The accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) of the Tokyo Electric Power Company (TEPCO) in March 2011 resulted in the dispersal of large quantities of radioactive materials. One of the main nuclides released was radioactive cesium-137 (¹³⁷Cs), which has a long physical half-life of approximately 30 years, most of which remains in forest soils. The ¹³⁷Cs accumulated in the soil move through the forest ecosystem along with the circulation of various substances, and some of it is washed away through the river ecosystem. The ¹³⁷Cs transferred to the animal community are causing serious problems, such as contamination of fishery resources. Although there have been studies on the radioactive contamination of animals in the Fukushima environment after the accident, there are still some issues that remain. In this study, we tried to answer the following questions: The first is, “What is the amount of ¹³⁷Cs that has been transferred approximately 10 years after the accident?”. The second is “How do the transfer routes and amounts of ¹³⁷Cs differ depending on the animal's habitat?”. The third is “How do the transfer characteristics of ¹³⁷Cs differ depending on the animal species and the food resources used?”. We set up sampling sites along several rivers in Fukushima Prefecture and collected samples of animals, such as insects and fish, as well as environmental samples. We measured the ¹³⁷Cs concentration of the samples and conducted various analyses, including carbon and nitrogen stable isotope ratio analysis, to understand the transfer and dispersion pathways of ¹³⁷Cs. ¹³⁷Cs were still being taken up by animals in the forest and river ecosystems. The concentration of ¹³⁷Cs in animals collected from highly contaminated areas was high, and the concentration also varied depending on the food resources used by the animals. We believe that the results of this study, which examined the factors that determine the circulation and migration characteristics of ¹³⁷Cs, will contribute to assessing radiation ecological risk.