¨ The flow of energy through an ecosystem is one of the most important
factors that determines the system’s capacity to sustain life.
¨ Sunlight is the main energy source for life on Earth.
¨ In a few ecosystems, some organisms obtain energy from a source
other than sunlight.
¨ The energy is stored in inorganic chemical compounds.
Ø mineral water that flows underground or boils out of hot springs
Ø undersea vents are loaded with chemical energy
¨ Autotrophs use energy from the environment to fuel the assembly of simple
inorganic compounds into complex molecules.
v plants, some algae, and certain bacteria are organisms that produce
¨ Organisms that make their own food are also called producers.
Energy From the Sun
¨ The best-known autotrophs obtain energy through the process of photosynthesis.
¨ Photosynthesis is the process by which light energy is used to convert carbon
dioxide and water into oxygen and energy-rich carbohydrates.
¨ Oxygen is added to—and carbon dioxide is removed from—the
¨ On land, plants are the main autotrophs
¨ In freshwater and the upper layer of the ocean, algae are the main
¨ In tidal flats and salt marshes, cyanobacteria is the most common
Life Without Light
¨ Some autotrophs can produce food in the absence of light.
¨ These autotrophs rely on energy within the chemical bonds of inorganic molecules.
¨ Chemosynthesis is the use of chemical energy to produce carbohydrates.
¨ Several types of bacteria represent a large portion of living autotrophs.
¨ Some organisms cannot make their food directly and must acquire it from other organisms.
¨ Heterotrophs are organisms that rely on other organisms for their energy and food supply.
v animals, fungi and some bacteria fit into this group
¨ Heterotrophs are also called consumers.
¨ Types of heterotrophs are:
Ø Herbivores obtain energy by eating plants.
Ex) cows, caterpillars,
Ø Carnivores eat animals.
Ex) snakes, dogs, and owls
Ø Omnivores eat both plants and animals.
Ex) humans, bears,
Ø Detritivores food on plant and animal remains and other dead matter.
Ex) mites, earthworms, snails, and
Ø Decomposers break down organic matter
Ex) bacteria and fungi
¨ Energy flows through an ecosystem in one direction, from the sun or inorganic compounds to autotrophs
(producers) and then to various heterotrophs (consumers).
¨ The energy stored by producers can be passed through an ecosystem
along a food chain.
¨ A food chain
is a series of steps in which organisms transfer energy by eating and being eaten.
Ex) grassàgrazing antelopeàcoyote
Ex) algaeàzooplanktonàsmall fishàsquidàshark
¨ A food web
is a feeding relationship among various organisms in an ecosystem that form a network of complex interactions.
¨ A food web links all the food chains in an ecosystem together.
¨ A trophic level
is each step in a food chain or food web.
¨ Producers make up the first trophic level.
¨ Consumers make up the second, third, or higher trophic levels.
¨ Each consumer depends on the trophic level below it for energy.
¨ The amount of energy or matter in an ecosystem can be represented
by an ecological pyramid.
¨ An ecological pyramid is a diagram that shows the relative amounts of energy or matter contained within each trophic level in a food chain
or food web.
¨ There is no limit to the number of trophic levels but, only part
of the energy that is stored in one trophic level is passed on to the next level.
¨ Only about 10 percent of the energy available within one trophic
level is transferred to organisms at the next trophic level.
first-level consumersà1% second-level consumersà0.1% third-level consumers
¨ The more levels that exist between a producer and a top-level consumer,
the less energy that remains from the original amount.
¨ Biomass is the total amount of living tissue within a given trophic level.
¨ Biomass is expressed in terms of grams of organic matter per unit
¨ A biomass pyramid represents the amount of potential food available
for each trophic level.
Pyramid of Numbers
¨ A pyramid of numbers is based on the numbers of individual organisms
at each trophic level.
¨ For some ecosystems, such as a meadow, the pyramid of numbers is
the same as that of the energy and biomass pyramids.
¨ For others, such as a forest, there are fewer producers than there
¨ A single tree has a large amount of energy and biomass, but it is
only one organism.