4.1 Ecosystems

4.1 Ecosystems

Essential Questions:

What is the relationship between humans and nature?

To what extent do human settlements depend on natural systems?


4.1 Lesson 1 – Analysing food webs

Task 1: Introductory activity. Engagement with essential questions. Consider quietly the essential questions for the topic. Write down one or two ideas on post-it note and stick it around the essential question. 


  • Ecosystems: the combination of a community with a habitat eg. a rainforest.
  • Species: a group of highly similar organisms that can interbreed to produce fertile offspring eg. Felis Leo.
  • Populations: a group of organisms of the same species living in the same place at the same time eg. a flock of parrots in a tree.
  • Communities: a group of populations living in the same place at the same time eg. the bird community in a park.
  • Decomposer: a living organism that breaks down dead and decaying matter into basic nutrients eg. an ant.
  • Detritivore: a decomposer that ingests dead and decaying matter eg. an ant.
  • Saprotroph: a decomposer that secretes digestive enzymes outside of the body eg. a fungus. 
  • Trophic level: the feeding level or stage in a food chain eg. primary consumer.
  • Autotrophs: obtain inorganic nutrients from the abiotic (non-living) environment, and synthesise their own organic compounds eg. plants
  • Heterotrophs: obtain organic nutrients from the biotic (living) environment eg. sheep
  • Consumer: a heterotroph that ingests material from other organisms eg. a human eating a burger.

Thought provoking discussion. Choose discussion 1, 2, or 3.

DeepEco1) NOS: Deep Ecology is a kind of philosophy, which proposes that humankind are an integral part of ecological systems, and that we should feel a part of the ecology around us – instead of acting as arbitrators or manipulators of ecosystems.Arne Naess (founder of the Deep Ecology Movement) 

 Q. Is this a useful suggestion? Discuss this with your peers

2.) Chico Mendes


Chico Mendes is one of the most famous environmentalists of modern times. He became the face of the campaign to save the rainforests in the 90s, after being assasinated in his own home in Acre, Brazil, in 1988.

3.) US pullout from climate change

NOS The 2017 Paris climate accords attempt to limit the forecast for global warming to within 1.5 degrees centigrade. The agreements bind countries to limit their own emissions of greenhouse gases. Only three countries remain outside the Paris climate change awards. Korea, Nicaragua, and now the US.


What possible viewpoints could be used to defend or oppose the 2017 decision of the current US president to exit the Paris climate accords? Briefly discuss with your peers.

Ecology is the study of living things, their environment, and the relationships between them

Food web and Food chains

A food chain shows the feeding relationships between a number of organisms. The arrows represent the flow of organic nutrients and energy. A food web is a branching food chain.

 Figure 1: Foodweb from Manuel Antonio park, Costa Rica

food web


A food chain consists of a series of trophic levels or feeding levels. The order of trophic levels is always the same:

  • Producer – Produces organic compounds, from inorganic ones.
  • Primary consumer – Feeds on, and thereby obtains the nutrients / energy from, a producer
  • Secondary consumer – Feeds on, and thereby obtains the nutrients / energy from, a primary consumer
  • Tertiary consumer – Feeds on, and thereby obtains the nutrients / energy from, a secondary consumer


Exercise: Extract two food chains from the Manuel Antonio food web, identifying a producer, primary consumer, secondary consumer, and tertiary consumer.

Further descriptions:

  • Predator – an organism that hunts and kills for food, it’s prey.
  • Top predator – an organism that is a predator, and has not predators itself
  • Detritivores and saprophytes – organisms that feed on dead and decaying organic material (eg. ants and fungi). These are not normally shown in a food web, but they are significant.
  • Autotrophs – obtain inorganic nutrients from the abiotic environment (from the non-biological surroundings) eg. Plants
  • Heterotrophs – obtain organic nutrients from other organisms.
  • Detritivores are different to saprophytes because their digestion of detritus is internal ( they eat it), like ants. Saprophytes secrete digestive enzymes and digest detritus outside of their bodies, like fungi.

Exercise: From the Manuel Antonio park, select a top predator, an autotroph, and two heterotrophs which are in competition with each other for the same food source.


Species and communities

Species are groups of organisms that can interbreed to produce fertile offspring. A population is a group of organisms of the same species, living together at the same time eg. the squirrel monkeys of Manuel Antonio park.

A community is a group of different species living together at the same time eg. the monkey community of Manuel Antonio park (Howler monkey population, Squirrel monkey population, and white-faced Cappucin population).

Ecology Birdwatching Task:

  • Watch the bird feeder time-lapse (you might want to choose x0.25 the speed).
  • Give each kind of bird a letter and write a brief description eg. Bird A is large and brown, with a brown beak
  • Make a simple table to record how many birds of each type you think appeared at the bird feeder in the time captured.
  • Discuss any ‘uncertainty’ you might have about your counting. Make a list of things that might cause your data to be inaccurate. These are called ´sources of error´.
  • Report back to the class.


 This also needs to be turned into a suitable graph.

Lesson 2 – Using Chi-squared

Fundamental Question: How can we tell if the distribution of organisms is random, or significantly altered by one or more factors?

Lesson Objectives: Learn how to apply the Chi-squared test.


  • Chi-squared test. A stastical test applied to find if the distribution of organisms (eg. moss on a tree) is significantly different with respect to a certain factor (eg. north or south).
  • Null hypothesis. The assumption that the distribution can be explained by chance and is not significantly varied with respect to that factor.
  • Alternative hypothesis. The assumption that the distribution with respect to a certain factor varies significantly.

The Chi-squared test (called X2 as X is the old greek letter Chi, pronounced ‘Kigh’) is a statistical test used to see if a particular distribution occurs by chance or not.eg. Is there really more moss growing on the North or South side of trees compared to other plants like Lianas, or is it just random?

In order to work the test ‘assumes’ is no significant difference (aka the null hypothesis), and then sets out to try and prove that there actually is a significant difference.

Question: Is the occurrence of moss significantly different from the growth of other epiphytic plants such as Lianas?

Note: an epiphyte is a plant which utilises another plant as a habitat, without taking anything away from the host plant. Orchids, bromeliads, and Llianas are all epiphytes. This distinguishes it from a parasitic plant, which will feed on the host tree.

  • Null Hypothesis (given the symbol Ho): Yes, it is just by chance
  • Alternative Hypothesis (given the symbol Ha): No, it is not just chance where the moss and other epiphytes grow.
image credit ptinerets.com

image credit ptinerets.com moss and lichen growing on the side of a tree


Step one: Set up your categories: North, South, how much moss, how many other epiphytes

Step two: Arrange the categories into a table, adding a row and a column for totals.

Screen Shot 2017-01-31 at 11.23.23 AM


Step three: Add the data, adding totals for each row and column. Notice the bottom right hand corner will contain the grand total of all the lichen and moss found. This data is called the observed values

Screen Shot 2017-01-31 at 11.24.03 AM

Step four: Calculate the expected values for each category , using the formula

Expected value = row total x column total / grand total.

For example a category would be Moss growing on the North side of the tree, and the expected value would be 13×10 / 20 = 6.5. For mathematical purposes we would leave the number without rounding to 7, even though half a plant doesn’t make sense. Add these to the table you have made, using parentheses to signify an expected value.

Screen Shot 2017-01-31 at 11.31.53 AM

Step five: Using the formula X2 = (the sum of all the Observed – Expected)squared, divided by the expected. Calculate Chi-squared. The formula is written below:

Screen Shot 2017-01-31 at 11.34.54 AM


Step six: Looking up the critical Chi-squared value in the Chi-squared table

Degrees of freedom is 1 (number of rows-1) x (number of columns -1).

We can see that for 95% confidence, a value of 3.81 is required for Chi-squared to reject the null hypothesis.

Therefore in this case as our calculated value of Chi-squared is less than 3.81, we retain the null hypothesis. According to this data, there is nothing different about the way that moss grows on a tree trunk compared to other epiphytes. 




Task  Class activities (to do in class, working in pairs)

Activity:  Learning to use a basic X2 test (pronounced Chi-squared). You will be expected to look for gender bias based on response to a valid question that will generate categorical data.

1) Choose a question that has only two answers eg. Do you prefer using an iPad or a laptop in class

2) Draw a table separating out the possible categories your data will fall under this is an example question.

eg.     IPad      Laptop



3) Collect the data from our class, asking your classmates for cooperation in our example

4) Calculate the expected values if the null hypothesis was true eg. there is no gender bias in students who prefer working with iPads or laptops). To do this you can use the formula row total x column total / grand total (intuitively, the expected result for female students who like iPads will be the total number of people who like iPads x the fraction of people who are female).

5) Calculate the Chi – squared value. You can use an excel spreadsheet to make this faster, as it will do some of the calculating for you.

6) Work out the degrees of freedom. Number of rows -1 x Number of columns -1. eg. 1 in my example.

7) Check the Chi-squared value against the tabulated values on the powerpoint slide. You can see that the values have different probability ratings. eg. if your value exceeds the published value at p > 0.05, this means that there is a 95% certainty that the null hypothesis is wrong and there is a significant difference.

Still lost? Download chi-squared worked example and go through it, asking questions if needed.


Lesson 3 – Chi-squared lab

Down load lab instructions: the quadrat lab

Revision powerpoint

4.1 Checkpoint.

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