Temporal Compensation Among Different Non-Native Prey Sustains the Food Supply of an Intraguild Predator Threatening a Native Species

Abstract

Intraguild predation (IGP) is common in nature, and understanding the temporal changes in IGP structure is important for ecosystem management and biodiversity conservation. However, partial IGP, where the apex predator consumes not only shared prey and the mesopredator (IG prey) but also other prey (alternative prey), has rarely been addressed in terms of its temporal structural dynamics. We used stable isotope analysis to investigate seasonal changes in the structure of partial IGP involving feral cats, two non-native rat species (Rattus norvegicus and Rattus rattus), and seabirds on Mikura-shima Island, Japan. We also estimated the temporal dynamics of these two rat species by conducting a trapping survey. The results showed that cats depended on seabirds (shared prey) as food when seabirds were present, but when seabirds were absent, shifted their diet to Norway rats and subsequently showed increased reliance on black rats. In addition, Norway rats appeared to depend more on seabirds as food than black rats. Furthermore, the catch per unit effort of Norway rats declined sharply from the first to the second half of the period when seabirds were absent. These results suggested that Norway rats are IG prey, whereas black rats tend to be an alternative prey with less dependence on shared prey. The seasonal decline in Norway rats (IG prey) may have shifted this system from IGP to partial IGP, increasing the dependence on alternative prey. This study highlights the importance of understanding the dynamics of partial IGP involving multiple non-native predators for their management.