2.1 Ecosystem Structure
IBESS
Trophic structure and food chains
• Trophic structure is shows the different
feeding relationships in an ecosystem.
• A food chain shows trophic levels and
indicates the flow of energy in the feeding
relationship.
• In a food chain, the arrows always point in the
direction of energy flow.
Food chain in a Taiga forest
Primary
consumer
Secondary
consumer
Producer
What would you call the trophic level of
something that ate the lynx?
Sun
Ringed seal
Baltic cod
Sprat
• Normally ecosystems
are made up of more
than one food
chain…they are food
webs.
We can’t forget decomposers!
• Decomposers have
the important role of
feeding on the
remains or wastes of
other organisms.
• Many bacteria and
fungi are
decomposers.
Decomposition of a tree
Aquatic Ecosystems
• When we eat fish, we tend to consume at a
higher trophic level than when we consume
other animals.
• The fish that we tend to eat is high on the
food chain…phytoplankton --> zooplankton-->
sprat--> cod--> us…
LE 54-2
Tertiary
consumers
Microorganisms
and other
detritivores
Detritus
Secondary
consumers
Primary consumers
Primary producers
Heat
Key
Chemical cycling
Energy flow
Sun
Energy flow in ecosystems
Energy
flow
• The energy in an
ecosystem generally
starts with the sun.
• Plants and other
organisms use that to
make food through
photosynthesis.
• Energy is then
transferred up the
food chain.
Why a pyramid?
Energy lost as heat
• Between the trophic
levels, about 90% of
the energy is lost to
the environment,
usually as heat.
More on pyramids
• Three types of pyramids you should know
– Pyramid of numbers
– Pyramid of biomass
– Pyramid of production
Pyramids of Numbers
• The size of each block is proportional to the
number of individual organisms present in
that trophic level.
• Because of the drop in energy at each trophic
level, the number of predators is generally
much lower than the trophic level below.
Pyramid of Numbers
Pros and cons
• Pros
– Easy method to give overview
– Good for comparing changes in population
numbers over different times/seasons
• Cons
– All organisms are included regardless of size or
energy contained (a beetle takes up as much room
as a hippo)
– Where to assign omnivores?
Pyramids of Biomass
• Each tier represents the standing crop (total dry
weight of all organisms) in one trophic level.
• Most biomass pyramids narrow sharply from primary
producers at the base to top-level carnivores at the
apex because energy transfers between trophic
levels are so inefficient. Shown in grams per meter
squared (gm-2) or sometimes in Joules (J).
Biomass Pyramid
Pros and cons
• Pros
– Takes into account size, so beetles don’t take up as much
room as a hippo, better representing the energy in each.
• Cons
– Only uses samples from populations, so measurement isn’t
exact.
– Organisms must be killed to measure dry mass.
– Organisms with the same mass might not have the same
energy available. (fats v. carbs….)
– Where to assign omnivores?
– Time of year biomass is measured can greatly affect mass.
Inverted Biomass Pyramid
Some aquatic ecosystems might show an inverted biomass pyramid because
phytoplankton grows, reproduces, and is consumed quickly. Because of this they can
support a zooplankton population larger than their own biomass…but the production
pyramid is still bottom heavy.
Pyramids of production
• The loss of energy with each transfer in a food
chain is represented by this pyramid.
• Trophic levels are stacked in blocks with
primary producers forming the foundation of
the pyramid.
• The size of each block is proportional to the
net production, expressed in energy units per
square meter per year (Jm-2yr-1)
Pyramid of Production
Joules per square meter per year: Jm-2yr-1
1
m-2yr-
m-2yr-1
m-2yr-1
m-2yr-1
Pros and cons
• Pros
– Productivity are rates of flow, where biomass are
stores at one time.
– Shows actual energy transferred.
– Energy from solar radiation can be added.
• Cons
– Difficult and complex to collect energy data as the rate
of biomass production is needed over time.
– Like other pyramids…where to assign omnivores???
Feeding the World
• Which takes more land, eating plants or
animals?
• Which takes more fuel?
• Which takes more water?
Toxins in the Environment
• Humans release many toxic chemicals, including
synthetics previously unknown to nature
• In some cases, harmful substances persist for long
periods in an ecosystem
• When organisms ingest toxins that they can’t excrete or
get rid of, they accumulate in their bodies over time
(bioaccumulation)
• One reason toxins are harmful is that they become more
concentrated in successive trophic levels
• In biological magnification, toxins concentrate at higher
trophic levels, where biomass is lower (biomagnification)
LE 54-23
Concentration of PCBs
Herring
gull eggs
124 ppm
Lake trout
4.83 ppm
Smelt
1.04 ppm
Zooplankton
0.123 ppm
Phytoplankton
0.025 ppm
Practice 1
• An ecosystem consists of one oak tree on which
10 000 herbivores are feeding. These herbivores
are prey to 500 spiders and carnivorous insects.
Three birds of the same species are eating these
spiders and carnivorous insects. The oak tree has
a mass of 4000 kg, the herbivores insects have an
average mass of 0.05 g, the spiders and
carnivorous insects have an average mass of 0.2 g
and the three birds have an average mass of 10 g.
– Construct, to scale, pyramids of numbers and biomass
• #s, 1, 10 000, 500, 3
• Mass, 4 000 000 g, 500 g, 100 g, 30 g
Practice 2
• Assuming an ecological efficiency of 10%, 5%,
and 20% respectively, what will be the energy
available at the tertiary consumer level (4th
trophic level, given a net primary productivity
of 90 000 kJ m-2 yr-1?
• What percentage is this figure of the original
energy value at the primary producer level?
• 9000, 450, 90
• 0.1
Practice 3
• In an aquatic ecosystem, plankton are eaten by
minnows, which are eaten by pickerel, which are
eaten by cormorants.
• DDT (a toxin) is introduced into the ecosystem
and after ten years is found in the following
concentrations. Water (0.00005), Plankton (0.04),
Minnow (0.23), Pickerel (1.33), Cormorant (26.4).
– How many trophic levels are there in this food chain?
– How many times more concentrated is the DDT in the
body of the cormorant than in the water?
• 4
• 528 000
Describe the habitat, niche, population, community, and
ecosystem for one of the species in this diagram.
Types of relationships between
populations
• Competition
• Predation
• Symbiosis
– Mutualism
– Commensalism
– Parasitism.
Competition
• When more than one
individual or
population tries to use
the same limited
resource.
• This picture is hyenas
and vultures on the
African savannah
competing for a
carcass.
• When one animal
captures and eats
another animal for
food.
• This owl is about to
make a meal of this
mouse.
Predation
Symbiosis
• Symbiosis is a close relationship between two
species.
• There are many types of symbiosis including
mutualism, commensalism, and parasitism.
Mutualism
• When both species
benefit from the
relationship.
• These yellow billed
oxpeckers are getting
a yummy meal of ticks
and other bugs, the
rhino is getting the
itchy, blood- sucking,
annoying bugs
removed.
Commensalism
• When one species
benefits and the other
is unaffected.
• Nemo is getting
protection from living
in the sea anemone,
the anemone doesn’t
benefit, but it also
isn’t bothered.
Parasitism
• Parasitism is when one
species benefits and the
other is harmed.
• Why is this songbird so
much smaller than the
baby it is feeding? Brownheaded cowbirds lay their
eggs in other birds nest so
they do the taking care
of…the baby cowbirds will
go as far as to push the
songbird’s babies out of
the nest for more food.