Ecological Roles and Forest Management Implications of Small Terrestrial Mammals in Temperate and Boreal Forests—A Review

Abstract

Small terrestrial mammals (STMs) are vital components of forest ecosystems. They serve as seed dispersers, herbivores, prey, and vectors of pathogens. The STM community structure responds dynamically to forest composition, disturbance, and management regimes. However, despite their central ecological functions and frequent occurrence, STMs remain underestimated. This narrative review aims to comprehensively synthesize existing literature on the reciprocal interactions between STMs, temperate and boreal ecosystems, and forest management. Specifically, we (1) define a group of STMs and their specificities; (2) discuss the influence of forest structure, disturbance, and management on STM populations; and (3) analyze the known direct and indirect effects of STMs on forest ecosystems and forestry.

1. Introduction

Small terrestrial mammals (hereafter referred to as ‘STMs’) play a key role in forest ecosystems. STM populations are often sensitive to changes in habitat structure and forest management, making them valuable bioindicators of forest ecosystem health [1,2]. The interactions between STMs and forests are complex, bidirectional, and still not fully understood [3,4].

Although STMs are distributed across forest biomes worldwide, this review focuses on boreal and temperate forests. These regions have been studied extensively and provide a robust foundation of long-term ecological data. A characteristic feature of temperate landscapes—especially in regions with intensive and long-term human activity, such as Central Europe, Japan, and parts of North America—is their mosaic structure. These landscapes typically comprise a patchwork of arable fields, grasslands, forests, water bodies, and human settlements, with large, continuous tracts of a single habitat type being relatively uncommon [5].

In such regions, coniferous tree species are often overrepresented compared to their natural occurrence. Consequently, many forests geographically located in the temperate zone exhibit structural and ecological features that are more typical of boreal systems [6]. For this reason, both temperate [7] and boreal [8] biomes are included in our synthesis.

This decision reflects both the greater availability of published research in these regions and the authors’ research expertise. The aim of this review is to provide a comprehensive synthesis of current knowledge on the ecological relationships between STMs and forest environments, while also identifying major knowledge gaps and areas of uncertainty. To our knowledge, no comparable review currently exists.

2. Materials and Methods

2.1. Geographical Scope

The focus of interest is on the temperate and boreal forest biomes. The definitions of these biomes vary, but for this review, we follow the definitions of temperate and boreal forests as outlined in the IUCN Global Ecosystem Typology [7,8]. Geographically, these are large areas of North America, Asia (especially China, Japan, Korea, and Russia), and Europe.

2.2. Methodology

This narrative review is based on a targeted, non-systematic search of peer-reviewed literature. Although it does not strictly follow formal guidelines for systematic reviews, the thematic structure of the manuscript is based on established ecological and forestry frameworks [9,10]. Particular attention was given to thematic relevance and comprehensive coverage of key issues emerging from this framework.

Relevant studies were identified using English keywords in major academic databases, including Web of Science, Scopus, and Google Scholar. The search was carried out between [October 2024] and [April 2025].

The search terms included combinations of the following keywords: “small mammals”, “rodents”, “insectivores”, “shrews”, “forest structure”, “forest management”, “forest stage”, “temperate forests”, “boreal forests”, “clear-cutting”, “deadwood”, “disturbance”, and “zoonoses”. Only peer-reviewed journal articles and review papers were included; gray literature and non-English publications were not.

In total, the initial search retrieved over 1200 records. After removing duplicates, screening titles, and abstracts for relevance, 325 articles were retained for full-text review and thematic analysis. While not all of these were ultimately cited in the manuscript, they informed the synthesis and framing of the reviewed topics.

2.3. Limits of the Approach

We acknowledge several fundamental limitations of our review. The most significant include the considerable variability in forest types, management practices, and the species composition of STM communities across regions. Furthermore, the availability of relevant data varies greatly among geographical areas. Research on small mammals is often not a primary focus and typically depends on the work of a limited number of dedicated researchers. As a result, the distribution of published studies is uneven.

Relatively well-studied regions include the border zone between the USA and Canada, Scandinavia, Lithuania, Central Europe, Japan, and, more recently, China. In contrast, we found only a limited number of relevant studies from Russia. While some topics—such as population ecology in Scandinavia—have received substantial attention, many other aspects remain understudied or entirely unexplored.

This heterogeneity, combined with the ecological diversity and vast geographic scope of the review, carries a substantial risk that patterns observed in well-studied regions may not be representative elsewhere or across all time periods. In other words, key processes may differ significantly in less-documented areas.

Additionally, relevant sources not indexed in the databases used, or published in languages other than English—such as Russian, Japanese, Korean, or Polish—may have been overlooked.

Despite these limitations, we believe that this review offers a coherent and informative synthesis of the current state of knowledge. Moreover, by highlighting underexplored thematic, taxonomic, and geographic areas, it may help guide future research where it is most needed.

3. What Are Small Terrestrial Mammals?

Answering this question is not straightforward. While mammals (Mammalia) are taxonomically well-defined, the terms “small mammals” and “small terrestrial mammals” lack a universally accepted definition. Most authors use these terms intuitively, without precisely specifying which species they include. The size of mammals spans a continuum, with no clear boundary. Where “small size” is defined, it is usually based on the typical adult body mass; some references set the upper limit at 3 kg [11], while others even use thresholds of 5 kg [12,13]. In both cases, such a group would encompass up to 75% of the approximately 6500 known mammalian species [14]. This would primarily include species from the orders Rodentia, Afrosoricida, Eulipotyphla, Lagomorpha, and Didelphimorphia. Whether to include the smallest members of Carnivora and Primates is subject to debate [13]. For this text, we define “small mammals” as those with an adult body mass of approximately 200 g. This choice is based on two practical considerations. First, animals up to this weight are typically captured using standard live traps such as Sherman or Longworth traps. Second, by applying the additional criterion of “terrestriality”—i.e., species that spend most of their lives on or just beneath the ground surface—we obtain a group that is relatively coherent from ecological, morphological, and taxonomic perspectives. In temperate forests of the Northern Hemisphere, these criteria are typically met by certain rodents (e.g., genera Apodemus, Clethrionomys/Myodes, Microtus, Peromyscus, and Tamias) and insectivores (e.g., genera Blarina, Crocidura, Neomys, Sorex, and Talpa).

4. What Makes Small Terrestrial Mammals Special?

STMs are, first and foremost, abundant in both species diversity and individual numbers. They are relatively similar in morphology (e.g., small body size, mouse-like body shape, elongated head, clawed limbs, long tail), physiology (e.g., reliance on similar senses such as smell, hearing, and touch; plantigrade locomotion; omnivorous diet [15,16]), and life history traits (e.g., high adaptability, high metabolic rate, short generation time, rapid reproduction, largely secretive lifestyle on or near the ground surface).

4.1. Small Size

A small size appears to be an evolutionary advantage [17], as most animal species are small [14]. Body size is an inheritable trait, and evolution tends to conserve or slightly reduce it [18]. Smaller species often have shorter generation times, allowing for faster adaptation and speciation [19]. They also require fewer resources per individual, permitting a greater number of individuals to survive in each area [20], which lowers the probability of extinction and increases the chance of finding mates [17]. If only these principles operated, they would lead to an unending trend toward smaller body sizes. However, optimal body size represents a trade-off, as advantages in one trait often come with disadvantages in others. Smaller species have relatively higher energy demands per unit of body mass and reduced mobility [21]. Furthermore, optimal body size varies with metabolic type (e.g., endotherms vs. ectotherms) and environmental conditions, which fluctuate across space and time [22]. STMs typically exhibit a very high metabolic rate, resulting in intense energy requirements per unit of time and mass [23]. Therefore, they must feed frequently, usually every few hours [12], and often consume food amounts equivalent to a large fraction of their own body weight per day [13]. To meet these energetic demands, most STMs do not enter hibernation or aestivation, unlike species such as dormice or hedgehogs [14].

4.2. Adaptability

Their evolutionary adaptability stems from their generalist nature, enabling them to exploit a wide range of resources, especially food sources [24]. Many rodent and insectivore species are physiologically capable of digesting both animal and plant matter [13,25]. Some species show remarkable adaptability to various habitats, including economically significant generalist species [12]. Another aspect of their evolutionary plasticity is the genetic diversity of certain groups, particularly the relatively young family Muridae, which represents over half of all rodent species and continues to exhibit increasing genetic variability [26].

4.3. Reproduction

High reproductive capacity is characterized by early maturity, short gestation periods, and frequent, large litters [12]. Under favorable conditions, these traits allow populations to grow rapidly, sometimes exponentially [27]. Typically, these dynamics are accompanied by short individual lifespans and marked fluctuations in population size over time, known as population cycles [28].

5. Why Study Small Terrestrial Mammals?

STMs are globally widespread and abundant [14]. This is reflected in their species diversity, population density [29,30], biomass, and biomass turnover [31,32]. Consequently, they play a key role in nutrient recycling (including nitrogen [33] and carbon cycles [34]) and energy flow, and significantly influence ecological processes and functions in forest ecosystems, including those under forest management [35,36]. They contribute to stabilizing food webs and trophic chains [35] and to regulating insect populations [37]. They serve as an indispensable food source for reptilian, avian, and mammalian predators [38]; aerate the soil [39]; increase water retention [40]; facilitate plant growth [41]; and aid in the dispersal of seeds and fungal spores [42,43]. Additionally, STMs are valuable model organisms and bioindicators [44,45,46].

5.1. Models

The short generation time of STMs allows researchers to track population changes over relatively short periods [47]. Their small body size simplifies handling, requires less food and space per individual, and facilitates both captive rearing and experimental study [13]. High population densities provide abundant data for robust statistical analysis [48]. These traits make STMs important subjects for biological and ecological research, as well as effective models for understanding processes relevant to larger mammals (including humans), such as genetics, physiology, and behavior [49].

5.2. Bioindicators

Because of these characteristics, STMs also serve as effective bioindicators. For example, they have limited mobility, are closely tied to their immediate environment, and have short lifespans with high reproductive rates [12,27]. Thus, their population trends can reflect current environmental changes quickly and accurately [50,51]. With the increasing intensity of anthropogenic influences and climate change, STMs are becoming a crucial group for assessing the status and trends of terrestrial ecosystems, including forests [1,3].

Observed changes include not only shifts in abundance, dominance, population dynamics, and species distributions [52], but also alterations in behavior and in the character and economic significance of damage caused by STMs in forestry [53,54], agriculture [55], and beyond [56]. Furthermore, the accumulation of hazardous substances in their tissues can be used to monitor environmental contamination [46,57,58]. Finally, STMs are significant reservoirs and vectors for a wide range of zoonotic diseases and pathogens transmissible to other animals, including humans [12,59,60].

6. Effects of Forests and Forestry on Small Terrestrial Mammals

STMs can be categorized into the following groups, based on their relationship with forests:

• Forest-dependent species (e.g., Apodemus flavicollis, A. argenteus, Clethrionomys/Myodes spp.).
• Forest-tolerant species that can persist in forested habitats during at least some stages (e.g., Apodemus sylvaticus, A. agrarius, Arvicola spp., Microtus pinetorum, M. oeconomus, Peromyscus spp., Sorex spp.).
• Forest-avoiding species, often synanthropic rodents or species of steppe origin (e.g., Apodemus microps, Microtus arvalis, M. pennsylvanicus, Mus musculus, Rattus norvegicus, Rattus rattus).

To survive in the long term, organisms require continuous access to basic resources [12]. For small mammals, forests must offer sufficient food and shelter for survival and reproduction [13]. Forests are generally stable environments [27], but their stability is periodically disrupted by disturbances [61,62]. These can be natural (windstorms, insect outbreaks, diseases, droughts, fires) or anthropogenic (thinning and logging), and they can lead to changes in ecosystem functions [63].

The resulting forest stages differ significantly in their suitability for STM, influenced by factors such as forest area [64], species composition [65], age structure [66], spatial structure of trees [67], undergrowth vegetation [68], woody debris quantity and type [69], soil characteristics [70], and landscape grain and connectivity [71,72].

6.1. Forest Area

Forest area determines whether it can support a viable STM population. In mammals, viable populations avoiding inbreeding depression are often estimated at 50–500 individuals [73], with higher numbers (around 5000 individuals) required to preserve evolutionary potential and stable genetic variability [74,75]. STMs have relatively low space requirements, typically 100–1500 m² per individual [76,77], depending on habitat productivity, territoriality, and dietary needs [78]. Consequently, forest areas of 1.5–25 ha are estimated to sustain short-term populations, while 15–250 ha would be needed to preserve evolutionary potential. The lower values apply to folivorous species (Microtus) and the higher values to granivorous-omnivorous species (Apodemus, Peromyscus). The spatial requirements of folivorous-omnivorous (e.g., Clethrionomys/Myodes) and carnivorous species (e.g., Sorex, Blarina) lie between the two previous groups. Smaller landscape grain should suit generalists (e.g., Apodemus sylvaticus, Peromyscus leucopus, Sorex araneus), and larger forest complexes should suit species specialized for a particular habitat (e.g., Apodemus flavicollis, Clethrionomys rutilus) [79].

6.2. Forest Stand Structure

Tree stand structure determines the character of the undergrowth [80] and, thus, the density of hiding places and food sources for STMs [13,67]. Developmental stages in natural and managed forests are similar [81], but natural forests exhibit slower progression, higher habitat heterogeneity, and greater biomass retention, leading to higher biodiversity [82]. Managed forests, in contrast, have accelerated stages, more homogeneous age structures, and a lower amount of woody debris [83].

6.3. Forest Stages

We adapted the classification according to Zenner et al. [84]; the stages were grouped according to their influence on the changes in STM communities.

6.3.1. Disturbance—Gap

During disturbances—whether natural or anthropogenic—the forest canopy was typically thinned or entirely removed, depending on the intensity and type of disturbance [85]. A fundamental change in light, thermal, and moisture conditions led to the reconstruction of the original understory communities [86] and, subsequently, of STMs [87]. Forest specialists (e.g., Apodemus flavicollis, Clethrionomys glareolus) declined.

6.3.2. Regeneration

Successional dynamics are tied closely to the development of the herbaceous layer [87], influenced by the dominance of monocots or dicots. STM community diversity tends to be relatively low [88,89]. Grass-dominated sites favor folivorous generalists (Microtus spp.) [90], while dicot-rich sites (e.g., Vaccinium spp., Rubus spp.) support omnivores (Clethrionomys spp.), with greater overall diversity and abundance [91,92]. Granivorous generalists (e.g., Apodemus agrarius, Peromyscus maniculatus) appear at both sites a little later [91]. The duration of the herbaceous dominance depends on the speed of canopy closure and typically lasts only a few years [85]. In colder or mountainous areas, it may extend to ten years or more. If tree regeneration fails, a continuous grassland or shrub layer may persist for decades, and with it, the described communities of STMs [93].

6.3.3. Establishment

As tree regeneration progresses, microclimatic conditions and the understory shift [94], supporting the return of forest specialists [87,88]. The diversity and abundance of STMs during the establishment stage are generally the lowest across the entire cycle [62]. This condition can last for several decades [95].

6.3.4. Optimum (Masting)

Once trees begin producing seeds and fruits, small mammal abundance and diversity rise again, favoring granivorous species (e.g., Apodemus spp., Peromyscus spp.) [96,97]. Fruit and seed availability influence reproductive success and overwintering survival [98,99] and, thus, the population dynamics of STMs [100]. Synchronization of mast years among tree species (e.g., Quercus, Fagus, Picea) causes pronounced small mammal population fluctuations [101,102], with peaks typically occurring in the year following a mast event [103,104].

6.3.5. Terminal and Decay

The fruiting stage of the adult stand concludes with its decay. If decay occurs suddenly over a larger area (due to logging, fire, wind, or insect calamity), then we are back to the beginning of the cycle described above. If individual trees die off gradually, a multi-layered stand is formed on the site [105,106]. Such forests provide many microhabitats, and the abundance—especially the diversity—of STMs tends to be high [4,107].

6.3.6. Summary of Forest Stages

Two stages at opposite ends of the forest development are ideal for most STMs, as follows [62]: Firstly, the open stand of young trees with dense undergrowth and left woody debris [4,69,107]; second, multi-layered stands of different ages with undergrowth and a lot of woody debris [108,109]. Thus, the denser and more opaque the vegetation and woody debris on the forest floor, the higher the expected abundance and diversity of STMs [110,111]. Conversely, dense, not-yet-fruiting monocultures without an understory are the least suitable for most small mammals [112,113].

6.4. Silviculture

The above impacts of forest stages on STMs apply whether the causes of change are natural or forest management [114]. However, natural forest development is often modified in specific ways by human activity. It is generally accepted that, in terms of overall biodiversity, primary forests are the most valuable [62], plantations are the poorest [115], and secondary forests lie in between [116]. There are relatively few papers on this topic that consider STMs [67,97,117]. In particular, the differences between primary and secondary forests are much less pronounced than for some other groups of organisms [116,118]. It is likely that other factors play a more important role in STMs than the human impact on the forest itself [62].

6.4.1. Silvicultural Practices

Since wood remains the primary output of most forest management, the main goal is typically to achieve higher, faster, or better-quality timber production. This objective is pursued through various silvicultural practices. STMs are particularly affected by practices aimed at the following:

• Speeding up forest regeneration (e.g., soil preparation, planting, sowing).
• Ensuring successful stand establishment (e.g., control of competing vegetation, application of repellents, or fencing to minimize browsing damage).
• Stand tending during growth (e.g., thinning).
• Harvest operations (e.g., size of harvested areas and the type and amount of retained wood and logging residues).

There is relatively little large-scale soil preparation in forestry. In the floodplain forest, full-scale soil milling had a clearly negative effect on STMs [119]. Virtually nothing is known about the direct impact of sowing, planting, fencing, or the application of repellents against large ungulate browsing on STM.

Mowing reduced STM abundance [120]; however, lower plantation damage could not be demonstrated [121]. Herbicide application caused a temporary reduction in abundance and changes in the dominance of the STM community in the treated areas [122,123]. This effect varied among taxonomic groups, that is, 3 years for carnivores (Sorex spp.); 2 years for folivores (Microtus spp.); and a single year for granivores (Apodemus spp.) [119,120]. Long-term effects of herbicide application have not yet been observed [124,125].

The effects of thinning on STMs have been studied intensively only in North America [126]. Depending on the intensity, the effect of thinning on STMs was either neutral (in the case of light thinning) or positive (in the case of more intensive treatments) [127]. STM populations have increased in abundance, biomass, and often diversity [128,129]. When thinning was repeated after a 10-year interval, the outcomes were similar (either neutral or positive, depending on the thinning intensity) [29,127].

There are different forms of forest harvesting, which differ in the size of the area, the intensity of the intervention, and the timing. The main types of forest harvesting are clear-cutting, seed-tree cutting, shelterwood cutting, and selective cutting. The impact of each form on STMs is expected to be different.

By far, the most attention has been paid to the impact of clear-cutting on STMs; for other harvesting methods, our knowledge is less. The community structure of STMs is always altered after clear-cutting. Typically, diversity and overall abundance will increase, and the representation of generalists and species preferring more open habitats (Microtus spp.) will increase. Forest specialists usually decrease and may even disappear completely [29,114,118]. Coppicing can be understood as a specific form of clear-cutting, where the described development occurs more quickly [130,131,132].

Shelterwood cutting, unlike clear-cutting, does not lead to a complete replacement of the STM community [133,134]. Shelterwood cutting usually results in a slight increase in STM diversity and abundance, but the species composition remains stable [135]. At low intervention intensity (up to 30%), there are changes in dominance; populations of granivorous species (Peromyscus maniculatus) remain roughly the same as before the intervention, but omnivorous species (Clethrionomys gapperi) increase [133]. Stronger interventions (up to 50%) lead to a temporary enrichment of the STM species spectrum with folivorous open-area species, a further increase in omnivore species abundance, and may even lead to a decline of strictly forest granivorous species [133,135]. Seed-tree cutting has a very similar effect on STMs as stronger intervention shelterwood cutting; the described phenomena (decline of forest granivores, emergence of folivores and omnivore generalists) are stronger and longer-lasting [136]. Selective cutting probably has the least impact on STM communities. The application of this method did not change the species spectrum of STMs; only minor changes occurred in the dominance of granivorous species [137].

STMs have relatively low mobility [21]; depending on the species, they migrate under their own power to a maximum of 100-300 m; the lower value applies to folivores, and the higher to granivores [138]. These distances are valid if there are suitable or at least neutral migration corridors between attractive sites. If the route passes through non-preferred habitats, these distances are significantly shorter in the low tens of meters [72,138]. Thus, the size and isolation of cleared areas are important for STMs, not only because of differing habitat conditions but also due to their ability to reach isolated sites and maintain a functional population there [139,140]. Therefore, it can be summarized that the dispersal of STMs between habitats will normally occur if fragments are less than 100–300 m apart and if connectivity through migration corridors is maintained. This information can be used to plan management strategies aimed both at supporting populations of STM species and, conversely, at limiting their spread, e.g., to protect plantations when planning forest harvesting.

6.4.2. Decaying Wood

Decaying wood is a key structural component supporting forest biodiversity [141]. For STM, it has provided shelter from predators and climatic extremes [142]. As decomposition progresses, the wood supports the growth of fungi and the accumulation of invertebrates, thereby indirectly enhancing food availability for STMs [143]. Several studies have shown that increasing the volume of deadwood positively affects STM abundance and diversity [144], with larger and more slowly decomposing logs providing more stable microhabitats [109]. STMs have been observed to use downed woody material more than standing snags [145].

Conversely, other research has found no significant relationship between STM community metrics and either the decay phase or spatial distribution of deadwood at a site [146,147]. While some studies highlight strong positive effects of coarse woody debris, others—especially those focused on generalist species such as Apodemus flavicollis or Clethrionomys glareolus—report weak or inconsistent responses. These discrepancies can be explained by multiple factors, such as species-specific responses, with shrews and habitat specialists benefiting more than habitat generalists; forest structure and disturbance history, which influence deadwood availability and its ecological role; time lag effects, where short-term studies may fail to detect delayed demographic or behavioral responses; and variability in wood type and decay stage, which affect its utility for nesting, shelter, and prey availability.

Given these interacting variables, the ecological role of deadwood for STMs cannot be universally generalized. Instead, its importance should be evaluated in relation to local forest conditions, species composition, and landscape context. This perspective supports the view that coarse woody debris is one of several interacting habitat features that shape STM communities, rather than a singular driver of diversity.

6.4.3. Effects on Diversity

The highest diversity and equitability of STMs appear to be in forests managed with small-scale clear-cuts (up to 1 ha maximum) with a wider range of interspersed fruiting tree species [148,149]. Small clearings of different ages are crucial in terms of diversity. They significantly increase the availability of microhabitats and, thus, the existence of STM species that would not appear at all or in much lower numbers in a closed canopy forest [150,151]. The same is probably true for coppicing [130]. The succession on coppiced sites tends to be much faster; the connectivity of similar elements is more important from the point of view of STMs [131,132]. Usually, arboreal small mammals find very suitable conditions here [130]. Similarly, rich communities of STMs are found on sites with a mosaic of different forest stages from decayed to overgrown to mature closed canopy stands (typically reserves in Central Europe) [66,152]. However, the dominance of individual species can vary considerably [128]. Lower diversity of STMs with a predominance of a few generalist species occurs in monocultures and large-scale managed stands [119,153]. The diversity of STMs is also lower where shelterwood or selective systems prevail [129]; species requiring more open habitats do not find sufficiently suitable conditions for maintaining sufficiently large populations in the long term.

6.4.4. Effects on Abundance

Apart from species diversity, the abundance of STMs and their year-to-year fluctuations are also important. More abundant populations fluctuate more [28,154]. Major fluctuations occur in young stands before canopy closure [96] and then in mature stands after the start of seed production [66]. At other stages, abundance usually fluctuates less or tends to be persistently low [66,155]. The greatest fluctuations are seen in granivorous species inhabiting mature, seed-producing stands dominated by a single tree species, especially if the seed production is irregular [100,156,157].

7. Effects of Small Terrestrial Mammals on Forests and Forestry

The impact of STMs on forestry and forest ecosystems, in general, is multifaceted and significant, but difficult to quantify [12]. They influence their environment by providing basic needs (e.g., building shelters and searching for food) [13]. As consumers and vectors of vegetation (including seeds, seedlings, and the bark of young trees) and fungi, rodents can significantly influence natural regeneration, species composition, and the health of the next generation of forests [158]. Insectivorous STMs (e.g., Sorex spp., but sometimes Apodemus spp. [15,159]) play an important role in regulating the populations of some insect species [160,161,162]. In terms of biomass, STMs are an essential component of the cycle of many nutrients in the forest ecosystem [163,164,165], and an essential food source for many predators [38,65], thus playing an irreplaceable role in ecosystem stabilization.

7.1. Soil

Forests provide STMs with a more varied environment than arable land or meadows. They have much more rugged terrain here, so they are not forced to dig complex underground burrows under all circumstances. However, they often do so. The underground tunnel complexes serve as shelters, storage areas, toilets, and nurseries for their young [166]. These structures are then often used by other underground animals [167]. But tunnels also have several benefits for the soil and forest ecosystem itself [39]. Physical relocation of the soil (bioturbation) affects soil development and creates different conditions that allow other species of soil organisms to exist [39,168]. Tunnels promote air penetration and water infiltration. Thus, they reduce surface runoff and erosion, increase the water and air content of the soil, and facilitate plant root growth [167]. By burrowing, STMs contribute to mixing soil horizons from different depths, acting as plowing and fertilization; this facilitates plant access to nutrients [33]. In this way, organic matter rich in carbon or nitrogen (fallout, urine, feces) reaches deeper layers where it can remain bound for longer periods of time; higher Ca and Mg (and higher pH) and different proportions of humic and fulvic acids were found in the humus compared to sites where STMs were denied access [33,169]. The extent and importance of these activities are unfortunately difficult to generalize; the few studies on the subject suggest that STMs can move up to 18 m³ of soil per hectare per year, i.e., up to 20 tons of material, and usually in layers up to 10-40 cm deep [170,171].

7.2. Vegetation and Fungi

Food sources vary in nutrient and energy concentrations [172]. Preference for food sources is determined by the physiological structure of the gastrointestinal tract (GIT); the spectrum of GIT types is exceptionally diverse in STMs [173,174]. Almost exclusively, plant food with a high cellulose content and low nutrient and energy concentration (grasses, etc.) is consumed, e.g., by the genus Microtus spp. [175]. The next group needs a higher concentration of nutrients in their diet, but plant foods still clearly predominate. Unlike the previous group, these are the more nutrient-rich parts of the plant [176] (mainly fruits and seeds, less flowers, roots, or bulbs), which they supplement with fungi if possible [177,178] and invertebrates [15,175,179]. By consuming biomass and seeds, they influence forest succession and dynamics [180]. Attractive plant and fungal species are limited by consumption, while STMs contribute to their dispersal [181]. This can occur both purposefully (dispersal and loss during consumption and food storage) and accidentally (spores or seeds in the fur) [182,183,184]. Research suggests interaction with other herbivores affects species richness and understory composition [180,185].

7.3. Woody Plants

7.3.1. Seeds

By collecting seeds, STMs affect the success of both natural and artificial forest regeneration (sowing) and cause damage to seed storage facilities and forest nurseries [186,187].

In seed years, STMs can harvest a significant part of the crop or sown seeds in this way; up to 20% has been reported [188,189]. In a normal year, the harvest is significantly lower or non-existent. Under these conditions, the predation rate of oak seeds was lower [190], while almost the entire crop of beech was removed [191]. Lower amounts of seeds are removed in areas inhabited by wolves [192]. When given a choice, rodents prefer seeds of specific plant species [193,194]. Intraspecifically, STMs prefer seeds of the highest quality, i.e., healthy and ripe, heavier and larger [193,195]. For seeds with significant size variability (e.g., Quercus), STMs tend to consume smaller seeds on the spot, whereas larger seeds are more likely to be carried away and stored [196,197]. By selectively choosing seeds, they can significantly influence the species composition of the emerging forest [195,196]. In addition to direct consumption, some rodents hoard several seeds for storage [195,198]. Each rodent has several storage sites in different places. If there are enough seeds, STMs hide several times more seeds than they can consume [186,187]. Sometimes they subsequently check and adjust their stores further, e.g., by biting off sprouts [199] or by removing empty, insect-infested, or decaying seeds [200,201,202]. As not all stored seeds are consumed for various reasons, this behavior also leads to the spread of woody plants, forming clumps of seedlings at the former storage sites [196,203]. Unlike the action of birds, water, or wind, the dispersal of seeds or plants is not fundamentally accelerated in this way, since rodents most often move seeds at a maximum distance of tens of meters from the source (the average distance of seed transfer is reported to be only about 4 m) [181,204]. Rodent-hidden seeds are more difficult for other harmful organisms to access [205]. In addition, seeds in storage tend to have a better and more stable microclimate, promoting dormancy, subsequent germination [206], or colonization by mycorrhizal fungi spores [207].

7.3.2. Seedlings

The last—and, from a forestry point of view, most problematic—role of STMs is the consumption of whole seedlings and subsequent gnawing of their roots and bark of young trees [208,209]. Damage from STMs occurs very unevenly in location, time, and extent [210]. Significant damage by rodents has been found on seedlings of most tree species [203]. This damage often escapes attention because it occurs during the first year after germination. The seedlings of some tree species (Carpinus, Quercus) are damaged by rodents many times more than by fungi and insects [203]; in areas where rodents were prevented from entering a part of the forest, the resulting regeneration (number of seedlings per area) was twice as high [211].

7.3.3. Saplings

Bark-gnawing damage on trees older than one year has been described as a significant problem in Scandinavia [212,213], Central Europe [54,214,215], North America [208,216], and Japan [217,218], with more marginal occurrences noted in Mongolia [219] and Western Europe [220,221]. This suggests that the damage likely occurs under similar conditions elsewhere. Bite damage is most often caused by folivorous species of the genera Clethrionomys/Myodes, especially Microtus [208,213]. Less frequently, the genera Arvicola or Aplodontia are involved [213,222]. Among one-year-old and older trees, the most damaged are the smooth-barked species of the genera Tilia, Fagus, Acer, Larix, and certain species of the genus Quercus [214,221,223]. However, there are large regional differences. In Scandinavia, for example, the most extensive damage is to conifers [224], while in Central Europe, conifers are rarely damaged [54,214]. The difference in attractiveness is not only between tree species but also between different clones or individuals of the same species (Betula, Salix) [224,225,226]. In mixed forests, damage from rodent bites tends to be lower [227]. Tree bark is not a preferred food for rodents; they probably start consuming it only when they have no better alternative [228]. More frequent damage by gnawing occurs during large-scale clear-cutting [224], in trees from artificial plantings [229], in clearings with rich and connected undergrowth [54,224], or where a large amount of coarse woody debris is left behind [224,230]. Damage can occur at any time [208], but larger-scale damage usually occurs once every few years when high autumn vole numbers coincide with a winter characterized by a higher layer of long-lying snow [231,232]. Relatively significant damage occurring during the growing season has also been identified a few years ago [233]. Trees are usually gnawed at the root collar, less often continuously, or underground [208]. After the canopy of young trees closes and the undergrowth recedes, no significant damage is recorded. Only a few percent of the damaged saplings die directly because of rodent gnawing, but very little is known about the effect of damage on sapling development [100]. The damage reduced the growth rate by about 10% [234]. We know almost nothing about the long-term health effects. Given that more than 75% of seedlings are currently damaged by rodents in some areas, this may be a significant problem in the future [234].

7.4. Invertebrates

STMs need relatively high quantities of good-quality food [12,235]. In some species (Sorex spp., Crocidura spp., and sometimes the Sicista spp.), the animal component of the diet clearly predominates in the form of invertebrates and their developmental stages (eggs, larvae, pupae, and adults of insects, earthworms, gastropods, etc.) [16,236,237]. For other species (Apodemus spp., Clethrionomys spp.), invertebrates are a variable but essential part of their diet [175,238]. This effect is mostly positive from a forestry perspective, as they also consume species that are harmful to trees (e.g., Diprion spp., Melolontha spp.) [179].

7.5. Vertebrates and Humans

7.5.1. Food Source

Due to their abundance and (almost) ubiquity, STMs play a crucial ecological role as a food source for carnivorous animals, including protected ones (owls, raptors, snakes, small, medium, and sometimes large carnivores, etc.). Predator pressure and support of their populations (e.g., nest boxes for raptors and owls, reduced hunting of foxes, martens, etc.) are the basis of biological control, i.e., efforts to keep populations of STMs below the damage threshold [12,38].

7.5.2. Zoonoses

Wild STMs play a key role as reservoirs and vectors of parasites and pathogens [239]. Their ecological traits make them ideal for maintaining and transmitting pathogens to other wildlife, domestic animals, and even humans (so-called zoonoses) [240]. Modern methods have enabled a much deeper understanding of this issue in the last 20 years. Zoonoses differ in their causative agent (e.g., bacteria, virus, protozoa), the vector or reservoir of infection (a species of STM), mode of infection, infectivity, geographical distribution, and, of course, the manifestations or disease produced. The spectrum of diseases varies geographically, especially between North America and Eurasia [241]. The most common or most dangerous zoonotic diseases transmitted by STMs of temperate forests include hemorrhagic fever with renal syndrome, tularemia, tick-borne encephalitis, Lyme borreliosis, toxoplasmosis, salmonellosis, and others [242,243,244]. The mode of infection varies between zoonoses; for some, inhalation or ingestion of infectious particles is sufficient; for others, a bite from an infected arthropod or direct contact is required [245]. A higher incidence of tick-borne diseases was shown to be associated with tick numbers [246] in the year following the population gradation of STMs [247]. Gradation usually occurs the year after the seed year [248,249]. A more detailed discussion of this topic is beyond the scope of this paper.

8. Conclusions

This review synthesizes current knowledge on the complex, reciprocal relationships between small terrestrial mammals (STMs) and forest ecosystems, with a focus on temperate and boreal biomes. STMs contribute significantly to nutrient cycling, seed dispersal, and the regulation of invertebrate populations. They also serve as prey for higher trophic levels and act as ecosystem engineers through soil bioturbation. By fulfilling these ecological functions, STMs play an integral role in the structure and functioning of forest ecosystems.

STMs are particularly sensitive to changes in forest structure, composition, and management, making them valuable bioindicators of ecological integrity and disturbance.

Our review draws primarily on English-language sources from Europe and North America. Consequently, relevant findings from boreal forests in underrepresented regions such as Russia and parts of Asia may be absent. These geographical and linguistic biases should be considered when interpreting the generalizability of our conclusions and applying them in broader forest management contexts.

Some ecological patterns identified here appear broadly applicable across temperate and boreal regions, such as the negative effects of large-scale clear-cutting and the benefits of structural complexity. However, many observed responses are highly context-dependent, shaped by forest type, local climate, STM community composition, and disturbance history.

Forest management strongly influences STM communities. Intensive practices like clear-cutting often favor generalist species adapted to open habitats, while less disruptive approaches—such as shelterwood or selective cutting—can preserve key habitat elements and support forest specialists. Techniques promoting structural heterogeneity, including retention forestry, preservation of understory vegetation, and maintenance of coarse woody debris, have been consistently associated with increased STM abundance and diversity.

Beyond their ecological functions, STMs can significantly influence forest regeneration, both positively, as seed dispersers and regulators of herbivorous invertebrates, and negatively, through predation on seedlings and bark damage. Their role in forest dynamics and succession is, thus, both functional and operationally relevant. Given their ecological importance and management implications, STMs should be more explicitly integrated into forest planning, conservation policy, and biodiversity monitoring frameworks. Understanding STM ecology can improve forest resilience, inform strategies to mitigate rodent-related damage, and contribute to public health efforts through early detection of zoonotic risks.