Unit Four - Ecology
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The study of the relationships that exist
between organisms and their environments.
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Ecological interactions are influenced by two
types of factors:
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Abiotic Factors
- Biotic Factors
Biotic Factors

These include all living organisms and their
effects both direct and indirect on other living
things

May include organisms that were recently
alive
Examples of Biotic Factors
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Predator prey relationships
disease causing bacteria
poisonous plants
a huge tree that has fallen across a path
a dead moose that is found in a bog
Abiotic Factors
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The nonliving factors in an environment that
effect ecological interactions
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These factors define what type of organisms
can live in a particular area
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There are several important abiotic factors
Important Abiotic Factors
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Light Availability
Temperature
Availability of Water
Composition of soil and nutrient/mineral
availability
Light Availability
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Light from the sun (solar energy) is the
ultimate energy source for all living things
The availability of light energy differs
greatly on different parts of the earth
Temperature
• As you change latitude or altitude on the earth
the temperature patterns change greatly
Soil composition and availability
of Minerals/Nutrients

The development of soil will determine such
factors as pH, amount of organic nutrients,
amount and type of minerals present
Water availability
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The amount of precipitation differs depending
where on the earth you are
Biosphere
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The area of land air and water on or around the
earth in which all living things can be found
Succession
•Various disturbances disrupt communities
-abiotic: fire, volcano, retreat of glacier or water
-biotic: e.g human activity (clearcutting, farming, etc...)
•New communities emerge after disturbances in a predictable
sequence.
•This predictable change is called Succession.
•Succession happens because organisms cause changes in their
surroundings that make the environment less suitable for
themselves and more suitable for other organisms.
Details on Succession...
Two Types:
 primary succession - sequence of events from barren area
to stable ecosystem. Soil building is key component.
E.g. barren rock -> soil with bacteria, moss, lichen -> shrubby
plants -> large trees

secondary succession - sequence of events from soil to
stable ecosystem. Usually follows human activity that
destroys a previous community (e.g. clearcutting).
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In each case succession starts with a few hardy invaders
called pioneers.

If undisturbed it continues until a diverse and relatively
stable climax community forms
Primary Succession
Primary Succession in Hawaii
How does primary succession
work?
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Pioneer species are first ones to grow
E.g. Lichens can grow on barren rock, together with bacteria can
generate organic material which later becomes soil.
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Early species often change conditions (e.g. making
soil) that makes them more suitable for later
successional stages.
Entire process is a set of competitive replacements of
one species by another.
Lake Michigan is receding…
How does this image illustrate
succession?
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No life on water’s edge - waves, sand
Foredune - pioneer community, beach
grass binds sand, supports insects
Shrubs - inhibit grass by stealing light,
improve soil quality by providing lots
of plant material.
Pine Woods - short stage because trees
shade own seedlings and inhibit
growth. Needles add to soil.
Hardwoods - seedlings can grow in
shade - reach equilibrium i.e. climax
community
Aquatic
Primary Succession
Yosemite Valley meadows are
an example of meadow
succession
•Herbaceous growth
•brush
•Lodgepole pine vs. Red fir
Secondary Succession
Fire (secondary) succession at Yellowstone
Pioneer herbs
Food Chains
Food Webs
and Energy Flow
Trophic:
Related to feeding
Trophic Level
Functional classification of organisms according to feeding relationships
Autotrophy
Ability to produce organic material from inorganic chemicals and some
source of energy (photosynthesis)
6 CO2 +
Heterotrophic
Food Chain
Food Web
6 H2O
 C6H12O6
+
6 O2
Requiring a supply of organic matter or food from the environment.
Movement of energy and nutrients from one feeding group of organisms
to another in a series that begins with plants and stops with carnivores and
decomposers
Interlocking pattern formed by a series of interconnecting food chains
Nutrient Cycles
Inorganic nutrients (Carbon, Oxygen, Nitrogen) are recycled continually through
ecosystems.
Plants and Animals build
structures from the
nutrients and inorganic
material.
When they die detritus
feeders and decomposers
break down these structures
into the base nutrients.
This is returned to the soil
and then taken up by plants
and the cycle begins again.
Food Chains
Energy is transferred in a system from one organism to another.
This transfer of energy from organism to organism makes up a
food chain.
Each level of consumption is called a trophic level.
Primary producers are therefore in the first trophic level.
who are eaten by… you get the idea.
who are eaten by the secondary consumers (Carnivores)
Next come the primary consumers (Herbivores)
At the bottom of the food chain are the photosynthetic producers
which range from single-celled bacteria to redwood trees.
Example
Food
Chains
1st order
carnivore
2nd order
carnivore
Energy and Food Chains
Of all the energy that the sun sends to the Earth, most is reflected or absorbed by
the atmosphere or Earth surface.
1% of the energy sent by the Sun is available to life on Earth.
Of this energy, 3% is trapped by green plants or algae.
All life on Earth is therefore due to the 0.03% of the energy absorbed from the
Sun.
All life depends upon energy and each trophic level above the first trophic level
gets energy from consuming lower trophic level organisms.
Only about 10% of the energy from one trophic level can get to the next one
through consumption. The rest of the energy is lost as heat.
Energy loss as you move from trophic level to trophic Level
There is usually no more than 5 links in a food chain. Why?
Energy and the Food Chain
If 10% of the energy can be transferred from one trophic level to the
one above it, each trophic level must have 10x the energy as the one
above it.
The number of trophic levels depends
upon the number of primary producers
in the first trophic level.
Biomes with small numbers of primary
producers have short food chains
Energy Pyramid: This shows the available energy at each trophic level.
What does this mean for the future of Homo Sapien Sapien?
Trophic Levels
Animals feeding wholly on plants occupy a single trophic level.
But most animals at higher trophic levels occupy several trophic levels simultaneously because of
variation in their diets.
The first trophic level belongs to producers or plants
The second trophic level belongs to herbivores or
first order consumers.
The third trophic level belongs to carnivores or second order
consumers.
Food Webs
Summary:
Primary Producer plant/ algae
Primary consumer
herbivores
Secondary consumer
primary carnivore
autotrophs
hetrotrophs
Tertiary consumertop carnivore
Usually no more than 5 links in a food chain. Why?
Detritivores, scavengers, and decomposers
Detritivores:
consume litter, debris, and dung
Scavengers:
clean-up dead carcasses
Decomposers:
microorganisms that complete final breakdown of
organic matter
Food Web
Population = all members of same species
(interbreeding organisms) within an
ecosystem.
Populations can grow
exponentially...
...If each organism has
multiple offspring.
For Example
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1 fly lays about 120 eggs
In one year, that one fly has
about 5 x 1012 great, great,
great, great grandchildren.
This fly population is
meeting its Biotic Potential
because it is increasing at
the maximum rate possible.
Biotic Potential
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Maximum rate at which a population can increase in
ideal conditions.
Biotic Potential is affected by the organism’s
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Lifespan
Age at first reproduction
Frequency of reproduction
Clutch size (how many offspring produced)
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Length of reproductive capability
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Exponential growth of 2 organisms
with different biotic potentials
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Bacteria divide every 20
minutes; it takes 220
minutes to reach a
population of about 2000.
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Eagles reproduce once a
year starting at age 4 (red)
or age 6 (green). It takes
about 23 years (red) or 32
years (green) to reach 2000
individuals.
Reality Check: There are limits
to exponential growth !
Population growth is limited by“environmental resistance”
Density - Dependent Factors (tend to be biotic)
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Limited resources (food, space, light for photosynthesizers)
Competition
Predation - increased prey means increased predation
Parasitism - spread more easily in high density pops
Density- Independent Factors (tend to be abiotic)
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Weather (e.g. plants, insects sensitive to extreme hot, cold)
Natural disasters - fire, hurricanes, earthquake, volcanos
Some populations expand until
they reach equilibrium at their
limit
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Exponential growth under
favorable conditions: food,
space available, little to no
predation, parasitism or
competition.
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Once the population size
matches the carrying
capacity of the ecosystem, its
growth slows and reaches
equilibrium.
Carrying Capacity
•Is the maximum population size that can be supported by an
ecosystem over the long term
•Is typically limited by the resources available in that ecosystem
What happens if a population
exceeds carrying capacity?
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Some populations
grow too fast…
Population overshoots
resources…
Population crashes
E.g. Gypsy Moth
caterpillars can defoliate
the trees they live on so
quickly that their larvae
have nothing to feed on!
Sometimes they overshoot but
are able to stabilize
Predator - Prey relationships can
cause cyclical population curves
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When prey populations increase, more predation occurs
because- (1) predators encounter prey more often and (2)
more prey support a bigger predator population.
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When predators get too numerous, they reduce the prey
population, thus depleting their food supply.
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A change in the prey population illicits change in the predator
population and vice versa.
Predator-Prey population
trends
Human Population Growth:
When will we hit carrying capacity?
Advances have increased the earth’s carrying capacity and pop size