Principles of Life
Hillis • Sadava • Heller • Price
Instructor’s Manual
Chapter 46: The Global Ecosystem
OVERVIEW
Chapter 46 explores linkages in the global ecosystem, including biogeochemical
cycling and climate warming. Climate and human impacts on ecosystem function,
especially net primary productivity, are described. After an introduction to global
systems and biogeochemical cycles, the global water, nitrogen, and carbon cycles
are described in detail, including the ways in which human activities impact each
cycle. The next section explains the greenhouse effect and how human activities are
enhancing the effect and causing warming. Some of the impacts of warming are
discussed, including impacts to species’ life cycles and interactions, distributions,
and extinctions. The final section discusses the challenges that humans face in
addressing climate warming.
KEY CONCEPTS/CHAPTER OUTLINE
46.1 Climate and Nutrients Affect Ecosystem Function
• NPP is a measure of ecosystem function
• NPP varies predictably with climate and nutrients
Ecosystems are linked by processes and material movements. Net primary
productivity (NPP) is one aspect of ecosystem function. NPP varies among
ecosystem types, based on climate and nutrient availability. Terrestrial NPP is
related most closely to temperature and moisture. Aquatic NPP is related most
closely to light penetration and nutrient availability.
46.2 Biological, Geological, and Chemical Processes Move Materials
through Ecosystems
• The form and location of elements determine their accessibility to organisms
• Fluxes of matter are driven by biogeochemical processes
Earth is an open system with respect to energy; a constant supply comes from the
sun. It is a closed system with respect to matter; global biogeochemical cycles
describe the movement and transformations of the elements. The different forms and
locations of elements can be represented as compartments. Pools are the total
amounts in a compartment; fluxes are the movement of elements between
compartments. Trophic interactions transform and move elements through
communities.
46.3 Certain Biogeochemical Cycles Are Especially Critical for
Ecosystems
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• Water transports materials among compartments
• Nitrogen is often a limiting nutrient
• Carbon flux is linked to energy flow through ecosystems
• Biogeochemical cycles interact
The global cycling of water is driven by solar-powered evaporation. Water is
essential for life. Water moves materials around the planet and also transports heat
energy. Humans impact the water cycle through land-use changes and climate
warming. The nitrogen cycle involves chemical transformations that are mainly
accomplished by microbial species. Nitrogen fixation is essential to provide living
organisms with forms of nitrogen they can take up and use. Humans fix nitrogen in
energy-intensive industrial processes to use for fertilizers and explosives. Nitrate
runoff from land can result in eutrophication of aquatic ecosystems and dead zones
in offshore areas. Carbon cycling is linked to energy flow through ecosystems.
Photosynthesis and respiration are important processes. The biggest carbon pools are
in fossil fuels and carbonate rocks. Human activities impact photosynthesis and
surface runoff. Fossil fuel burning and deforestation add CO2 to the atmosphere.
Rice cultivation, livestock, and microbial processes increase CH4 in the atmosphere.
The biogeochemical cycles are interconnected by many processes.
46.4 Biogeochemical Cycles Affect Global Climate
• Earth’s surface is warm because of the atmosphere
• Recent increases in greenhouse gases are warming Earth’s surface
• Human activities are contributing to changes in Earth’s radiation balance
Electromagnetic radiation from the sun is mostly in the visible range. Molecules in
Earth’s atmosphere result in the greenhouse effect: the Earth absorbs solar radiation
and re-emits energy as infrared radiation. Some of the infrared radiation is absorbed
by greenhouse gas molecules in the atmosphere (CO2, CH4, N2O), which also reemit infrared radiation, keeping the energy within the Earth system as heat. Direct
measurement, and analysis of gas bubbles in glacier ice, indicate that CO2 and other
greenhouse gases have increased since 1880, concomitant with a rise in global
average temperature. The increasing temperature results in changes to the water
cycle, alteration of precipitation regimes, and increased storm intensity. Human
activities are altering Earth’s radiation balance by increasing greenhouse gas
concentrations and adding black carbon to the atmosphere.
46.5 Rapid Climate Change Affects Species and Communities
• Rapid climate change can leave species behind
• Changes in seasonal timing can disrupt interspecific interactions
• Climate change can alter community composition by several mechanisms
• Extreme climate events also have an impact
Recent climate warming is faster than any changes organisms have experienced in
their past evolutionary histories. Rates of evolution may be too slow to keep up with
the rate of climate change. Timing of life cycle events has evolved to match
environmental cues, and warming is changing many of these cues. Other
environmental cues do not change (such as day length) resulting in mismatch of cues
that can affect species interactions. As populations go extinct or shift their ranges,
new species combinations will result.
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46.6 Ecological Challenges Can Be Addressed through Science and
International Cooperation
Climate change has occurred before in Earth’s history and has sometimes resulted in
mass extinctions. Other organisms have changed Earth’s atmosphere in the past
(e.g., the first photosynthesizers changed oxygen levels). Present climate change is
due to a single species: humans. But science equips us with ways to devise solutions
to problems, and humans have a great capacity for cooperation. Governments have
already cooperated on several global problems (e.g., the Montreal and Kyoto
Protocols). Major challenges include the ever-increasing human population and
economic systems that demand constant growth in spite of Earth’s finite resources.
LECTURE OUTLINE
Chapter 46 Opening Question
How did Keeling’s research contribute to our understanding of the global
ecosystem?
Concept 46.1 Climate and Nutrients Affect Ecosystem Function
Ecosystem—an ecological community plus the abiotic environment with which it
exchanges energy and materials.
Ecosystems are linked by processes and material movements.
It impossible to understand a local ecosystem completely without considering it in
the context of the larger systems of which it is a part.
(VIDEO 46.1 Portrait of a marine ecosystem: A coral reef in the Philippines)
One aspect of ecosystem function:
Net primary productivity (NPP)—rate at which an ecosystem produces primaryproducer biomass.
NPP can be estimated by instruments on satellites that measure wavelengths of light
reflected from the Earth’s surface.
NPP varies among ecosystem types, mostly due to variation in climate and nutrient
availability.
Tropical forests, swamps, and marshlands are the most productive.
Cultivated land is less productive than many natural ecosystems.
FIGURE 46.1 NPP Varies among Ecosystem Types
NPP varies with latitude, as solar input and climate vary with latitude.
Tropics are very productive; high latitudes and dry regions are less productive.
FIGURE 46.2 Terrestrial NPP Corresponds to Climate
Terrestrial NPP tends to increase with temperature and moisture.
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Activity of photosynthetic enzymes increases with temperature (up to the point at
which they denature).
At very high moisture levels, productivity may be inhibited by cloud cover or lack
of oxygen in saturated soils.
FIGURE 46.3 Terrestrial NPP Varies with Temperature and Precipitation
Aquatic NPP is strongly affected by nutrient availability and light penetration.
Nutrients are most abundant in near-shore areas and upwellings.
Hydrothermal vents are productive areas in the deep oceans, where
chemolithotrophs use chemical energy rather than sunlight.
FIGURE 46.4 Marine NPP Is Highest around Coastlines
Concept 46.2 Biological, Geological, and Chemical Processes Move
Materials through Ecosystems
Earth is an open system with respect to energy, but a closed system with respect to
matter.
The sun provides a steady input of energy.
There is a fixed amount of each element of matter, but biological, geological, and
chemical processes can transform it and move it around the planet in
biogeochemical cycles.
(LINK Concept 6.1 Oxidation–reduction (“redox”) reactions)
Different chemical forms and locations of elements determine whether they are
accessible to living organisms.
The different forms and locations can be represented as compartments.
FIGURE 46.5 Chemical Elements Cycle among Compartments of the Biosphere
Pool—total amount of an element or molecule in a compartment.
Flux—movement of an element or molecule between compartments.
All the materials in the bodies of living organisms are ultimately derived from
abiotic sources.
Primary producers take up elements from inorganic pools and accumulate them as
biomass.
Trophic interactions pass the elements on to heterotrophs.
Decomposers break down the dead and waste matter pool into elements that are
available again for uptake by primary producers.
Concept 46.3 Certain Biogeochemical Cycles Are Especially Critical for
Ecosystems
The global water (hydrological) cycle:
Water is essential for life; makes up 70% of living biomass.
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Flowing water is an erosion agent and transports sediment—moves material around
the planet.
Because of high heat capacity, water redistributes heat as it circulates through the
oceans and atmosphere.
(LINK Chapter 2 The molecular nature of water and its crucial role in the evolution
of life as we know it)
FIGURE 46.6 The Global Water Cycle
Solar-powered evaporation moves water from ocean and land surfaces into the
atmosphere.
The energy is released again as heat when water vapor condenses.
(ANIMATED TUTORIAL 46.1 The Global Water Cycle)
Humans affect the water cycle by changing land use:
• Reduced vegetation (deforestation, cultivation, etc.) reduces precipitation retained
in soil and increases amount that runs off.
• Groundwater pumping depletes aquifers, brings water to surface where it
evaporates.
• Climate warming will melt ice caps and glaciers and cause sea level rise and
increased evaporation. Water vapor is a greenhouse gas.
The global nitrogen cycle:
Involves chemical transformations.
N2 gas is 78% of the atmosphere, but most organisms cannot use this form.
Nitrogen fixation: some microbes can break the strong triple bond and reduce N2 to
ammonium (NH4+).
(ANIMATED TUTORIAL 46.2 The Global Nitrogen Cycle)
FIGURE 46.7 The Global Nitrogen Cycle
FIGURE 46.8 Where Does the Extra Nitrogen Come From?
Other microbial species convert ammonium into nitrate (NO3−) and other oxides of
nitrogen.
N-fixing reactions are reversed by yet another group of microbes in denitrification,
which returns N2 gas to the atmosphere.
(LINK Concepts 19.3 and 25.2 The metabolic processes by which microbes
transform nitrogen)
Human activities affect the nitrogen cycle:
• Burning fossil fuels, rice cultivation, and raising livestock releases oxides of
nitrogen to the atmosphere.
• These oxides contribute to smog and acid rain.
• Humans fix nitrogen by an industrial process to manufacture fertilizer and
explosives.
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• Topsoil and dissolved nitrates are lost from farm fields and deforested areas by
wind and water runoff.
• The nitrates are deposited in aquatic ecosystems and result in eutrophication—
increased primary productivity and rapid phytoplankton growth.
Decomposition of the phytoplankton can deplete oxygen; other organisms can not
survive, and dead zones form offshore in summer.
FIGURE 46.9 High Nutrient Input Creates Dead Zones
• Excess nitrogen in terrestrial ecosystems can change plant species composition.
Species adapted to low nutrient levels grow slowly, even when fertilized, and can
be easily displaced by faster-growing species that take advantage of additional
nutrients.
In the Netherlands, this has caused 13% of the recent loss of plant species
diversity.
The global carbon cycle:
Movement of carbon is linked to energy flow through ecosystems; biomass is an
important pool.
The largest pools occur in fossil fuels and carbonate rocks.
Photosynthesis moves inorganic carbon from the atmosphere and water into the
organic compartment; respiration reverses this flux.
(ANIMATED TUTORIAL 46.3 The Global Carbon Cycle)
FIGURE 46.10 The Global Carbon Cycle
Dissolved CO2 in the oceans: some is converted by primary producers, and enters
the trophic system.
Organic detritus and carbonates continually drift down to the ocean floor.
Some organic detritus in ocean sediments is converted to fossil fuels. Carbonates
can be transformed into limestone.
(APPLY THE CONCEPT Certain biogeochemical cycles are especially critical for
ecosystems)
Human activities affect the global carbon cycle:
• Any activity that impacts primary productivity can alter fluxes.
• Runoff brings carbon to aquatic ecosystems.
• Deforestation and fossil fuel burning increase atmospheric CO2.
• Atmospheric CH4 is increased through livestock production, rice cultivation, and
water storage in reservoirs (microbes in water-logged soils produce CH4).
Biogeochemical cycles are interconnected.
If carbon uptake by primary producers increases, uptake of P, N, and other elements
also increases.
If decomposition rates increase, flux of elements back to inorganic compartments
increases.
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Any nutrient can limit biological functions; the limiting one is the one that is in
lowest supply relative to demand.
Biogeochemical cycles can interact in hard-to-predict ways.
Increased atmospheric CO2 can increase water-use efficiency by terrestrial plants;
In a high CO2 environment, the plants have stomata open less, which reduces loss
of water vapor.
Concept 46.4 Biogeochemical Cycles Affect Global Climate
All objects that are warmer than absolute zero emit electromagnetic radiation.
Most of the incoming solar radiation is in the visible range of wavelengths.
Some is absorbed in the atmosphere, some is reflected back to space, and some is
absorbed by the Earth’s surface.
(ANIMATED TUTORIAL 46.4 Earth’s Radiation Balance)
Greenhouse effect:
Earth’s surface re-emits energy in longer, less energetic infrared wavelengths.
Some of this infrared radiation is absorbed by gas molecules in the atmosphere
(greenhouse gases).
The molecules are warmed and radiate photons back to Earth’s surface, keeping the
energy within the Earth system as heat.
FIGURE 46.11 Earth’s Radiation Balance
Greenhouse gases include H2O, CO2, CH4, N2O.
Without the atmosphere, Earth’s average surface temperature would be about 34°C
colder than at present.
Keeling’s measurements from atop Mauna Loa in Hawaii show a steady increase in
CO2 since 1960.
FIGURE 46.12 Atmospheric Greenhouse Gas Concentrations Are Increasing (Part
1)
Analyses of air trapped in glacial ice demonstrate that CO2 and other greenhouse
gases began increasing after about 1880.
Average annual global temperature has also increased.
(APPLY THE CONCEPT Biogeochemical cycles affect global climate)
FIGURE 46.12 Atmospheric Greenhouse Gas Concentrations Are Increasing (Part
2)
FIGURE 46.13 Global Temperatures Are Increasing
Higher global temperatures are affecting climate:
• Hotter air temperatures
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• A more intense water cycle, with greater overall evaporation and precipitation.
• Hadley cells are expected to expand poleward; warmer tropical air will rise higher
and expand farther toward the poles before sinking.
• Precipitation will increase near the equator and at high latitudes and decrease at
mid-latitudes.
• Warming is spatially uneven, so precipitation changes will be season- and regionspecific.
• In general, wet regions are expected to get wetter and dry regions drier.
Precipitation trends in the twentieth century support these expectations.
FIGURE 46.14 Global Precipitation Patterns Have Changed
Warming may also increase storm intensity.
Strong hurricanes (category 4 and 5) have become more frequent since the 1970s.
(VIDEO 46.2 El Niño and global oceanic circulation and temperature)
Human activities affect Earth’s radiation balance:
• Adding greenhouse gases to the atmosphere
• Deposition of dust and dark-colored soot particles (“black carbon”) from fossil
fuel burning increases amount of solar energy absorbed by snow and ice—
increases melting.
• Adding aerosols to the atmosphere increases reflectance of solar energy, less
reaches Earth’s surface.
When all human effects are added to climate models, climate scientists conclude
human activities have contributed significantly to recent climate warming.
Concept 46.5 Rapid Climate Change Affects Species and Communities
Recent warming and other climate changes are far more rapid than anything
organisms have experienced in their evolutionary histories.
Life cycles have evolved so that critical events occur at favorable times of year.
Climate change is altering the timing of environmental cues.
Rates of evolution may be too slow to keep up with an environment that changes too
rapidly.
In the short term, many species seem to be adapting (e.g., trees leaf out earlier in the
spring).
But some species may not respond to climate change or may not be able to continue
adaptive tracking.
Some environmental cues do not change, such as day length, so temporal
relationships among cues are shifting.
There may be timing mismatches among species in a community, which will disrupt
interactions (e.g., hatch of pollinators and opening of flowers).
One documented mismatch:
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In the Netherlands, winter moth eggs hatch too early—before the oak leaves they
feed upon have emerged. The caterpillars starve.
Great tits feed on winter moth caterpillars, but they are not nesting earlier because
their cue is day length; they have lower breeding success.
FIGURE 46.15 Climate Change Affects Life Histories
If populations cannot respond to changing environments, they may go extinct,
resulting in changing species compositions.
Shifts in the geographic distributions can lead to assembly of novel communities.
Species have moved up mountains and towards higher latitudes. Species shift at
different rates or not at all, resulting in different species combinations.
Increased frequency of extreme climate events will also alter species distributions.
A rapid shift in plant community boundaries occurred after a drought in northern
New Mexico: a ponderosa pine forest shrank abruptly and drought-adapted piñon–
juniper woodland expanded by more than 2 km in less than 5 years.
The new community persisted after the drought ended.
Concept 46.6 Ecological Challenges Can Be Addressed through
Science and International Cooperation
Climate has changed in Earth’s past, precipitating five major mass extinctions.
There is precedent for atmospheric changes induced by organisms: the first
photosynthetic microbes increased oxygen concentrations to a level that was toxic
to the anaerobic prokaryotes.
The first land plants caused another rise in oxygen concentrations 250 million
years ago.
Present climate change is due to activities of a single species: Homo sapiens.
But, science equips us to understand the natural world and devise solutions to
problems.
Homo sapiens also has a remarkable capacity for cooperative action.
Governments have cooperated to support large-scale initiatives, such as the IPCC.
International agreements include:
• Montreal Protocol to prevent depletion of UV-absorbing ozone
• Kyoto Protocol to reduce emissions of greenhouse gases
• Convention on International Trade in Endangered Species (CITES), to conserve
species by eliminating international trade.
A major challenge is that economic policies of every nation aim for continual
economic growth—ever-increasing production and consumption of goods and
services—despite the fact that Earth has finite resources.
A related challenge is the continued multiplicative growth of the human population.
On a crowded planet, cooperation becomes more difficult.
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Answer to Opening Question
Dave Keeling was dedicated to developing a long-term record of atmospheric CO2
measurement.
The Mauna Loa record is known as the “Keeling curve.”
His measurements contributed to a better understanding of the pools and fluxes of
the global carbon cycle, including the influence of fossil fuel burning.
Keeling’s results were noticed imediately, and climate scientists began to warn of
the increased greenhouse effect.
Average temperature increased by 0.7°C during the 20th century, an increase very
close to predictions of global climate models.
The IPCC was formed in 1988 by a scientific collaboration of governments. The 4th
assessment report predicts an increase in global average temperature between
1.8°C and 4.0°C by the end of this century.
KEY TERMS
biogeochemical cycles
dead zones
eutrophication
fluxes
fossil fuels
greenhouse effect
greenhouse gases
nitrogen fixation
pool
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