CHAPTER 7 - Environmental Systems and Ecosystems Ecology
I. Central Case: The Gulf of Mexico= s A Dead Zone@
A. In 2002, the dead zone grew to its largest size ever - 22,000 square km (8,500 square miles).
B. The dead zone is a region in the Gulf of Mexico so depleted of oxygen that it cannot support marine organisms, a condition called hypoxia.
C. The change from productive fishery to dead zone has occurred in the last 30 years. The spread of the hypoxic zone threatens the Gulf= s fishing industry, one of the most productive fisheries in the United States.
D. Scientists studying the dead zone have determined that fertilizer runoff from midwestern farms is a major cause.
E. Other important causes included urban runoff, industrial discharge, fossil fuel combustion, and municipal sewage.
II. Earth= s Environmental Systems
A. Systems show several defining properties.
1. A system is a network of relationships among a group of parts, elements, or components that interact with and influence one another through the exchange of energy, matter, and/or information.
2. Systems receive input, process it, and produce output.
3. Sometimes a system= s output can serve as input to that same system in a circular process called a feedback loop.
a. In a negative feedback loop, output driving the system in one direction acts as input that moves the system in the other direction.
b. In a positive feedback loop, the output drives the system further toward one extreme.
4. The inputs and outputs of a complex natural system often occur simultaneously, keeping the system constantly active. If they are in balance, it is called a dynamic equilibrium.
5. Processes in dynamic equilibrium contribute to homeostasis, where the tendency of the system is to maintain stable internal conditions.
6. It is difficult to fully understand systems by focusing on their individual components because systems can show emergent properties, characteristics that are not evident in the system= s components.
7. Systems rarely have well-defined boundaries, so deciding where one system ends and another begins can be difficult.
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8. A closed system is isolated and self-contained, having no interactions with other systems. This does not occur in nature but is a useful way to begin considering a simple version of a system.
9. An open system exchanges energy, matter, and information with other systems. All systems on Earth are open systems.
B. Understanding the dead zone requires considering Mississippi River and Gulf of Mexico systems together.
1. Hypoxia in the Gulf of Mexico stems from excess nitrogen from the Mississippi River watershed.
2. Excess nutrients are present in runoff from fertilized agricultural fields, animal manure, crop residues, sewage, and industrial and automobile emissions.
3. The nutrients reach the Gulf, where they boost the growth of microorganisms; this provides food for bacterial decomposers, which flourish.
4. The decomposers use the oxygen in the water; other organisms, such as fish and shrimp, suffocate and die.
5. The process of nutrient enrichment, algal bloom, bacterial increase, and ecosystem deterioration is called eutrophication.
C. Environmental systems may be perceived in various ways.
1. The atmosphere is comprised of the air surrounding our planet.
2. The hydrosphere encompasses all water in surface bodies, underground, and in the atmosphere.
3. The lithosphere is everything that is solid earth beneath our feet.
4. The biosphere consists of the sum total of all the planet= s living organisms, or biotic components, and the abiotic portions of the environment with which they interact.
A. Ecosystems are systems of interacting living and nonliving entities.
1. An ecosystem describes all interacting organisms and abiotic factors that occur in a particular place at the same time.
2. Energy for most ecosystems is input from the sun and is converted to biomass by producers through photosynthesis.
3. Most materials cycle within an ecosystem.
4. Ecosystem outputs include energy (often as heat), water, and waste products from plants and animals.
B. Landscape ecologists study geographic areas with multiple ecosystems.
1. Landscape ecology is the broad-scale study of geographical areas that include multiple ecosystems.
2. An ecotone is a transitional zone where two or more ecosystems meet; it contains some elements from each ecosystem.
C. Energy is converted to biomass.
1. Autotrophs capture the sun= s energy through photosynthesis. This is gross primary production.
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2. After autotrophs use some of their acquired energy for their own metabolism, the remainder is used to generate biomass. This amount is the net primary production.
3. The rate at which biomass is generated is called productivity. Ecosystems with rapid biomass production have high net primary productivity.
D. Nutrients can limit ecosystem productivity.
1. Nutrients are elements and compounds that organisms consume and require for survival.
2. In natural ecosystems, some nutrients always run off land into oceans. This nutrient input causes high primary productivity in nearshore waters along continents.
3. When human activities, such as farms, cities, and industry, increase this nutrient load, then eutrophication and hypoxia often occur. This creates dead zones.
IV. Biogeochemical Cycles
A. Nutrients circulate through ecosystems in biogeochemical cycles.
1. Nutrients move through the environment in cycles called nutrient cycles or biogeochemical cycles.
2. Most nutrients travel through the atmosphere, hydrosphere, and lithosphere and from one organism to another, moving between pools, or reservoirs, remaining in a reservoir for a residence time.
3. Nutrient movement between reservoirs is called flux; the rates of flux can change over time. Flux typically involves negative feedback loops that promote dynamic equilibrium.
4. Human activity has changed some flux rates.
B. The carbon cycle circulates a vital organic nutrient.
1. Carbon is an ingredient in carbohydrates, fats, proteins, bones, cartilage, and shells of all living things.
2. The carbon cycle describes the routes carbon takes through the environment.
3. Through photosynthesis, producers pull carbon dioxide out of the atmosphere to produce oxygen and carbohydrates.
4. During respiration, producers, consumers, and decomposers break down carbohydrates to produce carbon dioxide and water.
C. Humans are shifting carbon from the lithosphere to the atmosphere.
1. As we mine fossil fuel deposits and cut or burn vegetation, we remove carbon from reservoirs and increase the flux into the atmosphere.
2. This ongoing flux of carbon into the atmosphere is a major force behind global climate change.
D. The phosphorus cycle involves mainly lithosphere and ocean.
1. The element phosphorus is a key component of DNA, RNA, ATP, and cell membranes.
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2. Phosphorus is primarily found in rocks, soil, sediments and oceans; the weathering of rocks releases phosphates into water at a very low rate of flux.
3. The phosphorus cycle has no appreciable atmospheric component.
4. Concentrations of available phosphorus in the environment are very low; this is often a limiting factor for producers.
E. We affect the phosphorus cycle.
1. We mine rocks for phosphorus to make fertilizers, and our sewage discharge and agricultural runoff are high in phosphates.
2. These additions to the available reservoir of phosphorus in water and soil can cause rapid increases in biomass and eutrophication and hypoxia in waterways.
F. The nitrogen cycle involves specialized bacteria.
1. Nitrogen makes up 78% of the atmosphere and is the sixth most abundant element on Earth.
2. The nitrogen cycle involves chemically inert nitrogen gas, which most living organisms cannot use. This makes the atmosphere the major reservoir for nitrogen.
3. Lightning, highly specialized bacteria, and human technology are the only ways to fix nitrogen into compounds usable by the living organisms.
4. Nitrogen is frequently a limiting factor for producers and therefore limits populations of consumers, including humans.
5. There are two ways that inert nitrogen gas becomes A fixed@ so that plants can use it - nitrogen fixation and nitrification.
a. Nitrogen fixation occurs through lightning or nitrogen-fixing bacteria that live in mutualistic relationships with many leguminous plants.
b. Nitrification occurs through specialized free-living bacteria.
c. Denitrifying bacteria converts nitrates in soil or water to gaseous nitrogen.
G. Humans have greatly influenced the nitrogen cycle.
1. Nitrogen fixation has always been a bottleneck, limiting the flux of nitrogen out of the atmosphere reservoir.
2. The Haber-Bosch process allows humans to synthesize ammonia, accelerating its flux into other reservoirs within the cycle.
3. Burning forests and fields and fossil fuels all increase the amount of atmospheric nitrogen, as does bacterial decomposition of animal wastes from feedlots.
4. Strategies to limit the amount of damage caused by excess nutrients in the waterways are not successful at this time.
H. The hydrologic cycle influences all other cycles.
1. The oceans are the main reservoir, holding 97% of all water on Earth. Less than 1% of planetary water is usable by humans.
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2. Water moves into the atmosphere via evaporation and transpiration. It returns to the surface as precipitation, most of which flows into water bodies as runoff.
3. Some precipitation and surface water soaks down to recharge underground reservoirs known as aquifers. This is groundwater, and its depth underground is the water table.
I. Our impacts on the hydrologic cycle are extensive.
1. We have dammed rivers and created reservoirs, increasing evaporation.
2. We have changed vegetation patterns, increasing runoff and erosion and decreasing recharge.
3. Irrigation, industry, and other human uses have depleted aquifers and increased evaporation.
4. Atmospheric pollutants have changed the chemical nature of precipitation.
5. Water shortages are giving rise to conflicts throughout the world and are predicted to increase.
V. Geological Systems: How Earth Works
A. The rock cycle is a fundamental environmental system.
1. Over geological time, rocks are heated, cooled, broken down, and reassembled in the rock cycle.
2. Rocks that form when magma cools are called igneous rock. When magna is spewed from a volcano, it is known as lava.
3. Sedimentary rock is formed when dissolved minerals seep through sediment layers and crystallize and bind particles together in the process called lithification.
4. When great heat or pressure is exerted on rock, it is transformed into metamorphic rock; marble and slate are examples.
B. Plate tectonics shapes Earth= s geography.
1. Earth= s surface consists of the crust, mantle, and core. Earth= s internal heat causes the mantle to flow, pushing rock up and down.
2. In the distant past all the landmasses on the plates joined into a supercontinent called Pangaea.
3. New crust is formed where two plates are pushed apart at divergent plate boundaries.
4. When two plates meet, they may slip and grind alongside one another, forming a transform plate boundary.
5. When two plates collide at convergent plate boundaries, uplift or subduction can result.
A. Approaching questions holistically by taking a systems approach is helpful in environmental science.
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B. The case of the Gulf of Mexico= s hypoxic zone provides evidence that systems thinking can lead to solutions.
C. Renewable solar energy, nutrient recycling, dynamic equilibrium, and negative feedback loops are all hallmarks of unperturbed ecosystems that have survived the test of time.
D. Our industrialized civilization is very young in comparison; we should consider taking lessons in sustainability from these older, successful ecosystems.
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