I made these notes with the help of my teacher and obtained 96% (straight 7) on my final IB examinations thanks to these incredibly well-organized and clear notes that include diagrams, images, colours, and much more and exactly match what you will be asked on your IB biology exam.
Chapter 4: Ecology
4.1: Species, communities and ecosystems
Ecosystems consist of both the community and the environment. The abiotic environment
has a fundamental impact on the community it supports.
Key terms:
Ecology The study of relationships between living organisms and organisms and their
environment (between biotic and abiotic factors)
Population A group of organisms of the same species living in the same area at the same time
Ecosystem A community and its abiotic environment interacting
Community A group of populations living and interacting with each other in an area
Species A group of organisms that can interbreed to produce fertile offspring
Habitat The environment in which a species normally lives (the location of a living organism)
All organisms require organic molecules (e.g amino acids) for functions of life (metabolism,..)
Types of organisms
1. Autotrophs (ex: Plants)
● Convert carbon dioxide (from the atmosphere or dissolved in water) to synthesize their
own organic compounds → They are producers
● Plant initially synthesis sugars (e.g. glucose) which are then converted into other
organic compounds such as:
○ complex carbohydrates e.g. starch, cellulose
○ lipids
○ amino acids
● Inorganic nutrient compounds (e.g. water, carbon dioxide, nitrates, phosphorus
and oxygen) are obtained the abiotic environment, whether it be the soil, air or water.
2. Heterotrophs (ex: zebras)
● Obtain organic molecules from other living organisms
3. Mixotrophs
● Both autotrophs and heterotrophs at the same time
○ Ex: Euglena will photosynthesize if sufficient light (autotroph)
otherwise it will ingest food through phagocytosis (heterotroph)
, Consumers
Consumers: Heterotrophs that ingest other organisms to obtain their organic molecules
Herbivores Feed on producers & plants (ex: zebra)
Omnivores Feed on a combination of both producers and consumers (ex: mouse & humans)
Carnivores Feed on other consumers (ex: lions)
Scavengers Are specialised carnivores that feed mostly on dead and decaying animals (ex:
hyenas)
Detritivores Type of heterotroph that obtain nutrients by consuming non-living organic
sources like detritus or humus (ex: earthworms) by internal ingestion
Saprotrophs They are decomposers: live on, or in, non-living organic matter. They secrete
digestive enzymes onto the organic matter and absorb the products of digestion.
NOT consumers: They do not ingest food as digestion is external as enzymes are
secreted (ex: Bacteria and fungi). Facilitates breakdown of dead organic material
Testing for the association between two species using the Chi-squared test
Positive association: Species found in the same habitat (ex: herbivore & plant)
Negative association: Species occur separately in different habitats
No association: Species occur as frequently apart as together.
Quadrat sampling use (limitation: large and mobile animals cannot be measured):
● Estimation of population density/size
● Measuring the distribution of species
Chi-squared test:
1. Define hypothesis
○ Null hypothesis (H0): There is no significant difference between
the distribution of two species (i.e. distribution is random)
○ Alternative hypothesis (H1): There is a significant difference
between the distribution of species (i.e. species are associated)
2. Complete the contingency table of observed frequencies
𝑟𝑜𝑤 𝑡𝑜𝑡𝑎𝑙 𝑥 𝑐𝑜𝑙𝑢𝑚𝑛 𝑡𝑜𝑡𝑎𝑙
3. Calculate expected values using the formula: 𝐺𝑟𝑎𝑛𝑑 𝑡𝑜𝑡𝑎𝑙
○ Expected values are what you would expect to be found if there is
no association between the species.
𝑁 2
2 (𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑣𝑎𝑙𝑢𝑒 − 𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑣𝑎𝑙𝑢𝑒)
4. Calculate chi-squared value 𝑥 = ∑ 𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑣𝑎𝑙𝑢𝑒
𝑖=1
5. Add all chi-squared values together
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