Table of Contents
- 4.1.2 – Autotrophs & Heterotrophs
- 4.1.3 – Consumers, Detritivores & Saprotrophs
- 4.1.4 – Communities
- 4.1.5 – Ecosystems and nutrient cycling
- 4.1.6 – Chi-Square test
- Checklist:(Always check your syllabus!)
First off, What is ecology?
It is the study of organisms in relation to their environment. Or to put it in IB standards, “The study of living organisms in relations to the abiotic factors”.
4.1.1 – Species
What is a species?
It is a group of genetically similar living organisms that can breed and produce offsprings. Fertile offsprings can breed and pass their genes on. Species can be isolated reproductively by geographical factors (like islands parting), behavioural differences and niche partitioning.
4.1.2 – Autotrophs & Heterotrophs
What is an autotroph?
They are organisms that can make their own food (complex organic molecules) by using carbon dioxide and other simple compounds, also called photosynthesis. Almost all plants are capable of autotrophic nutrition.
What is a heterotroph?
They are organisms that obtain nutrition (organic compounds) through eating other organisms. Examples: Carnivores, omnivores and herbivores.
Food chain/ Food web
In a food chain, producers are always autotrophs and all consumers are heterotrophs. This is because producers can generate organic chemicals, whereas consumers must eat other living things.
4.1.3 – Consumers, Detritivores & Saprotrophs
There are 3 types of heterotrophs:
1. Consumers
2. Detritivores
3. Saprotrophs
Consumers feed or other living organisms or lately living organisms to gain nutrients by ingestion or absorption.
Pro tip!
Ingestion ≠ Digestion!
Ingestion is a term use to feed something. Digestion is the process of breaking down food once its ingested
Example of Consumers:
| Herbivores | Consumers that feed only on producers |
| Primary consumers | Feeds on producers |
| Secondary consumers | Feeds on primary consumers |
| Tertiary consumers | Feeds on secondary consumers |
| Carnivores | Consumers that feed only on other consumers |
| Omnivores | Feeds on both producers and consumers |
Detritivores are heterotrophs that obtain nutrients from waste or any dead organic matter. They break them into smaller organic molecules. This process is called ‘internal digestion’ which an organism digest the food inside the body with help of a digestive tract. Most dead organic matter still contains organic compounds that can be used to provide energy.
As living beings die, they leave large amounts of organic matter, therefore detritivores are important roles to recycle and use nutrients as they return them back to soils and to autotrophs as a additional source of nutrients.
Example: Earthworms
Saprotrophs (or more commonly called decomposers)are heterotrophs that obtain nutrients from dead organisms with the process of ‘external digestion’. They secrete digestive enzymes on the matter to break down the complex organic compounds to simpler molecules and minerals. Like detritivores, they too help the soil.
Example: Fungi & bacteria
Pro tip!
1. Knowing their diet should help you classify species as consumers, detritivores, or saprotrophs.
2. Detritivores ≠ Saprotrophs!
4.1.4 – Communities
Community is a group of populations of different kinds of species living in the same environment.
Ecology studies how living things interact with their surroundings. Some symbiotic relationships can have either benefits, toxic, or no change at all.
Some types of relationships are:
1. Mutualism: benefits both organisms
2. Commensalism: one benefits while the other does not have any impact
3. Parasitism: one benefits while the other suffer
4. Predator-prey: one hunts and feeds on another.
You should be able to define and differentiate species, populations, or communities.
4.1.5 – Ecosystems and nutrient cycling
Ecosystems consist of communities that always interact with their environment. They do not live in a vacuum. Factors such as rainfall, pH of the soil or water in which they live, temperature range, humidity, etc., all have an effect on living organisms. Scientists call these aspects the abiotic (non-living) environment. Abiotic environment are the non-living factors of the area. Therefore a community interacts with its abiotic environment to form an ecosystem .
Nutrient cycles
Nutrient cycling helps to move organic molecules and minerals through the food chain and back into the soil where they can be taken up by plants to re-enter the food chain. Its purpose is to help move organic molecules and minerals using the food chain and then back to the soil where
Mesocosm
It is an experimental tool that allows the ability to control the conditions in a small part of the natural environment. It can act as a smaller model of a larger ecosystem, to show how energy enters and leaves but some inorganic matters don’t. This is a great tool to evaluate how different organisms or communities might react to the change in variables. Those variables are such like temperature, carbon dioxide levels and pH levels.
Pros of using mesocosm as a tool:
- Sustainability of an ecosystem can be observed
- Replicating treatments is easy.
- Multiple environmental elements can be tested.
- Establish the food webs.
- Study direct and indirect impacts.
- Evaluate contamination influence.
- Controlling the variables can let you study one environmental aspect.
There are two main types of mesocosms: aquatic and terrestrial.
Mesocosms can be set up in open tanks, however sealed glass vessels are better because matter cannot enter or leave while light and heat can enter. You must create a sealed mesocosm to create a sustainable system. However, you must follow the IB animal experimentation policy regardless of the mesocosm you set up. This means that if you incorporate a living animal in your mesocosm, you must ensure that it will not suffer. Also, use recycled materials whenever possible!
4.1.6 – Chi-Square test
The chi-squared test estimates the probability that a distribution is attributable to chance. As a goodness of fit statistic, it evaluates whether the observed data distribution matches the independent variable distribution.
A chi-squared test analyzes counted and categorized data. It doesn’t work with continuous data like °C temperature change. If you want to examine whether sunshine influences plant growth, a chi-squared test should not use plant length (in cm). However, categorizing plants as ‘tall’ and ‘short’ works. A chi-squared grid should only contain frequency (count) data, not percentages.
How to calculate the Chi-Square Value?
(Having a scientific calculator is very handy here!)
Step 1: Make your hypothesis
H1 Null Hypothesis: Variables are independant
H2 Alternate Hypothesis: Variables are dependant
Step 2: Find the expected frequency
Expected frequency is the number of quadrats a population is expected to occupy in an area.
Step 3: Calculating the Chi-square value (X2)
Using scientific calculator (TI-nspire version):
https://youtu.be/f_4xXqy6F4M
Step 4: Finding the degree of freedom (df)
(Number of rows)X(Number of Columns) = df
Step 5: Compare the Chi-square value with the significant value
X 2 > Critical Value: H 0 is rejected and the variables are dependant
X 2 ≤ Critical Value: H 0 is accepted and the variables are independant
Checklist:
(Always check your syllabus!)
IB wants you to:
- Outline that species are groups of organisms that can potentially interbreed to produce fertile offspring.
- Outline that members of a species may be reproductively isolated in separate populations.
- State that species have either an autotrophic or heterotrophic method of nutrition, mentioning that a few species have both methods and be able to give and identify examples of each.
- Outline that consumers are heterotrophs that feed on living organisms by ingestion.
- Outline that detritivores are heterotrophs that obtain organic nutrients from detritus by internal digestion.
- Outline that saprotrophs are heterotrophs that obtain organic nutrients from dead organisms by external digestion.
- Distinguish between consumers, detritivores and saprotrophs.
- Explain how a community is formed by populations of different species living together and interacting with each other.
- State that a community forms an ecosystem by its interactions with the abiotic environment.
- State that autotrophs obtain inorganic nutrients from the abiotic environment.
- Outline how the supply of inorganic nutrients is maintained by nutrient cycling.
- State that ecosystems have the potential to be sustainable over long periods of time.
- Set up sealed mesocosms to try to establish a sustainable system.
- Test for association between two species by using the chi-squared test with data obtained by quadrat sampling.
- Recognise and interpret statistical significance.
Go ahead and test yourself! Here is a list of quiz and past papers you can do on your own
IB biology Topic 4.1-4.2 Ecology
Energy Level
Check out our other blogs!
- Past Papers, Specimen Papers… What? – The Best IGCSE Guide To Revising; Efficient Studying 101
- Topic 1 – Characteristics And Classification Of Living Organisms – Best IGCSE Biology Revision Guide Syllabus 2023-2025
- Topic 2 – Organization Of The Cells Of The Organism – Best IGCSE Biology Revision Guide Syllabus 2023-2025
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Resources Used:
Kognity
Khan Academy
English