Body size shift in sympatric insects in response to distinct selective forces in fragmented urban environments

Phenotypic plasticity, which encompasses the diversification of both irreversible and reversible traits, has long been considered an adaptive response by animals to varying environmental conditions. However, the process by which irreversible and reversible traits are coordinated to form an adaptive response to the changing environment has yet to be clarified. Here, we investigated the variation in body size of two urban insect species in the context of habitat fragmentation. These species were the Chinese cricket Gryllus chinensis and the stove grasshopper Diestrammena japonica, which are sympatric in urban housing estates. Results indicated that both species changed in body size in patches of urban environment. However, their body size shifts showed opposite tendencies and were influenced by distinct selective forces: Chinese crickets increased their body size with the patch history and predation risk, whereas stove grasshoppers decreased their body size with the degree of fragmentation of the patches. Territorial and competitive Chinese crickets rarely experience resource scarcity during urban environment fragmentation. Thus, a larger body size was preferred in response to intraspecific competition among Chinese crickets. By contrast, stove grasshoppers are group-living and scramble for resources as competitors, requiring a large territory to secure adequate food for supporting a group of individuals. Consequently, stove grasshoppers frequently experienced resource scarcity in the patchy habitat, favoring small body size to reduce individual requirements throughout the life cycle. Our findings indicate that the body size shift of sympatric insects may be subjected to distinct selective forces in fragmented habitats, depending primarily on their reversible traits.