Page 126 - Environment: The Science Behind the Stories

feedback loops. Negative feedback enhances stability, and over It is difficult to understand systems fully just by focus-
time only those systems that are stable will persist. ing on their individual components because systems can
Positive feedback loops have the opposite effect. Rather show emergent properties, characteristics not evident in the
than stabilizing a system, they drive it further toward an components alone. Stating that systems possess emergent
extreme. In positive feedback, increased output leads to properties is a lot like saying, “The whole is more than the
increased input, leading to further increased output. Exponen- sum of its parts.” For example, if you were to reduce a tree
tial growth in a population (pp. 84–85) is one such example. to its component parts (leaves, branches, trunk, bark, roots,
The more individuals there are, the more offspring can be pro- fruit, and so on) you would not be able to predict the whole
duced. Another example is the spread of cancer; as cells mul- tree’s emergent properties, which include the role the tree
tiply out of control, the process is self-accelerating. plays as habitat for birds, insects, fungi, and other organisms
One positive feedback cycle of great concern to environmen- (Figure 5.2). You could analyze the tree’s chloroplasts (pho-
tal scientists today involves the melting of glaciers and sea ice in tosynthetic cell organelles), diagram its branch structure, and
the Arctic as a result of global warming (p. 516). Ice and snow, evaluate the nutritional content of its fruit, but you would still
being white, reflect sunlight and keep surfaces cool. But if the cli- be unable to understand the tree as habitat, as part of a forest
mate warms enough to melt the ice and snow, darker surfaces of landscape, or as a reservoir for carbon storage.
land and water are exposed, and these darker surfaces absorb sun- Systems seldom have well-defined boundaries, so decid-
light. This absorption warms the surface, causing further melting, ing where one system ends and another begins can be difficult.
which in turn exposes more dark surface area, leading to further Consider a smartphone. It is certainly a system—a network
warming (Figure 5.1b). Runaway cycles of positive feedback are
rare in nature, but they are common in natural systems altered by EMERGENT PROPERTIES
human impact, and they can destabilize those systems.

FaQ But isn’t positive feedback “good”
and negative feedback “bad”?
Understanding negative and positive feedback in systems can
be difficult, because it goes against the way we use those
terms in everyday language. In daily life, positive feedback
(such as a complimentary comment on a writing assignment) Element of CO 2 sink Habitat
is something that makes us feel good, whereas negative feed- forest ecosystem
back (such as criticism on schoolwork) may make us feel bad.
In essence, we have been trained to view positive feedback
as a stabilizing force (“Keep up the good work, and you’ll
succeed”) and negative feedback as a destabilizing force (“You
need to change your approach if you’re going to succeed”).
In environmental systems, it’s the opposite! Nega-
tive feedback resists change in systems, and in doing so it SYSTEM
enhances stability, typically keeping conditions within ranges
beneficial to life. Positive feedback exerts destabilizing effects
that push conditions in systems to extremes, threatening
organisms adapted to the system’s normal conditions. Thus,
negative feedback in environmental systems typically aids liv-
ing things, whereas positive feedback often harms them. Tree

Systems show several defining properties CHAPTER 5 • Envi R onm E n TA l S y STE m S A nd E C o S y STE m E C ology
Leaves Chloroplast
In a system stabilized by negative feedback, when pro-
cesses move in opposing directions at equivalent rates so
that their effects balance out, they are said to be in dynamic
equilibrium. Processes in dynamic equilibrium can con-
tribute to homeostasis, the tendency of a system to main- Acorn Branches Trunk Water
tain constant or stable internal conditions. A system (such
as an organism) in homeostasis keeps its internal conditions COMPONENTS
within a range that allows it to function. However, the steady Figure 5.2 A system’s emergent properties are not evident
state of a homeostatic system may itself change slowly over when we break the system down into its component parts.
time. For instance, Earth has experienced gradual changes in For example, a tree serves as wildlife habitat and plays roles in
atmospheric composition and ocean chemistry over its long forest ecology and global climate regulation, but you would not
history, yet life persists and our planet remains, by most defi- know that from considering the tree only as a collection of leaves,
nitions, a homeostatic system. branches, and chloroplasts. 125

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