2. Ecological Pyramids
• There are 3 types of ecological pyramid:
– Pyramid of number
– Pyramid of biomass
– Pyramid of productivity
3. Pyramid of Number
• Either you count each individual, or if this is
not possible make a collection over a specific
limited area and multiply up
• They are not always pyramid shaped
• The first level may be smaller than the second
if the individual primary producers are very
large (eg. trees)
4. Pyramid of Biomass
• Units of g m-2 or J m-2 (if you use joules it is
technically a pyramid of energy)
• A limitation is that they don’t represent changes
over time (they are just a ‘snap-shot’ or a
representation of ‘standing stock’)
• For this reason they are not always pyramid
shaped. Eg if a field has been extensively grazed,
its biomass at one moment in time may be very
low (even though its productivity may be high)
5. Pyramid of Productivity
• Possibly the most useful type of pyramid
• Units of g m-2 yr-1 or J m-2 yr-1
• Represent the flow of biomass or energy over
time. Therefore not just a snapshot like the other
pyramids
• Represent the flow of energy or biomass through a
food chain, and therefore are always pyramid
shaped
7. Bioaccumulation and Biomagnification
• Which trophic level is most at risk to toxic
pollutants in the environment and why?
• The top predators
• This is due to 2 processes:
– Bioaccumulation – the retention of non-polar
compounds in body tissues (usually fat)
– Biomagnification – the increase in these compounds
in organisms as you move up the food chain. Each
animal retains the compounds from all of those in the
preceding trophic levels
Video clip - biomagnification
8. Case Study: DDT
• Dichlorodiphenyltrichloroethane
• An organochlorine insecticide
• Very non-polar
• Similar in structure to oestrogen and therefore has
´feminising’ effects on animals
• In birds it weakens egg shells so that the parents break
them as they try to incubate the eggs
• The birds at highest risk are those close to the top of the
food chain
• DDT was one of the main subjects of Rachel Carson’s
1962 book ‘Silent Spring’ which helped to kickstart
modern environmentalism
• It was banned in most developed countries in the 1970s
and 1980s
• It is now known to cause birth defects, damage to the
nervous system and cancers
• It is still used in many poorer countries to control insect
problems – why?
9. Case Study: Minamata Disease
• Caused by the heavy metal mercury
• First documented In Minamata, Japan in
1957
• A chemical factory was releasing methyl
mercury into Minamata Bay between
1932 and 1968
• This biaccumulated in shellfish and was
biomagnified in fish
• Both were the staple diet of the local
human population
• Mercury causes severe damage to the
nervous system and Minamata Disease
was first identified due to the high
incidence of cerebral palsy in children in
the area
11. Population Interactions
• Population Interactions occur due to:
1. Competition
2. Predation
3. Parasitism
4. Mutualism
• They are examples of biotic factors
12. 1. Competition
• This may be interspecific and/or
intraspecific
• Interspecific competition occurs
when niches overlap (this is
called competitive exclusion or
Gause’s Principle)
• Intraspecific competition drives
natural selection
• G.F. Gause (Russian ecologist)
demonstrated his theorem by
putting two species of
Paramecium in the same Petri
dish
• They can survive separately, but
when together P. aurelia
outcompetes and eliminates P.
caudatem
13. 2. Predation
• Simply the hunting of one animal by another
for food
• Controlled by negative feedback loops to
create a steady-state equilibrium
14. 3. Parasitism
• A relationship in which one organism benefits
at the expense of another
– Ectoparasites – on the surface of a host (eg lice)
– Endoparasites – inside a host (eg tapeworm)
15. 4. Mutualism
• A relationship in which two organisms live
together and both benefit (often refered to as
a symbiotic relationship)
