Isopod feces-mediated shifts in germination timing enhance seedling establishment

Seedlings are particularly vulnerable to herbivory because their defenses are underdeveloped and their capacity to tolerate damage is limited. However, how seedlings cope with such threats remains poorly understood. Animal feces may provide important chemical cues that influence plant responses to herbivory. We examined whether the presence of isopod feces affects the herbivory of Japanese plantain (Plantago asiatica) seedlings in the field. Laboratory experiments were also conducted to test the effects of herbivore feces on P. asiatica germination. Bioactive compounds in the feces that regulate seed germination were extracted and identified. The field experiments indicated that the presence of isopod feces induces seed germination on rainy days (when isopod activity is low), reduces herbivory, and triples the survival rate of P. asiatica seedlings. In the laboratory, feces from isopods that had recently fed on P. asiatica leaves suppressed seed germination. However, germination resumed after simulated rain washed them away. Chemical analyses revealed that trehalose and abscisic acid are the active compounds responsible for germination suppression. These findings demonstrate that the detection of chemical cues in animal feces by seeds alters their germination timing accordingly, thereby increasing their chances of survival in environments with high herbivore densities.