14 | Section 3
Science
Energy Cycling
Sunlight is the ultimate source of energy that keeps
Earth’s ecosystems functioning. Energy cycling in an
ecosystem is the movement of carbon-based compounds
through the tissues of live and dead plants and animals.
There are two forms of energy:
XX kinetic energy – the energy of an
object in motion
YY potential energy – stored energy that
is available for performing work
There are two fundamental laws that govern the use and storage of all energy.
XX The first law of thermodynamics states that energy cannot be created nor destroyed.
YY The second law of thermodynamics says that energy disperses from being localized to spread out
unless it is prevented from doing so.
EXAMPLE: Energy and matter are not created out of thin air, nor are they destroyed. A plant is
consumed and an herbivore grows. An herbivore dies, decomposes, and replenishes the organic matter
in the soil. As energy is transferred from one organism to another, a portion of that energy is stored
in living tissue, and a part is dissipated as heat to the surrounding air. In this way, the flow of energy
through an ecosystem is the story of how carbon cycles through the food chain.
Primary Production
¨¨ The rate that sunlight is converted by autotrophic organisms (organisms which synthesize their own
food; for example, plants on land and algae in water) via photosynthesis into organic compounds is
referred to as primary productivity.
¨¨ Gross primary productivity (GPP) is the total rate of photosynthesis, or the total energy obtained
by autotrophs.
¨¨ The rate of energy stored after accounting for the energy expended is referred to as net primary
productivity (NPP).
¨¨ The stored energy found at a given area at a given time is often referred to as biomass, which is
simply the amount of organic material that can be found at a given area at any given time.
In terrestrial ecosystems, temperature,
precipitation, and nutrients control rates
of primary productivity. Generally, NPP
increases with increasing mean annual
precipitation, mean annual temperature,
and a longer growing season.
In aquatic ecosystems, the major controls on
primary productivity are temperature, light, and
nutrient availability. Microscopic phytoplankton
perform the majority of the ocean’s primary
production, form the basis of the oceans’ food
web, and fix large amounts of carbon.