Forests, especially tropical forests, are home to thousands of species of trees—sometimes tens to hundreds of tree species in the same forest—a level of biodiversity ecologists have struggled to explain. In a new study published in the journal Proceedings of the National Academy of Sciences (PNAS), researchers at the International Institute for Applied Systems Analysis (IIASA) and their colleagues in Australia are now providing a first model that elucidates the ecological and evolutionary mechanisms underlying these natural patterns.