General Knowledge

Feedback loops: How nature gets its rhythms

Testing, testing, one, two, three. When your band is trying to perform, feedback is an annoying obstacle, but in the grand orchestra of nature, feedback is not only beneficial, it’s what makes everything work. What exactly is feedback? The key element, whether in sound, the environment or social science, is a phenomenon called mutual causal interaction, where x affects y, y affects x, and so on, creating an ongoing process called a feedback loop. And the natural world is full of these mechanisms formed by the links between living and nonliving things that build resilience by governing the way populations and food webs respond to events. When plants die, the dead material enriches the soil with humus, a stable mass of organic matter, providing moisture and nutrients for other plants to grow.

The more plants grow and die, the more humus is produced, allowing even more plants to grow, and so on. This is an example of positive feedback, an essential force in the buildup of ecosystems. But it’s not called positive feedback because it’s beneficial. Rather, it is positive because it amplifies a particular effect or change from previous conditions. These positive, or amplifying, loops can also be harmful, like when removing a forest makes it vulnerable to erosion, which removes organic matter and nutrients from the earth, leaving less plants to anchor the soil, and leading to more erosion. In contrast, negative feedback diminishes or counteracts changes in an ecosystem to maintain a more stable balance.

Consider predators and their prey. When lynx eat snowshoe hares, they reduce their population, but this drop in the lynx’s food source will soon cause their own population to decline, reducing the predation rate and allowing the hare population to increase again. The ongoing cycle creates an up and down wavelike pattern, maintaining a long-term equilibrium and allowing a food chain to persist over time. Feedback processes might seem counterintuitive because many of us are used to more predictable linear scenarios of cause and effect. For instance, it seems simple enough that spraying pesticides would help plants grow by killing pest insects, but it may trigger a host of other unexpected reactions. For example, if spraying pushes down the insect population, its predators will have less food.

As their population dips, the reduced predation would allow the insect population to rise, counteracting the effects of our pesticides. Note that each feedback is the product of the links in the loop. Add one negative link and it will reverse the feedback force entirely, and one weak link will reduce the effect of the entire feedback considerably. Lose a link, and the whole loop is broken. But this is only a simple example, since natural communities consist not of separate food chains, but networks of interactions. Feedback loops will often be indirect, occurring through longer chains.

A food web containing twenty populations can generate thousands of loops of up to twenty links in length. But instead of forming a disordered cacophany, feedback loops in ecological systems play together, creating regular patterns just like multiple instruments, coming together to create a complex but harmonious piece of music. Wide-ranging negative feedbacks keep the positive feedbacks in check, like drums maintaining a rhythm. You can look at the way a particular ecosystem functions within its unique habitat as representing its trademark sound. Ocean environments dominated by predator-prey interactions, and strong negative and positive loops stabilized by self-damping feedback, are powerful and loud, with many oscillations.

Desert ecosystems, where the turn over of biomass is slow, and the weak feedbacks loops through dead matter are more like a constant drone. And the tropical rainforest, with its great diversity of species, high nutrient turnover, and strong feedbacks among both living and dead matter, is like a lush panoply of sounds. Despite their stabilizing effects, many of these habitats and their ecosystems develop and change over time, as do the harmonies they create. Deforestation may turn lush tropics into a barren patch, like a successful ensemble breaking up after losing its star performers. But an abandoned patch of farmland may also become a forest over time, like a garage band growing into a magnificent orchestra.

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