Influence of body size on the dual role of isopod Porcellio scaber in seed dispersal and predation of the mycoheterotrophic plant Monotropastrum humile

1. Endozoochorous seed dispersal by invertebrates, although infrequently documented, is gaining attention for its feasibility in plants with small seeds. Recent studies have highlighted terrestrial isopods as potential dual agents, acting both as seed dispersers and predators, especially for herbaceous plants with fleshy fruits and tiny seeds. However, the determinants of these contrasting roles are not well understood. 2. The present study explores intraspecific variation in Porcellio scaber, examining its dual role as a seed predator and disperser for Monotropastrum humile (Ericaceae), a plant with fleshy fruits and minute seeds (0.30–0.35 mm in length). We particularly focus on the impact of the sex and body size (5.44–11.99 mm) of the isopod P. scaber on seed dispersal or predation. 3. Our results indicate a significant correlation between seed dispersal effectiveness and isopod body size, with some variation attributed to sex differences. Smaller P. scaber individuals predominantly prey on seeds, whereas larger individuals are more likely to facilitate seed dispersal, maintaining seed viability. 4. Overall, our findings reveal that body size is a crucial factor in defining the ecological role of P. scaber. This challenges the conventional approach of using species-level average data in seed dispersal studies, emphasising the need to consider intraspecific variations for a comprehensive understanding of seed dispersal.