Compensatory abundance dynamics between an omnivore and a herbivore in agricultural landscapes

Population ecology grew from a question about how competition governs animal abundance, and its classic models center on intraspecific competition, by which a population is regulated toward its own carrying capacity. But abundance also responds to competition between species. Where two species draw on a shared carrying capacity, density compensation can occur, a decline in one offset by an increase in another. Well documented within a guild, it has rarely been examined across guilds, for instance between a herbivore and an omnivore, whose diets are expected to be too distinct for their numbers to be coupled. Whether single‐population models should be extended to such cross‐guild regulation remains open. Here we report a pattern suggesting that such coupling may arise in agricultural landscapes. We tracked the ungulate community of one region in central Japan from 2006 to 2024 using two catch‐per‐unit‐effort indices, from trapping near farmland and from gun hunting in forest interiors. In the farmland‐associated index, wild boar Sus scrofa declined while sika deer Cervus nippon increased over about two decades, yet their summed index changed little. After a classical swine fever outbreak sharply reduced wild boar, the summed index temporarily fell and then returned toward its earlier level. A simulation‐based null model indicated that this stability was unlikely to arise by chance, occurring in only 2.3% of simulations. The forest‐associated index showed no comparable constancy. The mechanism remains unknown, but the constancy in farmland and its absence in forest reveal a pattern that calls for explanation. If crops bring trophically distinct species into a shared resource space, compensatory dynamics and the models built to describe them may extend beyond conventional guild boundaries.