Ecological Pyramids
A key to understanding the structure and function of
ecosystems

What are Ecological
Pyramids?
• Ecological pyramids are graphical
representations of the trophic structure of
ecosystems.
• Trophic levels are the feeding position in a
food chain such as primary producers,
herbivore, primary carnivore, etc.

Types of ecological
pyramid
Three types of ecological pyramids can
usually be distinguished namely:
• Pyramid of numbers
• Pyramid of biomass
• Pyramid of productivity

Pyramid of numbers
• Is the graphic representation
of number of individuals per
unit area of various trophic
levels
• Large numbers of producers
tend to form the base
• Lower numbers of top
carnivores occupy the tip Pyramid of Numbers in a
Grassland Ecosystem

Pyramid of numbers -
example
• The shape of the pyramid of
numbers vary from
ecosystem to ecosystem.
• In aquatic ecosystems and
grassland communities,
autotrophs are present in
large numbers per unit area.
• They support a lesser
number of herbivores, which
in turn support fewer Pyramid of Numbers in a Aquatic Ecosystem
carnivores.

‘inverted’ + ‘spindle-
shaped pyramids

Evaluating the pyramid of numbers
ADVANTAGES DISADVANTAGES
Numbers of a specific species
Simple easy method (number
can be too great to measure
counting) of giving an overview
accurately
Good for comparing changes to
Does not take into account
the ecosystem at different times
‘juveniles’ or immature forms
of year e.g. between seasons
All organisms are included
regardless of size, hence
‘inverted’ pyramids

Pyramid of biomass
• Is the graphic representation of
biomass present per unit area of
different trophic levels, with
producers at the base and top
carnivores at the tip.
• Biomass is calculated by the mass
of each individual x number of
individuals at each trophic level (g
m-2 or g m-3)

Inverted pyramid of
biomass
In an aquatic habitat the pyramid
of biomass is inverted or spindle
shaped where the biomass of
trophic level depends upon the
reproductive potential and
longevity of the member.

Evaluating a pyramid of biomass
ADVANTAGES DISADVANTAGES
Overcomes the problems of pyramids Only uses samples from populations, so
of number difficult to measure biomass exactly
Time of year that biomass is measured
influences result e.g. trees in summer
Organisms of the same size do not
necessarily have the same energy
content
Inverted’ pyramids may result from
producers with high turnover rate

pyramid of productivity
Pyramid of productivity is a
graphic representation of the
flow of energy through each
trophic level of a food chain
over a fixed time period. The
input of solar energy may be
indicated as an extra layer at
the base.

pyramid of productivity
The energy level of each trophic level
Only 10% of the energy is available to
has two arts i.e. Net Production (NP)
next trophic level (as per Lindemann's
and Respiration (R) and is measured in
ten percent rule).
KJ m-2 yr-1

pyramid of productivity
Advantages Disadvantages
It is difficult and complex to
No inverted pyramids are
collect energy data (rate of
obtained
biomass production over time
Shows actual energy Problem required)
is always exists in
transferred and allows for assigning a species to a
rate of production specific trophic level,
Can compared different especially omnivores
ecosystems based on relative
energy flows

consequences of ecological pyramids and
ecosystem function
• Bioaccumulation - when plants / animals take up a
chemical from the environment and do not excrete it.
The chemical builds up in the organism over time to a
potentially lethal level.

consequences of ecological pyramids and
ecosystem function
• Biomagnification - refers to the sequence of processes
that results in higher concentrations of the chemical in
organisms at higher levels in the food chain (at higher
trophic levels). In this way the chemical’s concentration is
magnified from trophic level to trophic level. The
concentration of the chemical may not affect lower levels
of the food chain but the top levels take in so much it
can cause disease or death.

There are many pollutants like the DDT that
are not bio-degradable. These accumulate in the
organisms and cause serious health problems.
The contamination of water with these
pollutants results in their entry into the
microscopic plants and animals. These
organisms are fed upon by higher aquatic life
like the fish. The fish in turn are fed upon by the
land animals including man.
Thus, the pollutant reaches the body of man. At each step in the food chain, the contaminant
increases in quantity. This is because a fish feeds on large quantity of smaller plants and man eats
fish. These contaminants like DDT remain in the fats and are not degraded in the body. Over the
years the amount of DDT increases in the body. This is called biomagnification.