When I did physics in school we learned that energy is “the ability to do work”. That’s all I can remember about it from way back then, but it was a simple definition and easy to parrot back to an examiner. Nowadays, as I get older, I would be tempted to say, “energy is what I wish I had more of.”
Many years later when studying ecology, it came to mean a whole lot more and the subject took a fascinating turn, because energy flows through ecosystems are what determines life on this planet and how it is organised.
With a few exceptions, all life on the earth gets its energy from the sun. Green plants take in carbon dioxide from the atmosphere and water from the soil and using the sun’s energy break down these compounds and reform them into glucose and oxygen. Oxygen is a waste product and vents through the leaves back into the atmosphere and the glucose is converted into more complex carbohydrates like starch and ultimately into proteins and fats. The whole process is called photosynthesis, literally, ‘putting together with light’.
For a given amount of sun energy, green plants use 90% of that energy just doing what plants do—growing, metabolising and producing flowers and seeds for the next generation. The remaining 10% is stored in the plant as chemical energy. Plants are the primary producers in an ecosystem. An animal that eats the plant (a herbivore), breaks it down and extracts the energy to drive its own metabolism, growth and reproduction. The same thing happens with that energy, i.e. 90% of the energy stored in the plant is used to fuel the herbivore’s activities and the remaining 10% is stored in the herbivore’s body. Along comes a carnivorous predator to eat the herbivore and again 90% of the energy is used to fuel the carnivore’s life and 10% is stored in the carnivore’s body.
This is called the ten percent law and it gives us a basic understanding of the cycling of energy through food chains. Each level in the food chain is called a trophic level (from ‘troph’, a feeder). There can be 3 or 4 trophic levels in a food chain: primary producers, herbivores, first order carnivores and (sometimes) second order carnivores.
For example :
What we get is a trophic pyramid like so :
And looked at in energy storage terms :
This is the energy pyramid that would result from 100,000 kcal of sunlight energy being collected by green plants. The amount of stored energy decreases by a factor of 10 as energy moves up the pyramid and only 10 kcal of that original 100,000 kcal remains at the top of the pyramid. The rest has been lost in the processes of metabolism, growth and repair and reproduction.
Furthermore, the ten percent law shows the inefficiency of energy capture at each successive trophic level. The rational conclusion being, energy efficiency is best preserved by sourcing food as close to the initial energy source as possible. As food writer Michael Pollan has observed : “eat food, not too much, mostly plants.”
The really interesting thing to understand is that the energy pyramid is also a biomass pyramid (biomass is the amount of living material at each level). So in the grass—>gazelle—>lion ecosystem, there is always more grass than gazelles and more gazelles than lions. If you think about it, it couldn’t be any other way. If it was the other way up and there were more lions than gazelles and more gazelles than grass, the lions would quickly eat out all the gazelles and starve to death, while at the same time the gazelles would eat out all the grass and starve to death.
For me, that understanding is one of the most fascinating aspects of how life on earth works and all thanks to that big ball of energy up in the sky.
However, I think it’s also important to point out that the pyramid is now top-heavy with the most efficient and resourceful top predator on the planet, namely us, Homo sapiens.
The predator-prey, pyramid-type systems described above have evolved to be that way, that’s why they work. Evolution has worked on these systems for 3.5 billion years. Spend that much time on something and eventually you end up with a system that works. So why have we humans ended up unbalancing the pyramid the way we have? Because in the naturally evolved systems, the availability of energy (via food) is controlled largely by competition from organisms at the same level as you and by the availability of food at the level below you. In turn, your numbers are controlled by predation by organisms on the pyramid above you. By adopting agriculture, humans made more food and energy available to themselves than the system would have provided naturally. Killing everything that competes with you for that food goes a long way to getting even more food for yourself and when you discover a wonderful energy source like fossil fuels and you can put that towards producing even more food/energy, well, you’re home and hosed. Except that you’ve messed up a perfectly good working system and somewhere along the way, when those fossil fuels have run out and your numbers have exploded beyond what the systems below you on the pyramid can naturally support, you can expect a rather nasty crash.
In the real world, when civilizations exhaust their resource bases and wreck the ecological cycles that support them, they fall.
~~~~~~John Michael Greer (The Archdruid Report)
Every creature born in the biological community of the earth belongs to that community. Nothing lives in isolation from the rest; nothing can live in isolation from the rest. Nothing lives only in itself, needing nothing from the community. Nothing lives only for itself, owing nothing to the community. Nothing is untouchable or untouched. Every life in the community is owed to the community–and is paid back to the community in death. The community is a web of life, and every strand of the web is a path to all the other strands. Nothing is exempt. Nothing is special. Nothing lives on a strand by itself, unconnected to the rest.
~~~~~~Daniel Quinn (Providence: the story of a fifty-year vision quest)