Many cancers resist therapeutic intervention. This is fundamentally related to intratumor heterogeneity: like species in an ecosystem, multiple cancer populations with different phenotypic signatures coexist inside a tumor. In this brief talk I will review recent methods from theoretical ecology that could help us better understand the complexity of heterogeneous tumors. I will also discuss the interaction between cancer and two other complex systems, the immune system and the gut microbiome, with implications across a wide range of diseases. The goal is to propose open questions and a potential roadmap for further research in cancer, immune and microbial ecology.
