25.0 Introduction

1. Ecosystems Are the Most Complex Level of Biological Organization
1. Include Living and Nonliving Factors
2. Transfer of Energy Is Regulated, Nutrients Are Cycled fig 25.1
1. Earth is a closed system with respect to nutrients and chemicals

3. System is open with respect to energy

25.1 Chemicals cycle within ecosystems

1. Biogeochemical Cycles
1. All Substances in Organisms Cycle Through Ecosystems
1. Cycle via biogeochemical cycles
2. The bulk of these are not contained within the bodies of organisms
1. Contained within the atmosphere: Carbon, nitrogen and oxygen
2. Contained within rocks: Phosphorus, potassium and other minerals

2. Substances Are Incorporated from Nonliving Sources into Organisms
1. Minerals enter organisms through drinking water
2. Returned to non-living world through decomposition

2. The Water Cycle
1. The Most Familiar Biogeochemical Cycle fig 25.2
1. All life depends directly on the presence of water
2. Water is source of hydrogens used in generating ATP

2. The Path of Free Water
1. Energy from sun powers the evaporation of water into atmosphere
1. Most falls back into the oceans or subsurface bodies of water
2. Most of water over land evaporates from plants through transpiration

2. 98% of earth's water is free, only 2% is fixed

3. The Importance of Water to Organisms
1. All organisms require water to live
1. Plants obtain water from the earth
2. Animals drink water or obtain it by eating plants

2. Free water determines nature and abundance of life in any particular place

4. Groundwater
1. Water occurs as surface and ground water
1. Aquifers are permeable saturated layers of rock, sand and gravel
2. Ground water is an important reservoir of water
3. Water table: Upper, unconfined portion of ground water

2. Ground water flows more slowly than surface water
1. Rate of use is increasing enormously
2. Many aquifers are threatened with depletion

3. Pollution in groundwater is a serious problem
1. Estimated that 2% of the U.S. groundwater is polluted
2. Includes pesticides, herbicides, fertilizers, chemical waste pits

4. Removing pollutants from aquifers virtually impossible
1. Large volume of water with slow turnover rate
2. Generally inaccessible

5. Breaking the Water Cycle
1. Cycling in forest ecosystems is mediated by plants
1. 90% of moisture taken up by plants, transpires back into air
2. Vegetation is primary source of rainfall

2. Removal of forests affects water cycle
1. Moisture not returned to air
2. Drains away from region

3. von Humbolt study in Columbia in 1700s
1. Stripping trees from rainforest prevent water return to atmosphere
2. Area became semiarid desert

4. Transformation occurring today in the name of "development" fig 25.3
1. Much of Madagascar was tropical forest, now semiarid desert
2. No practical way to reforest land

5. Once broken, hard to reestablish water cycle

3. The Carbon Cycle fig 25.4
1. Based on Atmospheric Carbon Dioxide
1. Carbon dioxide comprises 0.03% of atmosphere, 700 billion metric tons
2. Synthesis of organic compounds fixes 10% of atmospheric carbon dioxide yearly
1. Accomplished by various photosynthesizers
2. All heterotrophic nonphotosynthesizers depend on their activity

3. Carbon dioxide released into atmosphere when organisms decompose

2. Most Carbon-Containing Compounds Ultimately Break Down
1. Some carbon compounds are accumulated
1. Cellulose is more resistant to breakdown
2. May eventually be incorporated into fossil fuels or minerals

2. 1 trillion metric tons of CO₂ are dissolved in the ocean
1. Fossil fuels contain 5 trillion metric tons
2. 600 billion to 1 trillion metric tons contained within organisms

3. Processes of respiration and photosynthesis are roughly balanced
1. Carbon dioxide increasing as a result of burning fossil fuels
2. May be altering global climates

4. The Nitrogen Cycle fig 25.5
1. Organisms Depend on Nitrogen
1. Nitrogen gas constitutes 78% of the atmosphere
2. Very little nitrogen (0.03%) is fixed in the soil, oceans and organisms
3. Nitrogen cycles between organisms and reservoirs

2. Nitrogen Fixation
1. Few organisms convert atmospheric nitrogen into biologically useful forms
1. All are nitrogen-fixing bacteria
2. Triple bond linking nitrogen atoms makes the gas very stable
3. Process is enzyme catalyzed and utilizes ATP
4. N₂ + 3H₂ ® 2NH₃

2. Some nitrogen-fixing bacteria are free-living
3. Some form symbiotic relationships with roots of legume plants
1. Fix enough nitrogen to be of significance
2. Large reservoir of ammonia and nitrates now exists

4. Bacteria and fungi rapidly decompose nitrogen-containing compounds
1. Use products to synthesize own proteins, release excess as ammonium (NH₄⁺)
2. Process called ammonification

5. Fixed nitrogen is lost to the atmosphere by denitrification
1. Accomplished by certain anaerobic bacteria
2. Nitrate converted to N₂ gas and N₂O (nitrous oxide), both return to air

5. The Phosphorus Cycle fig 25.6
1. Exemplifies Other Mineral Cycles
1. Most biogeochemical cycle reservoirs in minerals, not atmosphere
2. Phosphorus plays critical role in plant nutrition
3. Phosphates exist in the soil in only small amounts
1. Are relatively insoluble and contained in only certain kinds of rocks
2. Weather out of rocks, transported to oceans
3. Brought up by natural uplift of land masses or by marine animals
4. Form rich natural deposits of guano from sea birds that eat marine animals

4. Examples of phosphorous fertilizers
1. Guano deposits
2. Crushed phosphate-rich rocks
3. Seas are only inexhaustible source, value of seabed mining

2. Value of Phosphorous Fertilizers
1. Millions of tons of phosphates added to farm land each year
1. Calcium dihydrogen phosphate [Ca(H₂PO₄) ₂]is called superphosphate
2. Made by treating calcium phosphate with sulfuric acid

2. Not leading to proportional increase in crops

6. Biogeochemical Cycles Illustrated: Recycling in a Forested Ecosystem
1. Studies of Hubbard Brook Experimental Forest
1. Reveals details about overall recycling patterns in an ecosystem
1. Studied since 1962
2. Provides much data about nutrient cycling in forest ecosystems
3. Basis of development of experimental methodology

2. Description of region and methodology
1. Central stream of large temperate deciduous forest watershed
2. Measure water and nutrient flow made through concrete weirs on six streams
3. Determined minerals present in the streams

3. Conclusions: Undisturbed forests efficiently retained nutrients
1. Amount gained in snow and rain equaled amount lost in runoff
2. Quantities low compared to nutrients within system
3. Small net loss of calcium
4. Small net gain of nitrogen and potassium

4. Instructive with regard to loss of rain forest area to crops

2. Experiment to Determine Effect of Loss of Trees
1. Cut down of trees and shrubs in one of six watersheds
2. Prevented regrowth by use of herbicides
3. Deleterious effects
1. Amount of water runoff increased by 40%, not taken up by plants
2. Amount of nutrients lost was greatly increased
3. Ten times greater loss of calcium
4. Phosphorous not in run-off water, locked up in soil
5. Nitrogen lost at rate of 120 kilograms per hectare per year fig 25.7
6. Nitrate level in water increased to level unfit for drinking
7. Generated massive blooms of cyanobacteria and algae

4. Conclusion: Fertility lost, danger of flooding increased

25.2 Ecosystems are structured by who eats who

1. Trophic Levels
1. Ecosystems Contain Autotrophs and Heterotrophs
1. Autotrophs capture light energy and manufacture own food
2. Heterotrophs obtain organic molecules synthesized by autotrophs
3. Autotrophs are primary producers, heterotrophs are consumers

2. Cycling of Metabolic Energy
1. Energy captured is slowly released through metabolic processes
1. Green plants convert 1% of the sun's energy
2. Less energy converted by the animals that eat plants

2. Levels of consumers
1. Primary consumers: Herbivores, feed on green plants
2. Secondary consumers: Carnivores and parasites feed on herbivores
3. Decomposers: Break down matter accumulated in bodies of organisms
4. Detrivores: Live on dead organisms and cast-off parts of organisms

3. Such trophic levels exist in all complicated ecosystems
1. Organisms from each level compose food chain fig 25.8
2. Relationships are more accurately branching food webs fig 25.9

4. Some energy ingested is lost at each successive trophic level
1. Much goes to heat production, some lost for digestion and work
2. Less than 40% goes toward growth and reproduction

25.3 Energy flows through ecosystems

1. Primary Productivity
1. Productivity Terms Defined
1. Primary production
1. Also called primary productivity
2. Amount of organic matter produced from solar energy per area per time

2. Gross primary productivity (GPP)
1. Total amount of organic matter produced
2. Includes what is used by photosynthetic organisms for respiration

3. Net primary productivity (NPP)
1. Amount of organic matter produced in a community at a given time
2. Available to be used by heterotrophs
3. NPP = GPP - amount of energy expended in photosynthesizers metabolisms

4. Biomass
1. Net weight of all organisms living in the ecosystem
2. Increases as a result of net productivity

2. Productive Biological Communities
1. General trends tbl 25.1
1. Wetlands have high net primary productivity
2. Tropical rain forest has high net productivity, with larger biomass
3. Net primary productivity of rain forest lower in relation to biomass

2. Comparative net primary productivity: Biomass ratios
1. Tropical forest and wetlands = 1500 to 3000 grams/year
2. Temperate forest = 1200 to 1300 grams/year
3. Savanna = 900 grams/year
4. Desert = 90 grams/year

3. Secondary Productivity
1. Rate of production by heterotrophs
1. Cannot manufacture own biomolecules
2. Obtain by eating plants or other heterotrophs

2. About one magnitude less than primary productivity in same system
3. Loss of energy from plants to herbivores fig 25.10
1. Much of biomass not consumed, supports decomposers
2. Some energy not assimilated, passed out as feces to decomposers
3. Some energy assimilated is lost in heat

2. The Energy in Food Chains
1. Consist of Only a Few Steps
1. Much energy lost at each step
2. Little remains after four successive steps

2. Community Energy Budgets
1. Cole's experimental studies of freshwater ecosystem in upstate New York fig 25.11
2. For each 1000 calories of energy fixed by photosynthesizers
1. 150 calories transferred to small heterotrophs
2. 30 calories of that transferred to smelt
3. 6 calories of that transferred to trout (or humans)
4. 1.2 calories from trout transferred to humans

3. Eating meat to harvest energy is inefficient
4. Organisms that have a vegetarian diet have more food energy available
5. Important concept to feed hungry and overcrowded world

3. Factors Limiting Community Productivity
1. Higher productivity supports longer food chains in theory
2. Amount of sunlight is ultimate limit of a community's productivity
1. Primary productivity of deciduous forests increases as growing season increases
2. NPP higher in warm climates than cold ones
1. Longer growing season
2. More nitrogen available via activity of nitrogen fixing bacteria

3. Ecological Pyramids
1. Less Energy Available at Successive Levels
1. Comparison of values at different levels
1. Plant fixes about 1% of the energy received by its leaves
2. Heterotrophs process 10% of the energy available from their food sources
3. More individuals at lower levels of food chain than higher levels
4. Successive levels have lower biomass
5. Large animals are usually found at higher trophic levels

2. Diagrammatic representations of such relationships form pyramids fig 25.12

2. Inverted Pyramids
1. Some aquatic ecosystems have inverted biomass pyramids
2. Planktonic ecosystems dominated by small organisms floating in water
1. Turnover of lowest level, photosynthetic phytoplankton, very rapid
2. Cannot develop large population size
3. Phytoplankton reproduce rapidly
4. Community supports larger population of heterotrophs fig 25.12b

3. Top Carnivores
1. Loss of energy at each level limits number of top carnivores
1. One-thousandth of energy from photosynthesis passes to third stage
2. No predators for high level organisms, not enough biomass to support it

2. Top-level predators tend to be large animals
3. Small residual biomass concentrated in small number of individuals