The centrifugal theory of species diversity

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.

Literature Cited

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.


Two notes:

(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 (sheard@unb.ca) 1991 (original piece) and October 29 2015 (commentary)

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20 thoughts on “The centrifugal theory of species diversity

  1. Elizabeth Moon

    This is better than any of the weird and funny (?) things I thought up as an undergraduate (either time.) Got a morning chuckle out of me, in fact, on a day in which, so far, not anything else has been funny.

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  2. jeffollerton

    I appreciate that this was meant to be tongue in cheek, but isn’t the physics wrong? Under the effect of “centrifugal force” the animals wouldn’t accumulate at the equator as the effect is perpendicular to the axis of spin. So they would either stay where they were (assuming gravity is the greater force) or, in the absence of gravity, fly off into space. Perhaps that explains how life got here from Mars….? 🙂

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    1. ScientistSeesSquirrel Post author

      Well, Jeff, it didn’t go through peer review, so your claim is possible! But I’ve done the experiment (NOTE: no actual animals were tied to little strings and whirled around my head) and it seems to work…

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          1. jeffollerton

            Ah, but that’s different to having them attached to the surface of a sphere. Whirl them round your head and they are all coming from one fixed point.

            ________________________________

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            1. ScientistSeesSquirrel Post author

              True. Hence the importance of friction; that’s what resists their slippage to the equator (or as you put it, that’s why they’re “attached to the surface of a sphere”). I see the ambiguity – strings have two end. When I say “the surface of the Earth is imagined to lie at the end of the extended strings”, I mean that one end is at the Earth’s centre, and the other at the Earth’s surface. Remember, the force of gravity acts as if it were coming from the Earth’s centre.

              I have very, very sharp-eyed readers!

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              1. jeffollerton

                Yes, gravity acts towards the centre, but centrifugal force acts from the axis of spin, which would throw the strings/animals outwards, not towards the mid-line of a sphere. Imagine if it was a broom handle rotating (the handle is equivalent to the axis of the earth) – then strings don’t all point to the same place, they go out perpendicular to the handle/axis.

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                1. ScientistSeesSquirrel Post author

                  Correct – “they go out perpendicular to the handle/axis”. So the objects form a disc, if you like, which is perpendicular to the broom handle. They do not form a sphere centred on the centre of the handle! And if they started in such a sphere, the rotation would fling them into a disc. (This, as I understand it, is why Saturn has rings rather than a cloud, but I could be off base here EDIT: So very far off base… this may be the stupidest thing I’ve ever written on my blog. Of course this is not why Saturn has rings. D’oh!). So you are seeing the same behaviour I am.

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                  1. jeffollerton

                    I wonder if we are talking at cross purposes here? The thing I don’t get is how species at higher latitudes would end up at the equator, because moving out perpendicular to the axis would not result in an accumulation of species at the equator. There’d have to be some other force acting at an angle to the axis, not perpendicular, to create motion in that direction.

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                    1. ScientistSeesSquirrel Post author

                      Jeff: you are right, I’m pretty sure we’re making totally different arguments. I have been arguing that my physical model (things on strings whirled around) behaves the way I say (things in a plane perpendicular to spinning). I am right about that. You have been arguing that this is a lousy physical model for objects on the surface of the Earth. You are right about that! Gravity is not really a string, and if you took the Earth’s surface away objects sitting on it would fall toward the centre, not slip to the equator; and if you whirled them fast enough to overcome gravity, without the string they’d be flung outwards. Good thing, as you observed to start with, the whole thing is tongue in cheek, eh? 🙂

                      Liked by 1 person

                    2. jeffollerton

                      On the other hand, I have seen an argument that there are more species in the tropics because there’s more land area in that belt than at higher latitudes. Could the accumulation of continental mass at the equator be a function of centrifugal force….? For the purposes of History of Science I suggest that we call this the Ollerton Modification of the Heard Conjecture 🙂

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