Extreme drought of 1975 in the forest zone of the Southern Urals (Ilmen State Nature Reserve): dynamics of rodent populations and consequences

Climate change consequences assessing for environment and humans is one of the main tasks facing ecology and has great practical significance. Long-term studies of small rodent populations have contributed fundamentally to the development of population ecology. Extreme drought in the forest zone of the Southern Urals in 1975 is one of the rarest phenomena, a climatic event (once in a century), and it is considered as a contrasting background that provides for especially clear manifestation mechanisms of adaptive resistance in animals populations. In the paper, we present for the first time the 52-year dynamics of species structure of the rodent community and population abundance data, obtained in the Ilmen State Nature Reserve (Chelyabinsk Region, Southern Urals, Russia) before and after the drought of 1975 and its remote consequences. We tested the hypothesis about the similar population responses to both an extreme drought and normal autumn-winter conditions. The drought was verified by a dendrochronological indicator (radial growth of Pinus sylvestris trunks) and two climatic parameters, namely soil humidity (for the first time) and precipitation amount. Population dynamic phase portraits demonstrate species-specific features concerning mainly abundance levels before and after the extreme episode: dynamic stability of Clethrionomys glareolus and C. rutilus; abundance increase of Sylvaemus uralensis; collapse of population abundance of Microtus agrestis, M. arvalis, and Alexandromys oeconomus. Species composition (local species lists) turned out to be a reliable indicator of drought. Based on both original functional-ontogenetic approach and individual marking (CMR-method) data, the species populations' response patterns to drought were studied. In summer 1975, the Clethrionomys species have implemented a response strategy, which historically developed for regular winter conditions, reflected in complete blocking of yearlings sexual maturation (minimisation of metabolic processes) as we hypothesised originally. The next year, we observed population adaptations included breeding period prolongation of overwintering females and intergenerational crossing (age cross), which provide effective utilisation of possibilities for population growth along with preservation of yearlings and improvement of genetic heterogeneity. The Sylvaemus uralensis population demonstrated a similar response pattern to drought, reflected in partial blocking of yearlings' sexual maturation, then a gradual increase in the number of individuals up to a high dynamic level in recent decades. In contrast, Microtus agrestis, M. arvalis, and Alexandromys oeconomus populations are characterised by the usual participation in reproduction of both overwintering individuals and yearlings in drought year (i.e. our hypothesis has not been confirmed). As a consequence, there was a mass mortality and abrupt transition of the populations to the critically low abundance level of its oscillations and elimination. An invasion of an alien species (Apodemus flavicollis) was recorded for the first time in 2020. The weather anomaly triggered rapid long-term changes in the biotic community structure, by reflecting in the increase in population abundance of granivorous/seed-eating species (Clethrionomys glareolus, C. rutilus, Sylvaemus uralensis), the finding of a new species (Apodemus flavicollis), and changing of co-dominant (Sylvaemus uralensis instead of previous Microtus agrestis), and, finally, disappearance of herbivorous species (Microtus agrestis, M. arvalis, and Alexandromys oeconomus). Thus, long-term monitoring of rodent populations in forest ecosystem of the Ilmen State Nature Reserve revealed a real possibility of the rapid population rearrangements on evolutionary scale in populations of the local fauna as a result of extreme drought. Drought exposure consequences in the studied Rodentia species are markedly extended in time, being fixed in a series of generations.