1. Introduction
Habitat is one of the most important factors determining the distribution and diversity of small mammal species and communities. On a global scale and over the last decades, the habitat factor operates in conjunction with climate change [
1]. On a smaller scale, structural components of habitat influence the abundance of small mammals and the diversity of their communities [
2]. The scale effect is important in defining habitat association with small mammals. On islands, habitat complexity increases the influence of interspecific competition on small mammals [
3]. Coexistence in limited space requires a reduction in competition, leading to niche partitioning [
4]. Habitat preferences of different small mammal species are not the same at patch and landscape scales [
5], and, therefore, habitat preferences are scale-dependent [
6]. As a result, different small mammal assemblages are associated with specific habitat types [
7].
Two of the studies mentioned above really cover a wide range of material from both habitat and species perspectives [
5,
7]. However, these two studies do not address the fitness of small mammals, unlike our study of the relationship between body condition index (BCI) and habitat [
8].
Research on the diversity and abundance of small mammals in Europe has focused on several habitat groups. Based on 35 years of data, M. Zárybnická et al. [
9] found changes in small mammal populations based on landscape heterogeneity and forest management practices. The stability of small mammal communities was maintained by diverse habitats and influenced by both local biotic and abiotic factors. Forest habitats in Central Europe are the best studied as habitats supporting small mammals in terms of management practices, such as clear-cutting [
10,
11,
12].
Farmland habitats have been the focus of small mammal studies due to conflicts over crop damage and food security [
13,
14] and the global conflict between agriculture and biodiversity conservation [
15]. Some small mammal species, such as the greater white-toothed shrew (
Crocidura russula) and the wood mouse (
Apodemus sylvaticus), have been found to benefit from changes in agricultural land use, such as increases in grassland and fallow land [
16]. These agri-environmental management practices are recommended by the EU. Fallow land and crops with long growing seasons provide cover for small mammals and the predators that prey on them, thus maintaining the diversity and abundance of their communities [
17].
Knowledge of small mammal habitat associations can be translated into habitat management and restoration projects at local and landscape scales [
18]. On a broader scale, studies of small mammals still do not provide a sufficient basis for their conservation strategies [
19]. It should be noted, however, that none of the above studies assessed the fitness or body condition of small mammals, only their diversity and abundance. One of the most extensive studies, based on the analysis of owl prey, showed geographic variation in average prey weight, but this was not related to the body mass of specific individuals within a species or their body condition [
20].
Similarly, most of the previous studies of small mammals and their habitats in Lithuania and other Baltic countries focused on their diversity and abundance [
21,
22,
23,
24]. Coastal wetlands, hemi-boreal forest-farmland landscapes, successional stages from grassland to forest, and commercial orchards were analyzed, but again, the biomass and not the mass of an individual was evaluated [
25,
26,
27,
28]. Thus, there are no publications that can be directly compared with our data, i.e., the extremal BCI values of various small mammal species and their distribution in habitats.
Undoubtedly, the link between habitat and body condition is through food resources and diet. We did not follow the general dietary classification presented in [
29], but we analyzed BCIs of insectivores, omnivores, granivores, and herbivores. The relationship between small mammal diets and habitats has been analyzed in different habitats and at different latitudes [
30,
31,
32], while in Lithuania, the focus was on commensal habitats [
33], providing access to human-related foods. Our dietary studies, unfortunately, cover a much shorter period than the BCI study and are, therefore, not comparable without further research.
The Chitty effect, a common phenomenon in both the Americas and Europe, is related to the body condition of small mammals, as one of the manifestations of the effect is the presence of large-bodied individuals [
34]. Changes in body mass are a common phenomenon in cyclic rodent populations [
35], but the drivers of the Chitty effect are still incompletely understood. It is also not clear whether these large-bodied individuals have higher BCIs. Cyclicity in herbivores is one of the ecosystem functions [
36]. Collapses in this function have been observed since the 1980s in different species and countries [
37]. Regular cycles of abundance are being replaced by irregular fluctuations, sometimes leading to large-scale outbreaks [
38].
Habitat has been reported as one of the factors modulating the abundance of large-bodied common voles (
Microtus arvalis asturianus) [
35]. Larger individuals may have an advantage in resource use [
39], but there is evidence that small individuals may also use a large proportion of resources [
40,
41].
Extra-large individuals of the field vole (
Microtus agrestis) and sibling vole (
M. rossiaemeridionalis) were observed in agricultural habitats of Sweden [
42], those of the root vole (
Alexandromys oeconomus) in marshy habitats of Norway [
43]. Large individuals in non-cyclic populations of common hamsters (
Cricetus cricetus) were found in agricultural fields in the Czech Republic [
44]. In North America, large individuals of Townsend’s vole (
Microtus townsendii) were recorded in grasslands [
45], and those of meadow vole (
M. pennsylvanicus) in old fields and former agricultural areas [
46]. Information on the habitat distribution of extra small individuals is lacking.
The aim of this study was to analyze the habitat distribution of extreme (highest and lowest) values of the body condition index in different species of small mammals in Lithuania, representing mid-latitude countries with continental climates. We tested whether the distribution of these extreme values correlated with the proportions of individuals of each species caught in each habitat, i.e., whether the proportions of poorly and well-conditioned individuals were associated with specific habitats.
4. Discussion
A review of the literature reveals that no study has previously compared the variability of individual fitness across different habitats and species. Our study, which examines the distribution of BCI thresholds across species and habitats, therefore, makes an original contribution to our understanding of the relationship between fitness and habitat.
Outlying values in body condition indices (BCI < 2 and BSI > 4) were observed in all investigated habitats and in all species, with the exception of M. minutus, which was never under-fit. At BCI values greater than 5, however, no small mammals were captured in mixed habitats, with only a few individuals observed in shrub, wetland, riparian, and agricultural habitats. These highest BCI values were not observed in N. fodiens and A. uralensis, and were observed in only a few individuals in M. arvalis and M. agrestis.
Thus far, other investigators have documented the presence of extra-large individuals in a range of habitats, including agricultural settings [
42,
44,
46], grasslands [
45], and wetlands [
43]. However, there is a paucity of information regarding underfit individuals. The high number of
M. musculus with BCI > 5 observed in commensal habitats can be attributed to the availability of rich food sources and species adaptations [
50], as is the case for
M. minutus in meadows (and also in riparian and agricultural habitats, as illustrated in
Figure 3b,c) due to their scansorial lifestyle and preference for rich and protective habitats, such as reedbeds [
51].
The presence of over-fit
A. flavicollis and
C. glareolus in disturbed habitats, such as landfills and colonies of great cormorants (
Phalacrocorax carbo), is associated with elevated concentrations of nitrogen, phosphorus, carbon, and other biogens [
52]. As a consequence of elevated nitrogen concentrations in the basal resources of small mammals, already evident in the first year of the cormorant colony’s presence, there is a distortion of the trophic level of these small mammals. For instance, the isotopic δ
15N signatures of granivorous
A. flavicollis and omnivorous
C. glareolus in the cormorant colony are higher than those of insectivores in other habitats [
53]. Additionally, the concentration of biogenic elements originating from food waste and industrial discharges is elevated in landfills, resulting in environmental consequences [
54].
Theoretically, several mechanisms can contribute to species-specific overfitness, including genetic, ecological, behavioral, and physiological factors, but again, so far, we have only analyzed individual fitness. What could be tested at the site level by other authors is ecological release (reduced competition in environments where the number of competing species is reduced, allowing them to increase in number and increase individual fitness. Such a situation might occur in newly colonized areas after a strong disturbance.
What is the role of BCI in the context of broader issues in species biology? Individuals of the same species exhibit variation in size and other characteristics, which is a prerequisite for natural selection to occur [
55]. The impact of individual heterogeneity operates at multiple scales, from the individual to the species and ecosystem level [
56]. However, there is still a dearth of knowledge regarding the large-scale investigation of co-occurring species in the same habitats within BCI. Improved body condition can serve as a buffer against adverse environmental conditions and changes, enhancing the likelihood of survival [
57].
The impact of habitat on the body condition of small mammals has been documented, with habitat quality identified as the most crucial factor [
58]. The body condition of these animals is found to be associated with habitat type [
59], as well as with various forms of habitat alteration, including agricultural practices [
60] and habitat loss [
61]. Human activity can exert a detrimental (reducing activity and occurrence) or beneficial influence on small mammals, with more than half of the species demonstrating a positive response [
62]. Therefore, the mechanism is not straightforward, and further insight into BCI and habitats is necessary. Commensal and non-commensal small mammal species may adapt to urban environments by modifying their behavior [
63]. In our study, small mammal BCIs did not demonstrate adaptation to commensal habitats, with the exception of
M. musculus, a typical synanthropic species.
The intra- and inter-species dietary differences may be attributed to both body size [
64] and trophic group [
65], with herbivores exhibiting the highest risk. In low-latitude regions, the decline of small mammals is likely to be most pronounced as a consequence of deforestation [
66], which can be defined as the destruction of and the subsequent fragmentation of remaining patches. In Lithuania, the decline in meadows over the past three decades [
8] may have been a significant factor, as this habitat supports a higher-than-expected population of small mammals with the best body condition.
Nevertheless, the capacity of small mammal species to adapt is not confined to urban environments [
63]. It was demonstrated that Tullberg’s soft-furred mouse (
Praomys tullbergi) is capable of responding to fluctuations in resource availability by adjusting its individual body condition [
67]. Consequently, an understanding of the adaptive strategies employed by different species in diverse habitats is crucial for the development of effective conservation strategies. As stated by J.W. Moore and D.E. Schindler, “Adaptation ultimately underpins the resilience of Earth’s complex systems; species, communities, and ecosystems shift and evolve over time.” [
68]. This underscores the importance of long-term trends and baselines, as well as the utilization of body condition indicators that can be obtained retrospectively in other countries.
5. Conclusions
Based on long-term BCI variability, outliers in the body condition were present in all investigated species and habitats, with the exception of M. minutus, which exhibited no under-fit individuals.
The presence of the highest BCI levels can be attributed exclusively to habitat characteristics, particularly the resources provided in some cases: A. flavicollis and C. glareolus in disturbed habitats and M. musculus in commensal habitats.
The relative proportions of under- and over-fit small mammals of different species indicate that mixed/fragmented and commensal habitats may be considered the least favorable, while meadows and disturbed habitats may be considered the most favorable.
Given the possibility of retrospective assessment of the BCI in question, the index may prove useful for investigating adaptations to human influence and climate change.