Ecosystem transition due to deer overabundance: Insights from long‐term studies and future considerations

The overabundance of certain deer species is emerging as a critical issue in many forested regions across the Northern Hemisphere, including those in Japan. In the field of deer impact studies, the prevailing notion has been that overabundant deer populations could cause drastic changes of ecosystem states. This study comprises a review of the historical discussion surrounding ecosystem changes caused by deer overabundance, from theoretical frameworks to in situ observations and experiments. The synthesis highlights the potential for state transitions, shifting ecosystems from forest to nonforest states, and in some scenarios to a so‐called alternative stable states. However, detecting these transitions poses challenges due to the enduring impacts of past deer activity and the nonequilibrium nature of forest dynamics. Furthermore, this study reveals additional multifield complexities arising from the interactive effects of deer overabundance and global changes on future forest dynamics. To address these challenges, new avenues for research are proposed, emphasizing the importance of sustained efforts in conducting valuable long‐term studies.