Lecture example
An ecosystems and energy flow lecture, ready to teach from
Week 3 of an introductory environmental science course, written for lecturers and teaching assistants. It explains why energy flows while matter cycles, compares grazing and detritus food chains, and sets up the week's pond lab, reading and quiz.
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01Cover
Ecosystems and energy flow
ENV 101 · Week 3 · Dr Samuel Adeyemi · Ashcombe University
02Agenda
This week
- Recap: what is an ecosystem?
- Producers and consumers
- The 10% rule
- Lab: pond food webs
- Readings and assessment
03Section
Energy flows, matter cycles
Sunlight passes through an ecosystem once and leaves as heat.
04Key numbers
Only a sliver of sunlight becomes food
That is why food chains are short, and why any ecosystem has far fewer predators than plants.
- ~1% — Sunlight fixed by plants
- ~10% — Passed up each level
- 4–5 — Trophic levels at most
- 0.1% — Reaching level 4
05Chart
Plants burn up to two-thirds of the energy they capture
Chart: Gross production, Net production across Desert, Grassland, Temperate, Rainforest.
- Gross production: Desert 0.2 · Grassland 1 · Temperate 1.5 · Rainforest 3.5
- Net production: Desert 0.1 · Grassland 0.5 · Temperate 0.7 · Rainforest 1.2
kg of carbon per m² per year. Only net production feeds herbivores: half in deserts, a third in rainforest.
06At a glance
Week 3 at a glance
- Energy enters as light and leaves as heat — Producers fix about 1% of incoming sunlight. Each step up the food chain passes on roughly 10%; the rest is lost in respiration.
- Lecture: Tue 10:00 — Hall B, recorded.
- Lab: Thu 14:00 — Pond study, wear boots.
- Reading: Ch. 4–5 — About 60 pages.
- Quiz 2 opens Fri — Closes Sunday 23:59.
07Points
Three ideas to take from this week
Each one returns in the mid-term and the lab report.
- Energy flows one way, from the Sun to producers to consumers, and leaves the ecosystem as heat.
- Matter cycles: decomposers return carbon and nitrogen to be used again. Energy is never reused.
- Roughly 10% of energy passes between trophic levels, which limits food chains to four or five links.
08Two columns
Food chains and food webs tell different stories
Food chain
- One path from plant to predator
- Easy to model and teach
- Misses most real feeding links
Food web
- Many linked chains at once
- Shows who depends on whom
- Predicts knock-on effects of loss
09Comparison
Grazing and detritus chains move energy differently
Grazing chain
- Starts with living plants
- Herbivores eat green tissue
- Dominant in grasslands and oceans
Detritus chain
- Starts with dead organic matter
- Fungi, bacteria and worms feed first
- Dominant in forests and soils
10Table
How your course grade is built
Assessment Weight Due Format Weekly quizzes 15% Fridays Online, 10 Qs Lab reports 25% Weeks 4 and 8 1,500 words Mid-term exam 20% Week 7 60 minutes Final exam 40% Week 12 2 hours 11Timeline
Where Week 3 sits in the term
- Weeks 1–2Introduction to ecology and the scientific method.
- Week 3Ecosystems and energy flow, this week.
- Weeks 4–6Nutrient cycles: carbon, nitrogen and water.
- Week 7Mid-term exam on Weeks 1–6.
12Process
How Thursday's pond lab runs
- SampleSweep-net the pond margin for three minutes.
- SortSort invertebrates into trays by feeding group.
- IdentifyKey out each species with the field guide.
- ModelBuild your group's food web from the counts.
13Composed
Each week of ENV 101 links lecture, reading, lab and quiz
- Lecture: Concepts and worked examples (Lecturer)
- Reading: Chapters 4 and 5 before the lab (Students)
- Lab: Pond sampling in groups (Students)
- Seminar: Questions answered on Friday (Lecturer)
- Quiz: Ten questions, due Sunday (Students)
Contact hours
- 2 h: lecture, Tuesday
- 3 h: lab, Thursday
- 1 h: seminar, Friday
Task Lead Deadline 1 Read Ch. 4–5 Each student Before the lab 2 Pond lab Grace Mwangi Thursday 14:00 3 Lab write-up Lab groups Week 4, Friday 4 Quiz 2 Online Sunday 23:59 Getting help
- Office hours: Wednesday 13:00–15:00, Room 2.14
- Discussion board: Answers within one working day
- Lab demonstrators: On hand in every lab session
This week's goal: Explain the 10% rule with numbers
14Image
The campus pond is our model ecosystem
In Thursday's lab you will sample the campus pond and build a food web from what you find, from algae to dragonfly larvae.
- Bring boots and a waterproof
- Nets and trays are provided
Image: Students sampling a pond with nets, field notebooks in hand
15Team
Your ENV 101 teaching team
- Dr Samuel Adeyemi, Course lecturer. Lectures and the final exam.
- Grace Mwangi, Lab demonstrator. Runs the Thursday labs.
- Tomás Rivera, Teaching assistant. Seminars and quizzes.
- Dr Hye-jin Park, Course coordinator. Extensions and access needs.
16Quote
If you remember one thing
“Energy flows through an ecosystem and is lost as heat. Matter cycles and is used again.”
17Closing
Before Thursday, read Chapters 4 and 5
Come to the lab ready to sample. Quiz 2 opens on Friday and covers this week's lecture and reading.
Make your own version
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Questions
A slide or two helps at the start of a unit: this example shows how the grade is built and where the week sits in the term. Keep it short and put the detail in the course handbook, so the lecture time goes on the ideas.