"A great many of the plants and animals we perceive as healthy and plentiful today are in fact relics and ghosts. This seeming contradiction is explained by the fact that species loss is not a simple linear process. Many decades can pass between the start of a decline and the collapse of a population structure, especially where moderate-to-long-lived life forms are involved.The above selection is taken from a terrific (and terrifically depressing) article by Stephen M. Meyer, professor of political science at MIT, published in the Boston Review. Meyer addresses ideas all too familiar to those of us following the extinction crisis, but he succinctly describes the current quandary far better than most writers have. Meyer focuses on the concept of trophic generalists (or weedy species), relics, and ghost species. He also explains how “extinction debt” works, a concept not foreign to economists or ecologists, but apparently difficult for almost everyone else to comprehend.
Conservation biologists use the term 'extinction debt' to describe this gap between appearance and reality. In the past century we have accumulated a vast extinction debt that will be paid, with interest, in the century ahead. The number of plants and animals we 'discover' to be threatened will expand out of control as the extinction debt comes due."
I highly recommend this article if you're interested in reviewing or learning the basics of the extinction crisis.