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Ecology, Ecosystems,
& Food Webs
Chapter 16
© Brooks/Cole Publishing Company / ITP
What is Ecology?
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study of relationships between organisms & their
environment.
Levels of organization:
–
–
–
–
biosphere- biotic (living) & abiotic factors (non-living)
ecosystem: community + non–living environment
community: populations of different species in given area
population: a group of interacting individuals of same
species
– organism (individuals): any form of life
Organisms (Individuals)
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organisms are classified into species.
species: groups of organisms that resemble each
other, and in cases of sexually reproducing
organisms, can potentially interbreed.
estimates of 5 to 100 million species, most are
insects & microorganisms; so far only about 1.8
million named; each species is the result of long
evolutionary history.
wild or native species: population that exists in its
natural habitat .
domesticated or introduced species: population
introduced by humans (= non–native species).
Populations
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population: a group of individuals of the
same species.
examples: sunfish in a pond, white oak trees
in a forest, people in a city;
habitat: the place where a population
usually lives.
genetic diversity: in natural populations
individuals vary in their genetic makeup.
Ex: blue eyes vs. green eyes
Communities

community: populations of different
species living together in a given area.
– a biological community is a complex
interacting network of plants, animals and
microorganisms.
– example: redwood forest community,
consisting of populations of redwoods &
other trees, shrubs and herbaceous
species, animals and microorganisms.
Ecosystems
ecosystem: communities & the non–
living parts of the environment.
 example:
 Ducks, fish, and insect larvae living
in/on a lake or pond.

What is an ecosystem?
 Ecosystem
is a system of
living things that interact with
each other and with the
physical world.
A
Biome is a collection of
related ecosystems.
Main Ecosystems:
 Desert
 Tundra
 Rainforest
 Chaparral
 Ocean
 Grassland
 Taiga
 Temperate
Forrest
Green:Grassland Purple: Taiga Orange:Tundra
Black:Temperate Forrest Yellow:Desert
Brown:Chapparal White:Ocean
However……..
 An
ecosystem can be as large as
the Sahara Desert, or as small as
a puddle!!!
 Ecosystems are more than just
the organisms they contain.
Geography, weather, climate and
geologic factors also influence the
interactions within an ecosystem.
Biodiversity
Bio What?
 Biodiversity
is the variety of all
life forms: the different plants,
animals and micro-organisms,
their genes and the
ecosystems of which they are
a part.
Biodiversity Continued
 Genetic diversity-the total genetic
information contained in the genes of all species.
 Species Diversity-The variety of species,
refers to the number of species and the number of
individuals in a species.
 Ecosystem Diversity- the variety of
habitats natural communities and ecological
processes.
Importance Of
biodiversity
Humans are dependent on biodiversity
for their sustenance, health, well-being
and enjoyment of life.
 Food- we rely on biodiversity to provide us with a

large and varied food source

Health- Humanity derives many of it’s medicines
from biodiversity.
Threats to biodiversity
Habitat loss and fragmentation
 Competition from invasive species
 Pollution
 Global climate change
 Desertification
 Population growth and over
consumption
 Unsustainable use of natural
resources.

Simple ecosystem model
Key Attributes:
Biotic and
abiotic
processes
Ecosystem Boundaries
 How
do we decide where to draw the
lines around an ecosystem?
 Depends on the scale of the question
being asked
– Small scale: e.g., soil core;
appropriate for studying microbial
interactions with the soil environment,
microbial nutrient transformations
– Stand: an area of sufficient
homogeneity with regard to vegetation,
soils, topography, microclimate, and
past disturbance history to be treated
as a single unit; appropriate questions
include impact of forest management
on nutrient cycling, effects of acid
deposition on forest growth
Ecosystem Boundaries
Natural Boundaries: ecosystems
sometimes are bounded by naturally
delineated borders (lawn, crop field,
lake); appropriate questions include
whole-lake trophic dynamics and energy
fluxes (e.g., Lindeman 1942)
Watershed: a stream and all the
terrestrial surface that drains into it
 rich history of watershed scale
studies in ecosystem ecology
(“Small Watershed Approach” e.g.
Bormann and Likens 1967)
 watershed studies use streams as
‘sampling device’, recording surface
exports of water, nutrients, carbon,
pollutants, etc., from the watershed;
deforestation impacts on water
supply to a city.
Succession
Ecological succession, the series of
changes in an ecological community
that occur over time after a
disturbance. It can be:
Primary Succession
 Secondary Succession

Primary Succession
Where plants have not
grown before.
Secondary Succession
Area where there has
been previous growth.
Succession and Wildlife
What Sustains Life?

Energy From Sun
– one–way flow of usable energy from sun 
through feeding interactions heat

Cycling of Matter
– the continual flow of matter between the
nonliving environment & living organisms
(biogeochemical cycles)

Gravity
– enables Earth to hold its atmosphere gases;
causes downward movement of matter in
nutrient cycles.
What Sustains Life?

Energy From Sun
– one–way flow of usable energy from sun 
through feeding interactions heat

Cycling of Matter
– the continual flow of matter between the
nonliving environment & living organisms
(biogeochemical cycles)

Gravity
– enables Earth to hold its atmosphere gases;
causes downward movement of matter in
nutrient cycles.
Energy Flow & Nutrient
Cycling
Life on Earth
depends
upon one–
way flow of
high–quality
energy from
sun &
cycling of
crucial
elements.
Energy Flow
The ultimate source of energy in most ecosystems is
the sun.
Nutrient Cycles

nutrient: any atom, ion, or molecule an organism
needs to live, grow, or reproduce.
– macronutrients needed in relatively large
amounts e.g., C, O, H, N, P, S, K, Ca, Mg, Fe
– micronutrients needed in relatively small
amounts e.g., Na, Zn, Cu, Cl,
– nutrient cycles (= biogeochemical cycles)
involve continual flow of nutrients from nonliving
(air, water, soil, rock) to living organisms (biota) &
back again.
– nutrient cycles driven directly or indirectly by solar
radiation & gravity.
– Major cycles: hydrologic (water), carbon, oxygen,
nitrogen, phosphorus and sulfur.
Vocabulary for
Ecosystems

Abiotic: non–living components. Ex:
water, air,sun

Biotic: living components Ex: plants,
animals, bacteria

Trophic level- feeding level for an
organism
Major components of aquatic ecosystems.
Major components of terrestrial ecosystems.
Key Players in
Ecosystems
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Autotrophs/ producers: make their own
food via photosynthesis (plants) or
chemosynthesis (bacteria in thermal vents
use hydrogen sulfide (H2S) & carbon
dioxide)
Heterotrophs/ consumers: can’t make
their own food, feed on other organisms or
their remains.
–
Ex: herbivores, carnivores, decomposers, etc.
Key Energy Processes
Photosynthesis: use of chlorophyll. Energy
storing process.

–
6 CO2 + 6 H2O + solar energy  C6H12O6 + 6 O2
Cellular Respiration:
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–
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Aerobic Respiration: energy releasing process.
C6H12O6 + 6 O2  6CO2 + 6 H2O + energy (ATP)
Anaerobic Respiration
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–
Ex. Fermentation: energy releasing process used by
yeast and bacteria
Categories of Consumers
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primary consumers: (=herbivores) feed
directly on producers;
secondary consumers: (=carnivores) feed on
primary consumers;
tertiary consumers: feed only on carnivores;
omnivores: consumers that feed on both
plants & animals;
scavengers: feed on dead organisms;
decomposers (saprobes): consumers that
complete the breakdown & recycling of organic
materials from the remains & wastes of other
organisms;
detritivores: feed on detritus (partially
decomposed organic matter, such as leaf litter
& animal dung).
The Importance of Decomposers
Fig. 4–16
Summary of Ecosystem Structure
Fig. 4–17
Food Chains
Food chains are a simple food path involving a sequence
of organisms, each of which is the food for the next.
Fig. 4–18
Food Webs
Food webs are multiple food chains that are
interconnected. More complex than food chains.
Ecological Pyramids
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Represent the flow of energy through an
ecosystem.
Typically each trophic level has a certain
amount of BIOMASS (dry weight of organic
matter)
Ecological efficiency- amount of usable
energy transferred as biomass. Usually
10% at each transfer.
Food chains and webs only have 4-5
trophic levels, because too little energy
left to support top consumers.
Energy Pyramid
In nature,
ecological
efficiency varies
from 5% to 20%
energy available
between
successive
trophic levels
(95% to 80%
loss). About 10%
efficiency is a
general rule.
Fig. 4–19
Another Energy Pyramid
Annual pyramid of energy flow (in kilocalories per
square meter per year) for an aquatic ecosystem in
Silver Springs, FL.
Fig. 4–21
Note: More individuals
can be supported at
lower trophic levels.
Less energy is lost.
Biomass Pyramids
Displays the biomass at each trophic level.
Pyramid of Numbers
Pyramid of numbers displays the number of individuals
at each level.
1 owl
25 voles
2000
grass plants
Primary Productivity of
Ecosytems
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1.Gross primary productivity (GPP) is the
rate at which an ecosystem's producers
convert solar energy into chemical energy
as biomass.
2. Net primary productivity (NPP) is the rate
at which energy for use by consumers is
stored in new biomass.
NPP = GPP – R [rate at which producers
use biomass]
Net Primary Productivity
Estimated annual net primary productivity of major
biomes & aquatic life zones, expressed as kilocalories
per square meter per year.
Fig. 4–24