Download Chapter 7

Chapter 7
This lecture will help you understand:
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The nature of systems
Ecosystem-level ecology
Nutrient cycles: C, P, N
The hydrologic cycle
Plate tectonics and the rock cycle
Central Case: The Gulf of Mexico’s “Dead Zone”
• Major fisheries off Louisiana were devastated by die-offs.
• Scientists found large regions of low oxygen in the Gulf.
• The recurring “dead zone” resulted from nitrogen pollution traveling down the
Mississippi River.
Earth’s environmental systems
• Our planet consists of many complex, large-scale, interacting systems.
• System = 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
Feedback loops: Negative feedback
Feedback loop = a circular process whereby a system’s output serves as input to
that same system
In a negative feedback loop, output acts as input that moves the system in the
opposite direction.
This compensation stabilizes the system
Feedback loops: Positive feedback
• In a positive feedback loop, output acts as input that moves the system further
in the same direction.
• This magnification of effects destabilizes the system.
Dynamic equilibrium, homeostasis
• Dynamic equilibrium = when processes in a system move in opposite
directions at equivalent rates so their effects balance out
This can contribute to
• Homeostasis = tendency of a system to maintain constant or stable internal
conditions
• Earth’s climate and an animal’s body are examples of homeostatic systems in
dynamic equilibrium.
Emergent properties
Properties of a whole system not evident in the system’s components
“The whole is more than the sum of its parts.”
A tree is an element of a forest, a sink for CO2, and habitat for birds.
Closed and open systems
• Closed system = isolated and self-contained
• Open system = exchanges energy, matter, and information with other systems
• It is useful to think of Earth as a closed system.
• But any system is open if we examine it closely enough or long enough.
An environmental system
Mississippi River as a system:
• Emergent properties
• Input of water, fish, pollution, etc.
• Output to Gulf of Mexico
Two systems or one?
• The Mississippi River system and the system of the Gulf of Mexico interact.
• Understanding the dead zone requires viewing the Mississippi River and the
Gulf of Mexico as a single system.
• This holistic kind of view is necessary for comprehending many environmental
issues and processes.
Increasing nitrogen inputs
Eutrophication
Key to the dead zone =
Eutrophication: excess nutrient enrichment in
water, which increases production of organic matter...
… which when decomposed by oxygen-using
can deplete water of oxygen.
microbes
Creation of the hypoxic dead zone
Nitrogen input boosts phytoplankton…
…which die and are decomposed by microbes that suck oxygen from water, killing
fish and shrimp.
Earth’s structural spheres
• Lithosphere = rock, sediment, soil below Earth’s surface
• Atmosphere = air surrounding the planet
• Hydrosphere = all water—salt and fresh, liquid, ice, and vapor
• Biosphere = all the planet’s living things, and the abiotic parts of the
environment with which they interact
Ecosystems
• Ecosystem = all the interacting organisms and abiotic factors that occur in a
particular place and time
• Energy and nutrients flow among all parts of an ecosystem.
• Conception of an ecosystem can vary in scale:
small pond
large forest
entire planet
Landscape ecology
• Studies adjacent or interacting ecosystems on a larger geographic scale
• Many animals move between ecosystems; thus they must be studied on the
landscape scale.
• Ecotones = transitional zones where ecosystems meet
Energy in ecosystems
• Energy from sun
converted to
biomass (matter in organisms)
by producers
through photosynthesis
• Rapid conversion = high primary productivity
(coral reefs)
• Rapid plant biomass availability for consumers = high net primary
productivity
(wetlands, tropical rainforests)
Net primary productivity
Different ecosystem types show varying net primary productivities.
Biogeochemical cycles
• Nutrients are elements and compounds that organisms consume and require for
survival.
• Nutrients stimulate production by plants, and the lack of nutrients can limit
production.
• Nutrients move through ecosystems in nutrient cycles or biochemical cycles.
Nutrients
• Macronutrients are elements and compounds required in relatively large
amounts and include nitrogen, carbon, and phosphorous.
• Micronutrients are nutrients needed in small amounts.
The carbon cycle
How carbon (C) moves through our environment
• Producers pull carbon dioxide (CO2) from the air and use it in photosynthesis.
• Consumers eat producers and return CO2 to the air by respiration.
• Decomposition of dead organisms, plus pressure underground, forms sedimentary
rock and fossil fuels. This buried carbon is returned to the air when rocks are
uplifted and eroded.
• Ocean water also absorbs carbon from multiple sources, eventually storing it in
sedimentary rock or providing it to aquatic plants.
The carbon cycle
Human impacts on the carbon cycle
• We have increased CO2 in the atmosphere by burning fossil fuels and
deforesting forests.
• Atmospheric CO2 concentrations may be the highest now than in 420,000 years.
• This is driving global warming and climate change.
The phosphorous cycle
How phosphorus (P) flows through our environment
• P is most abundant in rocks. Weathering releases phosphate (PO43–) ions from
rocks into water.
• Plants take up phosphates in water, pass it on to consumers, who return it to the
soil when they die.
• Phosphates dissolved in lakes and oceans precipitate, settle, and can become
sedimentary rock.
The phosphorous cycle
Human impacts on the phosphorus cycle
The nitrogen cycle
How nitrogen (N) moves through our environment:
• Atmospheric N2 is fixed by lightning or specialized bacteria, and becomes available
to plants and animals in the form of ammonium ions (NH4+).
• Nitrifying bacteria turn ammonium ions into nitrite (NO2–) and nitrate (NO3–) ions.
Nitrate can be taken up by plants.
• Animals eat plants, and when plants and animals die, decomposers consume their
tissues and return ammonium ions to the soil.
• Denitrifying bacteria convert nitrates to gaseous nitrogen that reenters the
atmosphere.
The nitrogen cycle
Human impacts on the nitrogen cycle
• Haber and Bosch during WWI developed the Haber-Bosch process, a way to
fix nitrogen artificially.
• Since then, synthetic nitrogen fertilizers have boosted agricultural production.
• Today we are fixing as much nitrogen artificially as the nitrogen cycle does
naturally.
• We have thrown the nitrogen cycle out of whack.
Human impacts on the nitrogen cycle
Nitrogen and the dead zone
Excess nitrogen flowing down the Mississippi River into the Gulf causes hypoxia,
worse in some regions than others.
Nitrogen and the dead zone
The size of the hypoxic zone in the northern Gulf of Mexico had grown since 1985,
and was largest in 2002.
The hydrologic cycle
How water flows through our environment:
• Water enters the atmosphere by evaporation and by transpiration from leaves.
• It condenses and falls from the sky as precipitation.
• It flows as runoff from the land surface into streams, rivers, lakes, and
eventually the ocean.
• Water infiltrates into aquifers, becoming groundwater, the upper limit of
which is the water table.
The hydrologic cycle
Impacts on the hydrologic cycle
Human activity affects the water cycle. Examples:
• Damming rivers increases evaporation and can cause infiltration of surface
water into aquifers.
• Altering vegetation increases surface runoff and erosion.
• Agricultural irrigation depletes water sources and increase evaporation.
The rock cycle—a key environmental system
Rocks change from one form to another over time.
• Igneous rock = of volcanic origin; cooled magma that may flow across Earth’s
surface as lava
• Sedimentary rock = mineralized sediments
(layers of mud, dust, or sand) formed by lithification
• Metamorphic rock = transformed by extreme heat or pressure
The rock cycle
Plate tectonics
• The process by which plates of crust move across Earth’s surface, atop its
malleable mantle and molten core
• Over millions of years, continents change position.
• Movement = only 2–15 cm (1–6 in) per year
• Plate tectonics underlies earthquakes, volcanoes.
Global map of tectonic plates
Plate boundaries
Tectonic plates can meet in several ways.
Viewpoints: The dead zone
Conclusion
• Physical systems and processes lay the groundwork for how life spreads itself
across the planet.
• They include the hydrologic cycle, rock cycle, and plate tectonics, among others.
• Life interacts with its abiotic environment in ecosystems, through which energy
flows and materials are recycled.
• Human activities are causing significant changes in the ways those cycles
operate.
Conclusion, continued
• Thinking in terms of systems will help us avoid disrupting its processes and
mitigation any disruptions.
• The systems model applied to the Gulf of Mexico can be adapted to many other
environmental issues.
• The natural systems of our planet may provide lessons for future sustainability.