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Introduction to
Ecosystems

Explore how living organisms and their physical environment interact to form dynamic, interconnected systems that sustain life on Earth.

AP Environmental Science

What Is an Ecosystem?

An ecosystem is a community of living organisms (biotic factors) interacting with their non-living environment (abiotic factors) as a functional unit. Ecosystems can be as small as a puddle or as vast as the Amazon rainforest.

Biotic vs Abiotic Factors

Biotic Factors
All living components: plants, animals, fungi, bacteria, and protists that interact within the ecosystem.
Abiotic Factors
All non-living components: sunlight, temperature, water, soil, minerals, pH, and atmospheric gases.

Levels of Ecological Organization

Organism Population Community Ecosystem Biosphere Individual Same species Multiple species Community + abiotic All ecosystems

Energy Flow in Ecosystems

Energy enters most ecosystems as solar radiation and is converted to chemical energy by producers through photosynthesis. It then flows through trophic levels: from producers to primary consumers, secondary consumers, and beyond.

The Energy Pyramid

Producers 10,000 kcal Primary Consumers 1,000 kcal Secondary Consumers 100 kcal Tertiary 10 kcal ★ heat → heat → heat → Energy decreases at each trophic level

Nutrient Cycling: Matter is Recycled

Unlike energy, which flows one way through an ecosystem and is ultimately lost as heat, matter is recycled. Elements like carbon, nitrogen, and phosphorus move through biogeochemical cycles, passing between living organisms and the abiotic environment.

Carbon Cycle
\( \text{CO}_2 \) fixed by photosynthesis, released by respiration and combustion, stored in fossil fuels.
Nitrogen Cycle
\( \text{N}_2 \) fixed by bacteria, assimilated by plants, returned by decomposition and denitrification.
Phosphorus Cycle
Weathered from rock, absorbed by roots, cycles through food webs, returns via decomposition.

Sunlight passes through a living scene; nutrients return

A forest-floor scene with leaves, consumer, litter, fungi and roots accompanies separate energy-flow and matter-cycle diagrams.Energy enters organisms through sunlight and food, then dissipates as heat; decomposers also receive chemical energy from organic matter. Nutrient atoms can return through soil and other abiotic reservoirs as ecosystems exchange matter with their surroundings.

Separate energy and matter pathways

Separate energy and matter pathways. Energy enters organisms through sunlight and food, then dissipates as heat; decomposers also receive chemical energy from organic matter. Nutrient atoms can return through soil and other abiotic reservoirs as ecosystems exchange matter with their surroundings.Energy enters organisms through sunlight and food, then dissipates as heat; decomposers also receive chemical energy from organic matter. Nutrient atoms can return through soil and other abiotic reservoirs as ecosystems exchange matter with their surroundings.

Energy Flow vs Nutrient Cycling

Energy Flow Nutrient Cycling
Direction One-way (linear) Cyclical (recycled)
Source Sun (solar radiation) Earth's crust, atmosphere
Lost as Heat (at every level) Not lost, just relocated
Conservation Degrades (2nd law of thermodynamics) Conserved (law of conservation of matter)
Key Process Photosynthesis → Respiration Biogeochemical cycles

Understanding Trophic Levels

1
Producers (Autotrophs): Convert solar energy to chemical energy. Examples: plants, algae, cyanobacteria.
2
Primary Consumers (Herbivores): Feed directly on producers. Examples: rabbits, grasshoppers, zooplankton.
3
Secondary Consumers (Carnivores): Feed on primary consumers. Examples: frogs, small fish, snakes.
4
Tertiary Consumers (Top Predators): Feed on secondary consumers. Examples: hawks, sharks, wolves.
5
Decomposers: Break down dead matter at every level, recycling nutrients. Examples: fungi, bacteria, detritivores.

Food Web vs Food Chain

Hawk Snake Frog Mouse Grasshopper Rabbit Grass Shrubs

How to Analyze an Ecosystem

1
Identify the boundaries: Define the spatial limits of the ecosystem you are studying (a pond, a forest, a coral reef).
2
List biotic components: Catalog the species present and classify them by trophic level (producers, consumers, decomposers).
3
List abiotic components: Identify the key non-living factors: temperature, precipitation, soil type, sunlight, pH.
4
Trace energy flow: Construct a food web and energy pyramid to show how energy moves through the system.
5
Map nutrient cycles: Identify how key elements (C, N, P) cycle between biotic and abiotic reservoirs.

Example: A Pond Ecosystem

Algae Zooplankton Small Fish Heron Bacteria Abiotic Factors: Water temperature Dissolved \( \text{O}_2 \) Sunlight depth pH level

Explain why ecosystems exchange material across a chosen boundary.

Use this illustration to explore the concept described in its caption.

An ecological landscape shows pond outline, conceptual drainage divide and a broader study boundary; a companion illustrates crossings.
Pond, catchment and landscape boundaries answer different research questions. Organisms, matter and energy can cross the selected ecosystem boundary.
Arrows carry sunlight, matter and an organism across the selected pond boundary.
Energy, matter and organisms can cross a chosen ecosystem boundary; this is not a closed quantitative budget.

Applying the 10% Rule

A grassland ecosystem has producers with a total energy of \( 20{,}000 \text{ kcal/m}^2\text{/yr} \). Calculate the energy available at each trophic level.

1
Producers: 20,000 kcal
2
Primary Consumers: \( 20{,}000 \times 0.10 = \) 2,000 kcal
3
Secondary Consumers: \( 2{,}000 \times 0.10 = \) 200 kcal
4
Tertiary Consumers: \( 200 \times 0.10 = \) 20 kcal

Keystone Species and Ecosystem Stability

A keystone species has a disproportionately large effect on its ecosystem relative to its abundance. Removing it triggers a trophic cascade: a chain of effects through multiple trophic levels.

With Wolves (Keystone) Wolves (few) Elk (controlled) Vegetation (thriving) Without Wolves No wolves Elk (overpopulated) Vegetation (depleted) →

Common Mistakes

✕ Energy is Recycled
Energy flows one way and is lost as heat. Only matter (nutrients) is recycled through biogeochemical cycles.
✕ Food Chain = Food Web
A food chain is a single pathway. Real ecosystems have complex food webs with many interconnected chains.
✕ Decomposers Only at Bottom
Decomposers work at every trophic level, breaking down dead matter from producers, consumers, and top predators alike.

Ecosystem Summary

Concept Key Point Example
Ecosystem Biotic + abiotic interact Pond, forest, desert
Energy Flow One-way, 10% rule Sun → grass → rabbit → hawk
Nutrient Cycling Matter is recycled Carbon, nitrogen, phosphorus
Food Web Interconnected feeding Multiple chains in one habitat
Keystone Species Disproportionate impact Wolves in Yellowstone
Trophic Cascade Chain of effects across levels Removing predators → overgrazing

Question 1 of 5

Which of the following best describes the difference between energy flow and nutrient cycling in an ecosystem?

Question 2 of 5

A grassland ecosystem has producers with \( 50{,}000 \text{ kcal/m}^2\text{/yr} \) of energy. Using the 10% rule, approximately how much energy is available to secondary consumers?

Question 3 of 5

In a food web, a disease eliminates the frog population. Which of the following is the most likely immediate effect?

Question 4 of 5

Which of the following is an example of an abiotic factor in an ecosystem?

Question 5 of 5

A keystone species is best defined as an organism that: