Feedback loops: How nature gets its rhythms – Anje-Margriet Neutel

 

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Testing, testing, one, two, three.
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When your band is trying to perform, feedback is an annoying obstacle,
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but in the grand orchestra of nature, feedback is not only beneficial,
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it’s what makes everything work.
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What exactly is feedback?
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The key element, whether in sound, the environment or social science,
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is a phenomenon called mutual causal interaction,
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where x affects y, y affects x, and so on,
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creating an ongoing process called a feedback loop.
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And the natural world is full of these mechanisms
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formed by the links between living and nonliving things
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that build resilience by governing the way populations
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and food webs respond to events.
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When plants die, the dead material enriches the soil with humus,
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a stable mass of organic matter, providing moisture and nutrients
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for other plants to grow.
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The more plants grow and die, the more humus is produced,
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allowing even more plants to grow, and so on.
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This is an example of positive feedback,
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an essential force in the buildup of ecosystems.
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But it’s not called positive feedback because it’s beneficial.
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Rather, it is positive because it amplifies a particular effect or change
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from previous conditions.
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These positive, or amplifying, loops can also be harmful,
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like when removing a forest makes it vulnerable to erosion,
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which removes organic matter and nutrients from the earth,
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leaving less plants to anchor the soil, and leading to more erosion.
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In contrast, negative feedback diminishes or counteracts changes in an ecosystem
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to maintain a more stable balance.
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Consider predators and their prey.
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When lynx eat snowshoe hares, they reduce their population,
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but this drop in the lynx’s food source will soon cause their own population to decline,
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reducing the predation rate and allowing the hare population to increase again.
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The ongoing cycle creates an up and down wavelike pattern,
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maintaining a long-term equilibrium and allowing a food chain to persist over time.
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Feedback processes might seem counterintuitive because many of us
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are used to more predictable linear scenarios of cause and effect.
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For instance, it seems simple enough that
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spraying pesticides would help plants grow by killing pest insects,
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but it may trigger a host of other unexpected reactions.
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For example, if spraying pushes down the insect population,
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its predators will have less food.
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As their population dips,
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the reduced predation would allow the insect population to rise,
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counteracting the effects of our pesticides.
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Note that each feedback is the product of the links in the loop.
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Add one negative link and it will reverse the feedback force entirely,
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and one weak link will reduce the effect of the entire feedback considerably.
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Lose a link, and the whole loop is broken.
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But this is only a simple example,
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since natural communities consist not of separate food chains,
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but networks of interactions.
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Feedback loops will often be indirect, occurring through longer chains.
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A food web containing twenty populations can generate thousands of loops
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of up to twenty links in length.
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But instead of forming a disordered cacophany,
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feedback loops in ecological systems play together,
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creating regular patterns just like multiple instruments,
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coming together to create a complex but harmonious piece of music.
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Wide-ranging negative feedbacks keep the positive feedbacks in check,
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like drums maintaining a rhythm.
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You can look at the way a particular ecosystem functions within its unique habitat
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as representing its trademark sound.
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Ocean environments dominated by predator-prey interactions,
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and strong negative and positive loops stabilized by self-damping feedback,
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are powerful and loud, with many oscillations.
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Desert ecosystems, where the turn over of biomass is slow,
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and the weak feedbacks loops through dead matter are more like a constant drone.
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And the tropical rainforest, with its great diversity of species,
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high nutrient turnover, and strong feedbacks among both living and dead matter,
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is like a lush panoply of sounds.
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Despite their stabilizing effects,
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many of these habitats and their ecosystems develop and change over time,
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as do the harmonies they create.
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Deforestation may turn lush tropics into a barren patch,
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like a successful ensemble breaking up after losing its star performers.
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But an abandoned patch of farmland may also become a forest over time,
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like a garage band growing into a magnificent orchestra.

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