Principles of Life
Hillis • Sadava • Heller • Price
Instructor’s Manual
Chapter 42: Organisms in Their Environment
OVERVIEW
This chapter serves as a broad introduction to the field of ecology and covers a wide
range of topics. The first section introduces the concept of ecosystems and their
complexity and the hierarchy of ecological systems. The next section provides an
overview of the physical geography of Earth, including global patterns of
temperature and precipitation and atmospheric and oceanic circulation.
Biogeography and distribution of species is then related to physical geography and
continental drift. Terrestrial and aquatic biomes are described. The penultimate
section discusses the impacts of human activities on global ecosystems. The final
section describes important tools used in the study of ecosystems.
KEY CONCEPTS/CHAPTER OUTLINE
42.1 Ecological Systems Vary in Space and over Time
• Ecological systems comprise organisms plus their external environment
• Ecological systems can be small or large
• Each ecological system at each time is potentially unique
Ecological systems include biotic (living organisms) and abiotic (nonliving)
components. Ecology is the scientific study of these systems. The hierarchy of
ecological systems goes from the individual to the whole biosphere. Ecological
systems are complex; the microbial community in the human gut is an example.
Ecosystems can change over time.
42.2 Climate and Topography Shape Earth’s Physical Environments
• Latitudinal gradients in solar energy input drive climate patterns
• Solar energy drives global air circulation patterns
• The spatial arrangement of continents and oceans influences climate
• Walter climate diagrams summarize climate in an ecologically relevant way
• Topography produces additional environmental heterogeneity
Physical processes determine broad climatic patterns on Earth. The uneven
distribution of solar radiation results in latitudinal gradients of temperature and
seasonality and drives the major atmospheric circulation patterns. These circulation
patterns determine prevailing winds and global precipitation patterns. Prevailing
winds and placement of continents drive ocean surface currents. Deep ocean
currents result from variation in water density. Oceans and large lakes can moderate
climate. Climate for a given region can be summarized by climate diagrams that
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show average temperature and precipitation over a year. Topography can also
impact climate (e.g., mountains form rain shadows).
42.3 Physical Geography Provides the Template for Biogeography
• Similarities in terrestrial vegetation led to the biome concept
• The biome concept can be extended to aquatic environments
Physical factors, in part, determine the distributions of species on Earth.
Biogeography is the study of species distributions and the underlying causes of the
observed distributions. Biomes are distinct physical environments with ecologically
similar species. Terrestrial biomes are determined largely by temperature and
precipitation patterns and have characteristic vegetation types. Organisms in the
same biome type in different parts of the world have similar adaptations, even
though they may not be related phylogenetically (convergent evolution). The biome
concept is extended to aquatic environments, with salinity, water depth, and current
being the defining characteristics.
42.4 Geological History Has Shaped the Distributions of Organisms
• Barriers to dispersal affect the distributions of species
• The movement of continents accounts for biogeographic regions
• Phylogenetic methods contribute to our understanding of biogeography
Species distributions are also influenced by geologic history and continental drift.
The distribution of phylogenetically related biotas can be understood based on the
configuration and movement of continents and Pleistocene glaciation history. This
was first recognized by Alfred Russel Wallace, whose observations led to the
concept of biogeographic regions. Phylogenetic trees can also be combined with
biogeographic knowledge to further our understanding of modern species
distributions.
42.5 Human Activities Affect Ecological Systems on a Global Scale
• Human-dominated ecosystems are more uniform than the natural ones they replace
• Human activities are simplifying remaining natural ecosystems
• Human-assisted dispersal of species blurs biogeographic boundaries
Human activities are now altering ecosystems on a global scale, leading some to
suggest a new geological period called the “Anthropocene.” Human-dominated
systems, such as urban and agricultural lands, now cover about half of Earth’s land
surface. These systems have fewer species and less complexity than natural
ecosystems. In agriculture, monocultures are planted and diversity of crop species is
low. Natural ecosystems are also impacted by habitat fragmentation, pollution, and
overexploitation of wild species. Humans move species around the globe, which is
causing homogenization of the biotas that evolved over long periods of isolation.
42.6 Ecological Investigation Depends on Natural History Knowledge
and Modeling
• Models are often needed to deduce testable predictions with complex systems
Ecologists use the same methods of scientific inquiry that are used in other fields,
but the complexity of ecosystems makes certain tools particularly useful. Natural
history observations (not part of formal hypothesis-testing investigations) are
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essential to our understanding of ecosystem functioning and are also useful in
formulating questions and hypotheses and designing ecological experiments.
Computer models are useful in modeling the complexity of ecosystems and making
predictions. The models incorporate many natural history observations.
LECTURE OUTLINE
Chapter 42 Opening Question
Why did the rangeland restoration method that worked in Europe fail to work in the
Borderlands?
Concept 42.1 Ecological Systems Vary in Space and over Time
Physical geography—study of the distribution of Earth’s climates and surface
features
Biogeography—study of the distributions of organisms
Abiotic components of the environment—nonliving
Biotic component—living organisms
An ecological system—one or more organisms plus the external environment with
which they interact
Ecology—term coined by Ernst Haeckel in 1866; made it a legitimate scientific
subject and emphasized its relevance to evolution because ecological interactions
drive natural selection.
System—a whole, comprising a set of interacting parts; neither the parts nor the
whole can be understood without taking account of the interactions.
Ecological systems can include any part of the biological hierarchy from the
individual to the biosphere.
Each level brings in new interacting parts at progressively larger spatial scales.
FIGURE 42.1 The Hierarchy of Ecological Systems
Population—group of individuals of the same species that live, interact, and
interbreed in a particular area at the same time.
Community—assemblage of interacting populations of different species in a
particular area.
Ecosystem—community plus its abiotic environment
Biosphere—all the organisms and environments of the planet
(LINK Concepts 15.2 and 15.3 The population is the unit of evolution)
Generally, large ecological systems tend to be more complex and have more
interacting parts.
But small systems can also be complex:
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The human large intestine is densely populated with hundreds of microbial
species.
The gut environment provides stable conditions and ample nutrients.
The microbial species interact with each other and with their environment in many
complex ways.
At any given time, an ecological system is potentially unique.
In the human gut, the microbial species vary from person to person and with diet.
The host’s genotype and diet affect the gut environment from the bacterial point of
view; and the bacteria influence their environment, which includes the host.
Some health disorders may be treatable by manipulating the gut bacterial
community.
(LINK Figure 12.7 Scientists can analyze complex microbial ecosystems by
sequencing DNA present in environmental samples)
FIGURE 42.2 The Microbial Community of the Human Gut Depends on the Host’s
Diet
Concept 42.2 Climate and Topography Shape Earth’s Physical
Environments
Variation in physical environments results from atmosphere and ocean circulation
patterns and geological processes.
Weather—the state of atmospheric conditions in a particular place at a particular
time
Climate—average conditions and patterns of variation over longer periods
Adaptations to climate prepare organisms for expected weather patterns.
Earth receives uneven inputs of solar radiation due to its spherical shape and tilt of
its axis as it orbits the sun.
Subsequent results in temperature variation:
• Air temperatures decrease from low to high latitudes.
• High latitudes experience more seasonality—greater fluctuation over the course of
a year.
FIGURE 42.3 Solar Energy Input Varies with Latitude
FIGURE 42.4 The Tilt of Earth’s Axis of Rotation Causes the Seasons
Solar energy inputs are always greatest in the equatorial region, which drives global
patterns of air circulation.
Hadley cells:
The tropical air is warmed, rises, and then cools adiabatically (an expanding gas
cools).
The rising warm air is replaced by surface air flowing in from the north and south.
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The cooling air sinks at 30°N and 30°S.
FIGURE 42.5 Global Atmospheric Circulation
Other circulation cells form at the mid-latitudes and at the poles.
The circulation patterns influence prevailing winds and precipitation patterns.
Rising warm tropical air releases lots of moisture as rainfall. The sinking air at
30°N and 30°S is dry—most of the great deserts are at these latitudes.
Prevailing winds are deflected by the rotation of the Earth—the Coriolis effect.
FIGURE 42.6 Direction of Prevailing Surface Winds
Prevailing winds drive the major ocean surface currents.
Example: northeast trade winds drive water to the west; when it reaches a continent
it is deflected northward until the westerlies drive the water back to the east.
(VIDEO 42.1 El Niño and global oceanic circulation and temperature)
FIGURE 42.7 Ocean Currents
Deep ocean currents are driven by water density differences.
Colder, saltier water is more dense and sinks to form deep currents.
Deep currents regain the surface in areas of upwelling, completing a vertical ocean
circulation.
Oceans and large lakes moderate climate because water has a high heat capacity.
Water temperature changes slowly as it exchanges heat with the air.
Poleward-flowing ocean currents carry heat from the tropics toward the poles,
moderating climate at higher latitudes.
Example: the Gulf Stream warms northern Europe.
Climate diagram—superimposed graphs of average monthly temperature and
precipitation throughout a year.
The axes are scaled so that precipitation is adequate for plant growth when the
precipitation line is above the temperature line.
The growing season occurs when temperatures are above freezing and there is
enough precipitation.
(APPLY THE CONCEPT Climate shapes Earth’s physical environments)
FIGURE 42.8 Walter Climate Diagrams Summarize Climate in an Ecologically
Relevant Way
Earth’s topography also influences climate.
As you go up a mountain, air temperature drops by about 1°C for each 220 m of
elevation.
When prevailing winds bump into mountain ranges, the air rises up, cools, and
releases moisture. The now-dry air descends on the leeward side.
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This results in a dry area on the leeward side, called a rain shadow.
(ANIMATED TUTORIAL 42.1 Rain Shadow)
FIGURE 42.9 A Rain Shadow
Topography also influences aquatic environments:
Flow velocity depends on slope.
Water depth determines gradients of many abiotic factors, including temperature,
pressure, light penetration, and water movement.
Concept 42.3 Physical Geography Provides the Template for
Biogeography
Organisms must be adapted to their physical environments.
For example, a plant that has no means of conserving water cannot thrive in a desert.
Species are found only in environments they can tolerate.
(LINK Concept 28.3 Some adaptations of plants to challenging climatic conditions)
Early naturalist–explorers began to understand how the distribution of Earth’s
physical environments shapes the distribution of organisms.
Their observations revealed a convergence in characteristics of vegetation found in
similar climates around the world.
Biome—a distinct physical environment inhabited by ecologically similar organisms
with similar adaptations.
Species in the same biome in geographically separate regions display convergent
evolution of morphological, physiological, or behavioral traits.
(ANIMATED TUTORIAL 42.2 Biomes)
Terrestrial biomes are distinguished by their characteristic vegetation.
Distribution of terrestrial biomes is broadly determined by annual patterns of
temperature and precipitation.
These factors vary along both latitudinal and elevational gradients.
FIGURE 42.10 Temperature and Precipitation Gradients Determine Terrestrial
Biomes
FIGURE 42.11 Global Terrestrial Biomes
Other factors, especially soil characteristics, interact with climate to influence
vegetation.
Example: Southwestern Australia has Mediterranean climate with hot, dry summers
and cool, moist winters. The vegetation is woodland/shrubland, but no succulent
plants are here.
The soils are nutrient-poor, and there are frequent fires. Succulents are easily killed
by fires.
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FIGURE 42.12 Same Biome, Different Continents (Part 3)
The biome concept is also applied to aquatic environments.
Aquatic biomes are determined by physical factors such as water depth and current,
temperature, pressure, salinity, and substrate characteristics.
TABLE 42.1 Major Aquatic Biomes
The primary distinction for aquatic biomes is salinity: freshwater, saltwater, and
estuarine biomes.
Salinity determines what species can live in the biome, depending on their ability to
osmoregulate.
In streams, current velocity is important. Organisms must have adaptations to
withstand flow.
Current also impacts the substrate—whether rocky, sandy, silty, etc. Substrate also
determines what species are present.
Still-water biomes (lakes and oceans) have zones related to water depth.
Nearshore regions (littoral or intertidal) are shallow, impacted by waves and
fluctuating water levels. Distinct zonation of species is common.
Photic zone—depth to which light penetrates; photosynthetic organisms are
restricted to this zone.
(VIDEO 42.2 Portrait of a marine ecosystem: A coral reef in the Philippines)
FIGURE 42.13 Water-Depth Zones
Aphotic zone is too deep for light penetration.
Benthic zone—lake or ocean bottom
Water pressure increases with depth. Organisms in the deepest oceans (abyssal
zone) must have adaptations to deal with high pressure, low oxygen, and cold
temperatures.
Concept 42.4 Geological History Has Shaped the Distributions of
Organisms
Alfred Russel Wallace studied species distributions in the Malay Archipelago and
observed dramatically different bird faunas on two neighboring islands, Bali and
Lombok.
The differences could not be explained by soil or climate.
FIGURE 42.14 Wallace’s Line
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He suggested that the deep channel between the islands would have remained full of
water (and a barrier to movement of terrestrial animals) during the Pleistocene
glaciations when sea level dropped.
Thus, the faunas on either side of the channel evolved mostly in isolation over a long
period of time.
Wallace’s observations led him to divide the world into six biogeographic regions.
They contain distinct assemblages of species, many of which are phylogenetically
related.
Many of the boundaries correspond to geographic barriers to movement: bodies of
water, extreme climates, mountain ranges.
FIGURE 42.15 Movement of the Continents Shaped Earth’s Biogeographic Regions
Boundaries of some biogeographic regions are related to continental drift.
Example: southern beeches (Nothofagus) are found in South America, New Zealand,
Australia, and some south Pacific islands.
The genus originated on the southern supercontinent Gondwana during the
Cretaceous and was carried along when Gondwana broke apart.
FIGURE 42.16 Distribution of Nothofagus
Biotas of the seven biogeographic regions developed in isolation throughout the
Tertiary (65 to 1.8 mya), when extensive radiations of flowering plants and
vertebrates took place.
(LINK Figure 18.12 The movements of the continents over geological history)
Continental movement has recently eliminated some barriers, allowing biotic
interchange.
Examples: when India collided with Asia about 45 mya, and when a land bridge
formed between North and South America about 6 mya.
(LINK Concept 16.1 Review effects of barriers to dispersal on speciation)
Biogeographers use phylogenetic information, along with the fossil record and
geological history, to study modern distributions of species.
Geographic areas are superimposed on phylogenetic trees. The sequence and timing
of splits in the phylogenetic tree are compared with sequence and timing of
geographic separations or connections.
(APPLY THE CONCEPT Geological history has shaped the distributions of
organisms)
Concept 42.5 Human Activities Affect Ecological Systems on a Global
Scale
Human activities are altering ecological systems on a global scale.
Some have suggested we are entering a new geological period, the “Anthropocene.”
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We are changing the distributions of organisms, vegetation, and topography, as well
as Earth’s climate.
Human-dominated ecosystems, such as croplands, pasturelands, and urban
settlements now cover about half of Earth’s land area.
These ecosystems have fewer interacting species and are less complex.
In agricultural lands, monocultures replace species-rich natural communities.
Diversity of crops planted is also very low: 19 crops comprise 95% of total global
food production.
Agricultural systems are more spatially and physically uniform than natural
ecological systems.
FIGURE 42.17 Human Agricultural Practices Produce a Uniform Landscape
Human activities also reduce complexity in natural ecosystems:
• Damming and channelization of rivers
• Pollution and habitat fragmentation
• Overexploitation of wild species
• Introductions of new species
(VIDEO 42.3 Portrait of a rainforest ecosystem: Amazon of South America and
other locations)
(VIDEO 42.4 The exotic invasive plant kudzu, Pueraria lobata)
Humans move species throughout the globe, sometimes deliberately, sometimes
inadvertently.
Human-assisted biotic interchange is homogenizing the biota of the planet, blurring
the spatial heterogeneity in species composition that evolved during long periods
of continental isolation.
Concept 42.6 Ecological Investigation Depends on Natural History
Knowledge and Modeling
Natural history—observation of nature outside of a formal, hypothesis-testing
investigation—provides important knowledge about ecosystems.
These observations are often the source of new questions and hypotheses and aid in
design of ecological experiments.
Computer models are important tools in the study of ecosystems.
Natural history knowledge is used to build these models.
Example: rangeland grasses are affected by other plants and herbivores, soil fertility,
climate, and fire. To predict the effect of removing cattle to bring back grasses, all
these interactions must be known.
Answer to Opening Question
Grasslands in different parts of the world differ in significant ways.
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Grasslands occupy a range of climatic conditions from moist to very arid.
In Europe and eastern North America, pastures were once forests; “resting the
range” and managing herd size can restore much of the ecosystem.
Temperate grasslands of midwestern North America, Eurasia, South America, and
African savannas have long histories of evolution with grazing mammals.
It is not clear why removal of cattle has not restored grasslands in the U.S.–Mexico
Borderlands; it is an area of active research.
(VIDEO 42.5 Portrait of a grassland ecosystem: The Serengeti Plains of Africa)
FIGURE 42.18 Harmonious Grazers
KEY TERMS
abiotic
abyssal zone
adiabatically
aphotic zone
benthic zone
biogeographic regions
biogeography
biome
biosphere
biotic
biotic interchange
climate
climate diagram
community
currents
ecological system
ecology
ecosystem
Hadley cells
intertidal
landscapes
limnetic
littoral zone
natural history
pelagic zone
photic zone
physical geography
population
seasonality
topography
weather
zone
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