Highly toxic seeds of the Japanese star anise Illicium anisatum are dispersed by a seed-caching bird and a rodent

Abstract

Many field studies on plant seed dispersal teach us that we cannot judge the effective dispersal mode of plants by examining only the morphologies of the fruits and seeds. In the present study, we explored the seed dispersal process of an evergreen tree, the Japanese star anise Illicium anisatum, which is highly toxic, containing neurotoxins in both the fruits and seeds. The fruits exhibit ballochory, a mode of seed dispersal characterized by explosive fruit dehiscence, and the extreme toxicity apparently seems to deter fruit and seed consumption by animals. However, we found that the dispersal distance afforded by this mode was very short (≤ 6 m). In the field, we confirmed that a passerine species, the varied tit Poecile varius, was the only consumer of the seed in foliage, and the bird actively transported seeds or fruits to either cache or consume them. Seeds setting on the forest understory were removed by the small Japanese field mouse Apodemus argenteus, and were also dispersed by this animal. Analysis of seedling spatial distribution revealed that seedlings were highly aggregated near standing trees or fallen logs, suggesting that caching facilitated seed dispersal. This study warns that plant toxicity and the ecological function thereof should not be evaluated based only on limited knowledge of the effects on humans and mammals. Our results pose further questions on the evolution of toxin tolerance in seed-caching animals and on the mutualism between toxic plants and animals.