Deer grazing drove an assemblage‐level evolution of plant dwarfism in an insular system

Abstract

1. Plant dwarfism, a syndrome characterised by a significant reduction in plantheight and organ size, is a widely observed pattern of stress-tolerant life-formevolution that results from local adaptation to harsh environmental conditions.The drivers of assemblage-level dwarfism have primarily been attributed to abi-otic factors, such as low temperature, aridity, poor soil fertility or frequent fires.While biotic factors such as grazing pressure from herbivores can contribute tothe establishment of plant dwarfism, these factors have rarely been tested at as-semblage levels. 2. Focusing on a dwarf plant assemblage comprising over 80 taxa on a small conti-nental island in Japan with a high deer density, we hypothesised that historicaldeer grazing could also be a factor contributing to the large-scale convergentevolution of dwarfism. 3. To test this hypothesis, we measured the size of 1908 individual plants of 40 taxapairs, comprising both palatable and unpalatable pairs from the island and theircounterpart taxa from neighbouring regions, and sought to assess which factors(i.e. low solar radiation, estimated divergence time, low nutrient conditions andgrazing pressure from deer) may have contributed to the formation of the dwarfplant assemblage on the island. We also performed genetic analysis to infer thetime frames for the establishment of dwarf taxa. 4. Statistical analyses revealed that plant size was significantly reduced mainlyamong the palatable taxa growing on the island, with preferential grazing by deerbeing identified as the most significant factor influencing plant size. Furthermore, genetic analyses revealed that dwarf ecotypes may have evolved over tens ofthousands of years. 5. Synthesis: To the best of our knowledge, this study is the first to demonstratethat interactions with herbivores can shape the assemblage-level convergence ofplant dwarfism. These findings enhance our current understanding of the forma-tion of plant functional diversity.