2.4 Ecosystems, Energy Flow & Food Webs

2.4 Ecosystems, Energy Flow & Food Webs

GED Exam Focus: Ecosystem questions require analyzing food webs, calculating energy availability across trophic levels using the 10% rule, interpreting population growth graphs (J-curves vs. S-curves), and predicting how environmental disturbances affect carrying capacity.

Ecological Hierarchy & Ecosystem Components

Ecology investigates the interactions between living organisms and their physical environment. These interactions are organized into hierarchical levels:

$\text{Organism} \rightarrow \text{Population} \rightarrow \text{Community} \rightarrow \text{Ecosystem} \rightarrow \text{Biome} \rightarrow \text{Biosphere}$

• Organism: An individual living entity (e.g., one red oak tree).
• Population: A group of individuals belonging to the same species living in the same geographic area at the same time (e.g., all red oak trees in a forest).
• Community: All the interacting populations of different species residing in a common area (e.g., oak trees, deer, squirrels, fungi, and birds in a forest).
• Ecosystem: The combined system of a biological community (biotic factors) interacting with their non-living physical environment (abiotic factors).
  • Biotic Factors: Living or once-living organisms (plants, animals, bacteria, fungi).
  • Abiotic Factors: Non-living physical and chemical components (sunlight, temperature, water availability, soil pH, mineral nutrients, dissolved oxygen).

Energy Flow & Trophic Pyramids (The 10% Rule)

Energy enters almost all Earth ecosystems as sunlight, which is converted into chemical energy (glucose) by autotrophs (primary producers) through photosynthesis.

Trophic Levels in an Ecosystem

Organisms are categorized into trophic levels based on their primary source of energy:

1. Primary Producers (Autotrophs): Photosynthetic organisms (green plants, algae, cyanobacteria) that create organic biomass from solar energy.
2. Primary Consumers (Herbivores): Heterotrophs that eat primary producers (e.g., rabbits, zooplankton, grasshoppers).
3. Secondary Consumers (Carnivores/Omnivores): Heterotrophs that eat primary consumers (e.g., frogs, snakes, small fish).
4. Tertiary Consumers (Apex Predators): Carnivores that eat secondary consumers (e.g., hawks, wolves, sharks).
5. Decomposers & Detritivores: Organisms (fungi, bacteria, earthworms) that break down dead organic matter and waste, recycling inorganic nutrients back into the soil.

Energy Pyramid & The 10% Rule:
+--------------------------------------------------------+  Level 4: Tertiary Consumer
|               Tertiary Consumers (10 Joules)           |  (Apex Predators - 0.1% Energy)
+--------------------------------------------------------+
|             Secondary Consumers (100 Joules)           |  (Carnivores - 1% Energy)
+--------------------------------------------------------+
|           Primary Consumers (1,000 Joules)             |  (Herbivores - 10% Energy)
+--------------------------------------------------------+
|         Primary Producers (10,000 Joules)              |  (Autotrophs - 100% Energy)
+--------------------------------------------------------+
  ===> 90% Energy Lost as Heat/Respiration at Each Step! <===

The 10% Rule of Energy Transfer

During energy transfer from one trophic level to the next, only approximately 10% of the energy stored as biomass is incorporated into the next level. The remaining 90% is lost to the environment as metabolic heat, cellular respiration, unconsumed plant material, and waste.

GED Worked Example: A field of clover captures $80,000\text{ kJ}$ of solar energy. Calculate the energy available to snakes (secondary consumers) that eat mice (primary consumers).

1. Primary Producers (Clover): $80,000\text{ kJ}$
2. Primary Consumers (Mice): $80,000\text{ kJ} \times 0.10 = 8,000\text{ kJ}$
3. Secondary Consumers (Snakes): $8,000\text{ kJ} \times 0.10 = 800\text{ kJ}$

Answer: $800\text{ kJ}$ of energy reaches the secondary consumer level.

Food Chains, Food Webs & Keystone Species

• Food Chain: A simple, linear model showing a single pathway of energy transfer from one organism to another.
• Food Web: A complex, interconnected network of multiple overlapping food chains within an ecosystem, reflecting real-world feeding relationships.

Critical Rule: Arrow Direction in Food Webs

In food chain and food web diagrams, arrows point FROM the organism being eaten TO the organism that consumes it (indicating the direction of energy and matter transfer). $\text{Grass} \longrightarrow \text{Rabbit} \longrightarrow \text{Fox}$ (Meaning: Energy moves FROM Grass TO Rabbit, and FROM Rabbit TO Fox).

Keystone Species & Trophic Cascades

A keystone species is an organism that plays a disproportionately large role in maintaining the structure, stability, and biodiversity of an ecosystem relative to its abundance.

• Example: Sea Otters in Pacific kelp forests. Otters prey on sea urchins. If otters are removed, sea urchin populations explode and overgraze kelp beds, destroying the habitat for hundreds of fish species. This dramatic ecosystem-wide collapse triggered by removing a top predator is called a trophic cascade.

Population Dynamics & Growth Models

Populations change in size over time according to birth rates, death rates, immigration, and emigration.

1. Exponential Growth (J-Curve)

Occurs when a population grows at an accelerating rate under ideal, unconstrained conditions with abundant resources, no competition, and absent predation. Represented graphically as a J-shaped curve.

2. Logistic Growth (S-Curve) & Carrying Capacity

In nature, populations encounter environmental resistance (resource limits, disease, space competition). Growth slows and levels off at the environment's Carrying Capacity ($K$)—the maximum population size that a specific environment can sustainably support long-term. Represented as an S-shaped curve.

Population Growth Curves:
Exponential (J-Curve)               Logistic (S-Curve)
  Population                          Population
     |         /                         |           - - Carrying Capacity (K)
     |        /                          |         /-----
     |       /                           |        / 
     |  ____/                            |  ____/   
     +-------------> Time                +-------------> Time

3. Limiting Factors

• Density-Dependent Factors: Factors whose impact intensifies as population density increases (e.g., competition for food/water, infectious disease, predation, waste accumulation).
• Density-Independent Factors: Factors that impact population size regardless of density (e.g., natural disasters, blizzards, floods, volcanic eruptions, human deforestation).

Symbiotic Relationships

• Mutualism ($+/+$): Both species benefit (e.g., bees pollinating flowers).
• Commensalism ($+/0$): One species benefits; the other is unaffected (e.g., barnacles riding on whales).
• Parasitism ($+/-$): One species (parasite) benefits at the expense of the host (e.g., ticks sucking blood from deer).