Bio Lessons 22 and 23
Lesson 22
Ecosystem Energetics and Productivity
Productivity:
The rate at which all organisms at all trophic levels synthesize new organic matter.
Primary Productivity:
The productivity of primary producers (plants, algae, etc.).
Gross Primary Productivity (GPP):
Total amount of photosynthesis by all primary producers in an ecosystem over a set time (usually a year).
Respiration:
Energy used by plants for their own metabolic needs.
Net Primary Productivity (NPP):
GPP minus the energy used for respiration.
Energy available for growth and reproduction of autotrophs.
NPP is the energy available for the second trophic level (herbivores).
Secondary Productivity:
The productivity of heterotrophs (herbivores, carnivores, etc.).
Each heterotrophic level has its own secondary productivity.
Energy transfer inefficiency:
Not all NPP is consumed or converted efficiently by herbivores.
Example with grasshoppers:
17% of ingested energy is invested in growth.
50% is not assimilated and is excreted as feces.
33% is used in cellular respiration (converted to heat).
Energy limitations for higher trophic levels:
Carnivores only access the 17% growth energy from herbivores.
Some prey (like grasshoppers) are not eaten, further reducing energy available.
Carnivores also lose energy through waste and heat.
Ecosystem-wide energetics:
Only about 1% of solar energy is captured by primary producers over a year.
In each trophic level:
Some energy becomes heat.
Some energy becomes feces or dead organic matter.
Only a fraction of the original solar energy passes through each trophic level.
10% Rule:
Only about 10% of the chemical bond energy available at one trophic level becomes available to the next level.
Energy eventual fate:
All chemical bond energy captured by photosynthesis eventually becomes heat.
Even energy in detritus (dead material) becomes heat through decomposer activity.
Primary Productivity Importance:
Sets the total available energy in an ecosystem.
Limits the number of trophic levels possible.
Factors affecting NPP:
Temperature
Precipitation
Nutrient Availability
Ecological Pyramids
Ecological Pyramid:
Stacked boxes representing trophic levels.
Width of each box = magnitude of an ecological property.
Pyramid of Energy Flow (A):
Shows decline of energy at each trophic level.
Can never be inverted.
Pyramids of Biomass (B and C):
Show the biomass (mass of living material) at each level.
Usually upright, but can be inverted (e.g., in aquatic systems with small phytoplankton).
Pyramid of Numbers (D):
Shows the number of individuals at each trophic level.
Generally upright because of energy constraints.
Trophic Interactions and Cascades
Trophic Cascade:
A situation where upper trophic levels influence lower levels.
Top-Down Effects: Upper levels control or affect the populations below.
Examples of Top-Down Effects:
Stream Experiment:
Trout (predators) added to enclosures.
Result:
Fewer invertebrates (due to trout eating them).
More algal biomass (because fewer invertebrates are grazing the algae).
Damselfly and Fish Experiment:
Without fish:
High damselfly nymph population (carnivores).
Low herbivore insect population.
High algae.
With fish:
Fewer damselfly nymphs.
More herbivores.
Lower algal biomass.
Large-scale Trophic Cascade:
Sea otters → sea urchins → kelp forests along the west coast of North America.
Bottom-Up Effects:
Primary productivity controls the ecosystem from the bottom.
Model description:
Low productivity = low herbivore and carnivore populations.
Increased productivity → more herbivores once a threshold is passed.
Further increased productivity → carnivores establish and control herbivores.
Producer biomass can increase again if herbivores are kept in check.
Experimental Support:
Bottom-up effects models have been supported by various studies.
Island Biogeography
Species-Area Relationship:
Larger islands have more species than smaller islands.
MacArthur and Wilson’s Equilibrium Model:
Species richness on islands is a balance between:
Colonization (new species arriving)
Extinction (species disappearing)
Model Dynamics:
Colonization Rates:
High initially, then decrease as fewer new species are available to colonize.
Extinction Rates:
Low initially, then increase as more species crowd the island.
Effects of Island Size and Distance:
Small islands:
Higher extinction rates (due to small populations).
Islands close to mainland:
Higher colonization rates (easier dispersal).
Predictions of the Model:
Large, near islands = highest species richness.
Small, far islands = lowest species richness.
Evidence:
Studies on bird species in the Asian Pacific:
Smaller islands = fewer bird species.
Islands closer to mainland = more bird species.
General Support:
Multiple studies support the dynamic equilibrium model of island biogeography.
Ecosystem Ecology Overview
Ecosystem ecology studies how all species in a community interact with the physical environment to obtain resources.
Ecosystem = living organisms (producers and consumers) + nonliving (abiotic) factors.
Nutrients cycle between biotic (living) and abiotic (nonliving) components.
Energy, mainly from solar energy, flows one-way through ecosystems (does not recycle).
Ecosystem boundaries are not fixed — inputs and outputs (gases, water, nutrients) can occur from outside.
Nutrient Cycles (Biogeochemical Cycles)
Biogeochemical cycles = movement of nutrients between living and nonliving components of ecosystems.
Water Cycle
Importance: Water makes up ~60% of human body weight; crucial for life.
Water can be:
Synthesized during cellular respiration.
Broken down during photosynthesis.
Rates of water production and usage by organisms are balanced.
Processes:
Evaporation: Water moves from Earth's surface (oceans, lakes, rivers, land) into atmosphere.
Transpiration: Water vapor exits plant surfaces (90% of atmospheric water from vegetated landscapes).
Condensation: Cooling causes water vapor to return to liquid.
Precipitation: Liquid or frozen water falls back to Earth.
Groundwater: Main freshwater reservoir (95% of freshwater).
Aquifer: Permeable underground rock, sand, or gravel layer storing groundwater.
Water table: Upper level of groundwater; accessible by streams and plant roots. Deeper layers require wells.
Carbon Cycle
Importance: Carbon is the framework of all organic molecules.
Source: Atmospheric CO₂ (0.03% of atmosphere) or dissolved as bicarbonate in water.
Processes:
Carbon fixation: Plants, protists, and some prokaryotes absorb CO₂ and use it to build organic molecules during photosynthesis (light-independent reactions).
Consumption: Animals eat plants to get organic carbon.
Respiration: Breaks down organic molecules to release energy; releases CO₂.
Decomposition: Breakdown of dead organisms releases CO₂.
Methane production: Anaerobic prokaryotes produce methane that eventually oxidizes to CO₂.
Fossil fuels: Long-term storage of carbon; burning fossil fuels releases stored carbon rapidly and disrupts the cycle.
Nitrogen Cycle
Importance: Nitrogen is needed for proteins and nucleic acids.
Limiting nutrient: Nitrogen often limits growth in ecosystems despite making up 78% of the atmosphere.
Problems: Plants and animals cannot use atmospheric nitrogen (N₂) directly.
Processes:
Nitrogen fixation: Certain prokaryotes (both free-living and symbiotic with plants like rhizobia) convert N₂ gas into ammonia (NH₃).
Also occurs abiotically through lightning, meteorites, and cosmic radiation.
Conversion: In soil, ammonia quickly becomes ammonium (NH₄⁺) due to hydrogen ions.
Nitrification: Microorganisms convert ammonium into nitrate (NO₃⁻) in two steps.
Decomposition: Releases ammonia and nitrate from dead matter and waste.
Denitrification: Another group of bacteria in anaerobic conditions converts nitrate back into atmospheric N₂.
Human Impact: Widespread fertilizer use has heavily altered the nitrogen cycle.
Phosphorus Cycle
Importance: Needed for phospholipids, ATP, nucleic acids, and other organics.
Unique Feature: No gaseous phase.
Processes:
Weathering: Releases phosphate (PO₄³⁻) from rocks into soil.
Uptake: Plant roots absorb phosphate; animals get it by consuming plants/animals.
Decomposition: Releases phosphate from waste and dead matter.
Runoff: Phosphate can be washed into streams/rivers and settle into sediments if not absorbed quickly.
Human Impact: Fertilizer runoff disrupts phosphorus cycle.
Limiting Nutrients
Limiting nutrient: The nutrient in shortest supply that limits ecosystem productivity.
Common limiting nutrients:
Nitrogen: Limits many terrestrial and aquatic ecosystems.
Phosphorus: Also a common limiting factor.
Iron: Limits phytoplankton (key ocean photosynthesizers); primarily supplied by wind-blown dust from places like the Sahara Desert.
Ecosystem Energetics
Key Point: Energy flows one-way, unlike nutrients which cycle.
First Law of Thermodynamics: Energy cannot be created/destroyed, only transformed.
Second Law of Thermodynamics: With each energy transfer, some energy becomes unusable heat.
Heat energy cannot power cellular activities, only maintain body temperature in endothermic (warm-blooded) animals.
Earth's energy supply is continuous thanks to the sun (open energy system).
Trophic Levels (Energy Organization)
Trophic level: Groups of organisms that get energy the same way (similar to food chains but broader).
Levels:
First trophic level: Autotrophs or primary producers.
Usually photoautotrophs (photosynthesis), sometimes chemoautotrophs (chemical energy).
Second trophic level: Herbivores (primary consumers eating autotrophs).
Third trophic level: Primary carnivores (eat herbivores).
Fourth trophic level: Secondary carnivores (eat primary carnivores).
Boundaries between levels are flexible — many organisms eat across levels (especially carnivores).
Detritivores and Decomposers
Detritivores: Organisms that physically ingest dead organic material (example: vultures).
Decomposers: Bacteria and fungi that secrete enzymes to break down dead organic material and then absorb nutrients.
Detritus: Dead organic material feeding detritivores and decomposers — comes from all trophic levels.
Could form a separate food system based entirely on dead organic matter!
Lesson 23
The Biosphere and Global Ecology
Biosphere: The global level of ecology that includes all living communities on Earth.
Objective: Understand climate, terrestrial biomes, and aquatic habitats.
Solar Energy and Earth’s Atmosphere
Electromagnetic radiation from the sun is modified before reaching Earth's surface.
Atmosphere: Reflects and absorbs about half of incoming solar energy.
Ozone layer: Absorbs most UV radiation.
Energy that reaches Earth:
Warms the surface.
Is re-radiated as infrared radiation (heat).
Greenhouse effect: Traps this re-radiated heat in the lower atmosphere, warming the Earth.
Factors Affecting Solar Energy Distribution
Curvature of the Earth:
Causes angle of incidence to vary.
Solar rays are more concentrated at the equator and spread out at higher latitudes.
Latitude: Impacts how much solar energy a region receives.
Tilt of the Earth’s axis:
Causes seasons as the Earth orbits the sun.
Summer solstice (Northern Hemisphere tilted toward the sun): more solar energy, longer days.
Winter solstice (Northern Hemisphere tilted away): less solar energy, shorter days.
Annual mean temperature:
Highest and most stable at the equator.
Lower and more variable at higher latitudes.
Global Air Circulation Patterns
Driven by uneven heating of the Earth.
3 Key Concepts:
Hot air rises.
Air cools as it rises.
Warm air holds more moisture.
🌧 Equator (0° latitude):
Air rises due to heat and moisture → High precipitation.
🏜 30° N & S Latitude:
Dry air descends → Low precipitation → Desert formation.
🌦 60° N & S Latitude:
Air rises again → Moderate precipitation.
❄ Poles (90° N & S):
Cold, dry air sinks back → Dry and cold conditions.
Air Circulation Cells
Each hemisphere has 3 air circulation cells.
Responsible for wind and precipitation patterns.
Surface winds caused by:
Air circulation patterns + Earth’s rotation = Coriolis effect.
Winds curve, not in straight lines.
Named for direction they come from.
Ocean Circulation Patterns
Driven by surface winds → indirectly by the sun.
Gyres: Large, circular surface currents.
Move warm and cold water around oceans.
Affect climate and ocean productivity.
Gulf Stream:
Warm current.
Moderates Eastern U.S. and British Isles climates.
Upwelling:
Process where deep, cold, nutrient-rich water rises to the surface.
Boosts marine productivity.
Regional and Local Climate Effects
🏔 Rain Shadow Effect
Occurs when moist air rises over a mountain.
Windward side:
Air cools.
Water condenses.
High precipitation.
Lush vegetation (e.g., giant sequoias in California).
Leeward side:
Dry air descends.
Air warms → causes evaporation.
Arid climate → Desert forms (e.g., Great Basin Desert).
🌡 Elevation Effects
Temperature drops ~6°C per 1000 meters of elevation gain.
Higher elevation mimics moving to higher latitude.
On a tropical mountain:
Base: Rainforest.
Peak: Tundra or polar ice.
Biomes: Definition and Characteristics
Biomes: Major types of terrestrial ecosystems.
Named by the structure of predominant vegetation, but also include characteristic animals.
Distributed over land regions defined by climate conditions (especially temperature and precipitation).
Similar climate regions support the same biome worldwide (e.g., taiga in North America and Eurasia).
Although the same biome type may exist in different places, the species composition will differ.
A biome = a vegetation type with structure and adaptations for a specific climate.
Climate and Biome Distribution
Temperature and precipitation are the best predictors of biome type.
Mean annual temperature and mean annual precipitation help determine biome placement.
Tropical rain forest = high temperature + high precipitation.
Primary productivity: The rate at which plants produce organic matter.
Strongly influenced by temperature and precipitation.
Higher in warm, wet biomes (e.g., tropical rain forest).
Other influencing factors:
Soil type.
Disturbance history (e.g., fire, logging, storms).
Tropical Rain Forest
Location: Equatorial regions (South America, Africa, Asia).
Climate: Warm year-round + high precipitation.
Highest Net Primary Productivity (NPP) of any terrestrial biome.
Vegetation:
Tall trees (up to 80 meters), with buttresses for support.
Multiple vertical layers of vegetation.
Vines and epiphytes (plants that grow on other plants).
Broad-leaved evergreen growth.
Soils:
Nutrients recycled quickly.
Without plant cover, nutrients leach quickly, leaving poor soils.
Animal Diversity:
Extremely high, especially insects, birds, and primates.
Tropical Dry Forests: Replace tropical rainforests further from the equator.
Savanna
Transitional ecosystem between tropical dry forest and desert.
Found 10–20° latitude, north and south of equator.
Terrain: Flat, often with rivers.
Climate:
Seasonal rainfall.
Frequent fires.
Vegetation:
Dominated by grasses.
Scattered fire-resistant trees.
Animals:
Large grazing mammals.
Migrate to find water.
Desert
Defined by water loss (evaporation and transpiration) exceeding precipitation.
Climate:
Low, sporadic rainfall.
Temperatures can be extremely hot or very cold.
Vegetation:
Sparse.
Soils lack organic layer.
Plants adapted to drought tolerance (e.g., cactus).
Adaptations:
Plants enter dormancy during dry seasons (often as seeds).
Animals may become inactive or dormant.
Temperate Grassland
More precipitation than deserts, but not enough for woody plants.
Climate:
Frequent drought.
Hot summers, cold winters.
Location: Often in continental interiors.
North American Example: Prairie—once the largest biome in North America.
Most now used for agriculture (fertile soils).
Vegetation:
Grasses adapted to fire and grazing.
Animals:
Historically supported many grazing mammals.
Temperate Deciduous Forest
Climate:
Warm summers, cold winters, plentiful rain.
Vegetation:
Deciduous trees (drop leaves in winter).
Allows for winter dormancy.
Biodiversity:
High diversity of plants and animals.
Layered forest structure.
Leaf litter supports birds, mammals, invertebrates, and fungi.
Temperate Evergreen Forest
Found along coastlines with temperate climates.
Common Trees: Spruces, pines, redwoods.
Pacific Coast (North America):
Most rain falls in fall, winter, spring.
Summer drought favors needle-leaved evergreens.
Taiga / Boreal Forest
Location: Broad band across North America and Eurasia (50–65° latitude).
One of the largest biomes on Earth.
Climate:
Harsh, long winters.
Short growing season.
Vegetation:
Dominated by evergreen conifers.
Low plant diversity.
Thick canopy → little light below → sparse undergrowth.
Landscape:
Shaped by glacial activity → many lakes and bogs.
Permafrost common in northern parts.
Animals:
Moose, caribou, deer, wolves, bears, wolverines, lynx.
Many birds migrate from the tropics.
Montane Coniferous Forests:
Found in mountain ranges (e.g., western U.S.).
Similar to taiga, just at higher elevation.
Arctic Tundra
Location: North of taiga; treeless.
Precipitation: Low.
Permafrost: Year-round frozen layer that:
Impairs drainage.
Creates soggy soils.
Inhibits biological activity.
Vegetation:
Perennial herbaceous plants (grasses, sedges, mosses, lichens).
Some dwarf woody plants.
Animals:
Lemmings, caribou, Arctic hare, muskox, abundant insects.
Alpine Tundra
Found at high elevations on mountains.
Similar climate and vegetation to Arctic tundra.
Caused by elevation, not latitude.
Freshwater Habitats
Freshwater habitats cover only a small percentage of Earth's surface.
Large still bodies of freshwater = Lakes
Smaller still bodies of freshwater = Ponds
Oxygen in Lakes and Ponds
Most aquatic organisms require oxygen from the water.
Oxygen enters water via:
Photosynthesis (by aquatic plants and phytoplankton)
Diffusion from the atmosphere
These processes mostly occur near the surface, so oxygen is most plentiful near the surface.
Light and Photosynthesis
Light is required for photosynthesis.
The photic zone = the depth of water where light is sufficient for photosynthesis.
Varies with particulate matter (cloudier water = shallower photic zone).
Phytoplankton = the primary producers in open freshwater (photosynthetic organisms).
Temperature and Stratification
Water is heated by sunlight striking the surface.
Temperature varies with depth, depending on lake size and location.
Thermal stratification = layering of water by temperature.
Occurs seasonally in many lakes.
Wetlands
Freshwater wetlands form where freshwater meets land.
Wetlands have:
High Net Primary Productivity (NPP)
Serve as buffers between land and water (protect shorelines, filter water).
Lake Productivity and Eutrophication
Oligotrophic Lakes
Low nutrients
Low algal density
Clear water → deeper light penetration
Generally high oxygen levels
Eutrophic Lakes
High nutrients
High algal growth → can lead to algal blooms
Algal blooms:
Block sunlight
Reduce water clarity
Lead to dead organic matter, which fuels decomposition
Decomposition uses oxygen, especially in deeper water
Low oxygen levels can alter animal species composition
Eutrophication
Process where a lake becomes eutrophic
Often caused by human activity (e.g., fertilizer runoff)
Can lead to algal blooms and oxygen depletion
Reversible if nutrient inputs stop
Other affected bodies of water: Chesapeake Bay
Marine Habitats
Marine Variation Factors
Marine habitats vary by:
Light
Temperature
Nutrients
Four Ocean Ecosystem Types
Open Ocean
Deep water, away from the continental shelf
Primary producers: phytoplankton
Low NPP due to nutrient limitations
Continental Shelf Ecosystems
Shallow marine areas near continents
Higher productivity due to more nutrients
Include:
Estuaries (mix of salt and freshwater)
Banks
Coral reefs
Upwelling Regions
Areas where nutrient-rich deep water rises to the surface
Leads to high phytoplankton productivity
Support abundant animal life
Major upwelling zones:
Coast of California
Coast of Peru and Ecuador
Deep Sea
Largest habitat on Earth
Conditions:
Cold
Dark
High pressure
Seasonless
Food-poor, but:
Diverse organisms
Some are bioluminescent
El Niño and Ocean Circulation
Ocean Circulation Influences Climate
El Niño Southern Oscillation (ENSO) = periodic warming event
Caused by:
Weakened trade winds, which normally push surface water westward
Effect:
Cold, nutrient-rich water is replaced by warm, nutrient-poor water
Causes:
Drop in phytoplankton
Fish populations decline
Sea bird and sea lion populations crash (especially around Galápagos Islands)
Global El Niño Effects
Pacific Ocean becomes warmer
Widespread weather changes:
Southeast U.S.: cooler, wetter weather
Asia: often drier
Chile: more precipitation → higher plant productivity → increased animal population growth (trophic cascade)