The island rule is not valid in terrestrial isopods (Crustacea: Oniscidea)

The island rule posits that small animals grow larger on islands, whereas large-bodied ones are expected to dwarf, but remains largely unexplored in invertebrates. We tested the rule at an inter-specific level on terrestrial isopods (Oniscidea). We also tested the influence of island area on body size variability. We assembled female size and distribution data from the literature for 901 species of Oniscidea. We compared lengths of insular endemics (N = 296 species) and congeneric or confamilial mainland species (N = 605). We repeated the same analyses for species within climatic zones. We found that isopods do not follow the island rule in either the family or genus level, either globally or within climatic regions. While sizes of island isopods are often different than those of their mainland kin, they show no overall tendency towards dwarfism or gigantism on islands. Island area has no discernible effect on body size. Size variation is similar among insular and mainland species, but the size range is greater in insular species. Globally, the island rule is not supported. Oniscidea are phytosaprophagous feeding on a wide variety of plant material and are preyed upon by a wide range of predators, so that island environment does not exert strong selection pressures on their body size.