Cascading effects driven by population recovery of sika deer on habitat use of sympatric mammals under heavy snow conditions

Abstract

Unlike the top-down cascading effects driven by apex predators, the bottom-up effects on biological interactions originating from herbivory by large mammals, especially within mammalian assemblages, have received less attention. This study aimed to identify the multifaceted impacts driven by sika deer (Cervus nippon) during midwinters, when their herbivory impacts were expected to be strong due to deficient dietary resources. We focused on interference and exploitative competition caused by deer on the habitat use of four sympatric mammal species, namely two herbivores (Japanese serow, Capricornis crispus, and Japanese hare, Lepus brachyurus angustidens) and two mesocarnivores (red fox, Vulpes vulpes, and Japanese marten, Martes melampus melampus). To estimate the wintering behaviors of these mammals and deer herbivory pressure, we performed snow tracking and counted the fresh feeding marks of deer during midwinter in 2022 and 2023. We conducted surveys on 201-km transects with different stages of deer population recovery (i.e., different deer population densities) in heavy snowfall regions of Japan. We then systematically quantified the deer-driven effects on sympatric mammals using piecewise structural equation modeling (SEM). We recorded 1700 tracks of target mammals and 1327 trees with deer feeding marks. Based on these records, we constructed an SEM with acceptable performance (Fisher's C = 27.1, P = 0.30). The SEM suggested some possibilities that exploitative competition mediated by deer herbivory constrained the habitat use of mammals with dietary habits similar to that of deer, i.e., serow (effect size, −0.13) and hares (−0.14). In addition, through constricting the habitat use of hares, the deer-driven cascading effects reduced the occurrence of their key predators, i.e., foxes (−0.12) and martens (−0.10). Thus, our observations provide novel evidence that food webs are regulated by herbivore-driven bottom-up cascading effects in cases where plant primary productivity is limited by heavy snowfall.