224
BURNING ALL FUEL
RESERVES WILL
INITIATE THE RUNAWAY
GREENHOUSE
ENVIRONMENTAL FEEDBACK LOOPS
A
ll parts of an ecosystem
are interdependent. Any
species or habitat change
will feed back into the system, and
affect the whole of that system,
including the part where it all
started. In other words, the
feedback travels around in a loop.
In some situations, change
is kept in check by the loop. For
example, if aphids suddenly
multiply, they provide more food
for ladybugs, leading to an increase
in the number of ladybugs. But
with more ladybugs feeding on the
aphids, aphid numbers drop again.
This is negative feedback and it
helps maintain the status quo.
In other cases, feedback can
accelerate change. Shrubs, for
example, may begin to take over
from grass on newly colonized land,
casting their shade over the grass,
depriving it of sunlight, and slowing
its growth. The shrubs now have
more water and nutrients, so they
prosper at the expense of the grass.
This is positive feedback and it is
inherently destabilizing.
Ideas about feedback loops first
developed early in the 20th century.
They were based on the work of
two mathematicians—Alfred Lotka
(1880–1949) in the US and ItalianIN CONTEXT
KEY FIGURE
James Hansen (1941–)BEFORE
1875 In the book Climate and
Change, Scottish scientist
James Croll describes the
climate-warming feedback
effect of melting ice.1965 Canadian biologist
Charles Krebs discovers the
“fence effect,” showing vole
populations protected from
foxes rocketing, then crashing.1969 American planetary
scientist Andrew Ingersoll
highlights the “runaway
greenhouse effect” that caused
the planet Venus to heat up.AFTER
2018 Ecologists in Alaska
predict that the accelerating
release of methane from
formerly frozen lakes will
increase global warming.They keep populations
under control.They damp
down change.Negative feedback loops
regulate ecosystems.Negative
feedback loops
provide stability.US_224-225_Environmental_loops.indd 224 12/11/18 6:25 PM