All of nature‑‑the environment’s cornucopia of lakes, forests, rivers, oceans, mountains, meadows, deserts and plains, and the flora and fauna of local and foreign ecosystems and you and me representing humanity, just to name a few of the interlocking parts of the natural world‑‑is affected every day by trophic cascades. It is a term and topic not without controversy, both within the scientific community and among those prone to conflating science and politics or at least the economic interests that buy politicians. Nevertheless, we are all well served by contemplating and trying to understand (and observe) trophic cascades in the world in which we live.
Author, scientist and environmentalist Aldo Leopold (1887-1948) is credited with first describing the dynamic as early as the 1930s and ‘40s in connection with his observations of wolves and the effects on the ecosystem when they were removed. Just mentioning wolves, as everyone reading this well knows, invites controversy. Perhaps if Leopold had been observing mountain pine bark beetles, sea otters, wolverines or blue green algae and their trophic cascading relationship to the changing interactions of entire ecosystems, instead of wolves, the term ‘trophic cascades’ would rest more comfortably in popular discussions and debates about the world’s environment.
Leopold literally turned ecologists’ understanding of the environment upside down. Before him, it was generally perceived that every ecosystem was regulated from the bottom up by resource availability: that is, plants at the bottom take energy from the sun; herbivores take energy from (eat) the plants and carnivores (predators) at the top take energy from (eat) the herbivores. The food chain of nature is far more complicated than this simplistic description, of course, but Leopold noted that when wolves were removed from a particular environment the deer population increased which in turn reduced the vegetation which negatively affected every part of the ecosystem connected to that vegetation. That is, all of it and its regulation worked both top down as well as bottom up. Brian Silliman and Christine Angelini of the Nature Education Knowledge Project describe it as, “When ecosystems are green, predators are often holding grazers in check, while, when they are overgrazed, predator loss or removal is often responsible for elevated grazer densities and plant loss. This tri-trophic interaction, where predators benefit plants by controlling grazer populations, is known as a trophic cascade.”
By the early 20^th century the sea otter of southeast Alaska and the Aleutian Islands were hunted to near extinction for their pelts, called by one wholesale distributor “the most luxurious and exclusive fur in the world.” Sea urchin populations exploded as their primary predator the sea otter vanished, and, as a consequence, kelp beds, a staple of healthy seabed ecology in Alaska diminished drastically. In recent years sea otters have been reintroduced to the oceans around the Aleutians and “…predictable changes in the density of sea urchins, kelp, and the organisms that utilize the habitat created by healthy kelp beds, have been observed, demonstrating the potential for whole-ecosystem recovery with the reinstatement of predator populations (Estes & Duggins 1995).”
That is, trophic cascades can decimate entire ecosystems from bottom to top of the food chain and vice-versa, and they can also reverse the damage in both directions and restore ecosystems to the dynamic balance that is a healthy natural world.
Think of that.