The Ecology Book

136

biological and physical elements

of the environment. The different

types of ecosystem can be defined

by their physical environments.

There are four categories of

ecosystem: terrestrial, freshwater,

marine, and atmospheric. These

can be further subdivided into

various types according to different

physical environments and the

biodiversity within them. Terrestrial

ecosystems, for example, can be

subdivided into deserts, forests,

grasslands, taigas, and tundras.

Dynamic feedback

Tansley’s most important insight

was that these discrete communities

of living and nonliving components

form dynamic systems. In a

terrestrial ecosystem, for example,

the organisms interact to recycle

matter: plants absorb carbon

dioxide (CO 2 ) from the atmosphere

and nutrients from the soil to grow.

These plants release life-sustaining

oxygen into the atmosphere by

respiration, and provide food for

animals. Animal excreta and dead

matter also release carbon, and

provide material to be decomposed

by bacteria and fungi, in turn

providing soil nutrients for plants.

Arthur Tansley also argued

that these internal processes

within an ecosystem conform to

what he described as “the great

universal law of equilibrium.”

Self-regulating, these processes

have a natural tendency toward

stability. The cycles within an

ecosystem contain feedback loops

that correct any fluctuations from

a state of equilibrium.

THE ECOSYSTEM

A small glacial lake, or tarn, in the

English Lake District. Each tarn has

an ecosystem that varies according

to many factors, including the degree

of nutrient enrichment in the water.

Each ecosystem is located in a

particular area, with characteristics

unique to its environment, and

behaves as a self-contained and

self-regulating system. Together,

the patchwork of ecosystems

across the globe form what

Austrian scientist Eduard Suess

called the biosphere—the sum

total of all ecosystems.

External factors

Various external factors, such as

climate and the geological makeup

of the surrounding environment,

can affect an ecosystem. One

constant external force that affects

all ecosystems is the Sun. The

supply of energy that it provides

enables photosynthesis and the

capture of CO 2 from the atmosphere;

some of this energy is distributed

through the ecosystem and through

the food chain. In the process, some

of this energy dissipates as heat.

Other external factors, however,

can arise unexpectedly to create

pressures on ecosystems. All

ecosystems are subjected to

external disturbances from time

to time, and must then go through

a process of recovery. These

disturbances include storms,

In this ecosystem, plants use the Sun’s energy

for photosynthesis. As shown by the pale arrows,

this energy is passed on—to herbivores, who eat

plants; to predators, who eat herbivores; and to

saphrophytes, who take energy from decomposing

remains and transfer nutrients to the soil.

At each stage, some energy is lost as heat.

The dynamic transfer of energy

Frog

(predator)

Heat

Heat

Heat

Fish

(herbivores)

Plants

(producers)

Saphrophytes

and parasites

(decomposers)

Nutrients

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