Photo: Jaguar – a large-bodied tropical mammal. U.S. Fish and Wildlife Service Digital Library System, public domain.
It’s frequently claimed that a person can expect to have only a handful of truly good ideas in a lifetime. One should, therefore, use them well! I had one of my best ideas (with tongue in cheek, that is) as a grad student – but I buried it in an issue of the Ecological Society of America Bulletin. If blogs had existed back then, perhaps things would have been different; but they do now, so I thought I’d resurrect this piece it from its obscure tomb.
I’m pleased that the ESA Bulletin occasionally indulges itself in a bit of levity, and am grateful for permission to repost it here. I hope you’ll enjoy the piece. Here it is, as originally published (ESA Bulletin (72(1):13, 1991):
The Centrifugal Theory of Species Diversity
It is well known that the Earth turns on its axis, completing one rotation in approximately 24 hours (Halladay and Resnick 1978); the essence of the centrifugal theory is that this results in species simply being flung to the equator. A simple analogy should make this clear. Imagine a very small ball to which a large number of objects are fastened by strings of uniform length. The objects, of course, are species; and the surface of the Earth is imagined to lie at the ends of all the extended strings. The strings are analogous to the force of gravity. Now imagine that the ball is spun very rapidly; the result would clearly be the accumulation of the objects around the equatorial plane of the spinning system. Clearly, the latitudinal diversity gradient must result from the same process.
This mechanism also provides an explanation for Bergman’s rule (that animals often increase in size towards the poles). Friction provides a force opposing the tendency of animals to be flung to the equator, and the larger the animal, the greater the frictional force holding it in place. Furthermore, the larger the animal, the less likely it is to jump clear of the ground, eliminating frictional restraint completely (albeit briefly). Heavier animals do not slip to the equator as quickly.
The centrifugal theory of species diversity represents by far the simplest, most parsimonious explanation for tropical species richness of those suggested to date. The conventional use of parsimony to evaluate hypotheses would dictate acceptance of this mechanism in place of all other, overly elaborate constructs.
Begon M, JL Harper and CR Townsend. 1986. Ecology: individuals, populations, and communities. Sinauer, Sunderland, MA.
Dobzansky T. 1950. Evolution in the tropics. American Scientist 38:209-221.
Halliday D and R Resnick. 1978. Physics. 3rd ed. John Wiley and Sons, New York, NY.
Kusnezov M 1957. Numbers of species of ants in faunas of different latitudes. Evolution 11:298-299.
Pianka ER. 1983. Evolutionary Biology. 3rd ed. Harper and Row, New York, NY.
Stehli FG, AL McAlester and CE Heisley. 1967. Taxonomic diversity of recent bivalves and some implications for geology. Geological Society of America Bulletin 78:455-466.
(1) For a while this was my most requested reprint (yes, in those days we still sent requests for paper reprints). Fortunately, it isn’t any more, as I’ve published some actual science.
(2) When I wrote this I was (embarrassingly) unaware that actually was a “centrifugal theory of speciation” (Brown 1957 Quarterly Review of Biology 32:247–277) as well as a “centrifugal theory of community organization” (Keddy and MacLellan 1990 Oikos 59:75-84). Neither is directly related to tropical diversity, so I hope the authors of those papers realize I wasn’t making fun of them!
I promise that my next post will be original, and not funny.
© Stephen Heard (firstname.lastname@example.org) 1991 (original piece) and October 29 2015 (commentary)