Earwigs and Woodlice As Some of the World's Smallest Internal Seed Dispersal Agents: Insights from the Ecology of Monotropastrum Humile (Ericaceae)

Societal Impact StatementThis study illuminates the underappreciated role of invertebrates in seed dispersal, extending beyond the well-documented contributions of ants. Focusing on Monotropastrum humile ((sic)(sic)(sic) [silver dragon plant] or (sic)(sic)(sic) [crystal orchid]), a non-photosynthetic plant known for its minuscule, dust-like seeds, the present research uncovers their seed dispersal roles of woodlice and earwigs in Japan. Remarkably, these invertebrates include the smallest known endozoochorous seed dispersers. Summary Endozoochory, or internal seed dispersal through the digestive tracts of animals, has been less studied in invertebrates compared with vertebrates. Nonetheless, endozoochory is plausible whenever seeds are small enough for ingestion by frugivorous animals, suggesting a potential role for invertebrates in seed dispersal, especially for plants with minute seeds. Monotropastrum humile (Ericaceae), characterized by its fleshy fruits and dust-like seeds, is known to utilize invertebrate agents such as camel crickets and cockroaches for seed dispersal. Here, we investigate this seed dispersal mechanism using time-lapse photography, feeding experiments, and seed coat anatomy analysis, particularly focusing on interactions among undocumented invertebrate internal seed dispersers. Field observations indicated that in the studied population, M. humile fruits were primarily consumed by camel crickets, woodlice, and earwigs. Their effectiveness as seed dispersers varied, with camel crickets primarily acting as dispersers, whereas earwigs and woodlice were more inclined toward seed predation. Nonetheless, some seeds defecated by earwigs and woodlice remained intact, suggesting that they could also function as dispersal agents. The woodlouse Porcellio scaber is now recognized as the world's smallest internal seed dispersal agent. Combined with earlier discoveries, such as seed dispersal by camel crickets, cockroaches, and ants, and the fact that P. scaber is an exotic species in the study site, M. humile likely depends on a broad spectrum of local invertebrates. The engagement of multiple invertebrate dispersers may enhance seed dispersal across diverse habitats.