Chapter 3 Ecosystems What Are They and How Do They Work?
Core Cases Study
Have You Thanked the Insects Today?
 Insects have a bad reputation as pests
o Compete for food
o Disease vectors
o Invade houses, lawns, gardens and homes
 Natural services (Part of earth’s natural capital, Fig. 3.1)
o Pollination
o Pest control
 Insect attributes
o Been around for 400 my
o Many have high reproductive rate
o Rapidly evolve when faced with new environmental conditions
o Resistant to extinction
 Environmental lesson
o Although insects can thrive without newcomers such as us, we and most other land organisms
would perish w/o them.
I.
The Nature of Ecology
A. What is ecology?
1. Ecology – study of how organisms interact with one another and with their non-living
environment.
2. Levels of organization of matter (Fig. 3.2)
B. Organisms and Species
1. Organism – any form of life and the fundamental unit of ecology.
2. Species – groups of organisms that resemble each other in appearance, behavior, chemistry, and
genetic makeup
a. Classification is based common ancestry – sciences called Systematics and Taxonomy
b. How many species are their?
i. 1.4 million identified
ii. Estimated to be 4 to 100 million
C. Case Study: Which Species Run the World?
1. Microbes – bacteria, protists, fungi
2. Bad reputation justified?
a. Handful are pathogenic
b. Most play key roles in ecosystem functioning
i. Nitrogen cycle
ii. Decomposers
iii. Produce oxygen
c. Benefit humans directly
i. Help produce food – bread, yogurt, cheese, vinegar, tofu, soy sauce
ii. Intestinal bacteria
iii. Antibiotics
iv. Control plant diseases and populations of insects that attack crops.
D. Populations, Communities, and Ecosystems
1. Population – group of interacting/interbredding individuals of the same species occupying a
specific area.
a. Genetic diversity – variation in genetic make-up (Fig. 3.5)
b. Habitat – place where organism or population lives and can have access to food, shelter,
water, and mates.
c. Distribution, or range – area over which we can find a species
2. Community (biological) – group of interacting populations of species in an area
3. Ecosystem – community and its interaction with non-living matter and energy
a. Range from a small puddle to streams, woods, to the gut of an animal.
b. May be natural or artificial (farm, reservoir, crop field, urban area)
4. Biosphere – all of earth’s interconnected ecosystems.
II.
The Earth’s Life-Support Systems
A. The Earth’s Life-Support Systems: Four Spheres
1. Biosphere consists of several spherical layers (Fig. 3.6).
a. Atmosphere
i. Troposphere – to 17 km (11 mi) above sea level; mostly N2 (78 %) and O2 (21 %)
ii. Stratosphere – 17-48 km (11-30 mi) above sea level; contains O3
b. Hydrosphere
i. Liquid water, surface and underground
ii. Ice, as ice caps, icebergs, and permafrost
iii. Water vapor
c. Lithosphere
i. Crust and upper mantle
ii. Contains nonrenewable fossil fuels and minerals
iii. Renewable soil, important for plant life
B. What Sustains Life on Earth?
1. Flow of high-quality energy
a. Sun  ecosystem w/ much lost to heat dispersed  radiated back into space
i. No round trips because high-quality energy can’t be recycled
2. Cycling of matter or nutrients
a. Atom, ions, or compounds needed by organisms
3. Gravity
a. Holds the atmosphere
C. What Happens to Solar Energy Reaching the Earth?
1. About 1-billionth of sun’s output reaches the Earth.
a. Either reflected or absorbed by chemicals, dust, and clouds.
b. Amount reaching the earth equals the amount reflected or radiated back to space (Fig. 3.8).
2. Some rough numbers
a. 80% that gets into the atmosphere warms the troposphere and evaporates and cycles water
through the biosphere.
b. 1% generates the wind.
c. 0.1% used by green plants, algae, and bacteria for photosynthesis.
3. Most radiation making it through the atmosphere is degraded into loner-wavelength infrared
radiation (IR).
a. Encounters greenhouse gases – CO2, H2O, CH4, N2O, and O3
i. Molecules vibrate and release even loner IR increase the KE of molecules in the
atmosphere.
ii. Warms atmosphere causing a natural greenhouse effect.
III. Ecosystem Components
A. Biomes and Aquatic Life Zones: Where Organism Live
1. Biomes – terrestrial portions of biosphere with distinct climates and species.
a. Forests, deserts, and grasslands
2. Aquatic life zones
a. Freshwater – lakes, steams
B.
C.
D.
E.
F.
b. Marine – coral reefs, coastal estuaries, deep ocean
Nonliving and Living Components of Ecosystems
1. Two types of components (Fig. 3.10):
a. Abiotic
i. Water
ii. Air
iii. Nutrients
iv. Sunlight
b. Biotic
i. Producers
ii. Consumers
iii. Decomposers
2. Different species and their populations thrive under different physical and chemical conditions.
a. Each pop. has a range of tolerance to variations in physical and chemical conditions (Fig.
3.11).
i. Individuals in a pop. have slight diff in genetic makeup allowing differences in tolerance
ranges.
(a.) Optimum range
(b.) Physiological stress zone
(c.) Intolerance zone
b. A species may have a wide rang of tolerance to one factor but narrow for another.
i. Highly tolerant species can live in a wide variety of habitats w/ widely diff conditions.
c. Physical conditions can limit the distribution of a particular species (Fig. 3.12).
Factors That Limit Population Growth
1. Limiting factor – factor important in regulating population growth.
a. Limiting factor principle – too much or too little of any biotic factor can limit of prevent
growth of a population, even if all other factors are at or near the optimum range of
tolerance.
i. What types of conditions limit organisms in aquatic ecosystems? Terrestrial?
Producers: Basic Source of All Food
1. Producers, or autotrophs
a. Green plants, algae, types of bacteria
2. Photosynthesis
a. Light used to fix carbon into high energy chemical bonds
b. Plants, phytoplankton, cyanobacteria
c. Carbon dioxide + water + solar energy  glucose + oxygen
3. Chemosynthesis
a. Specialized bacteria convert simple inorganic compounds from their environment into more
complex nutrients compounds without sunlight.
b. Chemosynthetic bacteria near hydrothermal ocean vents, obtain energy from H2S gas, which is
used to produce carbs.
Consumers: Eating and Recycling to Survive
1. Consumers, or heterotrophs
a. Primary consumers, herbivores
b. Secondary consumers, carnivores
c. Tertiary consumers or higher
2. Omnivores
3. Decomposers (Fig. 3.13)
4. Detritivores
5. In natural ecosystems there is little or no waste.
Aerobic and Anaerobic Respiration: Getting Energy for Survival
1. Aerobic respiration – energy is released from organic compounds such as glucose using oxygen.
a. Process turns organic molecules back to CO2 and water.
2. Anaerobic respiration, or fermentation – breaking down organic molecules in absence of oxygen
to obtain energy.
a. Instead of CO2 and water, products may include methane, ethanol, acetic acid, lactic acid, and
hydrogen sulfide.
G. Two Secrets of Survival: Energy Flow and Matter Recycling
1. Individual organisms have a one-way energy and matter flow.
2. But, ecosystems are sustained by matter recycling and one-way energy flow (Fig. 3.14).
IV. Biodiversity
A. The Diversity of Life: A Crucial Resource
1. Biological diversity, or biodiversity – one of earth’s most important renewable resources.
2. Four components (Fig. 3.15):
a. Genetic diversity
b. Species diversity
c. Ecological diversity
d. Functional diversity
B. Biodiversity Loss and Species Extinction: Remember HIPPO
1. Human impact is reducing biodiversity through habitat loss and premature extinctions at rates 100
to 10 000x higher than the world’s natural rate.
2. Five major causes, HIPPO
a. Habitat destruction and degradation
b. Invasive species
c. Pollution
d. Population growth
e. Overexploitation
C. Why Should We Care about Biodiversity?
1. Wealth of natural capital
a. Natural resources – food, wood, energy, medicines, etc.
b. Natural services – air and water purification, recycling soil, waste disposal, and pest control,
etc.
2. Goals, Strategies, and Tactics for protecting biodiversity (Fig. 3.16)
V.
Energy Flow in Ecosystems
A. Food Chains and Food Webs: Who Eats and Decomposes Whom?
1. There is little matter wasted in ecosystems.
2. Food Chain (Fig. 3.17)
a. Shows conceptually how energy and matter flow from one organism to another.
b. Trophic level – feeding level
i. First trophic level – producers
ii. Second trophic level – primary consumers, or herbivores
iii. Third trophic level – secondary consumers, or carnivores
iv. etc
3. Detritivores and decomposers “feed” from all levels
4. Food Web (Fig. 3.18)
a. Conceptually represents the truer network of interconnected food chains.
b. A “map of life’s interdependence.”
B. Energy Flow in and Ecosystem: Losing Energy in Food Chains and Webs
1. Each trophic level in a food web contains a certain amount of biomass – the dry mass of all
organic matter contained in a trophic level.
a. Chemical energy in biomass, transferred from one trophic level to next.
i. Process not efficient, 2 – 40%, but typically 10%
ii. Ecological efficiency – percentage of usable energy transferred as biomass from one
trophic level to the next.
2. Pyramid of energy flow (Fig. 3.19)
a. Explains how earth can support more people if they eat at lower energy levels.
b. Limits the number of trophic levels that can occur in an ecosystem
C. Productivity of Producers: The Rate Is Crucial
1. Gross primary production (GPP) – rate at which and ecosystem’s producers convert solar energy
into biomass.
2. Net primary productivity (NPP) – rate at which producers use photosynthesis to store energy
minus the rate at which they use some of this stored energy for respiration.
NPP  GPP  R
a. Various ecosystems and life zones differ in their NPP (Fig. 3.22).
b. Only biomass (NPP) is available as food
c. The planet’s entire NPP limits the number of consumers (including humans) that can survive
on earth.
3. Humans use, waste, or destroy:
a. 27% of earth’s total potential NPP
b. 10-55% of the plant’s terrestrial NPP
c. main reason habitats and species are lost
d. Interestingly (or should I say, Disturbingly, Alarmingly, Unsustainably, etc.), humans, their
livestock and pets now make up 98% of total vertebrate biomass.
i. An ecological footprint that’s crowding out the other 2% of birds, mammals, reptiles, and
amphibians.
VI. Soil: A Renewable Resource
A. What Is Soil and Why Is It Important
1. Soil – thin covering over most of land, components: eroded rock, mineral nutrients, decaying
organic matter, water, air, and billions of living organisms (most microscopic decomposers).
2. Weathering – physical, chemical, and biological process that can lead to soil formation.
a. Can take long periods of time.
i. One centimeter can take 15 -100 y depending on climate.
3. Function
a. Nutrients for producers
b. Cleans water
c. Decomposes and recycles waste
d. Water recycling and storage
4. Human activities have accelerated soil erosion.
B. Layers in Mature Soils (Fig. 3.23)
1. Mature soils are arranged in to layers called horizons.
2. Soil profile – cross-sectional view of horizons.
3. The soil horizons:
a. O horizon – surface litter layer; contains mostly undecomposed or partially decomposed
leaves, twigs, crop wastes, animal waste fungi and other organic material.
b. A horizon – top soil layer; a porous mixture of partially decomposed bodies of dead plants and
animals, called humus, and inorganic materials such as clay, silt and sand.
i. Helps hold water and nutrients taken up by plants.
ii. As long vegetation anchors these layers, a soil stores water for use by plants.
iii. The top two layers have bacteria, fungi, earthworms, and small arthropods that interact in
complex food webs.
iv. Color of topsoil suggests how useful it is for growing crops.
v. Dark brown or black, rich in nitrogen and organic matter.
vi. Gray, bright yellow, and red, are low in organic matter and nitrogen.
c. B horizon – this and the layer below contain mostly inorganic matter, broken-down rock as
varying mixtures of sand, silt, clay, and gravel.
d. C horizon – lies on the base of unweathered parent material, often bedrock.
4. Spaces in the soil allow plant roots to “breath.”
a. Downward movement of water into the soil pores, called infiltration.
i. Leaching
C. Some Soil Properties
1. Soils are mixtures of different-sized particles (Fig. 3.25).
a. Sand – 0.05-2 mm
b. Silt – 0.002-0.05 mm
c. Clay – <0.002 mm
2. Soil texture
VII. Matter Cycling In Ecosystems
A. Nutrient Cycles: Global Recycling
1. Nutrients – elements and compounds that organisms need to live, grow, and reproduce.
a. They move through the air, water, soil, rock, and organisms – called biogeochemical cycles or
nutrient cycles.
b. Driven directly or indirectly by the sun and gravity.
c. Include the carbon/oxygen, nitrogen, phosphorus, sulfur, and water cycles.
B. The Water Cycle
1. Properties of water
a. H-bonding
e. Universal solvent
b. Liquid over a wide range of
f. Filters UV
temperatures
g. Cohesion
c. High specific heat
h. Expands when it freezes
d. High vaporization energy
2. The cycle (Fig. 3.26)
a. Precipitation may be stored in glaciers, infiltrates soil and percolates to aquifers, most
becomes surface runoff back to ocean.
b. Primary sculptor of earth’s landscape and carries dissolved nutrient compounds.
c. Purification of water by evaporation, transpiration, and filtration during percolation.
d. The hydrologic cycle can be viewed as a cycle of natural renewal of water quality.
3. Only about 0.024% of earth’s water supply is available to us as liquid fresh water.
C. Effects of Human Activities on the Water Cycle
1. Withdraw fresh water from sources faster than nature replaces it.
2. Clear vegetation from land for agriculture, mining, roads and buildings.
a. Increases runoff and reduces infiltration and percolation
3. Eutrophication
4. Water cycle speeding up because of warmer temperatures.
D. The Carbon Cycle: Part of Nature’s Thermostat
1. Fig. 3.27
2. Based on CO2 , makes up 0.038% of troposphere and is dissolved in water.
a. A key component of nature’s thermostat.
b. Producers remove CO2 from atmosphere and water and convert it to organic compounds such
as glucose.
c. Producers, consumers, and decomposers break down these organic compounds with O2 during
aerobic respiration.
d. O and H cycle in step with carbon
3. Some carbon atoms take millions of years to cycle
E.
F.
G.
H.
I.
a. Oil and coal
b. In only the last 100 years we have extracted and burned fossil fuels that took millions of years
to form.
i. Is why fossil fuels are non-renewable.
4. Oceans hold some of the CO2 in solution.
a. Used by aquatic producers for photosynthesis.
b. Some critter have CaCO3 shells, which when they die are deposited and buried, then converted
to limestone.
Effects of Human Activities on the Carbon Cycle
1. Clearing forests
2. Burning fossil fuels (Fig. 3.28)
a. Increases the planet’s natural greenhouse effect.
b. Resulting global warming can shift climate.
The Nitrogen Cycle: Bacteria in Action
1. Fig. 3.29
2. N2 makes up 78% of the troposphere and is crucial to the formation of proteins, vitamins, and
nucleic acids.
a. Cannot be directly used by multi-cellular plants and animals.
3. Two natural processes fix N2 into compounds useful to plants.
a. Electrical discharges in the atmosphere
b. Nitrogen-fixing bacteria
i. Make NH3, then converted with NH4+ and taken up by plants
c. Nitrification, when NH4+ is not taken up by plants, if further converted by other bacteria to
nitrite and then nitrate, easily taken up by plant roots.
4. Plants and animals return nitrogen rich compounds as waste, cast-off particles, and dead bodies.
a. Ammonification by specialized decomposing bacteria.
5. Denitrification – bacteria in waterlogged soil, and bottom sediments of lakes, oceans, swamps,
and bogs convert ammonia and ammonium ions back to nitrate and nitrite, and then N2 and
nitrous oxide (N2O).
Effects of Human Activities on the Nitrogen Cycle
1. Add large amounts of nitric oxide (NO) to atmosphere during high temperature combustion.
a. Converted to NO2 and nitric acid (HNO3), causing acid deposition, or acid rain.
2. Add nitrous oxide (N2O) through action of anaerobic bacteria on livestock wastes and commercial
inorganic fertilizers.
a. A greenhouse gas and depletes the ozone in the stratosphere.
3. Nitrate ions (NO3-) in inorganic fertilizers leach through soil and contaminate groundwater which
is harmful to drink esp. for small children.
4. Destroy forests, grasslands, and wetlands releasing large quantities of nitrogen as gaseous
compounds.
5. Agriculture and lawn runoff, and municipal waste discharges upset nitrogen balance in aquatic
ecosystems.
6. Nitrogen is removed from topsoil by harvesting nitrogen-rich crops, irrigation, and burning or
clearing forests.
* Human activities, according to the 2005 Millennium Ecosystem Assessment, since 1950 have
more than doubled the annual release of nitrogen from the terrestrial portion of the earth (Fig.
3.30)
The Phosphorus Cycle
1. An important component of DNA and energy storage molecules cycles slowly (Fig. 3.31).
2. The cycle is slow and on human time scale is one way from land to oceans.
3. Typically found as phosphate (PO4-3) salts in terrestrial rocks and ocean sediments.
4. Once in ocean sediments, buried for millions of years until geologic uplifting.
Effects of Human Activities on the Phosphorus Cycle
1. Mine large quantities to make inorganic fertilizers and some detergents
2. Reduce available phosphate in tropical soils when forests are cleared
3. Disrupt aquatic ecosystems with runoff from animal wastes and fertilizers and discharges from
sewage systems
*Since 1900 humans have increased rate of phosphorus release into the environment about 3.7fold.
J. The Sulfur Cycle
1. And essential component of many proteins, sulfur cycles in the ecosystem (Fig. 3.32).
2. Much is stored underground in rocks and minerals, including sulfate (SO4-2) salts.
3. Volcanoes release hydrogen sulfide (H2S) and sulfur dioxide (SO2) gases.
4. Anaerobic bacteria release H2S
5. SO4-2 enter atmosphere through sea spray, dust storms and forest fires.
6. Certain algae produce volatile dimethylsulfide, or DMS (CH3SCH3)
a. Serves and nuclei for condensation of water into droplets in clouds
b. In atmosphere, converted to SO2, then SO3, then sulfuric acid (H2SO4)
7. In O2 deficient environments such as flooded soil, FW wetlands, and tidal flats, specialized
bacteria convert sulfate to sulfide (S2-) which reacts with metals to form insoluble metallic
sulfides.
K. Effects of Human Activities on the Sulfur Cycle
We add sulfur dioxide to the atmosphere by:
1. Burning coal and oil
2. Refining sulfur containing petroleum.
3. Convert sulfur-containing metallic ores into free metals such as copper, lead, and zinc releasing
sulfur dioxide into the environment.
L. The Gaia Hypothesis: Is the Earth Alive?
1. Some have proposed that the earth’s various forms of life control or at least influence its chemical
cycles and other earth-sustaining processes.
a. The strong Gaia hypothesis: life controls the earth’s life-sustaining processes.
b. The weak Gaia hypothesis: life influences the earth’s life-sustaining processes.
VIII. How Do Ecologists Learn About Ecosystems?
A. Field Research, Remote Sensing, and Geographic Information Systems (GIS)
1. Field Research, sometimes called muddy-boots biology
2. Remote sensing
3. GIS (Fig. 3.33)
a. Spatial data base
b. Computerized maps of forest cover, water resources, air pollution emissions, coastal changes,
relationships between cancers and sources of pollution, GPP, changes in global sea
temperature, etc.
c. Modeling of this data can be used to weather forecast, plan urban development, agriculture,
formulating international environmental policy, and judging the effectiveness of international
environmental treaties.
B. Studying Ecosystems in the Laboratory
1. Ecologists also use controlled indoor and outdoor chambers to study ecosystems
a. Culture tubes, bottles, aquaria, greenhouses, and indoor and outdoor chambers
b. Factors can be controlled such as temperature, CO2, light, humidity, etc.
2. Results must be interpreted with care.
C. Systems Analysis
1. Ecologists develop mathematical and other models to simulate the behavior of ecosystems.
2. Models can be used to
a. Project possible changes in the environment
b. Help anticipate environmental surprises
c. Analyze the effectiveness of various alternative solutions to environmental problems
D. Importance of Baseline Ecological Data
1. We need baseline data on the world’s ecosystems so we can see how they are changing and
develop effective strategies for preventing or slowing their degradation.
a. Scientists have less than half of the basic ecological data needed to evaluate the status of
ecosystems in the United Sates (Heinz Foundation 2002; Millennium Assessment 2005).