The Ecology Book

225

See also: Predator–prey equations 44–49 ■ Competitive exclusion principle

52–53 ■ Global warming 202–203 ■ Halting climate change 316–321

THE LIVING EARTH

Positive feedback interferes with

a balanced ecosystem. If there

is a surplus of resources, or a lack

of predators, a population can grow

freely. A bigger population leads to

more births, and so an acceleration

of the growth in population.

Equally, positive feedback can

result in an accelerated contraction

of a population. If fish stocks

decline in a lagoon, for instance,

local people may resort to importing

canned food. Pollution from the

dumps where the cans are thrown

away can seep into the lagoon,

killing the fish—and encouraging

the locals to import even more

of the damaging cans. And yet,

positive feedback loops can

sometimes set off a chain of events

that becomes a “virtuous” circle.

For example, if shrubs are planted

in unstable soil, their roots may

stabilize the soil, allowing both

the shrubs and soil to thrive. ■

Feedback loops and

climate change

In recent years, accelerating

and decelerating warming

trends have brought the idea

of feedback loops to the fore

in climate change science.

In 1988, the climate scientist

James Hansen spoke at a US

congressional hearing of the

rises in global temperature

caused by human activity.

He has since voiced the belief

that the continued burning

of fossil fuels could set in

motion a series of calamitous

positive feedbacks on Earth’s

climate, leading to the

“runaway greenhouse” he

describes in his 2009 book

Storms of My Grandchildren.

One warming feedback

loop is created by the melting

of polar ice caps, as newly

exposed land and water

absorb the heat that the ice

once reflected back into the

atmosphere. The melting of

Siberian permafrost is another

warming loop. As temperature

rises melt the permafrost,

huge amounts of methane,

a greenhouse gas, could be

released into the atmosphere,

accelerating global warming.

Arctic areas such as Greenland

have seen a reduction in summer

ice of 72 percent since 1980. The

warming of the atmosphere and

rising sea levels are part of the

resulting positive feedback loop.

Vito Volterra (1860–1940)—who

independently devised equations

based on the interaction between

predators and prey. Their equations

showed that a prey population will

grow rapidly when the number of

predators drops, while the predator

population will drop when prey

numbers drop, because the

predators go hungry. The result is

a constant cycle of falling and

rising predator and prey populations.

Balancing the system

The predator–prey cycles identified

by Lotka and Volterra were focused

on the interaction between single

predator and prey species. Since

their studies, the theory of feedback

loops has developed to embrace

entire ecosystems. Ecologists now

think that negative feedback loops

are of central importance for the

functioning of all ecosystems,

keeping every part of them naturally

within the bounds of sustainability.

Populations can never swell for long

beyond the carrying capacity of the

rest of the system to support them.

Thus, negative feedback regulates

an ecosystem and keeps it stable.

In a healthy ecosystem, a repeating

fluctuation in numbers between prey,

such as rabbits, and predators, such

as foxes, is an example of a negative

feedback loop balancing the system.

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