Chapter 54
Key Vocabulary Terms: trophic levels, primary producer, primary consumer, secondary consumer, detritus,
food chain, food web, decomposition, primary productivity, gross primary productivity, net primary
productivity, biomass, secondary productivity, biomass pyramid, biogeochemical cycle, nitrification,
denitrification, ammonification, biological magnification
Chapter Outline
INTRODUCTION
Ecosystems Are the Most Complex Level of Biological Organization
Include living and nonliving factors
Transfer of energy is regulated, nutrients are cycled
Earth is a closed system with respect to nutrients and chemicals
System is open with respect to energy
Ecosystems May Have Clearly Recognizable Boundaries
Ecosystems change over time and become new ecosystems
Changes are gradual and adapt to particular conditions
Overall characteristics of populations adjust to the new conditions
THE FLOW OF ENERGY
Ecosystems Contain Autotrophs and Heterotrophs
Autotrophs capture light energy and manufacture own food
Heterotrophs obtain organic molecules synthesized by autotrophs
Energy captured is slowly released through metabolic processes
Primary Productivity
Primary productivity: amount of organic matter produced from solar energy
per area per time
Gross primary productivity: total amount of energy converted to organic
compounds per area per unit time
Net primary productivity: total amount of energy fixed per unit of time
minus energy expended in organismal metabolic activities
Biomass: total weight of all organisms living in the ecosystem
Increases as a result of net productivity
High net productivity in cornfield ecosystem
High net productivity, low biomass in tropical rainforest
ecosystem
Comparative net primary productivity: biomass ratios
Tropical forest and marshlands = 1500 to 3000 grams/year
Temperate forest = 1100 to 1500 grams/year
Dry deserts = 200 grams/year
Estuaries, coral reefs, sugarcane fields = 3600 to 9100 grams/year
Intertidal zone = 14,600 grams/year
Trophic Levels
Green plants convert 1% of the sun`s energy
Less energy converted by the animals that eat plants
Levels of consumers
Primary consumers: herbivores, feed on green plants
Secondary consumers: carnivores and parasites feed on herbivores
Decomposers: break down matter accumulated in bodies of organisms
Detrivores: live on dead organisms and cast-off parts of
organisms
Such trophic levels exist in all complicated ecosystems
Organisms from each level compose food chain
Relationships are more accurately branching food webs
Some energy ingested is lost at each successive trophic level
Much goes to heat production, some lost for digestion and work
Less than 10% goes toward growth and reproduction
Experimental studies of freshwater ecosystem
For each 1000 calories of energy fixed by photosynthesizers
150 calories transferred to small heterotrophs
30 calories of that transferred to smelt
6 calories of that transferred to trout (or humans)
1.2 calories from trout transferred to humans
Organisms that have a vegetarian diet have more food energy available
There are more individuals at lower trophic levels than at upper levels
Diagrammatic representations of such relationships form pyramids
Biomass pyramids may occasionally be inverted
Energy pyramids cannot be inverted
BIOGEOCHEMICAL CYCLES
All Substances in Organisms Cycle Through Ecosystems
The bulk of these are not contained within the bodies of organisms
Contained within the atmosphere: carbon, nitrogen and oxygen
Contained within rocks: phosphorus, potassium and other minerals
Substances are incorporated from nonliving sources into organisms
Returned to non-living world through decomposition
The Water Cycle
All life depends directly on the presence of water
Energy from sun powers the evaporation of water into atmosphere
Most falls back into the oceans or subsurface bodies of water
98% of earth`s water is free, only 2% is fixed
All organisms require water to live
Plants obtain water from the earth
Animals drink water or obtain it by eating plants
Water occurs as surface and ground water
Aquifers are permeable saturated layers of rock, sand and gravel
Ground water is an important reservoir of water
Water table: the unconfined portion of ground water
Ground water flows more slowly than surface water
Rate of use is increasing enormously
Many aquifers are threatened with depletion
Pollution in groundwater is a serious problem
The Carbon Cycle
Based on atmospheric carbon dioxide
Synthesis of organic compounds fixes 700 billion metric tons yearly
Accomplished by various photosynthesizers
All heterotrophic non-photosynthesizers depend on their activity
Carbon dioxide released into atmosphere when organisms decompose
Some carbon compounds are accumulated
Cellulose is more resistant to breakdown
May eventually be incorporated into fossil fuels or minerals
1 trillion metric tons of CO2 are dissolved in the ocean
Fossil fuels contain 500 billion metric tons
600 to 1000 billion metric tons contained within organisms
Processes of respiration and photosynthesis are roughly balanced
Carbon dioxide increasing as a result of burning fossil fuels
May be altering global climates
The Oxygen Cycle (linked with the Carbon Cycle)
Only the earth possesses significant quantities of free oxygen
Free oxygen is a product of three billion years of photosynthesis
Without continued photosynthesis, respiration would deplete all
nonatmospheric oxygen in fifty years
The Nitrogen Cycle
Nitrogen gas constitutes 78% of the atmosphere
Very little nitrogen is fixed in the soil, oceans and organisms
Few organisms convert atmospheric nitrogen into biologically useful forms
All are nitrogen-fixing bacteria
Triple bond linking nitrogen atoms makes the gas very stable
Process is enzyme catalyzed and utilizes ATP
Some nitrogen-fixing bacteria are free-living
Some form symbiotic relationships with plants called legumes
Fix enough nitrogen to be of significance
Plants can grow in soils with low amounts of nitrogen
Bacteria and fungi rapidly decompose nitrogen-containing compounds
Use products to synthesize own proteins, release excess as
ammonium
Process called ammonification
Fixed nitrogen is lost to the atmosphere by denitrification
The Phosphorus Cycle
Most biogeochemical cycle reservoirs in minerals, not atmosphere
Phosphates exist in the soil in only small amounts
Are relatively insoluble and contained in only certain kinds of
rocks
Weather out of rocks, transported to oceans
Brought up by natural uplift of land masses or by marine animals
Form rich natural deposits of guano from sea birds
Millions of tons of phosphates added to farm land each year
Calcium dihydrogen phosphate is called superphosphate
Made by treating calcium phosphate with sulfuric acid
Biogeochemical Cycles Illustrated: Recycling in a Forested Ecosystem
Evidence in studies of Hubbard Brook Experimental Forest
Central stream of large temperate deciduous forest watershed
Measure water and nutrient flow made through concrete weirs
Conclusions: undisturbed watershed efficiently retained nutrients
Instructive with regard to loss of rainforest area to crops
Experimental felling of trees and shrubs in one of six watersheds
Amount of water runoff increased by 40%
Amount of nutrients lost was greatly increased
Conclusion: fertility lost, danger of flooding increased