Energy Flow
3-2
• Sunlight - main source of energy for life
on Earth
– Of all the sunlight that reaches Earth’s
surface, only <1% is used by living things
• Some organisms rely on energy stored
in chemical compounds
Producers
• Autotrophs - organisms that capture energy
from sun or chemicals to produce their own
food
– Ex. Plants, some algae, and certain bacteria
– a.k.a. producers
• All producers are essential to the flow of
energy through the biosphere
Energy from the Sun
• Photosynthesis - autotrophs use sun’s energy
to convert CO2 and H2O to O2 and C6H12O6
– This process is responsible for adding O2 to
atmosphere and removing CO2
• Plants - main autotroph on land
• Algae- main autotroph in sunlit layer of ocean
• Cyanobacteria (photosynthetic bacteria) important in tidal flats & salt marshes
Life without Light
• Bacteria can produce food without
sunlight
– Use energy stored in chemical bonds of
inorganic molecules like hydrogen sulfide
– Known as chemosynthesis
• Chemosynthetic bacteria live in very
extreme environments, like volcanic
vents on the ocean floor & hot springs
Consumers
• Organisms that cannot harness energy
directly need to acquire energy from
other organisms
• Ex: animals, fungi, & many bacteria
• a.k.a. heterotrophs
Types of Heterotrophs
•
•
•
•
Herbivores - plant eaters
Carnivores - meat eaters
Omnivores - plant and meat eaters
Detritivores - feed on plant and animal
remains (detritus)
– Ex: earthworm, snails, crabs
• Decomposers - break down organic matter
– Ex: bacteria, fungi
Feeding Relationships
• Energy flows in an ecosystem in 1
direction: sun-->producers-->consumers
• Relationships between producers &
consumers connect organisms into a
feeding network based on who eats
whom
Food Chains
• Diagrams that show energy being
passed through an ecosystem in series
of steps, in which organisms transfer
energy by eating & being eaten
• Require at least 3 organisms: producer,
herbivore, carnivore/omnivore
• Can also have detritivore &/or
decomposer
• Organisms are connected by arrows
• Arrows point to who is eating, not to
who is being eaten
• Imagine arrow head as an open mouth
• Raspberry-->rabbit-->coyote
• Grass-->mouse-->snake
• Grass<--mouse <--snake (WRONG)
• Sun-->mouse-->owl (WRONG)
• Dirt-->grass-->vole
Food Webs
• Diagram that shows all the complex
feeding relationships in an ecosystem
• Several food chains (from same
ecosystem) together
• Arrows still point to who is doing the
eating
• Trophic level - each step in food chain
or food web
– Producers - 1st trophic level
– Herbivores - 2nd trophic level
– Carnivores - 3rd trophic level
• Each consumer depends on the trophic
level below it for energy
Energy and Food
• Of all the sunlight that reaches Earth,
plants use less than 1% of it
• However, they are able to store over
30% of the energy they capture, and
produce 170 billion metric tons of food
each year
• The total amount of organic matter
present in a trophic level is called
biomass
• Biomass is the amount of energy, in the
form of food, available to the next
trophic level
• About 90% of the energy contained in a
trophic level is used by organisms in
that level
• Only 10% of the stored energy is
passed on to the consumers in the next
trophic level
• Known as the “10% law” or “rule of
10%”
– Main reason most food chains have 5 or
less links
Ecological Pyramids
• Diagrams that show relative amount of
energy or matter contained in each
trophic level
• There are 3 types of pyramids: energy,
biomass, and numbers
Energy Pyramid
• There is no limit to the number of trophic
levels a food chain can support, but
determined by # of producers
• However, only part of stored energy is passed
on to the next level
• Most energy consumed is used for
respiration, movement, & reproduction
• Some remaining energy is released into
environment as heat
• Out of all stored energy, only 10% of
that energy is available to the next
trophic level
• So, each succeeding trophic level can
only support 1/10 the amount of living
tissue as the level below it
• Units on energy pyramids are usually
calories
Biomass Pyramid
• Biomass - total amount of living tissue
within a given trophic level
• Units of biomass - grams or lbs/unit
area
• Represents potential food available for
each trophic level
Numbers Pyramid
• Based on number of organisms for each
trophic level
• May not be typical pyramid shape
depending on the ecosystem
• Energy moves between trophic levels in
the form of food
• Food also contains matter
– Carbon, hydrogen, nitrogen, & oxygen are
the most common element in food
• Molecules composed of these elements
store energy as they move from 1
trophic level to the next
• Without even 1 of these elements, food
cannot be produced
• The growth of producers in most
ecosystems is limited by the lack of one
or more of these elements, not by the
sun