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The Common hamster (Cricetus cricetus) has been intensively colonizing cities during the last decades. Changes in the burrow numbers and their distribution over an area might be important indicators of the population status for the Common hamster in an urban environment. In this study, we consider the character of the burrow distribution on the experimental plot (2.2 ha) situated in the park of Simferopol City, Russian Federation. The brushwood and tree vegetation on this plot had been previously mapped in detail. Hamster burrows were put on the map once a quarter during a year. The peak of burrowing activity is shown to be in November. Interestingly, the ground activity of hamsters continued even in the coldest month (January) of the year but the number of used burrows was very low. The squares where both trees and burrows were present (by average annual indicators) was met significantly more frequently than burrows in the tree-free squares (P = 0.02; χ 2 = 5.2) but this was not the case for the winter and spring seasons. We assume that the connection of burrows with arboreous vegetation facilitates digging, ensures better protection from predators and is a food source. All these factors ensure favorable conditions for the high abundance of the Common hamster in the urban environment.

Traditionally, urbanization has been seen as a negative phenomenon for biota. However, changes in the environmental parameters induced by urbanization might be favorable for some species. Over the past half‐century, the common hamster has actively populated cities, establishing populations in some European, Russian and Kazakhstan cities. Based on integrative methods, we investigated free‐range common hamsters inhabiting Simferopol from 2015 to 2018 to reveal possible adaptations to the urbanized environment across several parameters, including lifespan, hibernation period, reproductive activity and body mass. Results show that in urban areas, the common hamster demonstrates an extremely short hibernation period compared to other localities, possibly due to enhanced food resources from urban forestry (walnuts, locus and hazelnut), allowing the species to start breeding very early (February) and finish as late as October. We present the first evidence of polyandry for this species: mating of receptive females with several males and subsequent confirmation of multiple paternity. Despite high reproductive potential, the lifespan of the common hamster in urban conditions is generally very short (less than 1 year). We speculate that in the process of synurbization, the common hamster's innate plasticity across many life history traits permits it to successfully colonize throughout a wide range of habitats, with the ability to form novel adaptations to urban environments.

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The paper is devoted to an important problem of modern ecology, namely, the reaction of native ecosystems to the activity of invader species, with the European beaver (Castor fiber L) as an example. This species transforms the environment as a result of its foraging and constructive activities. Higher vascular plants (macrophytes) in water bodies and streams are one of the basic components of biotopes. They serve as a limiting factor for the spread of such invasive species as C. fiber, because macrophytes determine the basis of this animal's diet. On the other hand, under the influence of the constructive and foraging activities of C. fiber, the macrophyte communities undergo significant changes. As a result of this process, such indicators as trophicity and saprobity of natural waters change, whose quality is formed at the level of small reservoirs and watercourses. It is especially important for the forest-steppe zone characterized by relative poverty of water resources. The data collected during three field seasons (2015-2017) allow describing the current status of the highest aquatic vegetation in several small water bodies and watercourses located in the Volga regional forest-steppe State Nature Reserve. Other factors of environment transformation, such as the economic use of reservoirs and watercourses and their adjacent territories, and the recreational load, are practically excluded there. The presented data reflect the influence of C. fiber upon the biocenoses of small water bodies and watercourses.