Why are there so many kinds of animals?

In the first discussion for the “Biodiversity” group in the 2015 Critical Thinking in Ecology course at the University of Edinburgh, we tackled the theme of “The Diversity of Life”.

We read the papers:

Hutchinson, G. E. (1959) Homage to Santa Rosalia or why are there so many kinds of animals? American Naturalist, 145-159.
Hutchinson, G. & MacArthur, R. (1959) A theoretical ecological model of size distributions among species of animals. American Naturalist, 117-125.

And asked ourselves the question: “Why are there so many kinds of life out there on planet earth?”

We started the discussion off with introductions where we each described our favourite living organism to get us started: the list spanned the majestic killer whale to the often much maligned grey squirrel – that is really very cute despite its invasive tendencies.

Then we delved deeper into the science.  The major take-home messages of our discussion were as follows:

  1. If you want to start to understand the key concepts in the field of ecology, first go back and read a foundational paper or two like Hutchinson’s Address to the American Society of Naturalists from 1959 and then check out the wikipedia pages for concepts like ecological niche, limiting similarity, body size and species richness, or macroecology
  2. Theoretical papers should be made to be easier to interpret for a non-mathematical audience, but equally, those of us that are afraid of equations should work harder to understand them.  Hutchinson and MacArthur’s model of size distributions among species of animals isn’t so complicated at all, if it is explained well!


    From Hutchinson and MacAurthur, 1959

  3. Ecology was different back in the day with a different writing style in papers, fewer references, regression lines fitted “by eye” rather than by complex statistics and close connections between different researchers – Hutchinson was the PhD supervisor of MacAurthur and Slobodkin who he thanks in his acknowledgements for providing him with “their customary kinds of intellectual stimulation” and then he says: “To all these friends I am most grateful”.

So, why are there so many kinds of animals? Well, I am not sure that over 50 years on from Hutchinson’s paper, we know for sure, but the answer probably has something to do with niches, evolution and macroecological patterns of the diversity of life on planet earth.

It was a very jolly first discussion – thanks everyone for your participation.  I am already looking forward to our next tutorial group meeting!

by Isla


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