Energy and ecosystems

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 :

plant—>caterpillar—>sparrow—>sparrowhawk.

grass—>gazelle—>lion.

What we get is a trophic pyramid like so :

image

And looked at in energy storage terms :

Image result for energy pyramid ecology

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.

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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)

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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)

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2 Responses to “Energy and ecosystems”

  1. Chris Says:

    Very well written piece, on how our eco-systems work. Only as top predator, we shun the flow of energy to anything else. Which increasingly makes our living environment impossible to dwell in.

    This 4 day heatwave which hit QLD and some parts of NSW, has reminded me how much we need canopy cover, from the sun. The trees thrive in sunlight, so long as they have adequate moisture. We don’t thrive in that much sunlight, so the trees protect our hides. Literally.

    We don’t thank them very much, by removing their companions in under-storey, and stick them in overexpose concrete jungles, just to shade the footpaths. I read someone defend “tree lawns” as a sensible strategy for pedestrians and road traffic. All I could think is, how pandering to the ignorance of our artificial constructs, deemed “safer”, hides a more sinister threat. The trees are our only network of protection against the sunlight, and we treat barren spaces like they’re safer for us.

    We should be giving our trees their companion under-storey back, and use less concrete solutions. Or at least plan for the concrete in a more symbiotic relationship with ecosystems. Concrete grass pavers are a good example of that. They allow water to penetrate the ground, which maintains connection to the flow of energy in an eco-system. Instead of breaking it and diverting energy flow away from where concrete is laid.

    Not saying we avoid the use of concrete altogether, but that we stop treating nature like it’s not a stakeholder in our decision making processes. It was good to read your thoughts and thanks for highlighting what needs more emphasis, in our understanding of eco-systems.

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    • foodnstuff Says:

      Thanks, Chris. Yes, trees are definitely important for moderating the climate. I work at at a nearby friend’s house in her garden and there are no trees, just lawn and shrubs. I really notice the difference to when I’m working home here, and she’s only 5 mins drive away. It’s so much hotter there, even on a mild day.

      A single tree, on its own, without understorey is like a single chicken on its own. Very sad. The understorey provides habitat for all the little birds and other things that keep the tree healthy and free of predators. I always recommend when advising people about planting, to put a group of 3 or 5 trees close together with some understorey between and around. The tree roots interlock and support one another and the canopies do the same and make for greater stability. 3 trees sharing the same conditions of light, water and nutrients won’t grow much bigger than one tree alone and are less likely to blow over.

      Ecology should be taught in school from grade 1. I don’t know how else we combat the separateness of Man and Nature which seems to have permeated our present culture.

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