Chapter 3 Notes: The Biosphere, Energy Flow, and Cycles of Matter
3.1 What Is Ecology?
Ecology is the scientific study of interactions among organisms and between organisms and their physical environment.
The biosphere is the global sum of all ecosystems; it includes all life on Earth and all parts of Earth where life exists (land, water, air).
Extends from about 8 km above Earth’s surface to about 11 km below the ocean surface.
Key questions in ecology include: What is ecology? What are biotic and abiotic factors? What methods are used in ecological studies?
Biotic factors = living components of an environment (plants, animals, bacteria, fungi, etc.).
Abiotic factors = nonliving components (sunlight, heat, precipitation, humidity, wind, soil, water currents).
Interdependence in nature: organisms and their environments form a web of interdependent relationships; changes in one part affect others.
Levels of organization (from smallest to largest):
Organism (individual)
Population (group of individuals of the same species living in the same area)
Community (assemblage of populations living together in a defined area)
Ecosystem (community and its physical environment)
Biome (group of ecosystems sharing similar climates and organisms)
Biosphere (all life on Earth and all parts of Earth that support life)
The root word oikos (Greek) means “house.” Ecology = study of nature’s houses and the organisms that live in them.
Ecology and economics share a root (oikos): both deal with houses and flows of energy and nutrients, but ecology focuses on natural systems and energy/nutrient flows, while economics focuses on human trade.
Interdependence of humans within the biosphere: humans depend on ecological processes for essentials like food and drinkable water.
Methods ecologists use:
Observation: ask questions, count populations, note behaviors, etc.
Experimentation: test hypotheses in labs/greenhouses or in nature by changing conditions.
Modeling: build mathematical or computer models to understand complex, long-term, or large-scale phenomena.
Think about it: Lewis Thomas’s quote about Earth being alive invites reflection on how to interpret Earth as a living system and how to study it scientifically.
Key vocabulary: biosphere, species, population, community, ecology, ecosystem, biome, biotic factor, abiotic factor.
3.2 Energy, Producers, and Consumers
Living systems require energy for growth, reproduction, and metabolism; energy typically comes from the sun, but some organisms use chemical energy from inorganic compounds.
Autotrophs (primary producers): organisms that capture energy and convert it into usable organic molecules.
Primary energy source on land: sunlight (photosynthesis).
Aquatic primary producers: algae; in deep-sea or pigment-limited regions, photosynthesis may be limited, so chemosynthesis is used by some organisms.
Chemosynthesis: using chemical energy from inorganic molecules to produce carbohydrates, enabling life in environments without light (e.g., deep-sea vents, hot springs, tidal marshes).
Photosynthesis (typical land/sea producers):
General equation:
Produces oxygen and stores energy in carbohydrates.
Chemosynthesis: energy from inorganic bonds (e.g., with hydrogen sulfide, H2S) is used to synthesize organic molecules; example pathways occur in tide pools, hot springs, and deep-sea environments.
Heterotrophs (consumers): organisms that cannot directly capture energy from the environment and must obtain energy/nutrients from other organisms.
Major consumer types:
Herbivores: eat plants
Carnivores: eat other animals
Omnivores: eat both plants and animals (e.g., humans, bears, pigs, coati)
Scavengers: consume carcasses of other animals (e.g., vultures)
Detritivores: feed on detritus (dead organic matter) often by chewing/grinding detritus (e.g., giant earthworms, some snails, crabs)
Decomposers: break down dead organic matter chemically (e.g., bacteria, fungi), releasing nutrients back into the ecosystem
Detritus pathway: decomposers convert dead matter into detritus. Detritivores then consume detritus; decomposition also releases nutrients for producers.
Examples of consumer roles in ecosystems: a given ecosystem may host herbivores (leaf eaters), carnivores, omnivores, detritivores, scavengers, and decomposers, often with overlaps and flexible behaviors (e.g., some carnivores scavenging).
Experiments illustrating ecological interactions: a classroom activity with two jars—one with aphids only (plants grow, aphids reproduce) and one with aphids plus lady beetles (predation reduces aphids and impacts plant health). This demonstrates predator–prey dynamics and producer–consumer relationships.
Prefix note: auto- means “by itself”; trophic relates to feeding.
Important vocabulary: autotroph, primary producer, photosynthesis, chemosynthesis, heterotroph, consumer, carnivore, herbivore, scavenger, omnivore, decomposer, detritivore.
Think about it: energy is essential for all organisms; energy transfer through ecosystems shapes behavior, movement, and life-history strategies.
3.3 Energy Flow in Ecosystems
Energy flow in ecosystems is a one-way stream: from primary producers to various consumers, with energy dissipating as heat at each transfer.
Food chain: a linear sequence showing energy transfer through feeding relationships (producer → consumer → consumer …).
Example prairie chain: grass (producer) → antelope (herbivore) → coyote (carnivore).
In aquatic chains, producers include phytoplankton and attached algae; examples: phytoplankton → small fish (e.g., flagfish) → larger fish (e.g., largemouth bass) → wading birds (e.g., anhinga) → top predator (e.g., alligator).
Food web: a network of many interconnected food chains; feeding relationships are more complex than a single chain because organisms feed in multiple ways.
Example Everglades food web: algae/plants → various herbivores (grass shrimp, crayfish, etc.) → predators (fish, birds) → detritivores/decomposers feeding on detritus and recycling nutrients.
Detritus pathway and decomposers: most organisms die without being eaten; decomposers break down dead matter and release nutrients that primary producers can reuse; detritivores consume detritus particles, often feeding on decomposers too.
Disturbances and food webs: disturbances (e.g., oil spills, climate change) can alter feeding relationships and cascade through the web; e.g., changes in bacteria that transform nitrogen can be linked to changes in upstream energy/nutrient flows (Narragansett Bay case). Krill-antarctic web example illustrates how a single keystone food source (krill feeding on algae under sea ice) supports many predators; declines in krill or sea ice can cascade to many species.
Energy pyramids: visualize energy transfer across trophic levels with three types of pyramids:
Pyramids of Energy: show energy available at each trophic level; the amount of energy decreases with each step; typical transfer efficiency is about 10% from one level to the next.
Pyramids of Biomass: show the total mass of living matter at each level; environmental context can invert biomass pyramids in some ecosystems.
Pyramids of Numbers: show the number of individual organisms at each level; often, producers outnumber consumers, but not always (inverted pyramids possible).
The 10% Rule (energy pyramid): only about 10% of the energy at one trophic level is stored in the tissues of organisms at the next level.
Math representation: if producer level has energy E0, then the energy at the n-th consumer level is:
Example: 1000 units at the producer level → first-level consumers ~100 units → second-level ~10 units → third-level ~1 unit; thus, fewer top-level consumers can be supported as you go up.
Why are there fewer trophic levels? because energy transfer is inefficient and energy is dissipated as heat; this limits the length of food chains and the biomass/number of organisms at top levels.
3.4 Cycles of Matter
Matter moves through the biosphere via biogeochemical cycles (biological, geological, chemical, and human processes), whereas energy flows in a one-way stream.
Cycles of matter involve the following ideas:
Elements move through organisms and the environment; atoms are recycled, not created or destroyed, just transformed.
Each cycle is powered and connected to energy flow.
Major cycles discussed: carbon cycle, nitrogen cycle, phosphorus cycle; oxygen participates in these cycles by forming and cycling with carbon, nitrogen, and phosphorus.
The matter mill concept (biogeochemical cycles) is illustrated by Figure 3–13: nutrients are recycled through cycles driven by energy flow.
The Water Cycle (hydrologic cycle):
Processes include evaporation, transpiration, condensation, precipitation, runoff, and groundwater flow.
Water moves between oceans, atmosphere, and land; groundwater can feed plants via roots and re-enter the cycle via transpiration or evaporation.
A water molecule may take thousands of years to complete one cycle (roughly up to ~4000 years in some paths).
Key terms:
Evaporation: water from oceans/other bodies becomes water vapor.
Transpiration: evaporation of water from plant leaves.
Condensation: water vapor condenses into clouds.
Precipitation: rain, snow, sleet, hail returns water to the surface.
Runoff: surface water moves toward rivers/streams/oceans.
Groundwater: water that infiltrates soil and percolates downward; can feed plants or surface water.
The Carbon Cycle: carbon is a central element in organic molecules; reservoirs include the atmosphere (as CO2), oceans (as dissolved CO2), rocks/fossil fuels, forests, and living organisms.
Carbon dioxide is exchanged between atmosphere and oceans via chemical/physical processes.
Plants fix CO2 during photosynthesis to build carbohydrates; carbon moves through food webs to consumers; respiration releases CO2; decomposition returns carbon to the environment.
Geological processes can convert carbon into carbon-containing rocks and fossil fuels (coal, oil, natural gas).
Human activities (burning fossil fuels, deforestation) release CO2, affecting atmospheric levels.
Figure references illustrate the carbon reservoirs and the carbon cycle pathways.
The Nitrogen Cycle: nitrogen is essential for amino acids, nucleic acids (DNA, RNA, proteins).
Atmospheric nitrogen (N2) makes up about 78% of the atmosphere; most organisms cannot use N2 directly.
Nitrogen fixation: certain bacteria (in soil and on legume roots) convert N2 to ammonia (NH3). Lightning also fixes nitrogen in small amounts.
Nitrification: bacteria convert fixed nitrogen to nitrites (NO2−) and nitrates (NO3−) that plants can uptake.
Assimilation: plants take up nitrates/ammonia and build proteins; consumers reuse nitrogen by feeding on plants.
Ammonification/Decomposition: decomposers convert nitrogenous wastes into ammonia.
Denitrification: bacteria convert nitrates back to N2 gas released to the atmosphere.
Human impact: fertilizers add fixed nitrogen to the biosphere; excess can runoff into water bodies.
The Phosphorus Cycle: phosphorus is essential for DNA, RNA, ATP and other molecules; it does not cycle through the atmosphere in a major way.
Phosphorus is mined from phosphate rock, turned into fertilizer, and applied to crops.
Phosphates move with weathering and erosion into soils and waters; incorporate into organisms and cycle through the food web via excretion and decomposition.
Phosphorus in marine sediments can be recycled; geological activity can release phosphates back to rock.
Nutrient Limitation and Primary Productivity:
Primary productivity is the rate at which primary producers create organic matter.
When nutrients are limiting (even with ample sunlight and water), productivity is constrained by the scarce nutrient.
Common limiting nutrients:
In open oceans: nitrogen is often limiting.
In freshwater ecosystems: phosphorus is often limiting.
Runoff from fertilized fields can cause algal blooms by adding excess nutrients; algal blooms can disrupt ecosystem function and oxygen dynamics.
Interconnectedness of nutrients (Interlocking nutrients): movement of each nutrient depends on the others because all are needed for living systems; nutrient cycles are interdependent, like gears in a machine.
Global Ecology from Space: satellites monitor global patterns of plant growth, ocean temperatures, and ice cover; data (e.g., SeaWiFS) help assess productivity and ecosystem responses to climate change; observations show variations in polar ice and land/algae activity.
Guided inquiry and lab components:
Pre-lab: The Effect of Fertilizer on Algae — aims to study how excess nutrients affect algae growth.
Independent variable: amount/type of fertilizer or nutrient input;
Predictions and controls: predict algae growth over days; keep conditions constant except for the independent variable.
Real-world connections: nutrient cycles and nutrient limitation relate to agricultural practices, fertilizer use, and water quality.
Chapter review prompts and assessment topics (highlights):
Distinctions among energy flow vs. matter cycles; the four categories of processes that move matter (Biological, Geological, Chemical/Physical, and Human activities).
The water cycle, carbon cycle, nitrogen cycle, and phosphorus cycle in detail, including reservoirs, pathways, and human impacts.
How nutrient limitation shapes primary productivity in different ecosystems.
How to interpret biomass pyramids versus pyramids of numbers, and why energy pyramids emphasize energy flow and limits on trophic levels.
Interpretive questions about food webs, disturbances, and the interdependence of biotic and abiotic factors.
Key Equations and Concepts in LaTeX
Photosynthesis (simplified, land/sea producers):
Chemosynthesis (conceptual form):
Energy transfer between trophic levels (the 10% rule):
General ecology relationships and cycles are described conceptually, with emphasis on how energy flow and matter recycling shape ecosystems and their resilience to disturbances.
Connections to Foundational Principles
Energy and matter cycles demonstrate conservation and transformation: energy flows through ecosystems as work and heat, while matter is recycled through biogeochemical cycles.
Interdependence principle: organisms and environments influence each other; changes in abiotic conditions alter biotic communities and vice versa.
Systems thinking: ecosystems are dynamic, interconnected networks where perturbations can propagate through food webs and nutrient cycles, affecting productivity and stability.
Human-environment interactions: human activity alters cycles (e.g., nitrogen and phosphorus from fertilizers), climate, and the structure of food webs, with potential cascading ecological consequences.
Real-World Relevance and Ethical/Practical Implications
Nutrient management and pollution: understanding limiting nutrients helps prevent harmful algal blooms and protects water quality.
Climate change and ocean health: changes in temperature, ice cover, and productivity affect food webs from krill to top predators, with implications for global biogeochemical cycling.
Sustainable agriculture: balancing fertilizer input with ecosystem health to maintain productivity without triggering eutrophication.
Conservation planning: recognizing keystone species and critical links in food webs (e.g., krill–predator dynamics, decomposer pathways) to maintain ecosystem services.