The amphibians are undergoing a dramatic decline and are the most globally threatened class of vertebrates. Among the main causes of this problem, we have biological invasions and the spread of diseases. The most worrisome amphibian disease is chytridiomycosis, which predominantly affects the skin of adult amphibians. This is a fungal disease that can be spread by several amphibian and non-amphibian vectors. These include the bullfrog (Lithobates catesbeianus), which is a large voracious frog native to eastern North America, particularly known to be a highly invasive species and a tolerant carrier of Batrachochytrium dendrobatidis (Bd). The latter is one of the chytridiomycosis pathogens, specifically a chytrid fungus with two life stages, i.e., environmental and on the host. In this chapter, we consider an aquatic ecosystem invaded by bullfrogs. In particular, we are interested in studying the possible consequences of this biological invasion for native amphibian species. We propose and analyze a four-population model, considering the interaction dynamics between Bd carrier and noncarrier bullfrogs, noninfected native amphibians, and free fungus zoospores in the environment.