Energy Flow, Ecosystems and the Environment - Biology - Pearson A-Level

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Welcome to Energy Flow, Ecosystems, and the Environment!

Hi there! This chapter is absolutely central to understanding how life on Earth works. We are going to explore how energy moves through different living things, how key materials are recycled, and how biological communities change over time.

Don't worry if concepts like "trophic levels" or "denitrification" sound complicated right now. We'll break them down using simple steps and real-world examples. By the end, you’ll see that everything in nature is connected, just like a massive, sophisticated recycling system!

Section 1: Ecosystems, Trophic Levels, and Energy Capture

1.1 Defining the Ecosystem

An Ecosystem is simply the community of living organisms (the biotic components) interacting with the non-living parts of their environment (the abiotic components).

Biotic Components: Plants, animals, fungi, bacteria.
Abiotic Components: Sunlight, water, minerals, temperature, pH, soil structure.

Energy Entry: The Producers

Almost all energy entering an ecosystem comes from the Sun. This energy is captured by Producers (usually plants or algae) through photosynthesis.

Quick Review: Photosynthesis

Producers convert light energy into chemical energy (glucose). This chemical energy forms the base of almost every food web.

1.2 Trophic Levels and Food Webs

A Trophic Level describes the feeding position of an organism in a food chain. Think of it as a step on an energy ladder.

Trophic Level 1: Producers (e.g., grass, trees). They make their own food.
Trophic Level 2: Primary Consumers (Herbivores). They eat the producers (e.g., rabbits, cows).
Trophic Level 3: Secondary Consumers (Carnivores or Omnivores). They eat the primary consumers (e.g., foxes, snakes).
Trophic Level 4: Tertiary Consumers (Apex Predators). They eat the secondary consumers (e.g., eagles, sharks).

Food Chains show a simple, linear transfer of energy (Grass $\rightarrow$ Cow $\rightarrow$ Human). Food Webs show the complex, interconnected feeding relationships, which is much closer to reality.

The Essential Recyclers: Decomposers

Decomposers (like bacteria and fungi) break down dead organic matter and waste products. They are vital because they release trapped minerals and nutrients back into the soil, completing the cycles. Without them, nutrients would be locked up in dead bodies!

Key Takeaway (Section 1): Energy starts with the Producers and flows up the Trophic Levels. Decomposers ensure the materials are recycled.

Section 2: Energy Transfer and Ecological Efficiency

The flow of energy is one-way. Unlike nutrients, energy cannot be recycled. It is continuously lost as heat.

2.1 Why is Energy Transfer Inefficient?

Only a tiny fraction of the solar energy reaching Earth is ever captured by producers (often less than 3%). Even worse, when energy moves from one trophic level to the next, about 90% is lost! Only around 10% is successfully incorporated into the biomass of the next level.

Why is 90% Lost?

Not Eaten: Consumers might not eat the entire organism (e.g., bones, roots, fur). This is called uneaten material.
Not Digestible: Parts that are eaten but cannot be absorbed (e.g., cellulose) are passed out as faeces (waste).
Respiration: A large amount of energy is used by the organism itself for metabolism, movement, and staying warm. This energy is lost primarily as heat to the environment.

Analogy: Imagine trying to pass a bucket of water down a long line of people (trophic levels). The buckets have holes (respiration/waste), so the person at the end only receives a small splash.

2.2 Calculating Efficiency

We measure how effective the energy transfer is using the formula for Ecological Efficiency:

$$ \text{Efficiency} = \frac{\text{Energy incorporated into the next trophic level}}{\text{Energy available in the previous trophic level}} \times 100 $$

Don't Panic! This is just a percentage calculation. If 1000 kJ of energy is available at Trophic Level 2, and only 100 kJ is successfully passed to Trophic Level 3, the efficiency is (100/1000) $\times$ 100 = 10%.

Pyramids of Energy and Biomass

To visualize this loss, scientists use diagrams:

Pyramid of Energy: Shows the energy content (usually measured in kJ m$^{-2}$ year$^{-1}$) at each trophic level. Due to energy loss, this pyramid is always triangular (pyramid-shaped).
Pyramid of Biomass: Shows the total mass of living material (usually measured in g m$^{-2}$) at each trophic level. While usually pyramid-shaped, marine ecosystems can sometimes show an inverted pyramid of biomass (e.g., tiny phytoplankton reproducing very fast but being eaten faster than they accumulate mass).

Common Mistake: Pyramids of numbers can be inverted (e.g., one large oak tree supporting thousands of insects), but Pyramids of Energy can never be inverted. Energy must decrease at each step.

Key Takeaway (Section 2): Energy transfer is only about 10% efficient due to respiration, waste, and uneaten parts. This inefficiency limits the length of food chains.

Section 3: Nutrient Cycling (Carbon and Nitrogen)

While energy flows in one direction, essential chemical elements (like Carbon, Nitrogen, and Phosphorus) must be recycled continuously. They move between the atmosphere, living organisms, and the soil/water.

3.1 The Carbon Cycle

Carbon is the backbone of all organic molecules (carbohydrates, proteins, DNA). The reservoir for carbon is usually the atmosphere (as $CO_2$).

Step-by-Step Cycle:

Atmosphere to Biota (Living Things): Plants (Producers) take in $CO_2$ for photosynthesis, locking carbon into biomass (sugars, starch).
Biota to Atmosphere: All living things (plants, animals, decomposers) release $CO_2$ through respiration.
Biota to Soil: When organisms die, their carbon is transferred to the soil. Decomposers break this down, eventually releasing $CO_2$ back into the atmosphere via respiration (decomposition is fundamentally respiration).
Human Impact (Combustion): Burning fossil fuels (coal, oil, gas) releases carbon that was previously locked away in the Earth over millions of years, significantly increasing atmospheric $CO_2$.

Did You Know? The oceans are massive carbon sinks, absorbing a huge amount of atmospheric $CO_2$.

3.2 The Nitrogen Cycle

Nitrogen is essential for making proteins and nucleic acids (DNA/RNA). Nitrogen gas ($N_2$) makes up 78% of the atmosphere, but plants cannot absorb it directly. It must be "fixed" and converted into soluble nitrates.

The Nitrogen Cycle relies heavily on different types of specialized bacteria.

Step-by-Step Transformations (The N-Cycle):

Nitrogen Fixation: Atmospheric $N_2$ gas is converted into ammonium ions ($NH_4^+$). This is carried out by Nitrogen-fixing bacteria found in the soil or living symbiotically in the root nodules of leguminous plants (e.g., peas, beans).
$$N_2 \rightarrow NH_4^+$$
Ammonification: When dead organic matter (proteins, urea) decays, decomposers convert the nitrogen back into ammonium ions.
Nitrification: Ammonium is toxic, so it must be further converted. This is a two-step process carried out by Nitrifying bacteria (aerobic, requiring oxygen):
- Stage 1: Ammonium ($NH_4^+$) is converted to Nitrite ($NO_2^-$).
- Stage 2: Nitrite ($NO_2^-$) is converted to Nitrate ($NO_3^-$).
Nitrates ($NO_3^-$) are easily absorbed and assimilated by plant roots.
Denitrification: This process removes nitrogen from the soil, converting nitrates ($NO_3^-$) back into $N_2$ gas. This is carried out by Denitrifying bacteria (anaerobic, thrive in waterlogged soils where oxygen is scarce).
$$NO_3^- \rightarrow N_2$$

Memory Aid for the Nitrogen Cycle: FAN-A-D

Fixation ($N_2$ to $\text{Ammonium}$)
Ammonification ($\text{Dead Matter}$ to $\text{Ammonium}$)
Nitrification ($\text{Ammonium}$ to $\text{Nitrates}$)
AssImilation ($\text{Nitrates}$ absorbed by plants)
Denitrification ($\text{Nitrates}$ back to $N_2$)

Key Takeaway (Section 3): Carbon cycles quickly between the atmosphere and living things (photosynthesis/respiration). Nitrogen requires specialized bacteria to convert inert $N_2$ gas into soluble nitrates that plants can use.

Section 4: Succession and Environmental Change

Ecosystems are not static; they change over time. Succession is the directional change in the species structure of a community over time.

4.1 Primary vs. Secondary Succession

Primary Succession

This occurs in an area where life has never existed before or where the surface is devoid of soil (e.g., bare rock, newly cooled lava, bare sand dunes).

Pioneer Species: The first organisms to colonize the area (e.g., lichens and mosses). They are hardy and can survive harsh conditions.
Soil Formation: Pioneers break down rock (weathering) and, when they die, their organic matter adds humus, starting the formation of simple soil.
Seral Stages: As the soil thickens, larger plants (ferns, grasses) move in, followed by shrubs, and eventually trees.

Secondary Succession

This occurs in an area where a previous community existed but was removed by a disturbance (e.g., forest fire, abandoned farmland). Since soil is already present, this process is much faster.

The community skips the bare rock stage and moves straight to grasses and fast-growing herbs.

4.2 The Climax Community

Succession continues until a stable, self-perpetuating community develops. This final, stable stage is the Climax Community.

In the UK, the climax community is typically deciduous forest.
Characteristics: High species diversity, complex food webs, high biomass, and stability (it can handle small disturbances).

Human Influence: Plagioclimax

If succession is stopped or diverted by human activity (e.g., grazing by farm animals, repeated burning, deforestation), the stable community that results is called a Plagioclimax. A grassy field maintained by continuous mowing is a simple example of a plagioclimax.

Key Takeaway (Section 4): Succession is the natural progression of an ecosystem towards a stable Climax Community. Primary starts without soil (slow), Secondary starts with soil (fast).

Section 5: Pollution and Eutrophication (Connecting Cycles to Environment)

5.1 Eutrophication

This is a major environmental problem caused by the excessive release of nitrates and phosphates (nutrients) into water bodies (lakes, rivers). These nutrients often come from sewage or agricultural runoff (fertilizers).

Step-by-Step Process of Eutrophication:

Nutrient Overload: High concentrations of nitrates and phosphates enter the lake.
Algal Bloom: This rapid supply of nutrients causes a massive, rapid growth of algae on the surface (the "bloom").
Light Blocked: The dense algal layer blocks sunlight from reaching aquatic plants below the surface. These plants die.
Increased Decomposition: When the algae and plants die, decomposers (bacteria) increase rapidly to break down the large amount of dead organic matter.
Oxygen Depletion: The decomposers are aerobic (they respire) and use up vast amounts of dissolved oxygen in the water.
Fish Kill: The resulting lack of oxygen leads to the death of fish and other aerobic aquatic life, leading to an unbalanced, lifeless ecosystem.

Remember This: Eutrophication is a chain reaction where too much good stuff (nutrients) leads to a bad outcome (suffocation).

Conservation

Understanding energy flow and nutrient cycles is vital for effective conservation. Strategies often focus on managing biomass and controlling nutrient input:

Controlled Grazing: Managing animal numbers to prevent succession from being permanently diverted (avoiding undesirable plagioclimax).
Reducing Fertilizer Runoff: Implementing farming practices that limit the amount of nitrate and phosphate leaching into waterways to prevent eutrophication.

Final Encouragement: You now have a solid foundation in the mechanics of energy and matter in the environment. Remember that biology is all connected—the energy you learned about in photosynthesis drives every cycle we discussed here! Keep practicing those nutrient cycle diagrams!