Why Are The Trees Green?

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Why are trees green? It was this simple question, posed by Harvard ecologist Fred Smith in the late ’50s, that led Smith and his student Robert Paine, and others, to startling and profound discoveries about the kinds of delicate balance and styles of regulation characteristic of healthy ecosystems.

What Smith was asking, really, was this: Why haven’t herbivores eaten all the leaves, leaving the trees barren? After all, there’s so much good food, why don’t they eat it up?

The answer turns out to be astonishingly simple: The number of herbivores, and so the amount that they can eat, is governed (negatively regulated) by the number of their predators.

What made this simple thought hard to arrive at was the tendency to think of food chains as linear. Plants convert light to energy. Insects eat the plants. Birds eat the insects, and so on. But in fact, it is the existence of what Paine came to call “keystone predators” at the top of the food chain that allows for the flourishing of diversity and rich vitality lower down. Food chains exhibit feedback. The disappearance of sea otters led to barren ocean floors. Why? Because without the sea otters there was nothing to stop the sea urchins from eating all the kelp.

Similarly, if you remove starfish from the tidal pool areas, you can end up greatly diminishing the diversity of species flourishing in these intertidal regions. Why? Because with no starfish to keep their numbers in check, the mussels will eat up all the barnacles, algae, limpets and chiton. Who would have thought it? Want more kelp? Introduce sea otters. Want greater biodiversity in your tide pool? Be kind to starfish. This is the phenomenon that Paine named trophic cascade.

shared via: www.npr.org/sections/13.7/2016/05/27/479694232/why-are-the-trees-green
article: Alva Noë
photo: Hero Images/Getty Images

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