Abstract: Ecosystems are complex systems that include a large number of species and the surrounding environment, and their dynamics are driven by interactions among species and between the environment and organisms. Ecosystem stability is important to maintain ecosystem services, and much research has been accumulated. This seminar begins with a brief review of previous studies on ecosystem stability. We then explain the results of two of our recent studies. The first is on ecosystem stability in a fluctuating environment. We analyzed how ecosystems are affected by intensifying environmental fluctuations in a resource competition model consisting of plants and herbivores. As a result, we obtained a phase diagram varying the degree of species specialization and the intensity of environmental change. We also found a trade-off relationship between the ability to maintain a multispecies coexistence and the ability to prevent the total extinction of an ecosystem. The second is about the transformability of ecosystems. We analyzed a mutualistic ecosystem consisting of plants and pollinators and derived the conditions under which an ecosystem can be transformed from one state to another while maintaining its stability. The result shows a fundamental limit in ecosystems and may help determine effective interventions for real ecosystems.