Hello to all our future Class of '68, '69, and TCAS students!
Welcome to our A-Level Biology summary on the topic of "Ecosystems and Biomes"! This topic is a key part of the "Biodiversity and Environment" unit, which appears frequently on exams and is a great way to secure points if you understand the core principles.
If you feel like the content is overwhelming, you keep mixing up biome names, or you're confused about food chains, don't worry! In this guide, I’ll break down these complex topics into easy-to-understand chunks, provide memorization techniques, and highlight the common "exam traps." Let's dive in!
1. Biomes: The Homes of Life Around the World
A biome is a large-scale ecosystem classified by its geographical and climatic conditions (especially temperature and precipitation, like rain or snow), which dictate which plants and animals can survive there.
1.1 Terrestrial Biomes
We can classify the main biomes as follows:
- Tropical Rain Forest: High rainfall year-round. It has the highest biodiversity and is found near the equator (including Thailand!).
- Temperate Deciduous Forest: Distinct 4-season cycle. Trees "shed their leaves" before winter to reduce water loss.
- Taiga / Boreal Forest: Long, cold winters. Trees are cone-shaped (conifers) with needle-like leaves.
- Temperate Grassland: Examples include the Steppe or Prairie. Great for agriculture because of the fertile soil.
- Savanna: Tropical grassland with scattered trees. It has a distinct dry season (think The Lion King!).
- Desert: Very little rain (less than 25 cm/year). Organisms have specific adaptations, like camels or cacti.
- Tundra: The coldest biome! The subsoil remains frozen year-round (permafrost). No large woody plants.
1.2 Aquatic Biomes
These are easily divided by salinity:
- Freshwater: Salinity less than 1%, e.g., lakes, rivers.
- Marine: Salinity around 3%, e.g., oceans, coral reefs.
- Key Point: Estuaries are where freshwater meets saltwater (river mouths). They are excellent nurseries for marine life!
Important Note: Exams often ask, "What factors determine the type of terrestrial biome?" The answer is average temperature and average annual precipitation.
2. Ecosystem Components
An ecosystem consists of two parts working together:
1. Abiotic Components: Light, temperature, water, soil, minerals.
2. Biotic Components: Classified by their roles:
- Producer/Autotroph: Produces its own food via photosynthesis (e.g., plants, algae).
- Consumer/Heterotroph: Consumes other organisms (herbivores, carnivores, omnivores).
- Decomposer: Breaks down dead organic matter into minerals, returning them to the soil (e.g., fungi, bacteria). *Don't confuse them with detritivores, which just eat the dead matter!*
Did you know? Without "decomposers," the world would be filled with carcasses, and the soil's minerals would eventually run out!
3. Energy Flow and Nutrient Cycling
In an ecosystem, "energy flows in one direction," but "nutrients cycle."
3.1 Food Chain and Food Web
As organisms feed on one another, energy decreases at each trophic level.
Ten Percent Law:
Only 10% of the energy from the prey is transferred to the consumer. The other 90% is lost as heat and used for the organism's own metabolism.
\( \text{Energy gained} = \text{Energy from prey} \times 0.10 \)
3.2 Ecological Pyramids
- Pyramid of Numbers: Can be upright or inverted (e.g., one tree supporting 100 insects).
- Pyramid of Biomass: Measured by dry weight.
- Pyramid of Energy: Must always be upright (wide base)! Because energy decreases according to the 10% law.
3.3 Key Nutrient Cycles
- Water Cycle: Evaporation, transpiration, and condensation into rain.
- Carbon Cycle: Involves photosynthesis (intake) and respiration/combustion (release).
- Nitrogen Cycle: Focus on the role of bacteria in Nitrogen Fixation so that plants can use it.
Common Mistake: Many people think energy cycles in a circle. That’s incorrect! Energy is constantly lost as heat and does not return to producers, unlike minerals which are recycled.
4. Relationships Between Organisms
Memorize these symbols (+ means benefit, - means harmed, 0 means no effect):
- (+) (+) Mutualism: Cannot survive apart (e.g., lichens, termites and protozoa).
- (+) (+) Protocooperation: Can survive apart (e.g., oxpeckers and buffalo, bees and flowers).
- (+) (0) Commensalism: One benefits, the other is unaffected (e.g., sharks and remoras, orchids on trees).
- (+) (-) Parasitism: One benefits, one is harmed (e.g., ticks on dogs, intestinal worms).
- (+) (-) Predation: Predator (+) and prey (-).
- (-) (-) Competition: Both are harmed because they must fight for resources.
Memorization Technique: If the question says "If separated, they die or struggle to survive," look for Mutualism (+/+) right away!
5. Ecological Succession
This is the change in the community structure of an area over time.
1. Primary Succession: Starts from a "blank slate" with no prior soil, such as bare rock or cooled lava. The pioneer species is usually "lichens."
2. Secondary Succession: Starts from an area "where life previously existed" but was destroyed, like after a forest fire or slash-and-burn farming. This happens much faster than primary succession because soil and seeds are already present.
Eventually, it reaches a Climax Community, which is stable and balanced.
Key Takeaway
If it feels hard at first, don't worry! Just try to visualize the natural world:
1. Look at the climate (hot/cold/wet) -> You’ll know the biome.
2. Look at who eats whom -> You’ll see the energy flow.
3. Look at whether they are happy together -> You’ll know the relationship.
4. Look at whether the forest was damaged before -> You’ll understand succession.
Important for A-Level: Focus on food web diagrams, 10% law calculations, and comparing the distinct characteristics of each biome!
Keep going! I believe in all of you! ✌️