IB Environmental Systems and Societies

1. Topic 2
Ecosystems and Ecology
2.1 Species and Populations

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

6. Pyramid of Numbers Pyramid of Biomass Pyramid of Productivity

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
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

10. Population Interactions
• Make sure you have definitions of:
– Species
– Habitat
– Niche
– Community
– Ecosystem

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)