Responses of small mammals to habitat characteristics in Southern Carpathian forests

The positive response of small terrestrial and semi-aquatic mammals to beaver damming

Ecosystem engineers, such as the Eurasian beaver, Castor fiber, transform habitats, thereby creating favourable conditions for other species and increasing biodiversity. Multiple studies have revealed that beaver ponds are valuable habitats for invertebrates and vertebrates, including other mammals, but the impact of watercourse damming on the fauna of small terrestrial rodents and shrews has not yet been documented. We tested the hypothesis that the presence of beaver dams and consequent flooding enrich the small mammal assemblage both quantitatively and qualitatively. We live-trapped small mammals at nine beaver-modified sites on 300-metre transects alongside dammed watercourses, starting from the dam through to the pond to the sections with unmodified lotic conditions. The abundance and species richness of trapped small mammals were highest near the dams and declined with distance. Additionally, five out of 12 trapped species significantly decreased their abundance with linear distance along the shoreline from the dam and none revealed the opposite trend. Four species were more abundant on plots subjected to beaver-related inundation (especially Sorex minutus and Micromys minutus), while none were present solely on uninundated plots. Among the semi-aquatic species, two water shrews benefited from beaver activity in different ways. Neomys milleri occurred only in flooded sections, while N. fodiens preferred unmodified sections, but was the most numerous species closer to the dams, as per known patterns of competitive displacement observed in Central Europe. An important factor affecting small mammals, the herbaceous layer cover, appeared to be interdependent with damming. We provide the first unequivocal evidence that the presence of beaver dams facilitate the abundance and diversity of small mammals, presumably due to increased food abundance, availability of shelter and habitat connectivity. Beaver-created wetlands may act as potential refuges for species most susceptible to the consequences of anthropogenic climate change.

A Comparison of Small Rodent Assemblages after a 20 Year Interval in the Alps

Human-induced environmental alterations in the Alps may importantly affect small mammal species, but evidence in this sense is limited. We live-trapped small rodents in the Central-Eastern Italian Alps in three close-by habitat types (rocky scree, alpine grassland, and heath) at 2100 m a.s.l. during summer-fall, in 1997 and 2016. We compared small rodent assemblages through a Redundancy Detrended Analysis (RDA). In both surveys, we detected two specialist species, i.e., the common vole (Microtus arvalis) and the snow vole (Chionomys nivalis), and, unexpectedly, the forest generalist bank vole (Myodes glareolus). In 1997, grassland was mainly occupied by the common vole, while the bank vole and the snow vole were sympatric in the other habitats. In 2016, the snow vole was detected only in the scree, while other species did not show distribution changes. We discuss a series of hypotheses that might have driven the differences observed across decades, among which is a species-specific response to abiotic and biotic environmental alterations, with the alpine habitat specialist moving out of sub-optimal habitats. We encourage further research on this topic, e.g., via long-term longitudinal studies.

Size Matters: Diversity and Abundance of Small Mammal Community Varies with the Size of Great Cormorant Colony

Cormorant colonies are often viewed negatively by fishermen and foresters due to their extremely high impact on aquatic and terrestrial ecosystems. In forests, the habitats of nesting territories are destroyed, with concomitant impacts on the animal communities. In 2011–2022, investigating three colonies of Great Cormorants (Phalacrocorax carbo), we aimed to test whether their effect on small mammals depends on colony size. In the largest colony in Lithuania, a low species richness, lower diversity and relative abundance, as well as poorer body conditions of the most abundant species was found in the nesting zone. However, once the cormorants left the nesting site, all the parameters recovered. Two small colonies had a positive impact, with higher species richness in the territory of the colony (seven and ten species), diversity (H = 1.56 and 1.49), and relative abundance (27.00 ± 2.32 and 25.29 ± 2.91 ind. per 100 trap days) compared with the control habitat (three and eight species; H = 1.65 and 0.99; 12.58 ± 1.54 and 8.29 ± 1.05 ind./100 trap-days). We conclude that up to a certain colony size, cormorant pressure is a driver of habitat succession and has similar effects on the small mammal community as other successions in disturbed habitats.