This book is out of print, so you can only find it in libraries. If you can get your hands on it, it’s a great read. Ehrlich defines and describes the most fundamental cogs in nature’s machinery, and the relationships between those cogs. The final chapter on ecosystems is the creme of the book, and the creme of that creme is his discussion of how food chains are ruled by entropy (the 2nd law of thermodynamics).
Why is the total mass of top level predators a mere fraction of the total mass of lower level vegetation? Because as energy moves up the food chain, at each stop on the “energy escalator,” a portion of the available energy is (in practical terms) lost. Ehrlich writes:
Organisms at each [food chain] level do work in the course of maintaining their structure and metabolism, growing, and reproducing. The energy so used [at that level] is thus subject to the inexorable tax of the second law, and the portion taxed away is not available to the next trophic level. The significance of the second law here is that, in any ecosystem, the amount of energy available to each successive trophic level declines. Thus more energy is available to support plants than herbivores, more to support herbivores than carnivores, and so on.
In other words, were it not for entropy, our planet would have a total mass of lions and tigers equal to its total mass of trees and grasses. That would be a lot of cats. We don’t have them because, as Ehrlich makes clear, only 10% of the energy that flows into one level of a food chain is available to the next level. So if the grass on an African plain manages to capture 1000 calories of energy from the sun, only 100 of those calories will be available to support the zebras and wildebeest, and only 10 of the those 100 will be available to support the lions.
The corollary to this fact is that we humans would have far more calories available for chewing and swallowing if we decided to only chew and swallow plants. The further up the food chain we dine, the less food is available to us, all other things being equal. So…. vegetarians of the world unite! (I’m actually not one… and never will be.)
My favorite part of the book is Ehrlich’s discussion of how ecosystems become disturbed, and how seemingly tiny organisms can cause extreme changes in landscapes. For example, in the 1880s, the viral disease “rinderpest” was accidentally introduced into Africa’s Serengeti region by cattle imported from Russia. The pest quickly decimated the cattle of the local Masai and then began knocking off herds of native buffalo, wildebeest and giraffe. With less “natural” food, the lions of the region began dining on local humans instead (these were the famed Tsavo man-eaters made famous in the Val Kilmer/Michael Douglas flick “The Ghost and the Darkness”). Not wanting to end their lives as lion fodder, local farmers abandoned their fields, which soon overgrew with brush and woodland. It wasn’t until 50 years later, in the 1930s, that these woodlands were regained as plains and farmland again.
So in roughly a decade, a tiny virus, invisible to the naked eye, transformed a vast swathe of African landscape from a herbivore-filled plain and farmland into a brushy woodland forest replete with man-eating beasts. Amazing.