of the organic material they store and of their own bodies, are crucial for fertilising the soil and providing nutrients down to the lower layers, which are usually particularly poor in nutrients.
Despite the undeniable importance of the ecological role played by small mammals, most research efforts and also the allocation of economic funds have been directed mainly towards larger species, considered more charismatic. The public perception of small mammals is negatively influenced by some species of this group that are considered harmful to human interests (primarily to crop yields) such as the black rat (Rattus rattus), the Norway rat (Rattus norvegicus) and the house mouse (Mus musculus), although these represent less than 5% of the total species. The indiscriminate use of rodent control or suppression methods has often led to serious consequences, with cascading effects throughout the food chain and even to human involvement.
In Italy, rodents and insectivores currently include 48 species (40% of all Italian mammal species), although some of them have been introduced by man, such as the coypu (Myocastor coypus), the grey squirrel (Sciurus carolinensis) or the muskrat (Ondatra zibethicus). The Red List of the IUCN, the International Union for Conservation of Nature, considers most Italian small mammals as Least Concern, the lowest level of threat. This may be true for some cosmopolitan or widely spread species such as the house mouse (Mus musculus), several species of voles (genus Microtus) and wood mouse species (genus Apodemus), but in other cases these assessments are often arbitrary as they are not supported by experimental evidence on the real status of populations and on the main threats. As a consequence, some species are subjected to inadequate EU and/or national laws nowadays.
In most cases, population decline is due to habitat loss and fragmentation, the use of pesticides and the introduction of alien species that prey on or compete with our species. An emblematic example of the latter case is that of the red squirrel, threatened by the introduction by man of the American grey squirrel. A similar situation can be observed for Sorex alpinus, one of the few species among Italian small mammals classified as Near Threatened by the IUCN.