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Chapter 6
Ecosystems and
Communities
• Succession
– Series of regular
predictable
changes in
community
structure over
time
– Result in climax
community
Chapter 6
Primary Succession
– Total lack of organisms and bare rock or water
• Terrestrial
– Pioneer Community
• First to colonize bare rocks--lichens
– Successional or Seral Stage
• Multiple sequence of stages/communities
– Climax Community
• Relatively stable, long lasting, complex community
– Entire sequence of stages called a sere.
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Characteristics of Climax Communities:
• 1. able to maintain mix of species for a long
period of time
• 2. in energy balance
• 3. larger number and more kinds of organisms
and interactions between organisms
 complexity and energy efficiency
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• Secondary Succession
• Succession in area after destruction or major
disturbance, flood, fire, agriculture
• More common and rapid than primary
succession
• Usually some soil, seeds or root mass that can
re-establish
• Adjacent area can be source of seed, etc
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• Aquatic
• Except for oceans, most aquatic ecosystems are
considered temporary.
• Ever increasing entrapment of soil particles and
organic matter.
• Less depth means establishment of submerged
plants

terrestrial succession
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Types of Climax Communities
• Biomes
– terrestrial climax communities with a wide
geographic distribution.
– Variation of species exist within biomes but
exhibit similar pattern and processes
• Same general structure of ecosystem
• Same kinds of niches present
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2 primary factors which determine biome type:
• Precipitation:
– total amount
– form (rain, snow)
– seasonal distribution—concentrated at particular time
• Temperature:
– temperature swings
– length of seasons
• Other factors: fires, winds, organisms
• Altitude and latitude can mimic these effects
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Desert:
• Lack of water is primary factor
• Less than 25 centimeters (10 inches) precipitation
per year.
• Lack of clouds means no insulation
– Can be quite cold/hot
• Many types of plants and animals, most are quite
adapted to conditions.
– Large space between plants, dispersed animals too
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Grassland (aka: prairies or steppes):
• Between 25 and 75 centimeters precipitation per
year (10-30 inches).
• Windy, hot summers: cold or mild winters.
• Fire is important in maintaining biome
– Prevent invasion of trees
– Cycles nutrients
• Large herds of grazing mammals, many insects,
many birds.
• Most converted to agriculture
– 0.004% of Texas Blackland Prairie remains
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Savanna:
• Similar in appearance to grassland, but has more
trees.
• Precipitation is between 50 and 150 centimeters
per year (20 - 65 inches), but it comes in a few
heavy events, with drought in between.
• Plants and animals behave accordingly.
• Wildlife quite similar to grasslands.
• Fire still important so trees fire resistant
– Texas Post Oak Savanna
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Tropical Rain Forest:
• Near equator
– Africa, C&S America, SE Asia, some islands
• More than 200 centimeters precipitation per year
(80 inches).
– Some over 500 centimeters (200 inches).
• Warm temperatures/relatively constant.
• Many species.
• Soils are poor since nutrients are stored in biomass
of vegetation or washed out.
– Highly weathered so clayey—sun and heat make?
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Temperate Deciduous Forest:
• Distinct summer/winter seasons.
– Trees loose leaves in winter
• 100 centimeters (or more) of precipitation (40
inches).
– Mild winters, 6 month growing season
• Relatively few species of trees.
– Two or three major producers
• Many flowers, insects, birds and mammals.
– Texas—Eastern and Western Crosstimbers
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Taiga (aka: Northern Coniferous Forest or Boreal
Forest):
• Short cool summer, long hard winter.
– Up to 6 months, soil freezes
• Precipitation between 25 and 100 centimeters per
year (10 to 40 inches), with constant high
humidity.
• Trees see winters as dry due to precipitation in
form of snow.
– Needle shaped leaves.
– Flexible branches
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Tundra:
• No trees, permanently frozen soil (permafrost)—
north of Coniferous forests
• Low precipitation (less than 25 centimeters), but
comes in summer when snow melts.
• Permafrost does not allow water to soak
in....many small pools.
• Damage to systems takes long time to fix.
• Same conditions on high mountains (alpine
tundra) but different species
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Aquatic Ecosystems:
• Factors:
–
–
–
–
- sun light penetrability
- nature of bottom substrate
- water temperature
- amount of dissolved material
• Marine—high dissolved salts
• Freshwater—low dissolved salts
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Marine systems
• Pelagic Ecosystems
• Open ocean system, with free swimming organisms.
– Fish, whales, crustaceans,etc not attached to anything
•
•
•
•
phytoplankton (producer) in euphotic zone.
zooplankton
other consumers
nutrients are in short supply, except in few
concentrated areas.
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Benthic Ecosystems
• Systems at bottom of ocean.
– Critters attached or not
• If light can penetrate, plants exist.
• Substrate determines types of organisms which
can occur.
• No light: abyssal zone.
• Warm waters  coral reefs
Mangrove swamps
• Near shore region
– Can lead to terrestrial ecosystem development
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Estuaries
• Shallow partially enclosed area where freshwater enters
marines systems.
• Relatively few organisms capable of tolerating salt
fluctuations seen in this system.
• Large amount of nutrient inflow.
• Typically, entire area has sunlight penetrating to
bottom.
• Many organisms use as nursery.
• Trap sediment—salt marsh—terrestrial ecosystem
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• Freshwater Systems
• Lakes and Ponds:
– littoral zone (has rooted plants)
– limnetic zone (no rooted plants)
• Productivity:
– Temperature
– Depth
– Nutrients in water
• oligotrophic lakes—longer life
• eutrophic lakes—shorter life
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• Dissolved oxygen (DO) determines what
type of organisms will occur.
– Free oxygen O2
• Biochemical oxygen demand (BOD)
– A measure of how much decomposable organic
matter is in the water.
– Enter through critter waste in lake, inflows of
organic wastes, critter death, etc
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Streams and Rivers:
• periphyton - organisms attached to rocks and bottom
material.
• Not much chance for plants, so most energy comes
from material that falls in stream rather than from
photosynthesis.
• Many insects, fungi and bacteria use this organic
matter.
• Dissolved oxygen: less in slow, muddy rivers
• Wetlands: transition areas