Trophic Cascades and Keystone Species

Environmental & Science Education, STEM, Biological Evolution

Ed Hessler

In the spring of 1957, while teaching a class on the natural history of freshwater invertebrates at the University of Michigan, Professor Fred Smith made a substantial departure from the course material when he asked his class to consider a question about a tree outside the classroom window. "Why is that tree green?"

Obvious, no? No!

Smith was not interested in the obvious answers from biochemistry. His aim was a much more general question, one having to do with food chains. Smith later became the middle author of a now classic paper in vegetation ecology--the "green world," aka HSS (Nelson Hairston, Frederick Smith and Lawrence Slobodkin, 1960*).

It provoked considerable controversy and led to two general explanations: Top-down control or predators of herbivores. Bottom-up control or plants themselves limiting the effects of herbivores. In a well designed research investigation by a team of Duke University researchers and published in 2006 found that predators keep the world green.  However, the question has not been resolved and as Wilkiinson and Sherratt (2016**) note, the "dichotomy is probably too simple for understanding a complex system--such as vegetation at a site."

The video (19 m 29s) "Some Animals Are More Equal Than Others: Keystone Species And Trophic Cascades" from the Howard Hughes Medical Institute's (HHMI) series includes a re-enactment of that classroom. The film is narrated by Robert Paine who was a student in Smith's class when he asked this famous question. Paine describes the influence this question had on his subsequent career.

The film is about keystone species and trophic cascades which are two basic and important ecological concepts. The film tells the story of how these powerful concepts were first established through pioneering experiments of Robert Paine and James Estes. Trophic cascades occur when changes at the top of a food change result in changes do the rest of the chain. The keystone species is at the top of this kind of chain.

Paine's moving obituary about Frederick Smith was published in the Bulletin of the Ecological Society of America.

*Hairston, N. G., F. E. Smith, and L. B. Slobodkin. 1960. Community structure, population control and competition. American Naturalist 94:421–425.

** The Wilkinson Sherratt review may be found here. The authors note that after "having pointed out ... complexities and ...multiple processes involved in a full explanations, it appears to us that bottom-up processes are probably the most widely applicable explanations for why herbivores do not destroy all vegetation and so they provide an important part of the answer to the green world question."