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small and large
Thanks for all your responses on why there are so many more species of
small animals than large ones. Here are all the hypotheses that were
forwarded (I hope I haven't left anyone out):
1) Small animals have faster rates of speciation due to:
a) higher fecundity
b) shorter generation times
c) greater isolation between populations
2) Small animals have lower extinction rates due to:
a) higher fecundity
b) more dense populations
c) more limited needs
d) greater genetic diversity
I would like to take a little informal opinion survey. Please rank each of
the 7 hypotheses above on a scale from 0 to 5 (increasing importance)
according to how much you believe they have contributed to the exponential
decrease in species richness with decreasing body size. If you prefer yet
another hypothesis, advance it by all means and give it a similar ranking.
With regard to the question of how often large species give rise to small
ones, the terms large and small were admittedly vague. However, the fact
that we can indeed find lineages that show decreasing body size (sometimes
dramatically) does not change the fact that this trend is exceptional. It
stands to reason when we consider that at any given body size, there are
far fewer species twice as large than half as large. Even if the rate of
speciation were identical at every body size, and if the probability of
increasing size in lineages were identical to the probability of decreasing
size, the overwhelming predominance of small animals would produce an
overwhelming predominance of increasing-size speciation events. I believe
this is in fact what we observe.
Nevertheless, it is rather remarkable that no dinosaur has ever been
described that is known to have been smaller than a chicken as an adult.
But then, of all the mammals that are smaller than a chicken, I suspect
that not many have mammalian ancestors that are larger than a chicken.
Dinosaur herbivores, like large mammalian herbivores, had good reasons for
staying large, and their predators tracked them.