Ecosystems and material cycles

Welcome to Ecosystems and Material Cycles!

In this chapter, we are going to explore how nature is one big, interconnected machine. You will learn how living things interact with each other and their environment, how humans are changing these balances, and how nature "recycles" essential ingredients like water, carbon, and nitrogen. Think of this as the "How Nature Works" guide for your Biology Paper 2 exam!

1. How Nature is Organised

To understand an ecosystem, we have to look at it in layers. Scientists break it down into four main levels:

1. Individual organism: A single living thing (e.g., one rabbit).
2. Population: All the individuals of one species in a specific area (e.g., all the rabbits in a field).
3. Community: All the different populations of different species living together (e.g., the rabbits, the grass, the foxes, and the insects in that field).
4. Ecosystem: The community of living things plus the non-living (abiotic) parts of their environment, like the soil, water, and weather.

Abiotic vs. Biotic Factors

What determines if an organism can survive in a certain place? Two types of factors:

Abiotic Factors (Non-living):
- Temperature: Most things can't survive if it's too hot or too cold.
- Light intensity: Plants need light for photosynthesis.
- Water availability: No water = no life!
- Pollutants: Chemicals in the air or water can harm organisms.

Biotic Factors (Living):
- Competition: Organisms fighting for the same resources (food, space, mates).
- Predation: If there are too many predators, the prey population will drop.

Interdependence

In a community, every species depends on others for things like food, shelter, and pollination. This is called interdependence. If one species is removed, it can affect the whole web! For example, if all the bees die, many plants won't be pollinated, which means animals that eat those plants will go hungry.

Quick Review:
- Abiotic = Non-living (like 'A' for 'Away from life').
- Biotic = Living (like 'Bio' for 'Biology').

2. Living Together: Survival Relationships

Sometimes, two different species live very closely together. There are two types you need to know:

1. Parasitism: One organism (the parasite) lives on or in another organism (the host). The parasite takes what it needs, but the host gets nothing back and is often harmed.
Example: Fleas on a dog or tapeworms in a human.

2. Mutualism: Both organisms benefit from the relationship. It’s a "win-win" situation!
Example: Flowers and bees. The bee gets food (nectar), and the flower gets its pollen spread to other flowers.

Takeaway: In parasitism, only one wins. In mutualism, they both win!

3. Core Practical: Investigating Ecosystems

How do scientists count the number of organisms in a massive field? They don't count them all! They use sampling.

Using Quadrats

A quadrat is a square frame. You place it on the ground and count the organisms inside it. To be accurate, you must place them randomly so your results aren't biased.

The Calculation:
To estimate the total population of a species in a field:
\( \text{Total population} = \text{Mean number of organisms per quadrat} \times \frac{\text{Total area of the field}}{\text{Area of one quadrat}} \)

Using Belt Transects

If you want to see how the number of organisms changes as you move across an area (e.g., moving from a sunny field into a dark forest), you use a belt transect. You lay a tape measure down and place quadrats at regular intervals along that line.

Common Mistake: Students often forget that quadrats must be placed randomly for general population estimates, but systematically (in a line) for transects.

4. Human Impact and Biodiversity

Biodiversity is the variety of different species in an area. High biodiversity is great because it makes ecosystems more stable.

Human Interactions (The Bad News)

1. Fish Farming: While it provides food, it can lead to waste leaking into the wild, diseases spreading to wild fish, and predators getting trapped in nets.
2. Non-indigenous species: These are "invader" species that aren't native to the area. They often out-compete native species for food (e.g., Grey squirrels out-competing Red squirrels).
3. Eutrophication: This is a big exam favorite! Here is the step-by-step process:
- 1. Excess fertilisers wash into a lake/river.
- 2. This causes algae to grow rapidly (an "algal bloom").
- 3. Algae block the sunlight, so plants below die.
- 4. Bacteria decompose the dead plants and use up all the oxygen through respiration.
- 5. Fish and other animals die because there isn't enough oxygen.

Conservation and Reforestation (The Good News)

Humans can also help biodiversity by:
- Conservation: Protecting habitats or creating "seed banks" and captive breeding programs.
- Reforestation: Planting trees to restore forests that were cut down. This provides habitats and helps reduce CO2 in the atmosphere.

Did you know? High biodiversity helps us too! Many of our medicines are discovered in wild plants and animals.

5. Material Cycles

Materials like carbon, water, and nitrogen are "recycled" through ecosystems so they can be used over and over again.

The Carbon Cycle

Carbon moves between the air and living things through several processes:
- Photosynthesis: Plants take CO2 out of the atmosphere.
- Respiration: Animals and plants put CO2 back into the atmosphere.
- Combustion: Burning wood or fossil fuels releases CO2.
- Decomposition: Microorganisms (decomposers) break down dead matter and release CO2 as they respire.

The Water Cycle

1. Evaporation: Sun heats the water in oceans/lakes, turning it into vapor.
2. Transpiration: Water evaporates from the leaves of plants.
3. Condensation: Vapor cools to form clouds.
4. Precipitation: Water falls as rain or snow.
In areas of drought, humans use desalination to turn salty seawater into potable (drinkable) water.

The Nitrogen Cycle

Plants need nitrates to grow, but they can't take nitrogen gas straight from the air. They need help from bacteria:
- Nitrogen-fixing bacteria: Turn nitrogen gas from the air into nitrogen compounds in the soil.
- Nitrifying bacteria: Turn ammonia into nitrates.
- Denitrifying bacteria: Turn nitrates back into nitrogen gas (bad for plants!).
- Decomposers: Break down proteins in dead animals/plants into ammonia.

Farmers can increase nitrates by using fertilisers or crop rotation (planting peas or beans, which have nitrogen-fixing bacteria in their roots).

Memory Aid:
- Fixing bacteria = Fix the nitrogen into the soil.
- De-nitrifying = De-lete (remove) the nitrates from the soil.

Final Key Takeaway: Nature is a giant loop. Every atom of carbon or drop of water has likely been part of a dinosaur, a tree, or a cloud before it got to you!