Since Leopold's time, many scientists have studied food web relationships between top predators and their prey-called trophic cascades. In the 1960s and 1970s Robert Paine, working with sea stars, and James Estes, working with sea otters, showed that ecosystems without top predators begin to unravel. John Terborgh called the ensuing rampant species extinctions an "ecological meltdown." Paine created the metaphorical term keystone species to refer to top predators and noted that when you remove the keystone, arches and ecosystems collapse. Over the years ecologists found trophic cascades-also called top-down effects-ubiquitous from coral reefs to prairies to polar regions. However, William Murdoch and others have maintained that sunlight and moisture, which make plants grow, drive ecosystem processes from the bottom-up, making predators relatively unimportant. The Yellowstone wolf reintroduction provided the perfect setting to test these contrasting perspectives.
In the mid-1800s in his book The Origin of Species, Charles Darwin presciently described nature as a "tangled bank." Nature's complexity results from myriad species and their relationships with other species and all the things that can possibly affect them individually and collectively, such as disease, disturbance, and competition for food. Science works incrementally, taking us ever deeper into nature's tangled bank as we investigate ecological questions. Each study answers some questions and begets new ones. Sometimes we find contradictory results. Learning how nature works requires what Leopold called "deep-digging research" in which we keep searching for answers amid the clues nature gives us, such as the bitten-off stem of an aspen next to a stream where there are no wolves.