Ecosystem Components

Why This Matters

Every ecosystem question on your exam ultimately comes back to one core idea: how do living and non-living things interact to move energy and cycle matter? Whether you're analyzing a food web, explaining why a population crashed, or predicting the effects of habitat loss, you need to understand the building blocks that make ecosystems function. This topic connects directly to energy flow, nutrient cycling, population dynamics, and biodiversity—concepts that appear repeatedly in multiple-choice questions and form the backbone of ecology FRQs.

Here's the key insight: ecosystems aren't just lists of organisms and environmental factors. They're dynamic systems where every component plays a specific role in transferring energy or recycling nutrients. Don't just memorize definitions—know why each component matters and how it connects to the bigger picture. When you can explain the relationship between a decomposer and a biogeochemical cycle, or why removing a keystone species disrupts an entire community, you're thinking like an ecologist.

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The Living Players: Biotic Components

Every organism in an ecosystem falls into one of three functional categories based on how it obtains energy. This classification drives everything from food web analysis to understanding ecosystem productivity.

Producers

• Autotrophs that convert sunlight into chemical energy—primarily plants, algae, and cyanobacteria using photosynthesis ($6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$)
• Form the base of every food chain, determining the total energy available to all other trophic levels in the ecosystem
• Critical for carbon fixation—remove $CO_2$ from the atmosphere and produce the oxygen other organisms need to survive

Consumers

• Heterotrophs that obtain energy by eating other organisms—classified as herbivores (eat producers), carnivores (eat other consumers), or omnivores (eat both)
• Transfer energy up trophic levels, though only about 10% of energy moves from one level to the next due to metabolic losses
• Shape community structure through predation, competition, and herbivory—their feeding choices control population sizes throughout the food web

Decomposers

• Bacteria and fungi that break down dead organic matter—releasing nutrients back into the soil and atmosphere through decomposition
• Essential for nutrient cycling—without decomposers, dead matter would accumulate and nutrients would remain locked in organic forms
• Connect the end of food chains back to the beginning—making nutrients available for producers to use again

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The Stage: Abiotic Components

Non-living factors determine which organisms can survive in a given location and how productive the ecosystem can be. These physical and chemical conditions set the boundaries for life.

Abiotic Components

• Non-living environmental factors—including sunlight, temperature, water, soil composition, pH, and atmospheric gases
• Determine species distribution by creating conditions that favor certain adaptations over others (tolerance ranges define where species can survive)
• Changes cascade through the ecosystem—shifts in temperature or precipitation alter which producers thrive, affecting every trophic level above them

Habitat

• The physical environment where an organism lives—providing the resources needed for survival, growth, and reproduction
• Different habitats support distinct communities—a coral reef and a temperate forest have completely different species assemblages due to their abiotic conditions
• Habitat destruction is the leading cause of biodiversity loss—when the physical space disappears, so do the species that depend on it

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Ecological Roles and Relationships

Understanding what organisms do in their ecosystem—not just what they are—is crucial for predicting how changes will ripple through the system.

Niche

• The functional role an organism plays—including what it eats, when it's active, where it lives within the habitat, and how it interacts with other species
• Niche differentiation reduces competition—species evolve to use different resources or the same resources at different times, allowing coexistence
• Fundamental vs. realized niche—the fundamental niche is the full range of conditions a species could occupy; the realized niche is what it actually occupies due to competition and other interactions

Keystone Species

• A species with disproportionate impact relative to its abundance—its removal causes dramatic changes in community structure and ecosystem function
• Classic example: sea otters control sea urchin populations, which protects kelp forests; without otters, urchins overgraze kelp, collapsing the entire ecosystem
• Conservation priority—protecting keystone species maintains ecological balance more efficiently than trying to protect every species individually

Biodiversity

• The variety of life at multiple scales—species diversity (number and abundance of species), genetic diversity (variation within species), and ecosystem diversity (variety of habitat types)
• High biodiversity increases ecosystem resilience—more species means more functional redundancy, so the system can absorb disturbances without collapsing
• Biodiversity loss disrupts ecosystem services—fewer species means fewer interactions, reduced productivity, and weakened nutrient cycling

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Energy and Matter Movement

Ecosystems run on two fundamental processes: energy flows through (entering as sunlight, exiting as heat) while matter cycles within (atoms are recycled endlessly). This distinction is essential.

Energy Flow

• Unidirectional movement from sun to producers to consumers to decomposers—energy cannot be recycled; it must constantly enter the system
• Only ~10% of energy transfers between trophic levels—the rest is lost as heat through metabolic processes (the 10% rule)
• Limits food chain length—by the 4th or 5th trophic level, so little energy remains that top predators are rare and vulnerable to extinction

Trophic Levels

• Hierarchical feeding positions in an ecosystem—producers (1st), primary consumers (2nd), secondary consumers (3rd), tertiary consumers (4th)
• Biomass and energy decrease at higher levels—creating the characteristic pyramid shape of ecological pyramids
• Trophic efficiency varies by ecosystem—aquatic systems often have higher efficiency than terrestrial ones due to differences in producer types

Food Chains and Food Webs

• Food chains show linear energy transfer—simple sequences like grass → grasshopper → frog → snake → hawk
• Food webs show realistic complexity—interconnected chains revealing that most organisms eat multiple prey and have multiple predators
• Web complexity increases stability—if one food source disappears, organisms with diverse diets can switch to alternatives

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Nutrient Cycling and Biogeochemical Processes

Matter doesn't leave Earth—it just changes form. Understanding how elements move between living organisms and the physical environment is critical for addressing environmental issues.

Nutrient Cycling

• The continuous movement of elements between biotic and abiotic components—involving uptake, incorporation into biomass, death, decomposition, and release
• Decomposition and mineralization are rate-limiting steps—if decomposers slow down (cold temperatures, waterlogged soil), nutrients accumulate in dead matter
• Human activities disrupt natural cycles—fertilizer runoff adds excess nitrogen and phosphorus, causing eutrophication and dead zones

Biogeochemical Cycles

• Global-scale movement of elements through biological, geological, and chemical processes—the carbon, nitrogen, phosphorus, and water cycles are most commonly tested
• Carbon cycle connects to climate change—burning fossil fuels releases stored carbon as $CO_2$, increasing atmospheric concentrations and trapping heat
• Nitrogen cycle requires bacterial transformations—nitrogen fixation ($N_2 \rightarrow NH_3$), nitrification, and denitrification are all mediated by specialized bacteria

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Human Connections: Ecosystem Services

Ecosystems provide essential benefits that support human survival and well-being—understanding these services explains why conservation matters beyond intrinsic value.

Ecosystem Services

• Four categories of benefits—provisioning (food, water, timber), regulating (climate control, flood prevention, disease regulation), cultural (recreation, spiritual value), and supporting (nutrient cycling, soil formation)
• Economic value is enormous—global ecosystem services estimated at trillions of dollars annually, though most aren't captured in market prices
• Degradation has real consequences—losing wetlands increases flooding; losing pollinators threatens crop production; losing forests accelerates climate change

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Quick Reference Table

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Self-Check Questions

1. Trace the path: How does a carbon atom move from the atmosphere through a producer, a consumer, a decomposer, and back to the atmosphere? What processes are involved at each step?

2. Compare and contrast: What's the difference between energy flow and nutrient cycling? Why does energy require constant input while nutrients can be recycled indefinitely?

3. Apply the concept: If a keystone predator is removed from an ecosystem, what happens to biodiversity and why? Use trophic cascade reasoning in your answer.

4. Identify by function: Which ecosystem components are most critical for maintaining biogeochemical cycles—producers, consumers, or decomposers? Defend your answer.

5. FRQ practice: An ecosystem experiences a sudden temperature increase that kills most decomposers. Predict the effects on (a) nutrient cycling, (b) producer populations, and (c) overall ecosystem productivity over the following year.