Chapter 6

Ecosystems and Communities

• Succession– Series of regular

predictable changes in community structure over time

– Result in climax community

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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 reefsMangrove 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