Thursday, June 12, 2008

This Week's Citation Classic: The World is Green

This week's citation classic contains no data, no figures, no tables, no methods, i.e. none of the traditional trappings of the "Scientific Paper". Yet, judging by its 1,145 citations, its influence has been incalculable. The paper is commonly known as "the world is green paper" or "HSS 1960.

Hairston, N.G., Smith, F.E. and Slobodkin L. 1960. Community structure, population control and competition. American Naturalist 94:421-5, 1960.

Using simple logic, HSS take a simple observation (the world is green) and deduce that predators limit herbivore abundance, allowing plants to flourish. HSS summarize their deductions here:

In summary, then, our general conclusions are: (1) Populations of producers, carnivores, and decomposers are limited by their respective resources in the classical density-dependent fashion. (2) Interspecific competition must necessarily exist among the members of each of these three trophic levels. (3) Herbivores are seldom food-limited, appear most often to be predator-limited, and therefore are not likely to compete for common resources.

The paper was bold and controversial. Most ecologists either love it or hate it. (Personally I fall into the former category). It was not intended to be a be-all, end-all monograph, but rather as a starting point to synthesize ecological theory. Of course its points are simplistic, but the simplicity, brevity and generality are, to me, positive rather than negative aspects.

The paper was also one of the first to introduce Ecologists to the concept of the three terrestrial trophic levels: primary producers, herbivores and predators. It can also be seen as one of the progenitors of the food web concept. And if nothing else, it has spurred its detractors to experiments.


  1. "Mere numbers do not do justice to the bizarre condition of herbivore-impacted islets," the authors wrote. "The understory is almost free of foliage, so that a person standing in the interior sees light streaming in from the edge around the entire perimeter. There is almost no leaf litter, and the ground is bright red from the subsoil brought to the surface by leaf-cutter ants.

    This subject was a blast to learn about. Though ignorant of ecology I'm nevertheless sorry to admit that I had "thought" all taxa faced "malthusian" resource limitation. Fellow neophytes might enjoy the collection of ~20 ecology articles linked together in wikipedia, though I'm sure a textbook would be much better.

    The above quote refers to new-formed islands too small for top predators, and thus bearing multiple totally unchecked herbivore taxa. URL:

    I read one of the authors' papers on the subject. I don't have the knowledge to contend with possible confounding factors in the experiment, but it was a fascinating read.

    I read some other stuff and the one question I couldn't answer was why so many desert plants are armored (almost all of them really, at least in the Sonoran Desert).

    Also interesting to wonder why those plant taxa that are armored or toxic, are so, rather than some other taxa being so. And why so many grasses face major herbivory, while trees seem to face little. And why more, and more virulent, parasites of trees and herbs are not seen, especially those that are abundant with dense populations ("kill the winner" is another hypothesis relevant to that question).

  2. My guess is that these are all adaptations to herbivory. Desert plants are probably well defended because they are less able to regrow lost foliage in response to herbivory. A plant in the tropics may not need to invest in spines etc. because it can more easily regrow lost foliage, whereas desert plants may have considerable nutritive constraints.

    It may be that plants generally have more toxic defenses than do animals (I'm not absolutely sure this is true, but if so...) because they are unable to move away from the threat. So they invest in means to repel herbivores.

  3. This is very true, being a wildlife ecologist we often run into the problem of people wanting to put everything into simple categories. People like to argue about bottom-up or top-down, but the truth is, most populations are regulated by a certain degree of both. Research has shown however, that "top predators" ie the ones that have no natural predators (exclude humans)do regulate ecosystems to an extant we never expected. The problem is that by the time scienteists started studing this stuff, we had already wiped most of the big bad carnivores off the face of the earth.

    Some good reading on the subject would be the book "Where the Wild Things Were" by William Stolzenburg. "Sea Otters: Their role in restructuring near shore communities" by James A. Estes in Science (1974). Also, ecologists in Yellowstone are witnessing a complete trophic restructuring of the GYE since the reintroduction of the wolf. Problems we have been trying to solve for decades are "magically" being fixed by wolves. Fascinating stuff. Sorry for typos, I came across this blog while researching for a paper and really shouldn't have even taken the time to write this up. lol.