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

Plant dwarfism, a syndrome characterised by a significant reduction in
plant height and organ size, is a widely observed pattern of
stress-tolerant life-form evolution that results from local adaptation to
harsh environmental conditions. The drivers of assemblage-level dwarfism
have primarily been attributed to abiotic factors, such as low
temperature, aridity, poor soil fertility, or frequent fires. While biotic
factors such as grazing pressure from herbivores can contribute to the
establishment of plant dwarfism, these factors have rarely been tested at
assemblage levels. Focusing on a dwarf plant assemblage
comprising over 80 taxa on a small continental island in Japan with a high
deer density, we hypothesised that historical deer grazing could also be a
factor contributing to the large-scale convergent evolution of
dwarfism. To test this hypothesis, we measured the size of 1,908
individual plants of 40 taxa-pairs, comprising both palatable and
unpalatable pairs from the island and their counterpart taxa from
neighbouring regions, and sought to assess which factors (i.e. low solar
radiation, estimated divergence time, low nutrient conditions, and grazing
pressure from deer) may have contributed to the formation of the dwarf
plant assemblage on the island. We also performed genetic analysis to
infer the timeframes for the establishment of dwarf
taxa. Statistical analyses revealed that plant size was
significantly reduced mainly among the palatable taxa growing on the
island, with preferential grazing by deer being identified as the most
significant factor influencing plant size. Furthermore, genetic analyses
revealed that dwarf ecotypes may have evolved over tens of thousands of
years. Synthesis: To the best of our knowledge, this study is the first to
demonstrate that interactions with herbivores can shape the
assemblage-level convergence of plant dwarfism. These findings enhance our
current understanding of the formation of plant functional diversity.