Seventy-five years of change: Long-term reorganisation of helminth communities in the rook (Corvus frugilegus) in Ukraine

Population declines and extinctions affect all animal groups, yet long-term monitoring of helminths remains rare despite their ecological importance. This study analyses the helminth communities of the rook Corvus frugilegus Linnaeus, 1758 in Ukraine across three 25-year periods spanning over seventy-five years (1950-2024), using a total of 275 examined individuals, taking into account the effect of season on helminth communities. A total of 30 helminth species were identified (13 nematodes, 10 trematodes, 5 cestodes, and 2 acanthocephalans). Community composition differed significantly among historical periods (PERMANOVA, R2 ≈ 0.125, p < 0.001). Coverage-standardised Hill number diversity analyses revealed a pronounced depression of the helminth community in period 2 (1975-1999), probably linked with the peak of agrochemical intensification in Soviet agriculture. This period also characterised the complete absence of trematodes - a group particularly sensitive to agrochemical and other contaminations. Seasonal variation was also detected, mostly in the early period (1950-1974), but overall, the long-term pattern is much stronger than the seasonal one. Only four species persisted across all three periods, with host-specific and directly transmitted helminths showing the greatest stability. Interestingly, period 3 (2000-2024) is characterised by the emergence of nematodes Eucoleus frugilegi (Czaplinski, 1962) and Microtetrameres helix (Cram, 1927), which became dominant species. This likely reflects species range expansion or niche replacement following the collapse of earlier helminth communities. This study suggests a dynamic system rather than a simple decline and highlights the value of synanthropic species and institutional collections for reconstructing long-term dynamics of host-parasite systems under sustained anthropogenic pressure.