Influences on Scale of Living Things on Earth or Other Planets

[annyong]

I was just wondering what the exact influences are on the scale, and not the size, of living things on Earth. Plants and animals ranging in size from the smallest insect to the largest dinosaur or whale are all still relatively close in size. Why is it that there aren't creatures orders of magnitude larger scale? or smaller scale? On a planet with higher or lower gravity, would the overall scale of life change? Is a mile-wide living creature possible? Or is the scale of life inherently based on the size of the molecules we all start with?

[Mr Skeptic]

Yes, gravity plays a role in the sizes of things on earth. Other factors are food supply, volume:surface area ratio, heat production/loss, stresses due to moving, etc. For example, in islands, the limited food supply shrinks large animals, and the lack of predators makes small ones bigger, the rule being animals move closer to the size of rabbits. We have somewhat extended our surface area:volume ratio by using convoluted lungs and guts; otherwise we would never be able to get enough food and oxygen. Likewise, elephants have their big ears to increase their heat dissipation.

 

Remember that our metabolism is based on volume, implying food usage, oxygen usage, and heat production increase as the cube of the size, as does mass. Meanwhile, oxygen supply, food supply, and heat exchange occur at the surface, which increases as the square of the size. Bone strength increases as the cross section of bone, which increases as the square of size, while mass increases as the cube.

 

Then there are limits on actually finding food. This is more complicated, but if you get too big, you won't find enough to eat.

 

Basically, if you wanted to make a particular animal much bigger or smaller, you would run into some kind of problem. At some point a design change would need to be made to keep everything in balance, and there are limits in size that I don't think could be overcome.