Which term refers to organisms that thrive in stable, mature communities?

The term that refers to organisms thriving in stable, mature communities is climax species. These species represent the final stage of ecological succession, where the community has reached a stable state and maintains its structure and function over time. Climax communities are characterized by a diverse array of species that are well adapted to the prevailing environmental conditions.

In contrast, pioneer species are the initial organisms that colonize barren or disturbed habitats. They play a crucial role in preparing the environment for other species to invade and establish. Transitional species refer to organisms that exist within changing habitats and may not be specifically adapted to a stable community. Endangered species denote those at risk of extinction, which is unrelated to their role in stable communities.

Hence, among the provided options, climax species are the organisms that are best associated with stable, mature ecosystems.