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42
Organisms in Their
Environment
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
Concept 42.1 Ecological Systems Vary in Space and over Time
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
Concept 42.1 Ecological Systems Vary in Space and over Time
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.
Concept 42.1 Ecological Systems Vary in Space and over Time
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
Concept 42.1 Ecological Systems Vary in Space and over Time
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
Concept 42.1 Ecological Systems Vary in Space and over Time
Generally, large ecological systems tend to be
more complex and have more interacting parts.
But small systems can also be complex:
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.
Concept 42.1 Ecological Systems Vary in Space and over Time
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.
Figure 42.2 The Microbial Community of the Human Gut Depends on the Host’s Diet (Part 1)
Figure 42.2 The Microbial Community of the Human Gut Depends on the Host’s Diet (Part 2)
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.
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
The cooling air sinks at 30°N and 30°S.
Figure 42.5 Global Atmospheric Circulation
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
Figure 42.7 Ocean Currents
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
Figure 42.8 Walter Climate Diagrams Summarize Climate in an Ecologically Relevant Way
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
This results in a dry area on the leeward side,
called a rain shadow.
Figure 42.9 A Rain Shadow
Concept 42.2 Climate and Topography Shape
Earth’s Physical Environments
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.
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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.
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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.
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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.
Figure 42.12 Same Biome, Different Continents (Part 1)
Figure 42.12 Same Biome, Different Continents (Part 2)
Figure 42.12 Same Biome, Different Continents (Part 3)
Figure 42.12 Same Biome, Different Continents (Part 4)
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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.
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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.
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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.
Figure 42.13 Water-Depth Zones (Part 1)
Figure 42.13 Water-Depth Zones (Part 2)
Concept 42.3 Physical Geography Provides the Template for
Biogeography
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
Concept 42.4 Geological History Has Shaped
the Distributions of Organisms
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.
Concept 42.4 Geological History Has Shaped
the Distributions of Organisms
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
Concept 42.4 Geological History Has Shaped
the Distributions of Organisms
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 (Part 1)
Figure 42.16 Distribution of Nothofagus (Part 2)
Concept 42.4 Geological History Has Shaped
the Distributions of Organisms
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.
Concept 42.4 Geological History Has Shaped
the Distributions of Organisms
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.
Concept 42.4 Geological History Has Shaped
the Distributions of Organisms
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, Ch. 42, p. 837
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.”
We are changing the distributions of organisms,
vegetation, and topography, as well as Earth’s
climate.
Concept 42.5 Human Activities Affect Ecological Systems
on a Global Scale
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.
Concept 42.5 Human Activities Affect Ecological Systems
on a Global Scale
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
Concept 42.5 Human Activities Affect Ecological Systems
on a Global Scale
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
Concept 42.5 Human Activities Affect Ecological Systems
on a Global Scale
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.
Concept 42.6 Ecological Investigation Depends on
Natural History Knowledge and Modeling
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.
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.
Figure 42.18 Harmonious Grazers
Answer to Opening Question
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.