The world’s tropical forests are in “a critical state” in which the extinction of rare tree species could be a tipping point, say scientists who have developed an analytical method to map their biodiversity. “We are in the midst of an extinction crisis,” said Jayanth R. Banavar, provost and senior vice president at the University of Oregon and previously at the University of Maryland in College Park. “We are losing species perhaps more rapidly than ever before. It is the biodiversity of the species that keeps our planet the way it is. These species have evolved over many, many millennia. A species once lost is gone forever.” In a paper published Oct. 18 in the journal Science Advances, Banavar, a physicist, and co-authors from three other universities unveiled their findings, which are based on a mathematical framework relying on a mechanistic birth-death-immigration model of an ecosystem.